Chem 116 Lab Manual 3rd Edition

8/20/2019 Chem 116 Lab Manual 3rd Edition

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C L M

E A, P

I C – E

L.E. Laverman


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C L ME A, P I C – E

C

I 1

C AL

Unknown KHP Determination 11

Determination o Mn in Steel - Standard Additions 14

Determination o Mn in Steel - Volumetric Analysis 16

Determination o Cu and Pb in Brass - Chelation Ion Chromatography 20

Determination o Cu and Pb in Brass- Atomic Absorbance Spectroscopy 23

Determination o Pb in Brass- Gravimetric Analysis 25

Determination o Cu in Brass - Anodic Stripping Voltammetry 27

C BL

Synthesis and Characterization o Ruthenium Polypyridyl Complexes 30

Luminescence Quenching, Stern-Volmer Kinetics and Marcus Electron ranser Teory 37Construction and esting o Solid State Oxygen Sensors 46

C CL

Synthesis and Characterization o Mn(salpn) 54

Synthesis and Kinetics Studies o Wilkinon’s Catalyst 59

Synthesis and Characterization o CdSe Nanocrystals 68

Synthesis and ransient Absorbance Measurement o FeII(PPS)CO 72

Appendix – Working with Excel


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Lab notebooks

Orderly record keeping is essential in a laboratory. A well kept notebookmakes writing lab reports much easier. It also helps when trying totroubleshoot problems that may arise during an experiment. In anindustrial setting a laboratory notebook is a legal record o researchactivities. Inventions and ideas that are not properly documentedmay not hold up in patent litigation. A portion o your grade will bebased on your lab notebook. Some points to note are given below.

• Write legibly

• Your notebook should have printed numbered pages

• Use tables or data when appropriate

• Each page should be dated and signed

• Mistakes should be crossed out with a single line, initialed anddated

• Observations should be made as you go

• Changes to procedures should be noted

• Write legibly

Te lab notebook is not meant to be a picture perect record o whatyou did. Many students take 'notes' on scrap paper with the intento transering the data to their notebooks later. Tis practice shouldbe avoided at all times. Copies o the relevant notebook pages willbe turned in along with your report. A duplicating page notebookis preerable to photocopies. Te notebooks used or chem. 1A aresuffi cient.

Safety/Hazardous WasteIn any laboratory saety is paramount. ake note o the location osaety showers, eye wash stations and re extinguishers when enteringthe lab. Saety glasses are required and must be worn at all times inthe laboratory, no exceptions, ever. Lab coats are also recommended.Lab coats protect your clothes and offer an extra layer o protectionbetween you and the reagents you are working with. Te bookstoresells goggles and lab coats at a reasonable cost.

All chemical waste should be disposed o in a properly labeled waste

C ABCI


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container. I you ll up a waste container it is your responsibility to goto the stockroom and obtain a replacement. Be sure the label is corrector your experiment. Accidents can happen i wastes are improperlymixed. For example, in chemistry 116AL many samples are digestedin nitric acid. Nitric acid is an oxidizer and should never be mixedwith organic materials. Violent and explosive reactions may occur iwastes are mixed improperly.

Additional waste containers and labels are available in the

undergraduate stockroom. Do not leave solutions out in the lab. I asolution is lef out between lab periods it will be disposed.

Lab reports

Lab reports are the written synthesis o the work that you perormedin the laboratory. An outsider skilled in chemistry should be able toread your report and understand what you did, why you did it andwhat you discovered. You will nd that good written and verbalcommunications skills are keys to a successul career in the chemical

sciences. Tere are many ways to ormat and present data romlaboratory work. Te ormat depends upon the intended audience. Areport may be in the orm o a publication in a journal or your Ph.D.thesis in a research laboratory. Industrial reports may consist oprogress reports, patent applications or analytical results or a client.

When writing your lab reports, envision yoursel as graduatestudent writing up your research or publication in a journal. Te labreport should be divided into logical sections according the outlinediscussed below. Details on what should be included in each sectionare listed. As with all scientic reports, it should be written in pasttense (since you presumably are writing the report afer you did thework) and with passive voice. Grammatically, passive voice is a bit

awkward, however it lends objectivity to the report. For example,one may choose to write “I ound the weight percent to be 0.59%”.It is not important that you as an indivual made the measurement.Properly phrased one would simply write “Te weight percent wasound to be 0.59%”. Avoid the use o words such as “I” or “we”, theexperimental result should not depend on the experimentalist. I onlyone individual is capable o getting a certain result, then the result hasno meaning. Te ACS Style Guide is a great resource or scienticwriting in the chemical literature.1

Your reports should be prepared on a word processor. Spellingand grammar count. Additional gures and calibration curves not

included in the main lab report along with copies o your lab notebookshould be added in an appendix. Label your gures and tables. Keepin mind that the reader may not be amiliar with your experiment.Te ollowing sections should appear in your lab reports.

Experiment Title:

A short description o the experiment is suffi cient

1. “Te ACS Style Guide: A Manual or Authors and Editors” , 2nd Ed., Edited byJanet S. Dodd (ISBN 0-8412- 462-0)

Read the label beoreadding your waste to acontainer!

Even though we will beworking in groups, youare responsible or writing

your own unique labreports.


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Name:

Your name and the name o your group members. Identiy yourselas the primary author.

Sample ID:

For example, KHP 01-03 (Tis is only needed in Chemistry 116AL)

Experimental Methods:

Tis section describes the physical work that you perormed in the lab.Since the procedures are already written or you it is not necessary tore-write them in detail. Te exception is when you deviate rom thepublished method. Ofen times seemingly trivial points such as addingB to A instead o adding A to B are critical in explaining anomalousresults. You should write the procedure in brie orm without anyexperimental details. For example on may write “Carbonate reesodium hydroxide was prepared and standardized by titration withdried KHP using a phenolphthalein endpoint”.

Results:

Tis section contains, like the title suggests, your results and that’s it.What it does not contain is an in depth discussion o what the resultsmean. You should include data tables o raw data where appropriate(see table 1 below), calibration plots and gures (spectra, diagramsetc…). I you pooled data with group members, indicate which databelongs to you and which is rom your lab partners. Te resultssection is where you will report analytical results with condenceintervals and values you determined such as rate constants.

I the experiment analyzes a value by multiple methods, breakthe results into subsections according to analyte then method. Asentence such as “Te weight percent o KHP in sample # 01-03 wasound to be 25.28 ± 0.13 at 95% condence by titration with standardNaOH.” should be at the end o each subsection when appropriate.It is sometimes useul to put your results into a table. Tis makesit easy or the reader (or grader) to nd important values. ablesalways contain a caption above the table itsel. Grid lines should belimited to separations between headers (and ooters) and the data.Te table below illustrates some o these ideas. I you are unsure ohow to ormat a data table consult a style guide or reer to journals orexamples. An example table is shown on the ollowing page.


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able 1. Sodium hydroxide standardization results.

Mass KHP(g)

Vol NaOH (mL)

[NaOH](mol L-1)

rial 1 0.5426 28.04 0.09476

rial 2 0.6137 31.58 0.09514

rial 3 0.5973 30.63 0.09547

rial 4* 0.5011 25.83 0.09497

rial 5* 0.4973 25.16 0.09679

Final Result (± 95% C.I.) [NaOH] = 0.0954 ± 0.0001

* data rom lab partner

Figures are an effi cient way to present numerical data in an easy to visualize ormat. It is import to ormat your plots so that the data isclearly and easily identied. Te choice o onts, symbols and sizes othese eatures should be well proportioned. Use your best judgementhere. As with tables, reerence to a style guide or journals or examplesis a good way to learn how to do this. All gures should be captionedbelow the gure.

Figure 1. Calibration curve or zinc determined by AAS.

Discussion:

Tis section is a chance to comment on the experiment as a whole.In other words, what do the results signiy? What conclusionsor predictions can be made based on your results. I you obtainedinconsistent or anomolous results this is you chance to attempt toexplain why. Do your results agree with literature values?


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Te square o the standarddeviation is called the

variance.

Te RSD is also known asthe coeffi cient o variation.

References:

All reerences should be listed as endnotes according the AmericanChemical Society style.

Statistical Analysis of Data and Reporting Results

Presented below are useul ormulas and tables or the statisticalanalysis o data in the Chemistry 116 lab course. Reer to youranalytical chemistry textbook or in depth discussions.

Mean ( x )

Te arithmetic mean (or average) is calculated according to Eq. 1.1below,

1

n

i

i

x

xn

(1.1)

where x i are the values o the individual data points and n is the totalnumber o data points. Tis is the value that is to be reported as theresult or unknown data analysis along with appropriate condenceintervals.

Standard Deviation ( s )

Te standard deviation measures the how closely the individual datapoints are clustered around the mean. A large standard deviationrelative to the magnitude o the mean indicates a high level o noise.In experimental measurements we attempt to reduce the standarddeviation to the smallest possible value. Te standard deviation iscalculated according to Eq. 1.2

2

1

( )

1

n

i

i

x x

sn

(1.2)

I one could measure an innite set o data points we would have

a measure o the population mean (µ) and the population standarddeviation (σ ). As we increase the number o data points, x approachesµ and s approaches σ . Te value n – 1 in equation 1.2 is called thedegrees o reedom (DOF) or a sample set and is used when calculatingcondence intervals (see below). Standard deviations are sometimesreported as relative standard deviations (RSD) and are calculated as apercentage o the mean as in Eq. 1.3

100

s RSD

x

(1.3)


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Ofen data are simply reported as the mean plus or minus thestandard deviation, x ± s. Reporting the proper number o signicantgures is sometimes ambiguous. For example a data set may returna mean o 264 and a standard deviation o 18. One may report 2.64× 102 ± 0.18 × 102. However, since the standard deviation is in thesecond decimal o our mean the third signicant gure in the meanlooses signicance. In practice the number o signicant gures varies depending upon the intended use o the data. I 2.64 × 102

± 0.18 × 102

is the nal result or a series o measurements one mayreasonably report 2.6 × 102 ± 0.2 × 102 as a result. I however, your value is intended to be used in urther calculations it is desirable toretain more signicant gures than are “technically correct” in orderto avoid round-off errors. Generally in a series o calculations all othe decimal points (i.e. as many as your calculator holds) are useduntil the nal result is reached, at which time the proper number osignicant gures is determined and reported.

Probabilities and Reporting Results

When we report a result as x ± s we gain some insight into the noiselevel or a given set o measurements. What we seek in a measurementis in act the value o µ and σ . Since it is unreasonable to make aninnite set o measurements, we would like to express our mean ( x )in terms o probabilities that the value lies within a certain interval othe population mean (µ). Te condence interval is a value that statesthat the population mean lies within a certain interval relative to oursample mean with a given probability (certainty). For example, wemay report that the population mean (µ) lies within ±0.2 o 2.6 with95% certainty. In other words, we are 95% sure that the true value issomewhere within the range o 2.4 to 2.8. Te condence interval iscalculated according to Eq. 1.4

ts

xn

(1.4)

where x and s are the sample mean and standard deviation, n is thenumber o data points and t is the appropriate value o the student’s tat the desired condence at n – 1 degrees o reedom. An abbreviatedtable o student’s t values is given in table 2.

able 2. Selected values o Student’s t .

DOF 95% 99%

1 12.706 63.6572 4.303 9.9253 3.182 5.8414 2.776 4.6045 2.571 4.0326 2.447 3.7077 2.365 3.5008 2.306 3.3559 2.262 3.250


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Calibration Curves

Instrumental analysis methods involve a signal being measuredin response to an analyte o interest. For quantitative analysismeasurements the value o the signal must be proportional, preerablelinearly, to the amount o analyte present. One must measure theresponse o an instrument relative to known concentrations o analyteover an appropriate concentration range. It is not logical to calibrate

a response on the molar scale when the analyte in an unknownsample is in the milli or micromolar region. Solutions with knownconcentrations o analytes are called standard solutions or simplystandards. Most instrumental methods rely upon measurementso some electrical signal, e.g. current rom a photomultiplier tubeor potential o an electrode. Modern electronics allow us to makeaccurate and precise measurements o current, voltage or resistanceeasily.

Te limiting actor or many analytical measurements is theaccuracy o the standards. Careul preparation o standards isparamount in quantitative analysis. Poorly prepared standards

cannot be expected to give accurate results. A typical procedure orpreparing standards is to accurately weigh a quantity o a substancewith known composition. Note here that moisture can greatly alterthe composition o a substance. Te known mass is then dissolvedin a volumetric ask, diluted to the mark and thoroughly mixedresulting in a stock solution that can be used to prepare additionalstandards. Subsequent standard solutions are prepared by takingknown volumes (aliquots) o this stock solution and diluting them in volumetric asks. Many different concentrations can be prepared inthis manner. Volumetric pipettes are typically used to deliver known volumes o liquids. Beakers and graduated cylinders are not accurateor precise and should never be used in quantitative analysis.

Measurements o instrument response are made or each standardsolution. Each standard should be measured several times i timepermits. Te instrument response is then plotted as a unction oanalyte concentration resulting in a calibration curve. An examplecalibration curve is shown below in gure 2. Te error bars on eachdata point are the standard deviation o several measurements ateach known concentration. Te linear least squares t to the data isgiven along with the 99% condence intervals or the data. Note thatthe most accurate region is in the central portion o the calibrationrange. Extrapolations beyond the calibration range can result inunacceptable condence intervals.


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Figure 2. Example calibration curve. Te linear least squares t(solid), and the 99% condence intervals (dashed) or the data areshown.

Concentrations o analytes in unknown solutions can be determinedusing the slope and intercept values calculated rom the linear leastsquares analysis o the calibration data according to Eq.1.5

y b

xm

(1.5)

where x is the concentration o the analyte in an unknown solution, yis the instrument response or an unknown solution, and m and b arethe slope and intercept o the calibration curve respectively.

Condence intervals should be reported or results determinedusing a calibration curve. Te procedure can be quite lengthy to doby hand but can easily be incorporated into a spreadsheet. Students

are strongly encouraged to take advantage o the calculating power ospreadsheets when applicable. Te uncertainty in the x value rom acalibration curve (sx) is calculated according to Eq. 1.6.

s s

m k n

y y

m x x x

y

i

| |( )

( )

1 1 2

2 2

(1.6)

Here s y is the standard deviation o the y values, |m| is the absolute value o the slope, k is the number o replicate measurements o theunknown, n is the number o calibration points, x is the mean valueo x or the calibration points, x i are the individual calibration data x

values and y is mean value o y or the calibration points and y is themean y value o the unknown.

Te standard deviation o the y values (s y ) is calculated accordingto Eq. 1.7

2

2

i

y

d s

n

(1.7)

where d i is the y deviation o a data point rom the best t line and nis number o calibration points. Note two degrees o reedom are lostin determining the slope and intercept and thereor the denominator


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is n – 2. Te y deviations are calculated according to Eq. 1.8

( )i i id y mx b (1.8)

where x i and y i are the x and y values or a calibration data point.

Te calculated standard deviation in the x value is shown graphicallyin gure 3.

Figure 3. An unknown sample (open circle) is plotted on thecalibration curve. Te resulting uncertainty is shown graphically(vertical dashed lines).

Te condence interval or the unknown x value can be calculatedaccording to 1.9 where the student’s t value is taken rom n – 2 degreeso reedom.

x x ts (1.9)

Unknown results obtained using a calibration curve should bereported according to Eq. 1.9.

Standard Additions

Te method o standard additions is used when matrix effects roman unknown may interere with the determination o an analyte. Inthis method standards are prepared in which a constant amount ounknown has been added to each solution, see gure 4 below. Teresponse o each solution is measured and plotted in the usual way.

Te signal at zero added analyte is due entirely to the unknownanalyte and the sample matrix. Extrapolation to the x-intercept givesthe concentration o analyte in the unknown as shown in gure 4.Te x-intercept is calculated according to 1.10.

xb

I m

(1.10)


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A

B

C

Figure 4. Steps to prepare solutions or standard additions. (A) Addan aliquot o unknown to several volumetric asks e.g. 10 mL (B)Add various amounts o standard solution to all the asks but one (C)dilute to the mark and mix the solutions well.

Figure 5. Calibration curve or the method o standard additions.Te concentration o added analyte is plotted vs. instrument response.

Results obtained by standard additions should be reported with95% condence intervals. Te condence interval is calculatedaccording to Eq. 1.9 above, however the standard deviation o the xintercept (sx ) is calculated using Eq. 1.11 rather than equation 1.6.

s s

m n

y

m x x x

y

i

| | ( )

1 2

2 2 (1.11)


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Introducton

Volumetric analysis is a quantitative method in which an unknownamount or concentration is calculated by reaction with a knownamount o reagent. Tis type o analysis requires accuratemeasurements o delivered volume. Commonly available 50 mLburets are graduated in 0.1 mL increments. With practice, you shouldbe able to interpolate between the marks and read to the nearest 0.01mL. Te volume is read at the bottom o the meniscus o the liquidin the buret as shown in the gure below. An index card with a smallpiece o black electrical tape held behind the buret with the tape justbelow the meniscus aids in reading the volume. Your eye should beat the same level as the meniscus to avoid parallax errors. Do not

stand on chairs or stools to read burets. Tis a dangerous practice andshould be avoided. I you are not tall enough to read the buret at eyelevel, simply lower the buret over the edge o the lab bench while youread the volume.

32

33

Figure 1. Te reading o the buret above is 32.64 mL.

Clean you buret thoroughly beore use and rinse with your titrant.When water is drained rom a clean buret no drops should adhere tothe sides. A ew mL o solution should be drained rom the buret aferlling to remove any air pockets in the tip. Rinse the tip with distilledwater rom a wash bottle and record the volume beore beginning yourtitration. Te tip should be rinsed again into the Erlenmeyer askcontaining the titration solution beore recording the nal volume.

A drop o aqueous solution is approximately 0.05 mL. Sometimesless than a ull drop o titrant is required as the endpoint is approached.Partial drops can be delivered by orming a small droplet on the tipo the buret and rinsing the droplet into your Erlenmeyer ask witha wash bottle. It is also good practice to rinse down the sides o theash periodically during a titration with a small amount o waterrom a wash bottle.

U KHP D


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Preparation of NaOH solution

Preparing solutions o NaOH with known concentration is diffi cultbecause solid NaOH is very hygroscopic and absorbs CO2 romthe atmosphere. Tis makes it impossible to obtain an accuratemass reading under normal laboratory conditions. Tereore, it isnecessary to standardize NaOH against a primary standard acid suchas potassium hydrogen phthalate (KHP).

Carbon dioxide reacts with NaOH in solution to orm carbonateion (CO3

2-), a weak base. Carbonates are insoluble in concentratedNaOH so stock solutions o 50% NaOH can be used to preparecarbonate ree NaOH. Distilled water contains dissolved CO2 thatmust be removed. CO2 is removed by boiling distilled water or5-10 minutes and cooling. While you are preparing the carbonateree NaOH solution, add ~3 grams o potasium hydrogen phthalate(KHP) standard to a weighing bottle and your unknown KHP sampleto another weighing bottle and place them in a 110°C oven or uselater (dry your samples or 2 hours). Work in groups o two or thenext step. Each student should boil at least 1 liter o distilled water.You may choose to boil an extra liter o water at this time so that you

don’t run out during the titrations that ollow. One member o thegroup will transer 3-4 mL o 50% NaOH to one liter o cooled waterbeing careul not to disturb any carbonate precipitates at the bottomo the concentrated base. Te resulting solution will be approximately0.1 M NaOH. ranser the NaOH solution to a labeled polyethylenebottle and mix thoroughly. Te other group member will transertheir cooled water to a labeled glass bottle with a tight seal.

Standardization of NaOH solution

KHP is dried at 110°C and stored in a desiccator to prevent the

solid rom absorbing moisture. When perorming titrations usingKHP, phenolphthalein is suitable as an endpoint indicator. Tephenolphthalein endpoint is the rst pale pink color that persists or30 seconds. Rinse and then ll a clean 50 mL buret with the NaOHsolution prepared above (~0.1 M). Prepare a titration blank by addinga two drops o phenolphthalein indicator to 100 mL o water andtitrating to the endpoint as demonstrated by your A. Repeat theblank titration two additional times. Subtract the average volumerequired to titrate the blanks rom the subsequent titrations o NaOHsolution.

Weigh at least three 0.7 – 0.8 gram samples o dry KHP and placethem in three labeled Erlenmeyer asks. Record the mass to the

nearest 0.1 mg. Add 50 mL o reshly boiled and cooled water to oneo the KHP samples, dissolve the solid completely and add two dropso phenolphthalein indicator solution. itrate to the phenolphthaleinendpoint with your NaOH solution. Repeat this procedure with theremaining two KHP samples. Calculate the molarity o the NaOHsolution. Report the concentration with a 95% condence interval.

Caution! 50% NaOHis highly caustic, avoidcontact with your skin.

Place the boiled solutionsin an ice bath to speed upthe cooling process.

NaOH reacts with glassand solutions o NaOHshould not be storedin glass containers orextended periods o time.

KHP

F.W. 204.233 g mol-1


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Unknown KHP Determination

Your unknown KHP sample contains KHP and a neutral diluent.Using volumetric analysis you will determine the mass percento KHP in your unknown solid sample. Weigh three appropriatesize samples o your unknown KHP solid and add to three labeledErlenmeyer asks. Record the mass to the nearest 0.1 mg. Add 50 mLo reshly boiled and cooled distilled water to one o your unknown

samples, dissolve the solid and add two drops o phenolphthaleinindicator solution. Dissolve only one sample at a time to reduce theinuence o carbon dioxide. itrate to the phenolphthalein endpoint.Calculate the mass percent o KHP in your unknown sample.

Report your values to 0.01% with 95% condence intervals.

Te titration volumeshould be ~40 mL.

Unknowns labeled “LO”should use 2.9-3.1 gram

samples. Unknownslabeled “HI” should use

1.6-1.8 gram samples.


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Introduction

Steel is an alloy o iron with small amounts o transition metals suchas Mn, Cr, Cu etc.. Steel is digested in hot concentrated (4-5 M) nitricacid and analyzed or transition metals by a variety o techniques.Upon digestion manganese is converted to the colorless Mn2+ ion. Anoxidant such as the periodate anion is added to oxidize Mn2+ to theamiliar deep purple MnO4

– ion (Eq. 1 below). Te concentration othe MnO4

– ion is then quantitatively detected by visible spectroscopy.Other metals may interere with the analysis o Mn by this methodand must be removed or masked. Te primary constituent o steel isiron which can be masked by the addition o phosphoric acid (H 3PO4)orming a colorless phosphate complex in aqueous solution. Othermetals such as chromium and cerium can be oxidized with iodate

orming potentially interering colored complexes. o account orthese potential interering ions, the method o standard addition isused.

2+ - - +

4 2 3

-

42Mn + 5 IO + 3H O 5 IO + 6 H 2MnO (1)

Steel Digestion

Weigh a ~1 g sample o steel and transer it to a 250 mL beaker. Recordthe mass to the nearest 0.1 mg. Add 50 mL o 4 M HNO3 to the beaker

and boil gently or a ew minutes or until the sample is dissolved.Keep your sample covered with a watch glass during the digestionprocess to prevent loss o material through splattering. Slowly add 1.0gram o ammonium peroxydisulate [(NH4)2S2O8] and boil or 10 to 15minutes to oxidize any carbon in the sample. I your sample is pink orcontains a brown precipitate at this point, add ~0.1 grams o sodiumbisulte (NaHSO3) and heat or an additional 5 minutes. Allow thesolution to cool to room temperature and quantitatively transer thesolution to a 250 mL volumetric ask. Dilute with distilled water tothe mark.

Preparation of Standard Mn Solution

Dissolve ~ 100 mg o Mn metal in 10 mL o 4 M HNO 3 and boil ora several minutes to remove the nitrogen oxides that are generated.Record the mass o Mn to the nearest 0.1 mg. Quantitatively transerthe solution to a 1 L volumetric ask and dilute to the mark withdistilled water. Each group in the class should prepare one standardMn2+ solution.

D M SS A


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Standard Additions

ranser a 20 mL aliquot o steel solution to a 250 beaker usinga volumetric pipette. Add 5 mL o 85% phosphoric acid. Addaliquots o standard Mn2+ and solid KIO4 to the beaker accordingto table 1. Boil each solution or 5 minutes and allow to cool toroom temperature. Quantitatively transer the solutions to 50 mL volumetric asks and dilute to the mark. Measure the absorbance o

each o the purple solutions using the colorless blank as a reerence.Record the absorbance at λmax or the permanganate ion. Your A willdemonstrate the proper use o the UV-Visible spectrometer. A ploto absorbance vs. the concentration o added standard Mn2+ yieldsa straight line with an x intercept equal to the concentration o Mnrom the steel unknown. Te maximum absorbance or any solutionshould not exceed a value o 1.0.

Te extinction coeffi cient o permanganate1 at 525 nm is 2455M–1·cm–1. A ew simple calculations ahead o time can help preventmistakes in solution concentrations.

Table 1. Estimated sample volumes to use when preparing calibra-tion standards.

Sample Steel H3PO4 Std. Mn KIO4

1-blank 20 mL 5 mL 0 mL 0 g2 20 mL 5 mL 0 mL 0.4 g3 20 mL 5 mL 1 mL 0.4 g4 20 mL 5 mL 2 mL 0.4 g5 20 mL 5 mL 3 mL 0.4 g6 20 mL 5 mL 4 mL 0.4 g

7 20 mL 5 mL 5 mL 0.4 gReer to the discussion o the method o standard additions in

the lab manual introduction or in your analytical textbook. Youshould report the 95% condence interval using equation 1.11 in theintroduction.

Report the percent by mass of Mn in your steel sample to 0.01% alongwith the 95% condence interval.

1. Kalbus, G.E.; Lieu, V..; Kalbus, L.H., J. Chem. Ed., 2004,100

Te amount o steelsolution will vary

depending on the amounto Mn present in your

sample. Te absorbanceo the sample withoutadded Mn should be

approximately 0.1 a.u.


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1

Introduction

Tis method involves digestion o steel samples in hot nitric acidollowed by the selective oxidation o Mn ions to MnO4

– with sodiumbismuthate (NaBiO3) at reduced temprature. An excess o errousions are added to the sample which react with MnO4- according tothe redox reaction below.

- 2+ + 3+

4 2

2+MnO + 5 Fe + 8 H Mn + 5 Fe + 4 H O

(1)

Te excess errous ions are then back titrated with a standard solutiono potassium permanganate (KMnO4). Permanganate solutions arehighly oxidizing and react with any carbonaceous matter in solution

causing uncertainties in the concentration. Concentrated stocksolutions o permanganate will be provided or you. It is necessaryto lter these solutions to remove any precipitate o MnO2 that mayhave ormed prior to dilution. Te dilute solution o permanganateis then standardized against the primary standard sodium oxalate.Permanganate reacts slowly with oxalate at room temperature so thetitration must be carried out above 70°C. Te endpoint is signaled bythe persistence o a pale pink color due to the presence o unreactedpermanganate.

Required Solutions• 1:30 (v:v) HNO3 – free from nitrous acid

• 1:3 (v:v) HNO3

• 1:20 (v:v) H2SO4

• 0.9 F H2SO4 (150 mL of 6 F H2SO4 in 1.0 L)

• 0.01 M KMnO4 (see below)

• 0.03 M Fe2+ (see below)

Day 1

• Prepare all required solutions

• Standardized the KMnO4 solution

1. (a) Adapted rom Quantitative Inorganic Analysis, I.M. Kolthoff, E.B. Sandell,Te Macmillan Co., 1947, 709-710. (b) W. Blum, J. Am. Chem. Soc., 34, 1912,1395 (c) G.E.F. Lundell, J. Am. Chem. Soc. 45, 1923, 2600 (d) .R. Cunningham,Coltman, R.W., Ind. Eng. Chem., 16 , 1924, 58.

o prepare nitrous acid ree HNO3 careully boilconcentrated nitric acid orone to two minutes in the

ume hood. Allow the acidto cool beore dilution tothe desired concentration.

D M SV A


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Day 2

• Digest and workup steel unknown

• Standardize Fe2+ solution

• Titrate unknown

Work in groups o two or this experiment. Each student

will perorm all o the titrations indicated. You may pool yourstandardization results i you desire.

Preparation of standard KMnO4 solution

Dry ~0.5 grams o sodium oxalate at 105°C or one hour. Filter 100mL o ~0.1 M KMnO4 solution through a clean sintered glass lter. Donot use paper to lter the solution. Organic material is oxidized bythe permanganate ion. ranser the solution to a 1000 mL volumetricask and dilute to the mark with distilled water. Mix the solution well.

Weigh out several 100 mg samples o dried sodium oxalate andtranser to 250 mL beakers. Add 100 mL o 0.9 F H2SO4 and heat to80-90°C while stirring. It is not necessary to boil the solutions. Fill aburet with the permanganate solution. Te meniscus is diffi cult to readdue to the dark color o permanganate solutions. A light held behindthe buret will aid in volume readings. Record the initial volume osolution in your buret then add one drop o permanganate to thesolution oxalate while you are heating. When the color disappearsand all o the solid is dissolved you may begin the titration. itratethe oxalate solution until a pale pink color persists. Te reaction opermanganate with oxalate proceeds according to the ollowingreaction

- 2- +

4 2 4 2 2

2+2 MnO + 5 C O + 16 H 2 Mn + 10 CO + 8 H O (2)

Calculate the concentration of MnO4- in this solution. Tightly stop-

per the solution and store in your drawer until the next lab period.

The endpoint is very sharp in this titration so take care not to overti-

trate!

Preparation of Standard Ferrous Ammonium Sulfate

SolutionAdd ~12 grams o errous ammonium sulate hexahydrate(Fe(NH4)2(SO4)2 · 6 H2O) to a 1000 mL ask and dissolve in 1:20suluric acid. Dilute to the mark with 1:20 suluric acid. Mix well,tightly stopper the solution and store in your drawer until the nextsession. ake care to use the correct reagent or this step. Read thelabel! ranser 25 mL o 1:30 nitric acid to a 250 mL Erlenmeyerask with a volumetric pipette. Add 25 mL o the errous ammoniumsulate solution to the ask with another volumetric pipette. Fill a

Sodium oxalate is toxic.Handle careully and clean

up any spills promptly.Wash your hands

thoroughly afer handling.

Te color o the endpoint isvery similar in appearance

to the phenolphthaleinendpoint.


22/8818

buret with the previously standardized KMnO4 solution and titrate toa pale pink endpoint. Repeat this procedure several times. Calculatethe concentration o Fe2+ in the errous ammonium sulate solutionusing the stoichiometric relationships in equation 1.

Preparation of Steel Unknown Sample

Weigh a 1.0 gram o your steel unknown and transer to a 250 mLbeaker (record the mass to 0.1 mg). Do not dry the steel unknown.Add 50 mL o nitrous acid ree 1:3 HNO3 to the beaker in the umehood (do not use 1:30 HNO3 or this step!) Cover the beaker with awatch glass and gently boil or 5-10 minutes. All o the solid shouldbe dissolved. Remove the sample rom the heat and careully add~0.5 grams o NaBiO3 solid to the solution. Tis preoxidation stepremoves any carbonaceous reductants rom the solution. Return thesolution to the heat and boil or an additional 5 minutes. Te solutionshould turn purplish and/or a brown precipitate should appear. Inot, add an additional 0.2 grams o NaBiO3. Note, Te color changeis subtle so watch the solution careully. Do not add more than one

additional aliquot o NaBiO3. Remove the solution rom the heat andadd dropwise a solution o saturated sodium sulte (NaSO3) until thepurple tint and/or precipitate disappears. Return to the heat and boilan additional 5 minutes. Te solution should be rust orange in colorand contain no precipitates at this point. Allow the solution to coolor several minutes. Careully place the solution in an ice bath andcool until the temperature is 15°C. Add ~0.7 grams o NaBiO3 andstir or 5 minutes. A purple color should develop and solid NaBiO3 should remain in the beaker.

- + - 3+

3 4 2

2+ 2 Mn + 5 BiO + 14 H 2 MnO + 5 Bi + 7 H O

Wash a sintered glass lter with 1:30 nitric acid. Discard theltrate in the proper waste container. Filter your unknown solutioninto a clean lter ask. A olded paper towel placed so that the askis slightly tilted away rom the sidearm will help keep the solutionrom being pulled into the suction hose. Rinse the NaBiO3 solid withseveral 1–2 mL aliquots o cold 1:30 nitric acid. Add 3–4 mL o 85%phosphoric acid (H3PO4) to the ltrate and mix well. Quantitativelytranser the solution to a 100 mL volumetric ask. Rinse the lterask with several small portions o 1:30 nitric acid and add to the

volumetric ask. Dilute to the mark with additional 1:30 nitric acid.

Nitric acid generates NO(g) when oxidizing metals.NO is a reactive and toxic

gas and should only be generated in a ume hood.

85% phosphoric acid isvery viscous. Use a 10mL graduated cylinderto measure the volume oacid.


23/8819

itration of Steel Unknown

ranser 25 mL o the errous ammonium sulate solution to a cleanErlenmeyer ask using a volumetric pipette. ranser 25 mL othe steel unknown solution to the ask with a volumetric pipette.Te purple color will disappear as it enters the solution and reactswith the errous ions. Back titrate this solution with the standardpermanganate solution. Te endpoint is reached when a pale pink

color persists in solution.

Calculations

Tere are several steps in this procedure and good notebook practiceswill acilitate the many calculations required. Te Mn content othe unknowns varies rom approximately 0.10% to 1.00% All othe required stoichiometric relationships are given in the aboveinstructions. Reer to your analytical text or discussions o backtitrations.

Report the percent by mass of Mn in your steel sample to 0.01% alongwith the 95% condence interval.

Be sure to use the correctstoichiometry when goingthrough your calculations


24/8820

Introduction

Chelation ion chromatography can be used to determine trace amountso heavy metals in g/L concentrations. Te instrument used or thistype o chromatography is an HPLC system. Te column in thissetup consists o an ion exchange resin rom which transition metalsare separated using complexing eluents. An acidic solution o oxalicacid is used as the eluent or the determination o Pb2+, Cu2+, Cd2+,Mn2+, Co2+, Zn2+ and Ni2+. Te eluted metals are quantied by indirectspectrophotometric detection. Afer elution the stream is mixed witha solution containing (PAR). Te ligand changes color rom orangeto red when bound to transition metals metal. Tis technique is calledpost-column derivatization and allows or sensitive detection o traceamount o a variety o transition metals.

Preparation of Pb2+ Stock Solution

Add a weighed amount o Pb shot to a 150 mL beaker (do not dry thelead in an oven). In a ume hood add 20 mL o distilled water and 20mL o concentrated nitric acid. Cover with a watch glass and boilgently until the metal has dissolved and the solution is colorless. Ia white precipitate orms, cool the solution and add 20 mL o water.Remove rom the heat and allow to cool slightly. Rinse the watchglass and the sides o the beaker with a small amount o distilled waterand boil or an additional 10 minutes. Allow the solution to cool andquantitatively transer it to a 1000 mL volumetric ask. Te amount

o lead added should yield a nal concentration o lead o ~500 ppm(mg Pb2+ / L).

Preparation of Cu2+ Stock Solution

Weigh a ~0.5 gram sample o copper oil and digest with 50 mL odilute HNO3 (3 M) in a ume hood. Record the mass o copper tothe nearest 0.1 mg. Handle the copper sheet with gloves to avoidngerprints and do not dry in an oven beore weighing. Gentlyboil the solution or 10 minutes afer the last o the copper has beendissolved and the solution is blue and transparent. Keep the beakercovered with a watch glass and do not let the solution evaporate todryness. Cool the sample to room temperature and quantitativelytranser the solution to a 1000 mL volumetric ask. Dilute to themark with distilled water.

PAR

D C P B

C I C - HPLC


25/8821

Preparation of Unknown Brass Stock Solution

Do not dry the brass beore weighing. Weigh a 1.0 gram sample obrass and add it to a 150 mL beaker. Record the mass to the nearest 0.1mg. Add 10 mL o water ollowed by 15 mL o concentrated nitric acidin a ume hood. Afer the evolution o gas slows, add an additional10 mL o water. Heat the solution and continue to careully boil thecovered solution or an additional 20 minutes afer all o brass has

dissolved. Te solution will be reduced in volume but do not allow it toevaporate to dryness. Allow the solution to cool to room temperature.A uffy white precipitate may be visible in the blue solution. Teprecipitate is hydrated stannic oxide (H2SnO3). Tis precipitate may becollected with ashless lter paper and ignited to constant compositionor gravimetric determination o tin. Careully lter the solutionthrough a cone o lter paper into a clean beaker. Te ltration stepmay take a long time so plan accordingly. Quantitatively transer thesolution to a 500 mL volumetric ask.

Calibration Method

Using the stock solutions o lead and copper create calibrationsolutions containing both lead and copper. Te calibration solutionsshould contain both lead and copper over the range o 0 to 50 ppm.You will use these solutions to calibrate the instrument response. Tedetector or the HPLC does not obey Beer’s law when the absorbancemaximum is much above 1.0. I your higher calibration standardsexceed this level you will need to prepare additional solutions at lowerconcentration. Mix all o your diluted solutions well.

ranser a small amount o one o the standard solutions to a cleanand dry beaker. Fill the injection syringe several times with thissolution to thoroughly rinse the syringe and needle. Fill the syringe

with 1 mL o solution and slowly load the sample loop on the injection valve as instructed by your .A (when changing solutions you maywant to repeat this several times to thoroughly rinse the sampleloop). ake care not to inject air bubbles into the HPLC. Initiate thechromatographic analysis with the instrument sofware. A typicalchromatogram o a standard sample is shown below in Figure 1. Your.A. will demonstrate how to operate the HPLC sofware. You maywish to include a blank o distilled water to check or backgroundmetal contamination i time permits. You have one our hour labperiod to complete the calibration and analysis o your unknowns.Plan your HPLC injections accordingly.

Make sure to rinse the lter paper and precipitate

thoroughly with distilledwater.

Run each standard once,then your unknowns. You

may repeat injectionso the standards and

unknowns i time permits.


26/8822

Figure 1. Chromatogram showing the separation o lead and copperusing chelation ion chromatography.

Most o the brass samples contain zinc ions which elute at a muchlonger retention time than lead or copper. For runs with brass use thelonger method. Your A will demonstrate how to load new methodson the instruement. Completed chromatogram will automatically beprinted as a PDF le with the peak areas and heights listed.

Analysis

Construct a calibration curve by plotting the response vs. theconcentration o metal. Using linear regression methods determinethe slope and intercept or this data. Determine the concentrationo copper and lead in your injected unknown sample. Calculate themass percent o lead and copper in your unknown brass sample. Usethe equations given in the lab manual introduction to determinethe appropriate condence intervals. Te standard deviation o the

result or your unknown solution is ound by using equation 1.6 inthe introduction. Be sure to convert this to a relative deviation todetermine the deviation o the weight percent.

Report your results for the weight percent of copper and lead to0.01% along with 95% condence limits.

Use the peak area orcopper and peak height orlead i the lead peak is notresolved rom the copper.Te brass samples containa disproportionate amounto copper so accurate

peak areas or lead are

sometimes diffi cult toobtain.


27/8823

Introduction

Flame ionization atomic absorbance spectroscopy (AAS) consists oprobing electronic transitions o atoms in the gas phase. Samplesare atomized, introduced into a gas stream o acetylene and airand passed to a burner (Figure 1). Te resulting ame (2500K)decomposes the sample leaving atoms in the gas phase. Because theelectronic transitions o atoms are very sharp (bandwidth ~0.001nm), conventional lamps and monochromators cannot be used toprobe absorbance. Hollow cathode lamps containing the analyte ointerest are used as light sources with narrow bandwidth and correctenergy. Te light is passed through the ame and changes in intensityare monitored as the sample is introduced. Absorbance is dened as

log I o /I . We dene I o as the intensity o light reaching the detectorwhen a blank solution is being introduced to the ame. Te signal Iis measured when a sample containing the analyte is present and theabsorbance is calculated.

A B

C

D

I o

I

Figure 1. Schematic o a ame ionization atomic absorbancespectrometer. (A) hollow cathode lamp (B) acetylene ame (C) sampleintroduction / atomizer (D) monochromator/detector assembly.

Beer’s law holds or AAS just as it does in other spectroscopicmethods. A plot o absorbance vs. concentration should result in astraight line that can be used to calibrate the instrument response.Te calibration curve is then used in the determination o unknownanalyte concentrations.

Solution Preparation

Prepare stock solutions o copper, lead and brass as described in theprocedure or HPLC. Te HPLC method and AAS method requiredsimilar concentration ranges or calibration so you may use the samestandards and dilutions or both experiments. Adjust the dilution othe brass unknown so that the concentration o Cu and Pb all withinthe instrument calibration limits.

D C P B

A A S


28/8824

AAS Procedure

Te AAS has separate hollow cathode lamps or copper and lead.Monitor the absorbance o lead at 283.3 nm. Copper should bemonitored at 324.8 nm. Fill labeled test tubes with your calibrationsolutions and unknowns and place them in the autosampler. YourA will demonstrate how to use the instrument and sofware. I yourcalibration curve is not linear, rotate the ame so that the effective light

path is smaller. Repeat the analysis. Te sofware will automaticallycreate a calibration curve and report the concentration o analytes inyour unknown solutions.

Analysis

Construct a calibration curve by plotting the average absorbance vs. the concentration o metal. Include error bars or each point(standard deviations). Using linear regression determine the slopeand intercept or this data. Determine the concentration o Pb andCu in your unknown sample using the calibration curves.

Report your results for the weight percent of copper and lead to0.01% along with 95% condence limits.


29/8825

1

Introduction

Gravimetric analysis is one o the oldest analytical techniques. Itremains a very accurate method o analysis when a large sample isavailable. Modern techniques require much less sample but may notbe as accurate. Under the proper conditions, lead may be separatedquantitatively rom other metals as an insoluble sulate. Precipitationo lead sulate is accomplished by the addition o concentrated suluricacid. Copper and zinc sulates are soluble and do not coprecipitatewith lead. Te method is airly straightorward with two caveatsto the experimentalist. Te solubility o lead sulate increases withtemperature and is dependant upon the weight percent o suluric acidin solution (gure 1). In addition, PbSO4 is somewhat soluble in nitricacid. Evaporation o the solution to umes o SO3 effectively removes

nitric acid rom solution reducing the solubility o lead sulate.

Figure 1. Solubility o PbSO4 in suluric acid as a unction o wt%and temperature.2

1. Adapted rom Quantitative Inorganic Analysis, I.M. Kolthoff, E.B. Sandell, TeMacmillan Co., 1947, 697-7002. Data rom H.D. Crockord and D.J. Brawley J. Am. Chem. Soc., 56, (1934),2600

D P BG A


30/8826

Brass Digestion

Do not dry the brass beore weighing. Weigh three ~1.0 gram sampleso brass and add them to three 250 mL beakers. Record the mass to thenearest 0.1 mg. Add 10 mL o water ollowed by 15 mL o concentratednitric acid in a ume hood. Afer the evolution o gas slows, add anadditional 10 mL o water. Heat the solution and continue to careullyboil the covered solution or an additional 20 minutes afer all o

brass has dissolved. Te solution should be reduced in volume butdo not allow to evaporate to dryness. Allow the solution to cool toroom temperature. A uffy white precipitate may be visible in theblue solution. Careully lter the solution through a cone o lterpaper into a clean beaker. Add 4 mL o concentrated suluric acid tothe ltrate and heat the solution to umes o SO3. Cool the solutionand rinse down the sides o the beaker with distilled water. Returnto the heat and bring to umes o SO3 again. Cool the solution toroom temperature and dilute with 25 mL o distilled water. Heat thesolution to nearly boiling then add 50 mL o distilled water. Allow thesolution to stand at room temperature or one hour.

PbSO4 Collection

While you are digesting your brass samples prepare three ltercrucibles or later use. Add enough Celite to rom a layer on thebottom o the crucible about 5 mm deep. Rinse the Celite with dilutenitric acid ollowed by distilled water. Place the crucibles in a 200oCoven and allow to dry to constant mass. Store and cool the cruciblesin a desiccator. When the crucibles have reached a constant mass (besure to record the mass in your notebook), lter the brass solutionsbeing sure to transer all o the lead sulate solids. Wash the ltratethoroughly with cold 1:20 (v:v) suluric acid. Place the crucible in a

200°C oven and dry to constant mass. Calculate the weight percento lead rom the mass o lead sulate and the mass o the unknownsample.

Report your results for the weight percent of lead to 0.01% along withthe 95% condence interval.

Te volume at this pointshould be less than 50 mL

Celite is a ltering aidmade o diatomaceousearth


31/8827

Introduction

Anodic stripping voltammetry (ASV) is a useul technique ordetermination o very low levels o metals in solution. Te analyteso interest are concentrated at the working electrode by reductionin stirring solution that can last up to 30 minutes. Te voltage issubsequently swept towards more positive potential and the amounto current that ows during the oxidation waves is proportional to theamount o analyte in solution.

Figure 1. Anodic stripping voltammograms o copper at variousconcentrations using a mercury lm electrode.

A hanging mercury drop electrode (HMDE) is ofen used toacilitate a reproducible amount o analyte that is reduced. Mercuryorms amalgams with many other metals and is a very goodelectrode material or ASV. We will be using a modied mercurylm electrode (MFE) or our experiments. A MFE is constructed byreducing mercury ions to mercury metal at the surace o a workingelectrode to orm a thin lm o mercury. Te modied electrode isthen transerred to another solution to be used or the analysis. Caremust be taken with an electrode o this type so as not to dislodge themercury lm. An alternative method is to co-deposit mercury andthe analytes during the reduction step. Tis generates a reproduciblelm in situ and avoids the problem o having to transer the MFE toa separate solution.

D C BA S V


32/8828

Solutions

Prepare stock solutions o copper and brass as described in the methodA lab procedure. Use a solution o 4% (v:v) acetic acid to prepareall dilute samples or this procedure. Prepare several standards ocopper in the range o 1 to 50 ppm in 100 mL volumetric asks. As inall other calibration techniques you want your analyte concentrationto all within the calibration range. I your brass dilution does not all

within this range, adjust the dilution accordingly.

ASV Method

Pipette 25 mL o a copper or brass solution into a 50 mL beaker. Add aone inch stir bar and spike the solution with one drop o a 0.1 M Hg 2+ solution. Dispose these solutions in a properly labeled container only.Assemble the electrochemistry apparatus as directed by your A.Te green wire is the working electrode (glassy carbon electrode), thewhite wire is or the reerence electrode (Ag/AgCl) and the red wireis or the counter electrode (platinum wire). Reproducible stirring isrequired or this technique. Stir the solution at a setting o 125 rpm

on the electronic stir plate provided. Your A will demonstrate theuse o the electrochemistry sofware.

The linear sweep voltammetry method is used with the following

parameters:

Parameter ValueConditioning time 90 secConditioning voltage -0.800 VEquilibration time 30 secInitial potential -0.800 V

Final potential +0.500 VScan rate 10 mV/sec

Start the method with the solution stirring. A timer will countdown or 90 seconds. At the end o this time turn the stirring off.ry to stop the stirring as close to zero as possible. Te workingelectrode is held at –800 mV or 30 seconds to allow the solution tosettle. Te voltage will then be scanned to +500 mV at a rate o 10 mV/sec. Voltammograms like those shown in gure 1 will be recorded.Te sofware will allow you to measure the peak areas which will beused to construct a calibration curve. A plot o the peak area (mC) vs.copper concentration (ppm) should yield a straight line. Record the

voltammogram or your brass unknown and use the calibration curveto determine the concentration o copper in the unknown solution.Calculate the mass percent o copper in your brass unknown.

Hints for Better Results

Te reproducibility o this experiment can be improved with theollowing precautions. Te state o the electrode surace should beas identical as possible or each experiment. Te working electrodeshould be polished every ew runs to remove residual materials romthe previous experiment. Bubbles on the surace o the electrode

Mercury is toxic so handlethe solutions careully.


33/8829

will cause signicant problems in reproducibility. You may need todecrease the stirring rate to prevent bubbles rom orming on theelectrode surace. Bubbles prevent the solution rom reaching theworking electrode and effectively reduce the surace area or reductionto occur and lead to inconsistent results. Be sure to check all o theelectrical connections. I the electrodes are not connected properlyyou will see the current/potential overload indicator light up on thepotentiostat.

Report your results for the weight percent of copper to 0.01% alongwith the 95% condence interval.


34/8830

1

Introduction

Photo-induced electron transer reactions in metal complexes are animportant class o reactions in inorganic and biochemistry. Teseelectron transer reactions are typically initiated by light absorptioninto charge transer bands o metal complex. A metal to ligandcharge transer (MLC) is one type o reactive state that has beenextensively studied. [Ru(bpy)3]

2+, (bpy = 2,2 -́bipyridine), is aninorganic metal complex with an MLC absorption in the visibleregion o the spectrum. Tese excited state complexes are o interestor their insight into the electron transer process.

Te synthesis o [Ru(bpy)3](PF6)2 utilizes Ru(DMSO)4Cl2 as aprecursor, since it binds bidentate ligands more readily than otheravailable starting materials, such as RuCl

3·x H

2O. Once puried,

the Ru(DMSO)4Cl2 is reuxed in ethanol with 2,2´-bipyridine in theproper molar ratio to generate [Ru(bpy)3]Cl2. For this experimentseveral polypyridyl ligands will be available or you to choose rom.Following its synthesis, [Ru(bpy)3](PF6)2 will be characterized by UV- vis and uorescence spectroscopy, as well as cyclic voltammetry.

Synthesis of Ru(DMSO)4Cl2

Place 0.50 g RuCl3·3H2O in a 10 mL round-bottomed ask equippedwith a magnetic stirring bar. A reux condenser is attached tothe ask and the apparatus is placed on a heating mantle set on a

magnetic stirring hot plate. Bubble nitrogen or argon gas through 1.8mL o DMSO (DMSO = dimethyl suloxide, (CH3)2SO) placed in agraduated cylinder. Add the degassed DMSO to the round-bottomedask, and heat to reux with continuous stirring. Allow the mixtureto boil or ve minutes only. Do not overheat. Te mixture will turna brown-orange color when the reaction is complete.

Te solution is cooled and transerred, using a Pasteur lter pipet,to a 25 mL Erlenmeyer ask. Yellow crystals may separate out at thispoint; i this is the case, the crystals can be collected as stated below.Otherwise, the volume o the solution is reduced to 0.5-1.0 mL bypassing a gentle stream o nitrogen gas over the warmed liquid. 20.0mL o dry, reagent grade acetone is careully added to the DMSOsolution to orm two phases. Cool the mixture in an ice bath. Yellowcrystals should orm upon standing or 10-15 minutes. Alternativelythe solution may be stored covered until the next lab period. Collectthe crystals by suction ltration using a Hirsch unnel. Wash thecrystals with 1 mL o acetone ollowed by 1 mL diethyl ether. Allow

1. Synthesis Adapted rom S.M. Whittingham, A. Lees.; Chemistry 445:Inorganic and Materials Chemistry Laboratory, Binghamton University http://www.chem.binghamton.edu/chem445/ (accessed 4/4/03) “Te Preparation andCharacterization o [Ru(bpy)3](PF6)2”

S C

R P


35/8831

the crystals to dry then weigh to calculate a percent yield. Determinethe melting point and obtain an inrared spectrum using a KBr pellet.Prepare a KBr pellet by grinding 100-200 mg o dry IR grade KBr in amortar along with 1-3% by weight o your sample until a ne powderis produced. Load the press with enough o the powder to cover thebottom with a layer 1 -2 mm deep. Record the spectrum using air asthe background signal. Locate and identiy the absorption requencyo the S-O band

Synthesis of [Ru(L)3](PF6)2

[Ru(L)3(PF6)2 is synthesized rom Ru(DMSO)4Cl2 by reuxing witha polypyridyl ligand (L), in a stoichiometric ratio slightly less than1:3, i.e. a slight excess o ligand. Representative ligands are shown ingure 1. Check with your A or the ligands available. Each group ina section should choose a different ligand. Please note that there is alimited amount o certain ligands. Check your calculations with yourTA before proceeding.

A B

C D

Figure 1. Representative polypyridyl ligands. A) 4,4’-dimethyl-2,2’-bipyridine (dmbpy) B) 4,7-diphenyl-1,10-phenanthroline (dpphen) C)2,2’-bipyridine, (bpy) D) 1,10-phenanthroline (phen)

A 100 mL round-bottom ask is equipped with a magnetic stirringbar, into which 0.5 g o Ru(DMSO)4Cl2 and an appropriate amounto ligand are placed. 25 mL o reagent grade ethanol are then addedto the ask. Te mixture is heated at reux, with continuous stirring

overnight. Te solution changes in color rom yellow to dark red. Tesolution is rst dried by rotary evaporation to isolate the product.Te solid is collected by vacuum ltration and washed with tolueneto remove excess ligand. Te crystals are then dried under vacuum.Further purication may be achieved by column chromatography,with ethanol as the solvent and alumina as the stationary phase.

Te nal step in the synthesis is the conversion o [Ru(L)3]Cl2 toits hexauorophosphate (PF6

-) salt by metathesis. Heat 0.5 grams o[Ru(L)3]Cl2 in a beaker with 10 mL o distilled water. In a separatebeaker, a ve-old molar excess o ammonium hexauorophosphate

A ratio o 3.6 moles oligand per 1 mole o

Ru(DMSO)4Cl 2 should beused.


36/8832

(NH4PF6) is heated in 10 mL o distilled water. Check your calculationswith your TA before weighing the NH 4PF 6 , this salt is expensive.When both solutions are warm (not boiling), the NH4PF6 solutionis slowly added to the [Ru(L)3]Cl2 solution. Te color o the solutionchanges rom red to orange and a precipitate should orm. Continueto gently heat the resulting solution or 5 minutes (be careul not tobring the solution to a boil). Cool in an ice bath and collect the crystalsby suction ltration. Dry the crystals under vacuum and calculate apercent yield starting rom the Ru(DMSO)

4

Cl2

starting product.

Spectroscopic Characterization

Prepare 50 mL o an acetonitrile solution o your product that isapproximately 0.03 mM. Obtain the UV-Visible spectrum o Ru(L)3

2+ and calculate the extinction coeffi cient o the MLC band at max (400-500 nm). Literature values or complexes o this type are generally inthe range o 14,000 – 32,000 M-1·cm-1. Knowledge o the extinctioncoeffi cient you will also be able to determine the appropriateconcentration or an acetonitrile stock solution o Ru(L)3

2+ or use ina ollowing experiment. Te stock solution will be diluted to makeseveral solutions (25 mL each) that have an absorbance o 0.1 – 0.3 in

the MLC region.Te measured max in the MLC region will be used as the excitation

wavelength in the luminescence measurement. Te emission spectrumshould be recorded over the 500-900 nm range. Te absorption andemission spectrum o Ru(bpy)3

2+ is shown above in gure 2. Techarge transer bands have been assigned as ligand centered (LC, *), metal centered (MC, d d) or metal to ligand charge transer(MLC).2

In a later experiment we will need to obtain the E00 energy or thetriplet state in the ruthenium complex. Te E00 energy is the energydifference between an electron in the ground electronic state in the

lowest vibronic level and an electron in the excited triplet state inthe lowest vibronic level (see gure 3). Te energy or this transitioncan be estimated by the “10% rule”.3 Tat is, the point at which theemission intensity on the high energy side is 10% o that at max (gure4). Convert this wavelength into eV or use later (Remember that E =hc/ and 1 eV = 1.6022 10-19 J).

2. Kalyanasundaram, K. Coord. Chem. Rev., 46, 1982, 159-2443. Døssing, A, Ryu, C.K., Kudo, S., Ford, P.C. J. Am. Chem. Soc., 115, 1993, 5132.


37/8833

Figure 2. Absorption (blue line) and emission (red line) spectrum oRu(bpy)3

2+ in acetonitrile solution with assignments o the electronicabsorption bands. Te absorbance in the MLC region has beenmultiplied by a actor or our or clarity.

S0

S1

hν E00

k isc

T1

Figure 3. Energy level diagram depicting the triplet to singlet

phosphorescence transition (E00

energy).


38/8834

Figure 4. Luminescence spectrum o Ru(bpy)32+ in acetonitrile

solution. Te E00 energy is estimated to be where the emissionintensity is 10% that o the emission at max.

Electrochemical Characterization

You will be perorming several electrochemical measurements inacetonitrile (CH3CN) solution. A supporting electrolyte must beadded to the otherwise non-conducting solvent. Prepare 50 mL oan acetonitrile solution that is 0.1 M in n-tetrabutyl ammoniumhexauorophosphate (BAH) in a volumetric ask. Note: BAH ismoisture sensitive and exposure to the room air should be minimized.Te BAH salt is also expensive. Do not waste your solutions. Add5-10 mL o this solution to a clean and dry electrochemical cellcontaining a magnetic stir bar. Set up the electrodes on the potentiostatas instructed by your A. Bubble the stirring solution with nitrogenor argon or 5-15 minutes. Stop the stirring and remove the gas linerom the liquid but keep it suspended above the solution. Te gasrom the line should not disturb the surace o the solution. A slowow should gently blanket the cell with inert gas.

Record a blank cyclic voltammogram o the electrolyte solution overthe range o +1.75 to –2.25 V at a scan rate o 200 mV/sec. A reductionpeak near –0.8 V indicates the presence o oxygen and urther spargingis required. Add a small amount o your ruthenium complex to theacetonitrile solution (~50-100 mg), sparge or several minutes andrecord a cyclic voltammogram o this solution. Export your data asan ASCII le or inclusion in your lab report. Te voltammogramshould look similar to the one below in gure 5. Record the differentialpulse polarogram as instructed below beore discarding this solution.Repeat the above procedure to record the cyclic voltammogram ophenothiazine (PZ), N,N,N’,N’-etramethyl-p-phenylenediamine(MPD) and p-chloro nitrobenze (PCNB). Record the signal orthese compounds over the range o -0.250 to +1.0 volts at a scan rate o100 mV/sec. Te trace should resemble that shown in gure 6. Recordthe E1/2 value in your notebook or use in a later experiment.

Working electrode (green),Reerence electrode (white),Counter electrode (red).


39/8835

Figure 5. Cyclic voltammogram o Ru(L)32+ in acetonitrile solution

with 0.1 M BAH supporting electrolyte. Te E1/2 values or each waveare calculated as the potential halway between the cathodic (Epc) andanodic (Epa) waves as shown in gure 7.

Figure 6. Cyclic voltammogram o phenothiazine in acetonitrilesolution.


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Figure 7. Cyclic voltammogram or a quasi-reversible process. TeE1/2 value is the mean o the cathodic peak (E pc) and the anodic peak(E pa)

Differential Pulse Voltammetry

Reduction and oxidation potentials can also be measured usingdifferential pulse voltammetry. In this method a saw-tooth potentialwaveorm is scanned giving rise to greater sensitivity. Reer toyour analytical text or more inormation regarding various typeso electrochemical measurements. A representative differentialpulse polarogram is shown in gure 8. Te rst reduction in each voltammetry method corresponds to the reduction o Ru(L)3

2+ toRu(L)3

+. Te ground state reduction potentials and the E00 energyestimated above will be used to estimate the excited state reductionpotentials in a ollowing experiment.

Figure 8. Differential pulse polarogram o Ru(L)32+ in acetonitrile

solution with 0.1 M BAH supporting electrolyte. Te arrow indicatesthe direction o the scan.


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Introduction

Luminescence quenching o a molecule can be used to directlymeasure bi-molecular processes. In the ollowing experiment, we willinvestigate an electron transer induced quenching event rom anorganic electron donor to [RuII(L)3]

2+. In many cases this process isdiffusion controlled (kdiff ~10

10 s-1) and a bimolecular quenching rate(kq) will be determined by calibration o the quenching mechanismusing a series o prepared standards.

Te emission intensity o the uorophore can be quenched by anexcited-state uorophore quencher reaction (dynamic). Te ollowinggeneral scheme illustrates the nature o processes that deactivate anelectronically excited state o a molecule.

(excitation process)

(radiative)

(non-radiative)

(energy transer)

(electron transer)

Te absorption o a photon by A produces an excited state species

A*. Te excited species has several ways to get rid o the excessenergy – or example, emission o light (luminescence), conversiono energy to heat (radiationless deactivation), or interaction withanother species present in solution (bimolecular quenching). I onetakes the reciprocal o the radiative decay rate constant kr , the result isthe radiative lietime () o the lumophore. A relatively long lietimeo the excited state is important to acilitate an effi cient quenchingprocess. In other words kq must be much larger than kr . When this isthe case, the quencher molecule (Q) can interact with the excited statespecies and undergo energy or electron transer reactions.

A Stern-Volmer kinetics analysis measures the competition betweenthe bi-molecular quenching o a molecule in its excited state with the

unimolecular decay o the molecule rom its excited state. Te steady-state approximation assumes that the rate o ormation o A* is equalto the rate o its disappearance. Tis is a valid assumption or systemsin which an intermediate species reacts or returns to starting materialrapidly. For the system described here the ollowing expression isobeyed:

(1)

where I 0 is the integrated intensity o emission without a quencherpresent, I is the integrated intensity in the presence o quencher, [Q]

L Q

S-V K M E


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is the concentration o quencher, and kSV is the Stern-Volmer rateconstant dened as kq0. Here kq is the bimolecular quenching rateconstant and 0 is the natural radiative lietime in the absence oquencher. Reactions that are thermodynamically avorable (i.e. large-G) have kq values that approach the diffusion controlled limit, kD ~1010 s-1.

Te observed emission intensity is directly proportional to thelietime o the emitting species so that expression [1] above may be

redened in terms o lietimes as ollows.

(2)

Notice that in each case (1) and (2) the equation is in the orm ostraight line. I the ratio I 0 /I or 0 / is plotted vs. [Q] a straight lineshould result with the slope equal to the Stern-Volmer quenchingconstant,kSV and an intercept o 1. Very ast bimolecular rate constantsmay be determined by analysis o this type.

Electron ransfer Teory

Electron transer reactions are a class o reactions in which an electrondonor transers and electron to an electron acceptor. Tese simpleredox reactions are common in inorganic and biochemical systems.One may view electron transer processes along a reaction coordinatein which in which two overlapping potential energy suraces cross asshown in gure 1. Te electron transer reaction can occur through aninner sphere or outer sphere mechanism as in Eq. 3 and 4 respectively.In both cases there is no net change in the reactants or products so theree energy o the system remains unchanged, i.e. G = 0.

A

B C

D

E

A

B C

D

E

A B

Figure 1. Reaction coordinate diagrams or electron transer when A)G = 0 and B) G < 0.

NN RuIII(NH3)5(H3N)5RuII NN RuII(NH3)5(H3N)5Ru

III

5+ 5+

(3)

RuII(NH3)6

2+ + RuIII(NH3)63+ RuIII(NH3)6

3+ + RuII(NH3)62+

(4)

Te thermally activated electron transer pathway in gure 1Aproceeds through points ABCD with an activation barrier (Ea) equalto the difference in energy between points A and the crossing point B.


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Te electron transer rate is give by

(5)

where E is the requency actor that depends on the overlap o thetwo states, electron tunneling, or resonance between states. Tereaction coordinate diagram or an unsymmetrical electron transeris shown in gure 1B. In such a case the electron transer rate isrelated to the ree energy or the overall process and a reorganizationterm, , which takes into account changes in bond length and solventpolarization induced by changes in charge distributions, equation 6.

(6)

As with a symmetrical electron transer, the requency actor, A, isrelated to requency o the system reaching the crossing point. Te value or λ (gure 2.) can be estimated theoretically but is beyond thescope o this experiment. For a given related series o electron transer

reactions in the same solvent, the value o λ will be t to experimentaldata.

∆Gº∆G‡

λ

G

rxn

Figure 2. Reaction coordinate diagram or an unsymmetrical diabaticelectron transer with labeled ree energy terms.

Since many electron transer reactions are quite rapid in solution,we must take into account the theoretical limit or bimolecularreactions. For two species to react, they must be close enough tointeract with one another. In solution this is limited by the diffusionprocess. Imagine a large container ull o balls. A ew o the ballsare colored red and a ew are colored blue. Now shake the container.Reactions between red and blue balls can only occur i they collidewith one another. Te requency at which they collide is governedby the viscosity o the solution and the average kinetic energy o

the particles in solution (proportional to temperature). Te Debyeequation can be used to estimate the diffusion rate in solutions whenone or more o the reacting species is uncharged.

8

3000 D

RT k

(7)

Where is the viscosity o the solvent in poise and R is 8.31 107 ergmol-1 K-1. Solvent viscosities can be ound in the reerence books suchas the CRC Handbook o Chemistry and Physics.


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aking diffusion into account, the overall quenching rate constantis given in Eq. 8

k k K k K

q

D a ET

a = + ≈1 1

1 (8)

where K a is a pre-association constant, kD is the diffusion rate constantand kE is the electron transer rate constant given by Eq. 6. Becauseour quencher molecules are not charged we don’t expect any sort opre-association in our system and is set equal to 1.

Experimental

All measurements will be conducted in acetonitrile solutions underoxygen ree conditions (oxygen acts as a quencher). Te samples aresparged by inserting a needles into the septa o the cell and slowlybubbling N2 through the samples. Be sure to provide a second needleas a vent or the cuvette will build up pressure and crack. Spargingtimes o ~5 min are suffi cient or 2-3 mL o solution.

Prepare 100 mL stock solutions o ~1 mM o two o the ollowingquenchers: phenothiazine (PZ) , N,N,N’,N’-tetramethyl-p-phenylenediamine (MPD) or parachloronitrobenzene (PCNB). Besure to record the exact concentrations in your lab notebook. Addenough quencher solution to our 50 mL volumetric asks so thatthe quencher concentration will range rom 0.00 to ~0.1 mM aferdilution. Partially dilute the samples with 15-20 mL o acetonitrilebeore proceeding. Add an aliquot o [Ru(L)3]

2+ to one ask using avolumetric pipette so that the max is between 0.1 and 0.3 in the MLCregion o the visible absorbance spectrum. Check a single solutionbeore you make them all in case your calculated amount is incorrect.I the absorbance value is acceptable, proceed to add the [Ru(bpy)3]

2+

to the other asks diluting to the mark as required. It is importantthat the exact same volume o ruthenium stock solution is added toeach ask. Note: the TMPD solutions are photosensitive. Preparethem in dimmed room light. You should also prepare these solutionsone at time to minimize thermal reactions. There are a limited numberof volumetric flasks. Please, clean the flasks and replace them whenyou have finished your experiment so others may use them.

Fill a cuvette with ~2.5 mL o one o your solutions, cap the cuvettewith a septum and sparge with N2 or 5 minutes to remove oxygen. Besure there is a second needle to vent the gas. Record the uorescencespectrum o your sample over the 500 to 900 nm region. Using theavailable sofware record the integrated peak area or your sample.

Make a note o the emission max or use in the time resolved emissionexperiment. Te next step is to record the time resolved luminescenceo your sample. Excitation will occur using a pulsed N2 laser. Te laseremits short (< 4 ns) pulses o 337 nm light. Caution should be exercisedaround any laser, you only have one set o eyes. Laser saety gogglesare required as all times in the laser room – NO EXCEPIONS.Failure to ollow saety rules will result in a ailing grade. Your Awill instruct you how to operate the laser and data collection system.Set the monochromator on the instrument to the wavelength o theemission maximum that you recorded on the uorimeter. ranser

You may need to alter theconcentration range or thequenchers. ry the mostconcentrated solutions

rst. I the lietime is lessthan 100 ns you shoulduse a less concentratedquencher solution.

Your normal laboratorysaety goggles may be usedas laser saety goggles orthis experiment. Plastic

goggles absorb strongly inthe ultraviolet.


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the data to the computer when the data on the digital oscilloscopedisplays acceptable signal to noise. You should use the curve ttingroutine in the sofware to obtain the observed decay rate constant andthe lietime. You are required to enter a “guess” value or the ttingalgorithm. A guess value o 500 ns is a good place to start. Record thelietime o your sample in your lab notebook.

Data Analysis

Using EXCEL or another data analysis program (IGOR, SIGMAPLO, KALEIDAGRAPH etc…) plotI 0 /I and 0/ vs. the concentrationo quencher (inset gure 3). For each experiment t the data to astraight line using linear least squares regression and record theslope o each line. From these results determine the second orderbimolecular quenching constant kq. Report these results with alongwith their standard deviations. Include in your LAB report the plotsgenerated above, the values or the kq (with the standard deviation)rom each experiment. Compare your results to literature values iavailable. Many quenching rate constants can be ound in a reviewarticle by Hoffman et. al .1 Not all complexes will have data in thissource. Search the primary literature or your specic compound and

quencher combinations.

Class data will be compiled and posted on the course website oranalysis. Send your group results to your instructor using the reportorm available on the course website. Be sure to include all requestedinormation.

Figure 3. Emission intensity as a unction o quencher concentration.Inset: Stern-Volmer plot or the emission quenching by PZ.

Calculating ∆Gel

Te ollowing table contains example values or calculation o ∆Gel .Use the values you measured in the laboratory or your compound.Te boldface type indicates the calculation required to obtain the

1 Hoffman, M.; Bolletta, F.; Moggi, L.; Hug, G. J. Phys. Chem. Ref. Data, Vol. 18, No. 1, 1989. This lengthy papercan be downloaded directly from http://www.nist.gov/data/PDFfiles/jpcrd360.pdf

Te standard deviation othe slope o a line is givenin equation 4-21 in your

analytical text (Harris,Quantitative Chemical

Analysis, 8th Edition).


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correct value. Note the ree energy calculation gives you units o Jmol-1 , please convert the energy units to kJ. Te “n” in the ree energycalculation is the number o electrons transerred and is equal to 1 orthe reactions we are studying here.

riplet Energy

(rom emission data “10% rule”)

E 00 2.16 eV

Ground State reduction potential

(Ru2+/+ rom CV data)

GS -1.38 V

Excited State reduction potential(*Ru2+/+) (ES = GS + E 00 )

ES +0.78 V

Quencher reduction potential

(E1/2 o Q+/0 rom CV data)

Q +0.53 V

Potential or electron transer

(Eel = ES – Q) (cathode - anode)

E el +0.25 V

Free energy change or electrontranser

(∆Gel = -n F Eel )

-24.1 kJ mol-1

One step beyond

Tere is some work that takes place during the electron transerprocess.2 Tere is a work term that comes rom bringing two

molecules together to a radius o r , wR(r), and another or pullingthe two molecules apart afer the reaction, wP (r). Te work correctedpotential or electron transer is more properly given by the ollowing

E el = ES – Q + w P (r) - w R(r)

Each of the work terms can be calculated by the following equation

w P,R(r) = z A z B e2 N / [ r (1 + A r ½ )]

where z A and z B are the charges on molecules A and B , e is the charge

on an electron, is the dielectric constant, is the ionic strengthand A = (8Ne2/1000k )½ . Since one o the reactants (the organicquencher) is uncharged w R (r) = 0. Te remaining work term, wP (r) 0.03 eV. Since the correction is airly small we will neglect it in ourstudies, however you should be aware that these corrections do exist.

2. For more details see: D. Sandrini, M. Maestri, P. Belser, A. von Zelewsky and V.Balzani. J. Phys. Chem., 1985, 89, 3675-79


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Curve Fitting

Set up a spreadsheet in EXCEL as follows:

A B C D E F

1 ΔG

(J/mol)

log k q log k q

fit A 5.00E+10

2 -22730 9.98 10.03 λ 400003 -5510 9.71 9.33 k D 2.00E+10

4 -22580 10.03 10.03

5 -13140 9.68 9.75 χ 2 2.778965

6 -2400 9.78 9.11

7 -67540 10.17 9.72

8 -54060 10.32 10.08

9 -20340 9.92 9.98

10 -73080 10.02 9.43

11 -52300 10.26 10.10

12 2000 8.00 8.73

13 9000 7.50 8.04

14 11000 7.00 7.82

15 18000 6.80 6.98 Te data in columns A and B are the class experimental values orGEL and log kq provided on the course website. Te values in columnC are the calculated log kq values rom Eq. 8. Te experession or cellC2 is:

=LOG(1/(1/$F$3+1/($F$1*EXP(-1*(($F$2+A2)^2)/4*F$2*8.314*295)))))Copy and paste this expression in cells C3... or all data points. Te

values in cells F1, F2 and F3 are the guess values or A, λ and kD. Usethe values in the table above as starting points. Cell F6 is our measureo the goodness o t given by χ 2. EXCEL has a built in unction thatcalculates χ 2 values (SUMXMY2). Te expression or cell F6 is:

=SUMXMY2(B2:B100,C2:C100)

Change B100 and C100 to match the number o data points youhave on your spreadsheet. Te goal now is to vary the parameters

A, λ and kD in order to minimize the χ 2 value. Doing this by handwould be time consuming and we would never be sure we reached aminimum. We will use the SOLVER add-in or EXCEL to nd thebest values o A, λ and kD automatically or us. Te procedure orusing the SOLVER is outlined on the ollowing page.


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1. Under the ools menu (or Data tab in Offi ce v. 2007 or later) selectSolver. A pop-up window will appear. 3

2. In the box labeled “Set arget Cell” type in $F$5

3. Below this select “Equal o” the min unction since we are tryingto minimize the value in cell F5.

4. In the box labeled By “Changing Cells” type $F$1:$F$3 . Tis

allows the solver to vary the values or A, λ and kD to minimizethe sum o χ 2 .

5. Click the Options button and check the box labeled “Use automaticscaling” then click OK.

6. Now click on “Solve”. Te program will alter your guess values tot the data to minimize χ 2 .

7. A new pop-up will appear asking i you want to keep the new values or revert to your original values. Select keep solver solutionand click the OK button.

8. Te best t values or A, λ and kD will now be in cells F1, F2 and F3.Te tted values should match very closely to the experimentaldata (see below). I they do not, then you need to make betterguesses or A, λ and kD to start with. Enter new guess values andrepeat the procedure.

Figure 4. Plot o ΔGEL vs. log kq. Data is compiled class data rom2005-2010 and literature data. Te line is a t o the data to Eq. 8.

It is nice to show the tted line through the data points as a smoothunction. o accomplish this we will generate data using the tted values or A, λ and kD. In column G on your spreadsheet enter values

3. I you do not see Solver as an option, the add-in needs to be installed. o do thisselect “Add-Ins” under the “ools” menu and check the solver add-in. Note: Youmay need the Excel installation disk to add this eature.


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or DG rom 20 000 to -100 000 in increments o 2000. Column Hwill contain the new data or log kq. Enter the ollowing expressioninto cell H1

= L O G ( 1 / ( 1 / $ F $ 3 + 1 / ( $ F $ 1 * E X P ( - 1 * ( ( $ F $ 2 + G 1 ) ^ 2 ) /(4*$F$2*8.314*295)))))

Ten copy and paste this expression rom H2 to the end o the data incolumn G. Te resulting values in columns H and G are added to the

plot and shown as a smooth line (red line in gure 4). Compare thetted value or kD in cell F3 to the theoretical value calculated with Eq.7. Do these values agree with each other? Why or why not?


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1

Introduction

Electronic transitions induced by the absorption o UV or visiblelight result in an excited state molecule. Te energy absorbed must bereleased to return the excited species back to the original ground state.Tere are numerous unimolecular pathways or the excess energy tobe dissipated. A molecule may loose energy through nonradiativeprocesses such as vibrational cooling. Energy may also be lost throughradiative process such as uorescence o

Chem 116 Lab Manual 3rd Edition

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