Lecture 1&2

Lecture 1 & 2

• Introduction to chemical analysis• Basic step in an analysis• Stoichiometry• Error in chemical analysis

• Chemical analysis includes any aspect of the chemical characterization of a sample material.

• Analytical Chemistry - “Science of Chemical Measurements”

©Gary Christian, Analytical Chemistry, 6th Ed. (Wiley)

Qualitative analysis is what.

Quantitative analysis is how much.

Qualitative analysis is what.

Quantitative analysis is how much.

“Classical” and Modern Chemical Analysis of samples

Classical analysisClassical analysis• Based on the use of chemical reactivity and stoichiometry to

directly or indirectly measure amounts of analyte (the target of the analysis –a compound or element)

• The detection limit of classical analysis is limited by the need to establish and maintain equilibrium in the measurement reaction

• Generally used to measure mg or larger amounts of simple compounds

• Classical methods of analysis often have very high precision and good accuracy

Modern (Instrumental) Analysis• Based on the use of instrument transducers to relate a

physical property (e.g., absorption of light) to the amount of analyte

• Transducer must be calibrated by measurement of standards (with known amounts of analyte and matrix)

• The detection limit of instrumental analysis depends on the slope of the transducer-amount relationship for the analyte as well as any interferences.

• Often instrumental methods can detect very small amounts of analytes but with poorer precision than classical analysis

“Classical” and Modern Chemical Analysis of samples

Different methods provide a range of precision, sensitivity, selectivity, and speed capabilities.

Different methods provide a range of precision, sensitivity, selectivity, and speed capabilities.

©Gary Christian, Analytical Chemistry, 6th Ed. (Wiley)

The sample size dictates what measurement techniques can be used.

The sample size dictates what measurement techniques can be used.

©Gary Christian, Analytical Chemistry, 6th Ed. (Wiley)

• Quantitation:– How much of substance X is in the sample?

• Detection:– Does the sample contain substance X?

• Identification:– What is the identity of the substance in the sample?

• Separation:– How can the species of interest be separated from the

sample matrix for better quantitation and identification?

Fig. 1.1. Steps in an analysis

An analysis involves several steps and operations which depend on:

•the particular problem

• your expertise

• the apparatus or equipment available.

The analyst should be involved in every step.

An analysis involves several steps and operations which depend on:

•the particular problem

• your expertise

• the apparatus or equipment available.

The analyst should be involved in every step.

WHAT DO CHEMICAL ANALYST DO?

• Research Analytical Chemist Research Analytical Chemist

Applies known measurement techniques to well defined compositional or characterization questions.

Creates and /or investigates novel techniques or principles for chemical measurements.

Conducts fundamental studies of chemical/physical phenomena underlying chemical measurements.

• Senior AnalystSenior Analyst: Develops new measurement methods on existing principles to solve new analysis problems.

CHEMICAL ANALYSIS AFFECTS MANY CHEMICAL ANALYSIS AFFECTS MANY FIELDSFIELDS

• Physical-, Organic-, …, Chemistry:– “Theory guides but Experiment decides”

• Biotechnology:– Distinguishing isomers with differing

bioactivities.– Biosensors

• Materials Science:– High-temperature superconductors

• Manufacturing:– Quality control of packaged foods specifications

• Forensics:– Chemical features for criminal evidence

CHEMICAL ANALYSIS AFFECTS MANY CHEMICAL ANALYSIS AFFECTS MANY FIELDSFIELDS

Laboratory safety is a must!

Learn the rules.

Laboratory safety is a must!

Learn the rules.

©Gary Christian, Analytical Chemistry, 6th Ed. (Wiley)

• Definition – The word Stoichiometry comes from the Greek stoicheion, which

means to measure the elements to measure the elements – A good definition of the term’s meaning in the study of chemistry is

the “quantitative study of reactants and products in a chemical quantitative study of reactants and products in a chemical reactionreaction”.

– Stoichiometry allows one to calculate how much of a given product a reaction is expected to produce based on how much of the reactants are available

– Given the mass, volume and density, or the number of moles of reactants, one can calculate the mass, volume (if the density is known) or moles of product

Review of FundamentalsReview of Fundamentals

• Atomic, Molecular, and Formula Weights

• Moles: 1mole = 6.022 x 1023

(atoms, molecules or formula units)

How Do We Express Concentrations of How Do We Express Concentrations of Solutions?Solutions?

• MolarityMolarity (M)= moles/liter or mmoles/mL

• NormalityNormality (N) = equivalence/liter or meq/mL

• Molality Molality (m) = moles/1000g solvent

In normality calculations, the number of equivalents is the number of moles times the number of reacting

units per molecule or atom.

In normality calculations, the number of equivalents is the number of moles times the number of reacting

units per molecule or atom.

Example 1Example 1

•1 M sulfuric acid (H2SO4) is 2 N for acid-base reactions because each mole of sulfuric acid provides 2 moles of H+ ions. •1 M sulfuric acid is 1 N for sulfate precipitation, since 1 mole of sulfuric acid provides 1 mole of sulfate ions.

Example 2Example 2

•36.5 grams of hydrochloric acid (HCl) is a 1 N (one normal) solution of HCl. •Since hydrochloric acid is a strong acid that dissociates completely in water, a 1 N solution of HCl would also be 1 N for H+ or Cl- ions for acid-base reactions.

• Analytical MolarityAnalytical Molarity: gives the total number of moles of a solute in one liter of the solution.

• Example: a sulfuric acid solution that has an analytical concentration of 1.0M can be prepared by dissolving 1.0 mol or 98 g of pure H2SO4 in water and diluting to exactly 1.0L.

• Equilibrium MolarityEquilibrium Molarity: : expresses the molar concentration of a particular species in a solution at equilibrium.

• Example: The species molarity of H2SO4 in a solution with an analytical concentration of 1M is 0.0M because the sulfuric acid is entirely dissociated into a mixture H3O+, HSO4

- and SO42-

ion.

• Describe the preparation of 2.00L of 0.108M BaCl2 from BaCl2.2H2O (244.3 g/mol)

• The mass of BaCl2.2H2O is then

= 0.216 mol BaCl2.2H2O

= 52.8 g BaCl2.2H2O

244.3 g BaCl2.2H2O

mol BaCl2.2H2OX0.216 mol BaCl2.2H2O

0.108 mol BaCl2

L2.00L X 1mol BaCl2.2H2O

1 mol BaCl2

X

Solid Samples:Solid Samples:

• % (wt/wt) = (wt analyte/wt sample) x 100 %

• pt (wt/wt) = (wt analyte/wt sample) x 103 ppt

• ppm (wt/wt) = (wt analyte/wt sample) x 106 ppm

• ppb (wt/wt) = (wt analyte/wt sample) x 109 ppb

Liquid SamplesLiquid Samples

• % (wt/vol) = (wt analyte/vol sample mL) x 100 %• pt (wt/vol) = (wt analyte/vol sample mL) x 103 ppt• ppm (wt/vol) = (wt analyte/vol sample mL) x 106 ppm• ppb (wt/vol) = (wt analyte/vol sample mL) x 109 ppb

Liquid AnalyteLiquid Analyte

• % (vol/vol) = (vol analyte/vol sample mL) x 100 %• pt (vol/vol) = (vol analyte/vol sample mL) x 103 ppt• ppm (vol/vol) = (vol analyte/vol sample mL) x 106 ppm• ppb (vol/vol) = (vol analyte/vol sample mL) x 109 ppb

The units ppm or ppb are used to express trace concentrations.

These are weigh or volume based, rather than mole based.

The units ppm or ppb are used to express trace concentrations.

These are weigh or volume based, rather than mole based.

©Gary Christian, Analytical Chemistry, 6th Ed. (Wiley)

The equivalents (based on charge) of cations and anions are equal.

The equivalents (based on charge) of cations and anions are equal.

©Gary Christian, Analytical Chemistry, 6th Ed. (Wiley)

Definitions:

•The difference between a measured value and the “true” or “known” value.

•The estimated uncertainty in a measurement or experiment.

•Errors are caused by : Faulty calibrations Faulty standardization Random variations Uncertainties in results

• Measurement is influenced by many uncertainties.• Example:

Results for the quantitative determination of iron

• Reliability of the data can be assessed in several ways: Design experiments – reveal the presence of errors can be

performed Standard of known composition – analyze and the results

compared with the known composition Calibrating equipment – enhances the quality of data Statistical test

Replicates

•are samples of about the same size that are carried through an analysis in exactly the same way.• 2 to 5 replicates carry out in an experiment.•Results are seldom the same.

What should u do?

•Find the central value from the set of results.•Central value should be more reliable than any of the individual results.•Mean or median is usually used as the central value for a set of replicate measurements.

Mean,

•also called the arithmetic mean, or the average.•dividing the sum of replicate measurements by the number of measurement in the set.

where xi represents the individual values of x making up the set of N replicate measurement.

Median• The middle result when replicate data are arranged according to increasing or decreasing value.• For an odd number of results, the median can be evaluated directly• For an even number, the mean of the middle pair is used

Example:Calculate the mean and median for the data shown below:

19.4, 19.8, 19.5, 20.1, 19.6, 20.319.4, 19.8, 19.5, 20.1, 19.6, 20.3

• Describes the reproducibility of measurements

• The closeness of results that have been obtained in exactly the same way

• Determine by simply repeating the measurements on replicate samples

• Three terms are widely used to describe the replicate data:

Standard deviation Variance Coefficient of variation

•Standard deviations, describes the spread of individual measurements about the mean

•where xi is one of N individual measurements, and is the mean

•Variance, is the square of the standard deviation

Example :

• The closeness of the measurement to the true or accepted value and is expressed by the error.

• Accuracy measures agreement between a result and the accepted value, while precision describes the agreement among the several results obtained in the same way.

• Precision can be determine by measuring replicate samples

• Accuracy is more difficult to determine because the true value is usually unknown.

• Accuracy is expressed in terms of either absolute or relative error.

Absolute Error, E

where xt is the true or accepted value of the quantity.

Relative Error, Er

Relative error is also expressed in parts of thousand (ppt).

Example:

Examples:

Analyst 1 - good precision, good accuracy Analyst 2 - poor precision, good accuracy Analyst 3 - good precision, poor accuracy Analyst 4 - poor precision, poor accuracy

Chemical analyses are affected by at least two types of errors which are:

i.Random errorii.Systematic error

Random Error

•Causes data to be scattered more or less symmetrically around the mean value.

•Is reflected by its precision.

Systematic Error

•Causes the mean of a data set to differ from the accepted value.•Lead to bias in measurement results. Bias affects all of the data in a set in the same way and that it bears a sign.• Example: unsuspected loss of a volatile analyte while heating a sample.

Gross Error

•Differ from the previous 2 errors.•Usually occur only occasionally, are often large, and may cause a result to be either high or low.•Often the product of human errors.•Lead to outliers, results that appear to differ markedly from all other data in a set of replicate measurement

Example:

Three types of systematic error:

i.Instrumental errorsii.Method errorsiii.Personal errors

Instrumental ErrorsInstrumental Errors

•Caused by non-ideal instrument behavior, by faulty calibrations, or by use under inappropriate conditions.•Example : Pipets, burets, and volumetric flasks may hold or deliver volumes slightly different from those indicated by their graduations.•This measuring devices also maybe contaminated by contaminants on the inner surfaces of the containers.•Calibration eliminates most systematic errors of this type.

Method errorsMethod errors

• Arise from non-ideal chemical or physical behavior of the reagent and reactions.

• Some of the sources of non-ideality are: Slowness of the reactions Incompleteness of others Instability of some species Non-specificity of most reagents Possible occurrence of side reactions

Example: small excess of reagent required to cause an indicator to undergo the color change that signals completion of the reaction.

This type of error is often difficult to detect and thus the most serious of the three types of systematic error.

Personal errorsPersonal errors

• Result from carelessness, inattention, or personal limitations of the experimenter.

• Example: an analyst who is insensitive to color changes tends to use excess reagent in volumetric analysis.

• A universal source of personal error is prejudice, or bias.

• Most of us, have a natural tendency to estimate scale readings in a direction that improves the precision in a set of results.

• As a result, digital and computer displays on pH meters, laboratory balances, and other electronic instruments to eliminate number bias because no judgment is involved in taking a reading.

Detection of Systematic Instrument and Personal Errors

•Some instrument can be corrected by calibration.

• Periodic calibration is desirable because the response of most instrument changes with time as a result of wear, corrosion, or mistreatment.

• Personal errors can be minimized by care and self-discipline.

• It is a good habit to check instrument readings, notebook entries, and calculations systematically.

Detection of Systematic Method Errors

•Bias is particularly difficult to detect.

•One or more of these steps can be taken to recognize and adjust for a systematic error in analytical method.

a.Analysis of standard samples

•Standard reference materials are materials that contain one or more analytes at known concentration levels.

•Standard reference materials can be prepared by synthesis or can be purchased from a number of governmental and industrial sources.

b. Blank Determinations

• A blank contains the reagents and solvents used in a determination, but no analyte.

• All steps of the analysis are performed on the blank material.

• Blank determinations reveal errors due to interfering contaminants from reagents and vessels used in the analysis.

c. Variations in Sample Size

• As size of a measurement increases, the effect of a constant error decreases.

• Thus, constant errors can be detected by varying the sample size.