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Lecture Date: January 14th, 2008
Introduction to Analytical Chemistry
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What is Analytical Chemistry?
Qualitative:provides information about the identity ofan atomic, molecular or biomolecular species
Quantitative:provides numerical information as to the
relative amounts of species
Analytical chemistry seeks ever improved means ofmeasuring the chemical composition of natural and
artificial materials
The techniques of this science are used to identify
the substances which may be present in a materialand determine the exact amounts of the identified
substances
Definitions from www.acs.org
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The Role of Analytical Chemistry
-Friedrich Wilhelm OstwaldAnalytical Chemistry, or the art of
recognizing different substances and
determining their constituents, takes a
prominent position among the
applications of science, since the
questions which it enables us to answer
arise wherever chemical processes are
employed for scientific or chemical
purposes.
http://www.pace.edu/dyson/academics/chemistryplv/
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Analytical chemists work to improve the reliability of existing techniques to
meet the demands of for better chemical measurements which arise
constantly in our society
They adapt proven methodologies to new kinds of materials or to answer
new questions about their composition.
They carry out research to discover completely new principles ofmeasurements and are at the forefront of the utilization of major
discoveries such as lasers and microchip devices for practical purposes.
Medicine
IndustryEnvironmental
Food and Agriculture
Forensics
Archaeology
Space science
The Role of Analytical Chemistry
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History of Analytical Methods
Classical methods:early years (separation of analytes) via
precipitation, extraction or distillation
Qualitative:recognized by color, boiling point, solubility, taste
Quantitative:gravimetric or titrimetric measurements
Instrumental Methods:newer, faster, more efficient
Physical properties of analytes:conductivity, electrode
potential, light emission absorption, mass to charge ratio andfluorescence, many more
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Classification of Modern Analytical Methods
Gravimetric Methodsdetermine the mass of the analyte or somecompound chemically related to it
Volumetric Methodsmeasure the volume of a solution containingsufficient reagent to react completely with the analyte
Electroanalytical Methodsinvolve the measurement of such
electrical properties as voltage, current, resistance, and quantity ofelectrical charge
Spectroscopic Methodsare based on the measurement of theinteraction between electromagnetic radiation and analyte atoms or
molecules, or the production of such radiation by analytes
Miscellaneous Methodsinclude the measurement of suchquantities as mass-to-charge ratio, rate of radioactive decay, heat
of reaction, rate of reaction, sample thermal conductivity, optical
activity, and refractive index
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Analytical Methodology
1. Understanding and defining the problem
2. History of the sample and background of the problem
3. Plan of action and execution
4. Analysis and reporting of results
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1. Understanding and Defining
the Problem
What accuracy is required?
Is there a time (or money) limit?
How much sample is available?
How many samples are to be analyzed? What is the concentration range of the analyte?
What components of the system will cause an
interference?
What are the physical and chemical properties
of the sample matrix? (complexity)
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2. History of sample and background
of the problem
Background info can originate from many sources:
The client, competitors products
Literature searches on related systems
Sample histories:
synthetic route
how sample was collected, transported, storedthe sampling process
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Performance Characteristics: Figures of Merit
Which analytical method should I choose? How good is the
measurement, information content
How reproducible is it? Precision
How close to the true value is it?Accuracy/Bias
How small of a difference can be measured? SensitivityWhat concentration/mass/amount/range? Dynamic Range
How much interference? Selectivity (univariate vs. multivariate)
3. Plan of Action
2
1
1
N
xx
s
N
i
i
x
sRSD %100
x
sCV
N
sSm
s2
m
SSc
blm
m
bias = - xt
S = mc+ Sbl
Sm = Sbl+ ksbl
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4. Analyzing and Reporting Results
No work is complete until the customer is happy!
Analytical data analysis takes many forms: statistics,
chemometrics, simulations, etc
Analytical work can result in:peer-reviewed papers, etc
how sample was collected, transported, stored
technical reports, lab notebook records, etc...
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Components of an Analytical Method
Perform measurement
(instrumentation)
Handbook, Settle
Compare results
with standards
Pretreat and prepare sample
Obtain and store sample
Apply required
statistical techniques
Verify results
Present information
Extract data
f rom sample
Covert datainto informat ion
Transform
inform at ion into
knowledge
After reviewing results
mig ht be necessary
to modify and repeat
procedure
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Techniques
Separation Techniques
Gas chromatography
High performance liquid chromatographyIon chromatography
Super critical fluid chromatography
Capillary electrophoresis
Planar chromatography
Spectroscopic techniques
Infrared spectrometry (dispersive and fourier transform)
Raman spectrometryNuclear magnetic resonance
X-ray spectrometry
Atomic absorption spectrometry
Inductively coupled plasma atomic emission spectrometry
Inductively coupled plasma MS
Atomic fluorescence spectrometryUltraviolet/visible spectrometry (CD)
Molecular Fluorescence spectrometry
Chemiluminescence spectrometry
X-Ray Fluorescence spectrometry
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More Techniques
Mass Spectrometry
Electron ionization MSChemical ionization MS
High resolution MS
Gas chromatography MS
Fast atom bombardment MS
HPLC MS
Laser MS
Electrochemical techniques
Amperometric technique
Voltammetric techniques
Potentiometric techniques
Conductiometric techniques
Microscopic and surface techniques
Atomic force microscopyScanning tunneling microscopy
Auger electron spectrometry
X-Ray photon electron spectrometry
Secondary ion MS
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Technique Selection
Location of sample
bulk or surfacePhysical state of sample
gas, liquid, solid, dissolved solid, dissolved gas
Amount of Sample
macro, micro, nano,
Estimated purity of sample
pure, simple mixture, complex mixture
Fate of sampledestructive, non destructive
Elemental information
total analysis, speciation, isotopic and mass analysis
Molecular information
compounds present, polyatomic ionic species,functional group,
structural, molecular weight, physical propertyAnalysis type
Quantitative, Qualitative
Analyte concentration
major or minor component, trace or ultra trace
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An Example: HPLC vs. NMR
HPLC NMR
Location of sample
bulk or surface B B
Physical state of sample
gas, liquid, solid, dissolved solid, dissolved gas L,Ds L,S,Ds
Amount of Sample
macro, micro Ma, Mi Ma, Mi
Estimated purity of samplepure, simple mixture, complex mixture Sm,M P,Sm
Fate of sample
destructive, non destructive N,D N
Elemental information
total analysis, speciation, isotopic and mass analysis
Molecular informationCompounds present, Polyatomic ionic species, Cp,Io,St Cp,Fn,St
Functional group, Structural, MW, Physical prop
Analysis type
Quantitative, Qualitative Ql,Qt Ql,Qt
T,S (ion) limited
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Review of Background Material
Chemical equilibriumActivity coefficients Ionic strength
Acids and bases TitrationsOther simple chemical tests (spot tests)
Some important figures of merit Review of a few other helpful concepts
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Chemical Equilibrium
aA + bB cC + dD
K = [C]c[D]d/ [A]a[B]b
There is never actually a complete conversion ofreactants to product in a chemical reaction, there is onlya chemical equilibrium.
A chemical equilibrium state occurs when the ratio ofconcentration of reactants and products is constant. An
equilibrium-constant expression is an algebraic equationthat describes the concentration relationships that exist
among reactants and products at equilibrium
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Chemical Equilibrium
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Dissociation of water
2H2O H3O++ OH- Kw= [H3O+][OH-]
Acid base
NH3+ H2O NH4++ OH- Kb = [NH4+][OH-] / [NH3]
Solubility
PbI2(s) Pb2+ + 2I- Ksp = [Pb2+ ][I-]2
Oxidation-Reduction
IO3-
+ 5I-
+ 6H+
3I2(aq) + 3H20 Keq= [I2]3
/ [IO3-
][I-
]5
[H+
]6
Cl2(g) + 2AgI(s) 2AgCl(s) + I2(g) Keq= pI2/ pCl2
Typical Equilibrium Constant Expressions
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Activity Coefficients
Ions in solution have electrostatic interactions withother ions. Neutral solutes do not have suchinteractions.
When the concentrations of ions in a solution aregreater than approximately 0.001 M, a shielding effectoccurs around ions. Cations tend to be surrounded bynearby anions and anions tend to be surrounded by
nearby cations. This shielding effect becomessignificant at ion concentrations of 0.01 M and greater.Doubly or triply charged ions "charge up" a solutionmore than singly charged ions, so we need a standardway to talk about charge concentration.
The law of mass action breaks down
in electrolytes. Why?
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Activity Coefficients
Dilute solutions and concentrated solutions have slight differences and
a more precise method of calculating and defining the equilibriumconstant is needed:
ax= x[C]
IDEAL
[ ] < 10-3
NON-IDEAL
[ ] > 10-3
in dilute solutions-- = 1 < 1
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Effect of Electrolyte Concentration
Reason for deviation: The presence of electrolytes results in
electrostatic interactions with other ions and the solvent
The effect is related to the number and charge of eachion present - ionic strength ()
= 0.5 ( [A] ZA2 + [B] ZB
2 + [C]ZC2 + ..)
where Z = charge (ex. +1, -2, )
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Ionic Strength: Definitions
Dissociation of an electrolyte:
MxXm xMm+ + mXx-
Ionic Strength:
= 0.5 zi2Ci
Activity coefficient:
ai= i[X]I
Debye-Huckel limiting Law relates activity coefficient
to ionic strength
Mean ionic activity:
a = C (mm
xx
)1/(m+x)
z
i
i
i
28.31
51.0log
2
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What is the ionic strength for a 1.0 M NaCl solution?
I = 1/2(1*12 +1*12)
I = 1
What is the ionic strength for a solution whose concentrations
are 1.0 M La2(SO4)3plus 1.0 M CaCl2
for this solution the concentrations are:
[La 3+] = 2.0 M
[SO42-] = 3.0 M
[Ca 2+] = 1.0 M[Cl -] = 2.0 M
I = 1/2 (2*32+ 3*22+ 1*22+ 2*12)
I = 18
Ionic Strength Calculations: Examples
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Equilibria classified by reaction taking place
1) acid-base2) oxidative-reductive
Bronsted-Lowry definitions:
acid: anything that donates a [H+
] (proton donor)base: anything that accepts a [H+] (proton acceptor)
HNO2+ H2O NO2-+ H3O+
Aqueous Solution Equilibria
HA + H2O A-+ H3O+
Ka= [A-] [H3O
+] / [HA]
ACIDNH3+ H2O NH4++ OH-
Kb = [NH4+][OH-] / [NH3]
BASE
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Source: www.aw.com/mathews/ch02/fi2p22.htm
Strength of Acids and Bases
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p-Functions
The p- value is the negative base-10 logarithm of the molar
concentration of a certain species:pX = -log [X] = log 1/[X]
The most well known p-function is pH, the negative
logarithm of [H3
O+].
pH = - log [H3O+]
pKw= pH + pOH = 14
We can also express equilibrium constants for the strength
of acids and bases in a log formpKa= - log(Ka)
pKb= - log (Kb)
Kw= Ka* Kb
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Source: http://cwx.prenhall.com/petrucci/medialib/media_portfolio/text_images/TB17_03.JPG
Strength of Acids and Bases
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Amphiprotic Compounds
Amphiprotic solvents: a solvent that can act as either anacid or base depending on the solute it is interacting
with
methanol, ethanol, and anhydrous acetic acid are all
examples of amphiprotic solvents.
NH3+ CH3OH NH4++ CH3O-CH3OH+ HNO2CH3OH2+ + NO2-
Zwitterions: an amphiprotic compound that is produced
by a simple amino acids weak acid an weak basefunctional groups
Zwitterions carry both a positive charge (amino group)and negative charge (carboxyl group)
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Titrations
Advantages Disadvantages
great flexibility large amount of analyte requiredsuitable for a wide range of analytes lacks speciation (similar structure)
manual, simple colorimetric -subjective
excellent precision an accuracy sensitive to skill of analyst
readily automated reagents unstable
Definition: an analytical technique that measures
concentration of an analyte by the volumetric addition ofa reagent solution (titrant)- that reacts quantitatively with
the analyte
For titrations to be useful, the reaction must generallybe quantitative, fast and well-behaved
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Titration Curves
Strong acid - Strong base
Strong base - Weak acid
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Titration Curves
Strong base - polyprotic acid
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Buffer Solutions
Buffers contain a weak acid HA and its conjugate base A-
The buffer resists changes in pH by reacting with anyadded H+or OH-, preventing their accumulation. How? Any added H+ reacts with the base A-:
H+(aq) + A-(aq) -> HA(aq) (since A-has a strongaffinity for H+)
Any added OH-reacts with the weak acid HA:
OH-(aq) + HA (aq) -> H2O + A-(aq) (since OH-can
steal H+from A-)
Example:if 1 mL of 0.1 N HCl solution to 100 mL water, thepH drops from 7 to 3. If the 0.1 N HCl is added to a 0.01
M solution of 1:1 acetic acid/sodium acetate, the pH drops
only 0.09 units.
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Calculating the pH of Buffered Solutions
Henderson-Hasselbach equation
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Example 1
30 mL of 0.10M NaOH neutralised 25.0mL of hydrochloric acid. Determine the
concentration of the acid
1.Write the balanced chemical equation for the reaction
NaOH(aq) + HCl(aq) -----> NaCl(aq) + H2O(l)
2.Extract the relevant information from the question:
NaOHV = 30mL , M = 0.10M HClV = 25.0mL, M = ?
3.Check the data for consistency
NaOHV = 30 x 10-3L , M = 0.10M HClV = 25.0 x 10-3L, M = ?
4.Calculate moles NaOHn(NaOH) = M x V = 0.10 x 30 x 10-3= 3 x 10-3moles
5.From the balanced chemical equation find the mole ratio
NaOH:HCl
1:1
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Example 1 (continued)
6.Find moles HCl
NaOH: HCl is 1:1
So n(NaOH) = n(HCl) = 3 x 10-3moles at the equivalence point
Calculate concentration of HCl: M = n V
n = 3 x 10-3mol, V = 25.0 x 10-3L
M(HCl) = 3 x 10-3 25.0 x 10-3= 0.12M or 0.12 mol L-1
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Example 2
50mL of 0.2mol L-1NaOH neutralised 20mL of sulfuric acid. Determine the
concentration of the acid
1.Write the balanced chemical equation for the reaction
NaOH(aq) + H2SO4(aq) -----> Na2SO4(aq) + 2H2O(l)
2.Extract the relevant information from the question:
NaOHV = 50mL, M = 0.2M H2SO4V = 20mL, M = ?
3.Check the data for consistency
NaOHV = 50 x 10-3L, M = 0.2M H2SO4V = 20 x 10-3L, M = ?
4.Calculate moles NaOHn(NaOH) = M x V = 0.2 x 50 x 10-3= 0.01 mol
5.From the balanced chemical equation find the mole ratio
NaOH:H2SO4
2:1
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Example 2 (continued)
6.Find moles H2SO4NaOH: H2SO4is 2:1
So n(H2SO4) = x n(NaOH) = x 0.01 = 5 x 10-3moles H2SO4at the
equivalence point
7.Calculate concentration of H2SO4: M = n V
n = 5 x 10-3mol, V = 20 x 10-3L
M(H2SO4) = 5 x 10-3 20 x 10-3= 0.25M or 0.25 mol L-1
Notes on Solutions and Their Concentrations
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Molar Concentration or MolarityNumber of moles of solute in one Liter ofsolution or millimoles solute per milliliter of solution.
Analytical MolarityTotal number of moles of a solute, regardless of chemicalstate, in one liter of solution. It specifies a recipe forsolution preparation.
Equilibrium Molarity(Species Molarity)The molar concentration of a
particular species in a solution at equilibrium.
Notes on Solutions and Their Concentrations
Percent Concentrationa. percent (w/w) = weight solute X 100%
weight solution
b.volume percent (v/v) = volume solute X 100%volume solution
c.weight/volume percent (w/v) = weight solute, g X 100%volume soln, mL
S Oth I t t C t
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Some Other Important Concepts
Limit of detection (LOD): thelowest amount (concentration or
mass) of an analyte that can bedetected at a known confidence
level
Linearity: the degree to which a
response of an analyticaldetector to analyte
concentration/mass
approximates a linear function
Limit of quantitation (LOQ): the range over which quantitativemeasurements can be made (usually the linear range), oftendefined by detector dynamic range
Selectivity: the degree to which a detector is free frominterferences (including the matrix or other analytes)
Concentration
Detectorresponse
LOQ
LOD
Limit of linearity
Slope relates tosensitivity
Dynamic range
Si l Ch i l T t
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Simple Chemical Tests
While most of this class is focused on instrumental
methods, a very large number of simple chemical testshave been developed over the past ~300 years
Examples: Barium: solutions of barium salts yield a white precipitate with 2
N sulfuric acid. This precipitate is insoluble in hydrochloric acidand in nitric acid. Barium salts impart a yellowish-green color to
a nonluminous flame that appears blue when viewed through
green glass.
Phosphate: With silver nitrate TS, neutral solutions oforthophosphates yield a yellow precipitate that is soluble in 2 N
nitric acid and in 6 N ammonium hydroxide. With ammonium
molybdate TS, acidified solutions of orthophosphates yield a
yellow precipitate that is soluble in 6 N ammonium hydroxide.
Examples are from US Pharmacopeia and National Formulary USP/NF
A Colormetric Test for Merc r
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A Colormetric Test for Mercury
A modern example of aspot test: a test for
Hg2+developed usingDNA and relying on the
formation of a thymidine-
Hg2+-thymidine complex
LOD = 100 nM (20 ppb) inaqueous solution
Linearity from the highnanomolar to low micromolar
range
Selective for Hg2+andinsensitive to Mg2+, Pb2+, Cd2+,
Co2+, Zn2+, Ni2+, and other
metal ionsAngew. Chem. Int. Ed.,DOI: 10.1002/anie.200700269http://pubs.acs.org/cen/news/85/i19/8519news6.html
C /
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ppm:
cppm= mass of solute X 106 ppm
mass of solution
For dilute aqueous solutions whose densities areapproximately 1.00 g/mL, 1 ppm = 1 mg/L
ppb:
cppb= mass of solute X 109 ppb
mass of solution
Concentration in Parts per Million/Billion
D it d S ifi G it f S l ti
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Density and Specific Gravity of Solutions
Density: The mass of a substance per unit volume. In SIunits, density is expressed in units of kg/L or g/mL.
Specific Gravity: The ratio of the mass of a substance to
the mass of an equal volume of water at 4 degrees
Celsius. Dimensionless (not associated with units ofmeasure).
Oth H l f l I f ti
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Prefixes for SI Units
giga- G 109mega- M 106kilo- k 103deci- d 10-1centi- c 10-2milli- m 10-3
micro- u 10-6nano- n 10-9pico- p 10-12femto- f 10-15atto- a 10-18
Other Helpful Information