Lecture Date: January 14th, 2008

Introduction to Analytical Chemistry

What is Analytical Chemistry?

Qualitative: provides information about the identity of an atomic, molecular or biomolecular species

Quantitative: provides numerical information as to the relative amounts of species

Analytical chemistry seeks ever improved means of measuring 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 material and determine the exact amounts of the identified substances

Definitions from www.acs.org

The Role of Analytical Chemistry

-Friedrich Wilhelm Ostwald“Analytical 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/

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 of measurements and are at the forefront of the utilization of major discoveries such as lasers and microchip devices for practical purposes.

MedicineIndustry

EnvironmentalFood and Agriculture

ForensicsArchaeology

Space science

The Role of Analytical Chemistry

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 and fluorescence, many more…

Classification of Modern Analytical Methods

Gravimetric Methods determine the mass of the analyte or some compound chemically related to it

Volumetric Methods measure the volume of a solution containing sufficient reagent to react completely with the analyte

Electroanalytical Methods involve the measurement of such electrical properties as voltage, current, resistance, and quantity of electrical charge

Spectroscopic Methods are based on the measurement of the interaction between electromagnetic radiation and analyte atoms or molecules, or the production of such radiation by analytes

Miscellaneous Methods include the measurement of such quantities as mass-to-charge ratio, rate of radioactive decay, heat of reaction, rate of reaction, sample thermal conductivity, optical activity, and refractive index

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

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)

2. History of sample and backgroundof the problem

Background info can originate from many sources:

• The client, competitor’s products

• Literature searches on related systems

• Sample histories:• synthetic route• how sample was collected, transported, stored• the sampling process

Performance Characteristics: Figures of Merit

Which analytical method should I choose? How good is the measurement, information contentHow reproducible is it? PrecisionHow close to the true value is it? Accuracy/BiasHow small of a difference can be measured? SensitivityWhat concentration/mass/amount/range? Dynamic RangeHow much interference? Selectivity (univariate vs. multivariate)

3. Plan of Action

2

1

1

N

xxs

N

ii

x

sRSD %100

x

sCV

N

sSm

s2

m

SSc

blmm

bias = - xt

S = mc + Sbl

Sm = Sbl+ ksbl

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...

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 from sample

Covert data into information

Transform information into

knowledge

After reviewing results might be necessary to modify and repeat procedure

Techniques

Separation TechniquesGas chromatographyHigh performance liquid chromatographyIon chromatographySuper critical fluid chromatographyCapillary electrophoresisPlanar chromatography

Spectroscopic techniquesInfrared spectrometry (dispersive and fourier transform)Raman spectrometryNuclear magnetic resonanceX-ray spectrometryAtomic absorption spectrometryInductively coupled plasma atomic emission spectrometryInductively coupled plasma MSAtomic fluorescence spectrometryUltraviolet/visible spectrometry (CD)Molecular Fluorescence spectrometryChemiluminescence spectrometryX-Ray Fluorescence spectrometry

More Techniques

Mass SpectrometryElectron ionization MSChemical ionization MSHigh resolution MSGas chromatography MSFast atom bombardment MSHPLC MSLaser MS

Electrochemical techniquesAmperometric techniqueVoltammetric techniquesPotentiometric techniquesConductiometric techniques

Microscopic and surface techniquesAtomic force microscopyScanning tunneling microscopyAuger electron spectrometryX-Ray photon electron spectrometrySecondary ion MS

Technique Selection

Location of samplebulk or surface

Physical state of samplegas, liquid, solid, dissolved solid, dissolved gas

Amount of Samplemacro, micro, nano, …

Estimated purity of samplepure, simple mixture, complex mixture

Fate of sampledestructive, non destructive

Elemental informationtotal analysis, speciation, isotopic and mass analysis

Molecular informationcompounds present, polyatomic ionic species,functional group, structural, molecular weight, physical property

Analysis typeQuantitative, Qualitative

Analyte concentrationmajor or minor component, trace or ultra trace

An Example: HPLC vs. NMR

HPLC NMRLocation of sample

bulk or surface B B

Physical state of samplegas, liquid, solid, dissolved solid, dissolved gas L,Ds L,S,Ds

Amount of Samplemacro, micro Ma, Mi Ma, Mi

Estimated purity of samplepure, simple mixture, complex mixture Sm,M P,Sm

Fate of sampledestructive, non destructive N,D N

Elemental informationtotal analysis, speciation, isotopic and mass analysis

Molecular informationCompounds present, Polyatomic ionic species, Cp,Io,St Cp,Fn,StFunctional group, Structural, MW, Physical prop

Analysis typeQuantitative, Qualitative Ql,Qt Ql,Qt

T,S (ion) limited

Review of Background Material

Chemical equilibrium

Activity coefficients

Ionic strength

Acids and bases

Titrations

Other simple chemical tests (“spot tests”)

Some important figures of merit

Review of a few other helpful concepts

Chemical Equilibrium

aA + bB cC + dD

K = [C]c [D]d / [A]a [B]b

There is never actually a complete conversion of reactants to product in a chemical reaction, there is only a chemical equilibrium.

A chemical equilibrium state occurs when the ratio of concentration of reactants and products is constant. An equilibrium-constant expression is an algebraic equation that describes the concentration relationships that exist among reactants and products at equilibrium

Chemical Equilibrium

Dissociation of water2H2O H3O+ + OH- Kw = [H3O+ ][OH-]

Acid baseNH3 + H2O NH4

+ + OH- Kb = [NH4+][OH-] / [NH3]

SolubilityPbI2(s) Pb2+ + 2I- Ksp = [Pb2+ ][I-]2

Oxidation-ReductionIO3

- + 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

Activity Coefficients

Ions in solution have electrostatic interactions with other ions. Neutral solutes do not have such interactions. When the concentrations of ions in a solution are greater than approximately 0.001 M, a shielding effect occurs around ions. Cations tend to be surrounded by nearby anions and anions tend to be surrounded by nearby cations. This shielding effect becomes significant at ion concentrations of 0.01 M and greater. Doubly or triply charged ions "charge up" a solution more than singly charged ions, so we need a standard way to talk about charge concentration.

The law of mass action breaks down in electrolytes. Why?

Activity Coefficients

Dilute solutions and concentrated solutions have slight differences and a more precise method of calculating and defining the equilibrium constant is needed:

ax = x [C]

IDEAL

[ ] < 10-3

NON-IDEAL

[ ] > 10-3

in dilute solutions-- = 1 < 1

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, …)

Ionic Strength: Definitions

Dissociation of an electrolyte:MxXm xMm+ + mXx-

Ionic Strength: = 0.5 zi

2Ci

Activity coefficient: ai = i [X]I

Debye-Huckel limiting Law relates activity coefficient to ionic strength

Mean ionic activity:a = C (mmxx) 1/(m+x)

z

i

ii

28.31

51.0log

2

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)3 plus 1.0 M CaCl2

for this solution the concentrations are:

[La 3+] = 2.0 M[SO4

2-] = 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

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

Source: www.aw.com/mathews/ch02/fi2p22.htm

Strength of Acids and Bases

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 [H3O

+].

pH = - log [H3O+]pKw = pH + pOH = 14

We can also express equilibrium constants for the strength of acids and bases in a log form

pKa = - log(Ka)pKb = - log (Kb)

Kw = Ka * Kb

Source: http://cwx.prenhall.com/petrucci/medialib/media_portfolio/text_images/TB17_03.JPG

Strength of Acids and Bases

Amphiprotic Compounds Amphiprotic solvents: a solvent that can act as either an

acid 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 + HNO2 CH3OH2+ + NO2

-

Zwitterions: an amphiprotic compound that is produced by a simple amino acid’s weak acid an weak base functional groups

Zwitterions carry both a positive charge (amino group) and negative charge (carboxyl group)

Titrations

Advantages Disadvantagesgreat flexibility large amount of analyte requiredsuitable for a wide range of analyteslacks speciation (similar structure)manual, simple colorimetric -subjectiveexcellent precision an accuracy sensitive to skill of analystreadily automated reagents unstable

Definition: an analytical technique that measures concentration of an analyte by the volumetric addition of a reagent solution (titrant)- that reacts quantitatively with the analyte

For titrations to be useful, the reaction must generally be quantitative, fast and well-behaved

Chemical Stoichiometry

Stoichiometry: The mass relationships among reacting chemical species. The stoichiometry of a reaction is the relationship among the number of moles of reactants and products as shown by a balanced equation.

Mass MolesMoles Mass

Divide by molar massMultiply by stoichiometric

ratio Multiply by molar mass

Titration Curves

Strong acid - Strong base

Strong base - Weak acid

Titration Curves

Strong base - polyprotic acid

Buffer Solutions

Buffers contain a weak acid HA and its conjugate base A-

The buffer resists changes in pH by reacting with any added 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 strong affinity 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, the pH 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.

Calculating the pH of Buffered Solutions

Henderson-Hasselbach equation

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 reactionNaOH(aq) + HCl(aq) -----> NaCl(aq) + H2O(l)

2.Extract the relevant information from the question:NaOH V = 30mL , M = 0.10M HCl V = 25.0mL, M = ?

3.Check the data for consistencyNaOH V = 30 x 10-3L , M = 0.10M HCl V = 25.0 x 10-3L, M = ?

4.Calculate moles NaOHn(NaOH) = M x V = 0.10 x 30 x 10-3 = 3 x 10-3 moles

5.From the balanced chemical equation find the mole ratioNaOH:HCl1:1

Example 1 (continued)

6.Find moles HClNaOH: HCl is 1:1

So n(NaOH) = n(HCl) = 3 x 10-3 moles at the equivalence point

Calculate concentration of HCl: M = n ÷ V

n = 3 x 10-3 mol, 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

Example 2

50mL of 0.2mol L-1 NaOH neutralised 20mL of sulfuric acid. Determine the concentration of the acid

1.Write the balanced chemical equation for the reactionNaOH(aq) + H2SO4(aq) -----> Na2SO4(aq) + 2H2O(l)

2.Extract the relevant information from the question:NaOH V = 50mL, M = 0.2M H2SO4 V = 20mL, M = ?

3.Check the data for consistencyNaOH V = 50 x 10-3L, M = 0.2M H2SO4 V = 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 ratioNaOH:H2SO4

2:1

Example 2 (continued)

6.Find moles H2SO4

NaOH: H2SO4 is 2:1

So n(H2SO4) = ½ x n(NaOH) = ½ x 0.01 = 5 x 10-3 moles H2SO4 at the equivalence point

7.Calculate concentration of H2SO4: M = n ÷ Vn = 5 x 10-3 mol, V = 20 x 10-3L

M(H2SO4) = 5 x 10-3 ÷ 20 x 10-3 = 0.25M or 0.25 mol L-1

Molar Concentration or Molarity – Number of moles of solute in one Liter of solution or millimoles solute per milliliter of solution.

  Analytical Molarity – Total number of moles of a solute, regardless of chemical

state, in one liter of solution. It specifies a recipe for solution preparation. 

Equilibrium Molarity – (Species Molarity) – The molar concentration of a

particular species in a solution at equilibrium.

Notes on Solutions and Their Concentrations

Percent Concentration a. 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

Some Other Important Concepts

Limit of detection (LOD): the lowest amount (concentration or mass) of an analyte that can be detected at a known confidence level

Linearity: the degree to which a response of an analytical detector to analyte concentration/mass approximates a linear function

Limit of quantitation (LOQ): the range over which quantitative measurements can be made (usually the linear range), often defined by detector dynamic range

Selectivity: the degree to which a detector is free from interferences (including the matrix or other analytes)

Concentration

Det

ecto

r re

spon

se

LOQ

LOD

Limit of linearity

Slope relates to sensitivity

Dynamic range

Simple Chemical Tests

While most of this class is focused on instrumental methods, a very large number of simple chemical tests have 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 acid and 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 of orthophosphates 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 Mercury

A modern example of a “spot” test: a test for Hg2+ developed using DNA and relying on the formation of a thymidine-Hg2+-thymidine complex

LOD = 100 nM (20 ppb) in aqueous solution

Linearity from the high nanomolar to low micromolar range

Selective for Hg2+ and insensitive to Mg2+, Pb2+, Cd2+, Co2+, Zn2+, Ni2+, and other metal ions

Angew. Chem. Int. Ed., DOI: 10.1002/anie.200700269http://pubs.acs.org/cen/news/85/i19/8519news6.html

ppm: cppm = mass of solute X 106 ppm mass of solution

For dilute aqueous solutions whose densities are approximately 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

Density and Specific Gravity of Solutions

Density: The mass of a substance per unit volume. In SI units, 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 of measure).

Prefixes for SI Unitsgiga- G 109

mega- M 106

kilo- k 103

deci- d 10-1

centi- c 10-2

milli- m 10-3

micro- u 10-6

nano- n 10-9

pico- p 10-12

femto- f 10-15

atto- a 10-18

Other Helpful Information