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Introduction to Analytical Chemistry. Lecture Date: January 14 th , 2008. What is Analytical Chemistry?. Analytical chemistry seeks ever improved means of measuring the chemical composition of natural and artificial materials

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slide1

Introduction to Analytical Chemistry

Lecture Date: January 14th, 2008

what is analytical chemistry
What is Analytical Chemistry?

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

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

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

Definitions from www.acs.org

slide3

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/

slide4

The Role of Analytical Chemistry

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.

Medicine

Industry

Environmental

Food and Agriculture

Forensics

Archaeology

Space science

history of analytical methods
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…

slide6

Classification of Modern Analytical Methods

  • Gravimetric Methodsdetermine the mass of the analyte or some compound chemically related to it
  • Volumetric Methodsmeasure the volume of a solution containing sufficient reagent to react completely with the analyte
  • Electroanalytical Methodsinvolve the measurement of such electrical properties as voltage, current, resistance, and quantity of electrical charge
  • Spectroscopic Methodsare based on the measurement of the interaction between electromagnetic radiation and analyte atoms or molecules, or the production of such radiation by analytes
  • Miscellaneous Methodsinclude 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
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

slide8

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)
slide9

2. History of sample and background

of 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
slide10

3. Plan of Action

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? Sensitivity

What concentration/mass/amount/range? Dynamic Range

How much interference? Selectivity (univariate vs. multivariate)

bias =  - xt

s2

Sm = Sbl+ ksbl

S = mc + Sbl

slide11

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

Obtain and store sample

Pretreat and prepare sample

Perform measurement

(instrumentation)

Compare results

with standards

Apply required

statistical techniques

Present information

Verify results

Components of an Analytical Method

Extract data

from sample

Covert data

into information

After reviewing results

might be necessary

to modify and repeat

procedure

Transform

information into

knowledge

Handbook, Settle

techniques
Techniques

Separation Techniques

Gas chromatography

High performance liquid chromatography

Ion chromatography

Super critical fluid chromatography

Capillary electrophoresis

Planar chromatography

Spectroscopic techniques

Infrared spectrometry (dispersive and fourier transform)

Raman spectrometry

Nuclear magnetic resonance

X-ray spectrometry

Atomic absorption spectrometry

Inductively coupled plasma atomic emission spectrometry

Inductively coupled plasma MS

Atomic fluorescence spectrometry

Ultraviolet/visible spectrometry (CD)

Molecular Fluorescence spectrometry

Chemiluminescence spectrometry

X-Ray Fluorescence spectrometry

more techniques
More Techniques

Mass Spectrometry

Electron ionization MS

Chemical 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 microscopy

Scanning tunneling microscopy

Auger electron spectrometry

X-Ray photon electron spectrometry

Secondary ion MS

technique selection
Technique Selection
  • Location of sample
  • bulk or surface
  • Physical 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 sample
  • destructive, non destructive
  • Elemental information
  • total analysis, speciation, isotopic and mass analysis
  • Molecular information
      • compounds present, polyatomic ionic species,functional group, structural, molecular weight, physical property
  • Analysis type
  • Quantitative, Qualitative
  • Analyte concentration
  • major or minor component, trace or ultra trace
an example hplc vs nmr
An Example: HPLC vs. NMR

limited

T,S (ion)

review of background material
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
slide18

Chemical Equilibrium

  • 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

aA + bB  cC + dD

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

slide20

Typical Equilibrium Constant Expressions

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

activity coefficients
Activity Coefficients

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

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.

activity coefficients22

NON-IDEAL

IDEAL

[ ] < 10-3

[ ] > 10-3

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]

 < 1

in dilute solutions--  = 1

effect of electrolyte concentration
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 each

ion present - ionic strength ( )

 = 0.5 ( [A] ZA2 + [B] ZB2 + [C]ZC2 + …..)

where Z = charge (ex. +1, -2, …)

ionic strength definitions
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 (mmxx) 1/(m+x)

slide25

Ionic Strength Calculations: Examples

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

[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

slide26

ACID

BASE

HA + H2O  A- + H3O+

NH3 + H2O  NH4+ + OH-

Ka = [A- ] [H3O+ ] / [HA]

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

Aqueous Solution Equilibria

  • Equilibria classified by reaction taking place
  • 1) acid-base
  • 2) 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+

slide27

Strength of Acids and Bases

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

p functions
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

slide29

Strength of Acids and Bases

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

slide30

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

AdvantagesDisadvantages

great flexibility large amount of analyte required

suitable 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

Titrations
  • 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
slide32

Mass

Moles

Moles

Mass

Divide by molar mass

Multiply by stoichiometric

ratio

Multiply by molar mass

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.

titration curves
Titration Curves

Strong acid - Strong base

Strong base - Weak acid

titration curves34
Titration Curves

Strong base - polyprotic acid

slide35

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
Calculating the pH of Buffered Solutions

Henderson-Hasselbach equation

example 1
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 NaOH n(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 ratio NaOH:HCl 1:1
example 1 continued
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
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 reaction NaOH(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 NaOH n(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
example 2 continued
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 ÷ V n = 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

slide41

Notes on Solutions and Their Concentrations

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.

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

slide42

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 linearity

Slope relates to sensitivity

LOQ

Detector response

LOD

Dynamic range

Concentration

  • 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)
slide43

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

slide44

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.200700269

http://pubs.acs.org/cen/news/85/i19/8519news6.html

slide45

Concentration in Parts per Million/Billion

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

slide46

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).

slide47

Other Helpful Information

Prefixes for SI Units

giga- 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