Intro to Analytical Chemistry pptx

History of Analytical MethodsClassical methods: early years separation of analytes via precipitation, extraction or distillation Qualitative:recognized by color, boiling point, solubilit

Lecture Date: January 14 th , 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

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.

Medicine Industry Environmental Food and Agriculture Forensics Archaeology 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 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

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 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

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

S = mc + Sbl

S m = S bl+ 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

Atomic absorption spectrometry

Inductively coupled plasma atomic emission spectrometry

Inductively coupled plasma MS

Atomic fluorescence spectrometry

Microscopic and surface techniques

Atomic force microscopy

Scanning tunneling microscopy

Auger electron spectrometry

X-Ray photon electron spectrometry

Secondary ion MS

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

compounds present, polyatomic ionic species,functional group,

structural, molecular weight, physical property

Physical state of sample

gas, liquid, solid, dissolved solid, dissolved gas L,Ds L,S,Ds

Amount of Sample

Estimated purity of sample

pure, simple mixture, complex mixture Sm,M P,Sm

Review of Background Material

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

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

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

51 0 log

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)3plus 1.0 M CaCl2

for this solution the concentrations are:

Ionic Strength Calculations: Examples

Equilibria classified by reaction taking place

1) acid-base

2) oxidative-reductive

Bronsted-Lowry definitions:

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:

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)

great flexibility large amount of analyte required

suitable for a wide range of analytes lacks speciation (similar structure)

excellent precision an accuracy sensitive to skill of analyst

 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

Divide by molar mass

Multiply 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 reaction

2.Extract the relevant information from the question:

Example 2

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:

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

lowest amount (concentration or

mass) of an analyte that can be

detected at a known confidence

approximates a linear function

measurements can be made (usually the linear range), often

defined by detector dynamic range

interferences (including the matrix or other analytes)

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

nanomolar to low micromolar

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 Units