intro to analytical chemistry 15

8/10/2019 Intro to Analytical Chemistry 15

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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

intro to analytical chemistry 15

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