intro to analytical chemistry08
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Lectu re Date: January 14th, 2008
Introduction to Analytical Chemistry
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 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
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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
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, solubili ty, 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 somecompound chemically related to it
Volumetric Methods measure the volume of a solution containingsufficient reagent to react completely with the analyte
Electroanalytical Methods involve the measurement of suchelectrical properties as voltage, current, resistance, and quantity of
electrical charge
Spectroscopic Methods are based on the measurement of theinteraction between electromagnetic radiation and analyte atoms or
molecules, or the production of such radiation by analytes
Miscellaneous Methods include 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
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 propertiesof 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, 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)
3. Plan of Action
2
1
1
N
xx
s
N
i
i
x
sRSD %100
x
sCV
N
s
Sm s2
m
SSc
blmm
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...
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
Aft er rev iewing resul ts
might be n ecessary
to modify and repeat
procedure
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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 spectrometryChemiluminescence spectrometry
X-Ray Fluorescence spectrometry
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
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Technique Selection
Location of samplebulk 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
Analys is type
Quantitative, QualitativeAnalyte concen tratio n
major or minor component, trace or ultra trace
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
Amoun t o f Samp le
macro, micro Ma, Mi Ma, Mi
Estima ted purity of sample
pure, 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 information
Compounds pres ent, Poly atomic ionic species, Cp, Io, St Cp,Fn,St
Functional group, Structural, MW, Physical prop
Ana lysis type
Quantitative, Qualitative Ql,Qt Ql,Qt
T,S (ion) limited
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Review of Background Material
Chemical equilibriumActivity coefficients Ionic strengthAcids and bases Titrations Other simple chemical tests (spot tests) Some important figures of merit
Review of a few other helpful concepts
Chemical Equilibr ium
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 only
a chemical equilibrium.
A chemical equilibrium state occurs when the ratio ofconcentration 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
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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 bynearby cations. This shielding effect becomes
significant 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 downin 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
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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 each
ion 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 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)
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)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
Ionic Strength Calculations: Examples
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+
Aqueous Solut ion Equil ibr ia
HA + H2OA- + H3O+
Ka = [A- ] [H3O
+ ] / [HA]
ACID
NH3 + 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
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
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Titrations
Advantages Disadvantagesgreat 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
Definition: an analytical technique that measuresconcentration 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 generallybe 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 mass
Multiply by stoichiometricratio
Multiply by molar mass
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Titration Curves
Strong acid - Strong base
Strong base - Weak acid
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- cansteal 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.
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:
NaOH V = 30mL , M = 0.10M HCl V = 25.0mL, M = ?
3.Check the data for consistency
NaOH 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)
6.Find moles HCl
NaOH: 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
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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:
NaOH V = 50mL, M = 0.2M H2SO4 V = 20mL, M = ?
3.Check the data for consistency
NaOH 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:H2SO42:1
Example 2 (continued)
6.Find moles H2SO4NaOH: H2SO4 is 2:1
So n(H2SO4) = x n(NaOH) = 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
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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 chemicalstate, in one liter of solution. It specifies a recipe forsolution preparation.
Equilibrium Molarity (Species Molarity) The molar concentration of aparticular 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
Some Other Important Concepts
Limit of detection (LOD): thelowest amount (concentration or
mass) of an analyte that can be
detected at a known confidence
level
Linearity: the degree to which aresponse of an analytical
detector to analyte
concentration/mass
approximates a linear function
Limit of quantitation (LOQ): the range over which quantitativemeasurements can be made (usually the linear range), often
defined 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 to
sensitivity
Dynamic range
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Simple Chemical Tests
While most of this class is focused on instrumentalmethods, 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 Colormetr ic Test for Mercury
A modern example of aspot test: a test for
Hg2+ developed using
DNA 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: 10.10 02/anie.200700269
http://pubs.acs.org/cen/news/85/i19/8519news6.html
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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
ppbmass 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).
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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