Transcript Introduction to Analytical Chemistry

Introduction to Analytical Chemistry
Lecture Date: January 14th, 2008
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
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/
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
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 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
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)
N
s
 x   x 
i 1
2
i
N 1
s
RSD 
x
Sm 
s
N
bias =  - xt
s2
Sm = Sbl+ ksbl
s
CV 
100%
x
cm 
Sm  Sbl
m
S = mc + Sbl
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
Extract data
from sample
Pretreat and prepare sample
Perform measurement
(instrumentation)
Compare results
with standards
Covert data
into information
Apply required
statistical techniques
Verify results
Transform
information into
knowledge
After reviewing results
might be necessary
to modify and repeat
procedure
Present information
Handbook, Settle
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
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
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
HPLC
NMR
B
B
L,Ds
L,S,Ds
macro, micro
Ma, Mi
Ma, Mi
pure, simple mixture, complex mixture
Sm,M
P,Sm
N,D
N
total analysis, speciation, isotopic and mass analysis
T,S (ion)
limited
Compounds present, Polyatomic ionic species,
Functional group, Structural, MW, Physical prop
Cp,Io,St
Cp,Fn,St
Ql,Qt
Ql,Qt
Location of sample
bulk or surface
Physical state of sample
gas, liquid, solid, dissolved solid, dissolved gas
Amount of Sample
Estimated purity of sample
Fate of sample
destructive, non destructive
Elemental information
Molecular information
Analysis type
Quantitative, Qualitative
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


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
Chemical Equilibrium
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
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 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
in dilute solutions--  = 1
NON-IDEAL
[ ] > 10-3
<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 each
ion present - ionic strength ( )
 = 0.5 ( [A] ZA2 + [B] ZB2 + [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
log  i 
 0.51zi2 

1  3.28 i 
Mean ionic activity:
a =  C (mmxx) 1/(m+x)

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
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+
ACID
HA + H2O  A- + H3O+
Ka =
[A-
] [H3
O+
] / [HA]
BASE
NH3 + H2O  NH4+ + OHKb = [NH4+][OH-] / [NH3]
Strength of Acids and Bases
Source: www.aw.com/mathews/ch02/fi2p22.htm
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
Strength of Acids and Bases
Source: http://cwx.prenhall.com/petrucci/medialib/media_portfolio/text_images/TB17_03.JPG
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+ + CH3OCH3OH + 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

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
Advantages
great flexibility
suitable for a wide range of analytes
manual, simple
excellent precision an accuracy
readily automated
Disadvantages
large amount of analyte required
lacks speciation (similar structure)
colorimetric -subjective
sensitive to skill of analyst
reagents unstable
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
Moles
Moles
Mass
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 AThe 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)
affinity for H+)
(since A- has a strong
– Any added OH- reacts with the weak acid HA:
 OH- (aq) + HA (aq) -> H2O + A-(aq)
steal H+ from A-)

(since OH- can
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
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
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 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: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 ÷ 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
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
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
Detector response

Slope relates to
sensitivity
LOQ
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)
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 thymidineHg2+-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
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
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).
Other Helpful Information
Prefixes for SI Units
gigaG
megaM
kilok
decid
centic
millim
microu
nanon
picop
femtof
attoa
109
106
103
10-1
10-2
10-3
10-6
10-9
10-12
10-15
10-18