Page 55 - Modern Analytical Chemistry

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38 Modern Analytical Chemistry

A second class of analytical techniques are those that respond to the relative
amount of analyte; thus
3.2
S A = kC A
where C A is the concentration of analyte in the sample. Since the solutions in both
2+
cylinders have the same concentration of Cu , their analysis yields identical signals.
total analysis techniques Techniques responding to the absolute amount of analyte are called total
A technique in which the signal is analysis techniques. Historically, most early analytical methods used total analysis
proportional to the absolute amount of
techniques, hence they are often referred to as “classical” techniques. Mass, volume,
analyte; also called “classical” techniques.
and charge are the most common signals for total analysis techniques, and the cor-
responding techniques are gravimetry (Chapter 8), titrimetry (Chapter 9), and
coulometry (Chapter 11). With a few exceptions, the signal in a total analysis tech-
nique results from one or more chemical reactions involving the analyte. These re-
actions may involve any combination of precipitation, acid–base, complexation, or
redox chemistry. The stoichiometry of each reaction, however, must be known to
solve equation 3.1 for the moles of analyte.
Techniques, such as spectroscopy (Chapter 10), potentiometry (Chapter 11),
and voltammetry (Chapter 11), in which the signal is proportional to the relative
concentration techniques amount of analyte in a sample are called concentration techniques. Since most
A technique in which the signal is concentration techniques rely on measuring an optical or electrical signal, they also
proportional to the analyte’s are known as “instrumental” techniques. For a concentration technique, the rela-
concentration; also called “instrumental”
techniques. tionship between the signal and the analyte is a theoretical function that depends on
experimental conditions and the instrumentation used to measure the signal. For
this reason the value of k in equation 3.2 must be determined experimentally.

3 D Selecting an Analytical Method

A method is the application of a technique to a specific analyte in a specific matrix.
Methods for determining the concentration of lead in drinking water can be devel-
oped using any of the techniques mentioned in the previous section. Insoluble lead
salts such as PbSO 4 and PbCrO 4 can form the basis for a gravimetric method. Lead
forms several soluble complexes that can be used in a complexation titrimetric
method or, if the complexes are highly absorbing, in a spectrophotometric
method. Lead in the gaseous free-atom state can be measured by an atomic ab-
sorption spectroscopic method. Finally, the availability of multiple oxidation states
2+
4+
(Pb, Pb , Pb ) makes coulometric, potentiometric, and voltammetric methods
feasible.
The requirements of the analysis determine the best method. In choosing a
method, consideration is given to some or all the following design criteria: accuracy,
(a) (b) precision, sensitivity, selectivity, robustness, ruggedness, scale of operation, analysis
Figure 3.4 time, availability of equipment, and cost. Each of these criteria is considered in
Graduated cylinders containing 0.01 M more detail in the following sections.
Cu(NO 3 ) 2 . (a) Cylinder 1 contains 10 mL, or
2+
0.0001 mol, of Cu . (b) Cylinder 2 contains
2+
20 mL, or 0.0002 mol, of Cu . 3 D.1 Accuracy
© David Harvey/Marilyn Culler, photographer.
Accuracy is a measure of how closely the result of an experiment agrees with the ex-
pected result. The difference between the obtained result and the expected result is
accuracy usually divided by the expected result and reported as a percent relative error
A measure of the agreement between an
experimental result and its expected obtained result – expected result
value. % Error = ´100
expected result