D.6 Quantitative Applications

11 D.6 Quantitative Applications

Electrode

Quantitative voltammetry has been applied to a wide variety of sample

Figure 11.39

types, including environmental samples, clinical samples, pharmaceu-

Schematic showing the reactions by which

tical formulations, steels, gasoline, and oil.

an amperometric biosensor responds to glucose.

Selecting the Voltammetric Technique The choice of which voltammetric tech- nique to use depends on the sample’s characteristics, including the analyte’s ex- pected concentration and the location of the sample. Amperometry is best suited for use as a detector in flow systems or as a selective sensor for the rapid analysis of

a single analyte. The portability of amperometric sensors, which are similar to po- tentiometric sensors, make them ideal for field studies.

Pulse polarography and stripping voltammetry can frequently be used inter- changeably, although each has its advantages and disadvantages. Pulse polarography is better for analyzing a wider range of inorganic and organic analytes because the need to preconcentrate the analyte at the electrode surface restricts the application of anodic and cathodic stripping voltammetry.

When either pulse polarography or anodic stripping voltammetry can be used, the selection is often based on the analyte’s expected concentration and the desired

Chapter 11 Electrochemical Methods of Analysis

accuracy and precision. Detection limits for normal pulse polarography generally are on the order of 10 –6 –10 –7 M, whereas those for differential pulse polarography, staircase, and square-wave polarography are between 10 –7 M and 10 –8 M. Precon- centrating the analyte in stripping voltammetry lowers the detection limit for many analytes to as little as 10 –10 M. On the other hand, the current in stripping voltam- metry is much more sensitive than pulse polarography to changes in experimental conditions, which may lead to poorer precision and accuracy.

Anodic stripping voltammetry also suffers from occasional interferences when two metals, such as Cu and Zn, combine to form an intermetallic com- pound in the mercury amalgam. The deposition potential for Zn 2+ is sufficiently negative that any Cu 2+ present in the sample is also deposited. After deposition,

intermetallic compounds such as CuZn and CuZn 2 form within the mercury

amalgam. During the stripping step, the zinc in the intermetallic compounds strips at potentials near that of copper, decreasing the current for zinc and in- creasing the current for copper. This problem can often be overcome by adding a third element that forms a stronger intermetallic compound with the interfering metal. Thus, adding Ga 3+ minimizes this problem by forming an intermetallic compound of Cu and Ga.

Correcting for Residual Current In any quantitative analysis the signal due to the analyte must be corrected for signals arising from other sources. The total measured current in any voltammetric experiment, i tot , consists of two parts: that due to the

analyte’s oxidation or reduction, i a , and a background, or residual, current, i r .

i tot =i a +i r

The residual current, in turn, has two sources. One source is a faradaic current due to the oxidation or reduction of trace impurities in the sample, i i . The other source

is the charging current, i ch , that is present whenever the working electrode’s poten-

tial changes.

i r =i i +i ch

Faradaic currents due to impurities can usually be minimized by carefully preparing

the sample. For example, one important impurity is dissolved O 2 , which is reduced first to H 2 O 2 and then to H 2 O. Dissolved O 2 is removed by bubbling an inert gas

such as N 2 through the sample before the analysis. Two methods are commonly used to correct for the residual current. One method is to extrapolate the total measured current when the analyte’s faradaic current is zero. This is the method shown in the voltammograms included in this chapter. The advantage of this method is that it does not require any additional data. On the other hand, extrapolation assumes that changes in the residual cur- rent with potential are predictable, which often is not the case. A second, and more rigorous, approach is to obtain a voltammogram for an appropriate blank. The blank’s residual current is then subtracted from the total current obtained with the sample.

Analysis for Single Components The analysis of samples containing only a single electroactive analyte is straightforward. Any of the standardization methods dis- cussed in Chapter 5 can be used to establish the relationship between current and the concentration of analyte.

Modern Analytical Chemistry

EXAMPLE 11.10

The concentration of As(III) in water can be determined by differential pulse polarography in 1 M HCl. The initial potential is set to –0.1 V versus the SCE, and is scanned toward more negative potentials at a rate of 5 mV/s. Reduction of As(III) to As(0) occurs at a potential of approximately −

0.44 V versus the SCE. The peak currents, corrected for the residual current, for a set of standard solutions are shown in the following table.

[As(III)]

What is the concentration of As(III) in a sample of water if the peak current under the same conditions is 1.37 µ A?