Potentiometric measurements

The sensitivity of instruments using low resistance circuits is determined primarily by the sensitivity of the galvanometer (Figure 4.5). Electrode systems that have a high resistance, e.g. glass electrodes, require a high impedance voltmeter, which converts the potential generated into current which can be amplified and measured. Such instruments are commonly known as pH meters but may be used for many potentiometric measurements other than pH.

Titrations can often be conveniently followed potentiometrically and in many cases it is not the actual value of the electrode potential that is important but the pattern of changing potential as the composition of the solution varies - pH and redox measurements are particularly well suited to such methods. In many instances the equivalence point will be indicated by a significant change in potential (Figure 4.6) but sometimes the change at the equivalence point is difficult to detect and it might be more convenient to use a derivative plot. Instead of plotting potential (£) against volume of titrant (V), a graph is plotted of the change in potential for the unit change in volume AE/AV against volume. Such a first-derivative plot (Figure 4.7) indicates the equivalence point as a peak or spike.

electrodes

Figure 4.5 A potentiometer circuit. The voltage from the test circuit is balanced against a known voltage by means of a variable resistance using a galvanometer to indicate the position at which no current flows in either direction.

Volume NaOH (ml)

Figure 4.6 A titration curve. Acetic acid (10 ml of a 0.1 mol l-1 solution) was titrated with a sodium hydroxide solution (0.2 mol l-1) and the pH of the resulting solution plotted against the amount of alkali added.

Volume NaOH (ml)

Figure 4.6 A titration curve. Acetic acid (10 ml of a 0.1 mol l-1 solution) was titrated with a sodium hydroxide solution (0.2 mol l-1) and the pH of the resulting solution plotted against the amount of alkali added.

Figure 4.7

A first-derivative plot of a potentiometric titration curve.

Figure 4.7

A first-derivative plot of a potentiometric titration curve.

Volume of titrant

If two identical electrodes are placed in separate solutions that are similar in every way except for the concentration of the test ions, the potential developed between the electrodes will be related to the ratio of the two concentrations, e.g. Ag—AgCl electrodes in solutions containing chloride ions. A calibration curve of potential developed in a series of known concentrations of test ions can be used in the analysis of unknown samples. Often it is advisable to plot the graph as potential versus logarithm of concentration to give the straight line relationship as indicated by the Nernst equation. It is necessary to add a constant amount of a high concentration of a non-reacting electrolyte to give all solutions tested the same ionic strength. This is because the potential depends upon the activity of the ions rather than concentration.

Figure 4.8

The glass electrode.

Figure 4.8

The glass electrode.

Silver-silver chloride electrode

Internal reference solution of HCl

Glass membrane

Silver-silver chloride electrode

Internal reference solution of HCl

Glass membrane

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