(1) Both reactants behave reversibly. The curve is as shown in Figure 6.16(a) and is exemplified by the
titration of Fe(II) with Ce(IV) when a potential of 0.1 V is applied to the cell. At the outset, no current
flows because only Fe(II) is present, the only electrode reaction possible being Fe2+ → Fe3+ + e– at the
anode. Upon the addition of the first increment of titrant and up to the equivalence point, the
concentration of Fe(II) diminishes, whilst that of Fe(III) increases, and a current proportional to the
smaller of the two concentrations flows in the cell, resulting in a maximum at the half-way stage. At the
equivalence point only Fe(III) and Ce(III) are present and no current flows because neither electrode
reaction Fe(III) → Fe(II) nor Ce(III) → Ce(IV) can proceed at the potential of 0.1 V. After the
equivalence point, the current increases linearly with the rise in concentration of Ce(IV), that of Ce(III)
being constant.
(2) Only one reactant behaves reversibly. If the titrant alone behaves reversibly, no current can flow
until it is in excess (Figure 6.16(b)). This is the case if is titrated with I 2 at an applied potential of
0.1 V. After the equivalence point the current is linearly related to the concentration of excess I 2 , that of
I– being constant. In cases where only the analyte forms a reversible couple, e.g. I 2 titrated with
Na 2 S 2 O 3 , the current before the equivalence point follows a similar path to that in the Fe(II)/Ce(IV)
system, but afterwards remains at zero (Figure 6.15(c)).
A 'dead-stop' titration curve is produced if Ag+ is titrated with a halide using a pair of identical silver
electrodes. Only whilst both Ag+ and Ag are present will a current flow in the cell, and this is linearly
related to the Ag+ concentration. Bi-amperometric titrations require only simple equipment but
generally give poorer precision because the currents measured are not necessarily on the limiting
current plateau.
Amperometric titrations are inherently more precise than polarography and are more generally
applicable because the analyte need not itself be electroactive. Titrations involving the DME are not
affected by changes in capillary characteristics as are conventional polarographic determinations, whilst
working at a predetermined temperature is unnecessary provided that it remains reasonably constant
throughout the titration.
Applications of Polarography and Amperometric Titrations
Most metal ions are reducible at the DME, and multicomponent mixtures can often be analysed by
selecting an appropriate supporting electrolyte and complexing agent. Polarography is used for the
determination of trace metals in alloys, ultra-pure metals, minerals, foodstuffs, beverages and body
fluids although recently it has been largely superseded by atomic absorption spectrometry (Chapter 8).
It is ideally suited to the determination of metal impurities in AnalaR and other high-purity salts where
the sample matrix