The application of pure, symmetrical, bias-free square, sine and triangular wave periodic polarisation to all types of oxidationreduction titrations is reported. Electrode configuration and earthing and the destructive effect of bias are examined. Titration curve shapes are the same as those of classical d.c. differential electrolytic potentiometry, but the periodic current densities required are much higher than for d.c. The electrode response speed is greatly accelerated, unpoised potentials are steady, warning is given of the approach of the end-point in type I1 (b) titrations, electrodes retain full activity for very long periods and errors in titrations of iron(I1) with dichromate or cerium(1V) are eliminated in the periodic method as against the d.c. method. Discrimination in type I1 reactions is slightly attenuated in the periodic method, and in titrations a t low concentrations it is considerably attenuated. The nature and the conditions of the titration, the speed of the electrode charge-transfer process, the ballast load, the current density, the electrode area, the shape of the applied waveform, the applied frequency, and the deactivation of electrodes, are examined in detail. The benefit of the constant-current mode over the constsnt-potential mode is demonstrated.IN d.c. differential electrolytic potentiometry, two types of differential titration curve can occurY1 depending on the speed of electrode processes concerned. For fast or moderately fast reactions2 of comparable speed, a symmetrical peak of type I results, the differential potential falling near to zero on either side of the equivalence point. Slowness of the chargetransfer process produces an asymmetrical curve, although the peak remains sharp, and the differential potential fails to reach zero (by the amount of charge-transfer overpotential, y8) on one or both sides of the peak. When the charge-transfer process becomes very slow for one reactant, type I1 curves result; if the slowness is associated with the titrant species, a rising-Z curve of type I1 (a) occurs, while if the titrand species have a very slow chargetransfer rate, a falling-Z curve of type I1 (b) is obtained. The same shape of curves for type I reactions was expected for periodic p~larisation,~ and there was little reason to expect any difference in type 11 reactions. The investigation of polarisation with perfectly symmetrical periodic waveforms of two platinum indicator electrodes in oxidationreduction titrimetry is reported in this paper, together with an examination of the variables concentration, equilibrium constant, ballast load, current density, and the shape and repetition frequency of the applied waveform. It must be strongly emphasised that the investigation and results appertain only to perfectly symmetrical inputs. Previous work has been r e ~i e w e d , ~ and has been faulted on this matter of symmetry. EXPERIMENTALThe apparatus and instrumentation have been described and evaluated in the previous paper.4
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