Other Electrochemical Techniques Related to Polarography

The development of the fast potential sweep method, which was introduced by Matheson and Nichols and later improved by Randles and Sevcik, is a logical step in the evolution of polarography. Current-potential curves are determined with a stationary electrode, the potential of which varies either linearly or sinusoidally with time. The dropping mercury electrode can be used in this method provided that its area does not vary appreciably during the recording of the complete i-E curve. Because of the short duration 

Distinction should be made between the so-called single-sweep technique and the multisweep method.In the former method, a single current-potential curve is recorded and the system is brought back to its original condition before a second sweep is made. On the other hand, in the multisweep method, several consecutive i-E curves are recorded and the concentration gradient of the electroactive species changes from one sweep to another, particularly from the first to the subsequent ones. 

In cyclic voltammetry, the forms of the curves will be different from that for the first sweep since the concentration gradients will not be the same at various times during the first and subsequent sweeps. Each cycle gives a cathodic and an anodic branch and the comparison of these cycles can serve as an indication of the reversibility of the electrode process. The following relation between the half-wave potential E1/2, obtained from conventional polarography, and the peak potential Ep has been derived by Delahay for a reversible electrode process assuming linear diffusion to the electrode  

The peak current Ip for the reversible reaction cases is given by 

The current-potential curves for a totally irreversible process have also been derived by Delahay and the expression for the peak current is then 

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