Characteristics of diffusion controlled polarographic waves

1 Characteristics of diffusion-controlled polarographic waves The magnitude of diffusion-controlled currents are direct functions of depolarizer concentration and in this fact lies the quantitative analytical significance of polarography. All quantitative analysis by the technique is based on the direct proportionality between the limiting diffusion controlled current and depolarizer concentration as expressed in the Ilkovic equation viz., 

n is the number of electrons exchanged in the electrode reaction, D is the diffusion coefficient of the depolarizer (cm s ), m is the rate of mass flow of the mercury (mg s ), t is the drop time (s) and C is the depolarizer concentration. With the last quantity expressed as millimole litre , and the numerical constant 706, the maximum current during the drop-life is given in microamps. If, as is more usual, mean currents are measured, the numerical constant becomes 607, and the practical form of the Ilkovic equation becomes 

Equation (8.73) is seen to be very similar to the Nernst equation and may be regarded as its polarographic equivalent, 1/2 replacing E, to which it closely corresponds in magnitude. A plot of In f(j — i// versus may be used to determine both n and an accurate value of 1/2 (Fig. 8.14). 

For irreversible reductions, a wave has less than the theoretical slope for the number of electrons transferred and Equation (8.73) must be modified to 

Comparison of polarograms for (1) a reversible, and (2) an irreversible reduction the limiting currents in both cases being diffusion controlled and the number of electrons transferred equal. 

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