Capacitance of Electrode Impedance

Voltammetry [50] with superimposed ac current was used [33, 36—37] by the author to measure the impedance of platinum metals at 1000Hz in acid solutions containing methanol. The periodic i—U curves a and a in Fig. 46 were measured at 30mV/sec and 30°C in 1 M HCIO4 and 1 M HCIO4+I M CH3OH respectively. The shape of the curves is easily reproduced, and this was one of the main reasons for 

An attempt was made [33, 49, 51] to compute the coverage with CgHpOq from the capacitance data as on mercury. The validity of Frumkin s model [58] of the double layer was assumed in the simplified form  

Here the C s designate the double layer capacity for the coverage 0=0, and 1. The value of Cq=q at a given potential was taken [31, 46] as that in the absence of CH3OH, the value of Ce= as that at 1M. The 0 values computed from Eq. 27 at three different potentials in the double layer region are represented by open symbols in Fig. 47. 

On the basis of Eq. 27, Cq q has to be larger than any Cq since 6 is positive. The /coC —U curves show that this condition is not always fulfilled for the cathodic sweep. Therefore, the following relationship which is not based on the physical model was tried also (solid symbols in Fig. 47)  

The 0 values from the capacity data [33] are plotted as a function of in Fig. 47 together with the corresponding 0 values from Fig. 44 (solid line). Fig. 47 demonstrates that the 0 values according to Eq. 27 and Eq. 28 are always lower than the respective values of the direct determinations. Very approximate values are obtained. Eq. 27 and Eq. 28 are not at all applicable at potentials outside the double layer region. 

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