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Chemistry LibreTexts

Optional Experiment

Chronocoulometry (CC) is a technique in which the CA i-t is integrated to give charge, Q, vs. time, t, where Q = ∫ i dt. The current, i, is the rate of electrolysis at any given time, t, and Q is the amount of charge transferred to time, t. In the case of an electron-transfer reaction, we can integrate the Cottrell relationship, denoting the Q due to a diffusing species as Qdiff.

\[Q_{diff}=\dfrac{2nFAC^b\, D^{1/2}\,t^{1/2}}{π^{1/2}}\tag{6}\]

The total charge, QT, is comprised of two other components: the Qdl due to charging the double layer, as already discussed, and Qads, due to electrolysis of any electroactive species adsorbed on the electrode surface [i.e., QT = Qdiff + Q dl + Qads]. Since the latter two terms are time independent (involves charge only of the surface), a plot of Q vs. t1/2 should be linear with the extrapolated intercept at t = o due to Qdl + Qads.

With the i-t data from the chronoamperometry of ferricyanide reduction or FCA oxidation, obtain Q values at times of 50 mS, 100 mS, 200 mS and 300 mS. Plot Q vs. t1/2 and obtain the intercept value, which is due to Qdl. Is this value reasonable for the charging of the double-layer?