Capacitive charge, chronocoulometry

Sottomayor etal. [273] have investigated adsorption of n-hexanol on Au(lll) electrode from 0.2 M NaCl04 aqueous solution, applying chronocoulometry to measure dynamic capacitance and capacitive charge. They have found that in this case, adsorption may be described by the Frumkin isotherm and have determined the corresponding parameters. The obtained results were discussed and... 

Chronocoulometry — Figure 1. Waveform of a potential step experiment (a) and the respective chronocoulometric response (b). Dashed curves in b displayed for the illustration of the capacitive charge effect... 

Under ideal conditions, charge consumed by the double-layer capacitance and adsorbed reactants will follow the same time course as discussed earlier for ordinary chronocoulometry. Since the ratio of electrode area to solution volume is larger for thin-layer experiments, charge thus accounted for may represent a much greater proportion of the total. This fact points to an advantage of restricted diffusion experiments for studying some surface phenomena. 

The intercept of Q vs. is therefore Q + hFAYq. A common application of chronocoulometry is to evaluate surface excesses of electroactive species hence it is of interest to separate these two interfacial components. However, doing so reliably usually requires other experiments, such as those described in the next section. An approximate value of nFATo can be had by comparing the intercept of the plot obtained for a solution containing O, with the instantaneous charge passed in the same experiment performed with supporting electrolyte only. The latter quantity is Qdi the background solution, and it may approximate Q for the complete system. Note, however, that these two capacitive components will not be identical if O is adsorbed, because adsorption influences the interfacial capacitance (see Chapter 13). 

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