The electrical double layer of carbon and graphite electrodes. Part IV. dependence of carbon electrode dimensions and electrical capacity on pH

When the aqueous medium of a high surface carbon electrode is changed from neutral to acidic at potentiostatic conditions, the electrode contracts, consumes a significant amount of negative electric charge and its electric double-layer capacity increases mainly at the more positive potential range. The minimum in the curve of electrode length vs. potential which for a reversible system coincides with the potential of zero charge, is shifted negatively upon acidification, and the slope of the electrode length changes with respect to potential (LD) does not show the expected linear dependence on electric charge as it does in the case of a neutral solution. The effects are partially reversed when the solution is neutralized again. It is suggested that a single mechanism, the generalized surface quinone-hydroquinone (Q-HQ)-type transition that is catalysed in acidic medium is responsible for these phenomena. Thus, charge consumption is due to the HQ to Q conversion and the increase in the electric double-layer capacity results from the reversible couple. Electrode contraction and the loss of the linear LD-Q_e relation are due to the increase of the surface tension by the electron added to the conjugated system close to the interface. The shift in the length minimum arises from the decrease in surface oxide dipole because of the change of some keto- to enol-type surface groups. Monitoring the dimensional changes of the electrode may be employed for detecting the addition of π bonds to the graphitic conjugated system in the course of pH or potential changes.