Determination of the entropy change of the electrode reaction by an ac electrochemical-thermal method

An ac electrochemical thermal method with a polyvinylidene difluoride (PVDF) pyroelectric film sensor and the lock-in amplifier technique was developed for determining the entropy change Delta S of an electrode reaction. It was found that the Peltier heat could be determined separately from the polarization heat and Joule heat by use of a lock-in amplifier with the reference frequency the same as the frequency of the ac current. For two reversible electrode reactions the Delta S value for each electrode could be calculated by measuring the amplitudes of the ac electrochemical thermal signal V-0 ac current I-0, the phase shift between them and concerning the total entropy change of the whole cell reaction. The Fe(CN)(6)(4-)/Fe(CN)(6)(3-) electrode and its Delta S value could be used as a calibration rule to measure the Delta S for other single electrode reactions. The Delta S values for the reduction reactions in the systems of Fe2+/Fe3+, Cu/Cu2+ Br-/Br-2 and H-2/H+ were determined. The reaction mechanisms for Cu/Cu2+ and Br-/Br-2 electrodes were discussed from the view of entropy changes. Comparing with conventional and absolute ionic entropy systems, it was found that the conventional system is more convenient for practical use. (C) 1999 Elsevier Science S.A. All rights reserved.