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 develope d for determining the entropy change Delta S of an electrode reaction. It w as found that the Peltier heat could be determined separately from the pola rization heat and Joule heat by use of a lock-in amplifier with the referen ce frequency the same as the frequency of the ac current. For two reversibl e electrode reactions the Delta S value for each electrode could be calcula ted 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 ent ropy change of the whole cell reaction. The Fe(CN)(6)(4-)/Fe(CN)(6)(3-) ele ctrode 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 t he 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 elec trodes were discussed from the view of entropy changes. Comparing with conv entional and absolute ionic entropy systems, it was found that the conventi onal system is more convenient for practical use. (C) 1999 Elsevier Science S.A. All rights reserved.