Analysis of the temperature dependence of the α and β coefficients of the electrode reactions in the Cr(III)/Cr(II) and Cr(II)/Cr(Hg) systems

The temperature dependences of the four transfer coefficients for cathodic and anodic electrode reactions of two different types of systems, namely Cr(III)/Cr(II) and Cr(II)/Cr(Hg), were determined in 5.0 M NaClO4 solution. The results were obtained for a wide range of temperatures from -20 to 90 degrees C. The alpha parameter determined from the log k(fh) on E dependence corresponding to the Cr3+ + e(-) --> Cr2+ electrode process shows the highest dependence on temperature (partial derivative alpha/partial derivative T = 1.1 +/- 0.3 x 10(-3) K-1), whereas the beta parameter corresponding to the Cr2+ --> Cr3+ + e(-) electrode process is virtually independent of temperature (partial derivative beta/partial derivative T = 0). The same results were obtained when the transfer coefficients were determined from dependence of the enthalpy and entropy of activation on overpotential. The observed temperature dependence of alpha is attributed to the changes of the phi(2) potential. The transfer coefficients for the Cr2 + /0 electrode reaction also show some dependence on temperature (partial derivative alpha/partial derivative T = 8.7 +/- 2.0 x to 10(-4) K-1 and partial derivative beta/partial derivative T = 3.2 +/- 1.4 x 10(-4) K-1), which probably results from the influence of the phi(2) potential. (C) 1999 Elsevier Science S.A. All rights reserved.