TRANSIENT METHODS - COMPARISON OF 3 METHODS EMPLOYING THE CD2-BAR-CD(HG) COUPLE(VERTICAL)

A combination of three transient techniques (coulostatic, modified cou lostatic and galvanostatic) has been applied for the study of the elec trode reaction in aqueous solution, 1 M Na2SO4, pH 4. The techniques a re applied under optimal conditions as determined by the information c ontent curves. The modified coulostatic method, for which charging is performed by a galvanostatic pulse of a width of several tau(c), has b een applied for the first time. The classical coulostatic method has n ot been applied in its pure form, i.e. t(p) much less than tau(c), but with somewhat longer pulses and smaller current densities, in order t o avoid saturation of the measuring device during the charging step, a nd the evaluation of the results was performed using the equation for the coulostatic method with finite pulse width, derived for the modifi ed coulostatic method. The three methods complement each other: with t he classical coulostatic method the optimization of the experimental c onditions is simplest; the modified coulostatic method ensures the hig hest accuracy in the determination of j(0) and the galvanostatic metho d makes it possible to study the process on a longer time scale. With the three methods combined, the reaction can be studied in a time rang e of (1-40)tau(c) or more, depending on the ratio of tau(c)/tau(d). A two-indicator electrode cell was used in order to increase the overpot ential far which linearization of the overpotential/time plot is valid . The maximum value of the overpotential (not including IR(s)) was 6 m V. The exchange current density and the double layer capacitance were determined at different concentrations of Cd2+ and Cd(Hg). The stoichi ometric number nu was estimated to be 1. Cathodic and anodic transfer coefficients were calculated and corrected for the diffuse-double-laye r effect, according to Frumkin's theory. The transfer coefficients wer e found to be independent of temperature in the range 5-65 degrees C. alpha(a) = 1.59, alpha(c) = 0.23. The values corrected for diffuse dou ble layer were found to be alpha(a)(phi 2=0) = 1.69, alpha(c)(phi 2=0) = 0.12. The values of the standard rate constant at 25 degrees C are: k(sh) = 0.026 cm s(-1); k(sh)(phi 2=0) = 0.013 cm s(-1). The standard enthalpy and entropy of activation are: Delta H-eta=0uncorr(not equal 0) = 33 kJ mol(-1), Delta H-eta=0corr(not equal=0) = 32 kJ mol(-1); D elta S-eta=0uncorr(not equal 0) = 5 J (mol K)(-1), Delta S-eta=0corr(n ot equal 0) = -7 J (mol K)(-1).