APPLICABILITY OF DC RELAXATION TECHNIQUES TO MULTISTEP REACTIONS

The general theory of d.c. relaxation techniques has been developed ma inly for single-step electrochemical reactions and for multi-step reac tions with a clearly defined rate-determining step and no intermediate accumulation either on the electrode surface or in the solution. A fe w workers have considered the case of multi-step reactions, but, becau se of the complexity of these systems, different approximations and si mplifications were introduced in every treatment, limiting the general usefulness of the conclusions. Using numerical calculational methods, we have investigated the behavior of two-step (metal deposition/disso lution) reactions for potentiostatic and galvanostatic single- and dou ble-pulse relaxation experiments. We have carried out a large number o f numerical simulations using a wide range of variable values. The mai n purpose was to determine the conditions under which the techniques a re applicable for the measurement of the rate constant of the fast and the slow step of the reaction sequence. In particular, two 'critical times' were determined: (i) the time to reach 'steady-state' condition s with the transient techniques and (ii) the time available for the de termination of the fast-step kinetics at the beginning of the measurin g pulse. We have succeeded in representing these conditions in graphic al form as a function of parameters involving only a few (mostly known ) variables. We also found that the appearance of a maximum/minimum in the relaxation curves indicates that only the fast-step kinetics can be determined.