About 40 years ago, relaxation techniques were developed by Eigen and cowor
kers. These techniques provided a new and powerful method to investigate "f
ast" reactions in solution. To apply these techniques, the system, after pe
rturbation, should remain in a position close to equilibrium, so that the k
inetic equations can be linearized. If the system is brought to a position
far from equilibrium, the full system of dynamical equations should be cons
idered. Here (experimentally feasible), oscillation and chaos may appear. B
esides, the classical extension of relaxation techniques to stationary stat
es is not so straightforward as usually stated in the literature on relaxat
ion phenomena. Another possible problem of the classical approach to relaxa
tion techniques comes from the analysis of the total concentration change i
n normal modes of reaction. Normally, one or two of the relaxation times, c
an be determined from the kinetic trace. However, all of them are responsib
le for the total concentration change. Thus, a systematic error is associat
ed with this procedure. For these reasons, the purpose of this article is t
wofold. First, we point out these shortcomings and show the way to handle t
hem, and, second, we propose the use of sensitivity analysis as a new metho
d to extract thermodynamic information from relaxation experiments. (C) 200
0 Elsevier Science B.V. All rights reserved.