How much may the equilibrium be shifted in a chemical relaxation experiment? Part 2.—Two-step systems

Abstract

A simple method for determining the allowable maximum amplitude (a.m.a.) in a relaxation experiment is applied to systems with two kinetically uncoupled equilibria. This method is based on the use of factor r in the relaxation equation 1/v_ij=dΔ[X_i]//_dtτ^–1_jΔ[X_i](1 +r_j) for the ith component of the jth step. r is a thermodynamic parameter related to the extent of the equilibrium shift and determines the deviation from linearity. Expressions of r for the two equilibrium steps are given for the most common systems. The linearization condition, |r| 1, defining the allowable amplitude, is discussed in connection with the signal-to-noise ratio of experimental signals. In favourable cases large values of r can be used if the relaxation times are corrected. By applying this method to an acid–base equilibrium in a buffer, or coupled with an indicator, it can be shown that linearity can be excellent even for large perturbations in many practical cases.