THEORY OF MEASUREMENT OF WEAK MOLECULAR COMPLEXES .2. CONSEQUENCES OF MULTIPLE EQUILIBRIA

The consequences of multiple equilibria on the calculation of formation constants and other molecular constants for weak complexes are considered from a theoretical standpoint. A microscopic model is used to illustrate the averaging process involved in the assignment of formation constants for higher order complexes, and the slopes, and intercepts of various plotting forms usually applied to the evaluation of weak complexes are derived for a PX, PX2 system. It is shown that, except in the special case where all the microscopic constants of the system are closely similar, curved lines are to be expected on all of the usual plots. At very low saturation fractions, however, curvature is small, and in the limit as the concentration of the excess component approaches zero, the plotted curves are indistinguishable from straight lines. The slopes and intercepts of such lines have widely differing experimental meaning and cannot be interpreted simply as “K” or “Kє” except in cases of very little practical interest. From the shape of the theoretical curves, it appears that it is necessary to measure of the order of 75% of the entire saturation curve before a given stoichiometric model can be assigned to the fitted data. © 1969, American Chemical Society. All rights reserved.