Equilibrium Analysis, the Ionic Medium Method and Activity Factors

Abstract

Chemical models are used extensively to describe environmental systems, transport of pollutants, sorption phenomena, etc. The following chapter describes two different models for ionic medium/ionic strength corrections of equilibrium constants used in chemical speciation calculations. Published equilibrium constant data often refer to ionic media very different from those encountered in nature and in engineering systems, and they have therefore to be recalculated to the conditions relevant for the system under study. In addition, one may be interested to compare experimental data obtained in different ionic media in order to ascertain their quality. It is important to notice that models used in science and technology are provisional; they are used as tools, and it is their “practicality” that often decides which model one selects. The prime requirement of a model is that it has a scientific basis and that it is internally consistent. Before describing the scientific basis and structure of ion interaction theories, we will give a short discussion of equilibrium analysis, the method used to obtain precise information of the composition and equilibrium constants, i.e. the data used in speciation calculations.