Introduction

The purpose of this chapter is to outline the simplest methods of arriving at a description of the distribution of species in mixtures of liquids, gases and solids. Homogeneous equilibrium deals with single phase systems, such as electrolyte solutions (e.g., seawater) or gas mixtures (e.g., a volcanic gas). Heterogeneous equilibrium involves coexisting gaseous, liquid and solid phases.

Finding the distribution of components (‘speciation’) in all the phases of any system requires application of rather simple rules (Van Zeggeren and Storey, 1970):

1. (a) the mass conservation condition: unless radioactive decay is present, atoms and exchangeable particles like electrons must be conserved;

2. (b) the minimum Gibbs free energy condition: according to the Second Principle, a system must spontaneously evolve towards the lowest attainable energy condition, either stable or metastable;

3. (c) the phase rule, which tells us for a given choice of components (either atoms, such as O and Na, or particles such as protons and electrons) and for a number of externally imposed constraints (pressure, temperature, oxygen fugacity, …) how many phases are present.

Condition (a) has been addressed repeatedly in this book, and we will see from different examples how two contrasting approaches to condition (b) may be used, through either the mass action laws or direct minimization of the Gibbs free energy. Condition (c) is a consequence of (a) and (b) and for a thorough discussion the reader is referred to the books by (i) Van Zeggeren and Storey (1970) and Smith and Missen (1982) for general principles; (ii) by Morel and Hering (1993) and Michard (1989) for the case of electrolyte solutions.