ABSTRACT
Determining the concentrations of all the relevant chemicals that are present at thermodynamic equilibrium in a given system can, in principle, be carried out by knowing and then solving, the system of equations governing all of the pertinent chemical species. At equilibrium, all of the governing equations must be satisfied. They are of three types: equilibrium constant equations; mass balance equations; and electroneutrality equations. In natural water systems, there are four main sources of pressure: the weight of the overlying atmosphere; the weight of any overlying water; the weight of any overlying confining rock and soil; and surface tension pressurization of small droplets and crystals. Pure materials are like irresistible thermodynamic magnets for all other species. Achieving and maintaining near absolute purity is difficult and expensive. A variety of scales can be used to express solution concentration and activity.
