An equilibrium constraint is an a priori specification of the equilibrium chemical potential of a species. For ideal systems, this is equivalent to fixing its equilibrium composition. This unique feature of EQS4WIN allows it to solve a wide range of important types of problems arising in many areas. Typical examples are:

• specifying the pH of an aqueous solution
• specifying the concentration of ligand in a biochemical ligand-binding problem
• calculating the solubility of a solid in an aqueous solution
• specifying the partial pressure of one or more gaseous products to satisfy a pollutant specification
• specifying the presence of one or more solids in a geochemical or metallurgical system and calculating the partial presssures of species in a coexisting gas phase
• calculating equilibrium in osmotic systems

Several example problems are included with all versions of EQS4WIN which illustrate the feature of equilibrium constraints. In the Example Applications topics list, the Aquatic Chemistry, Biochemistry, and Geochemistry examples all involve the use of this feature.