Chemical Equilibria

Solids in Chemical Equilibria

Solids have names ending with either:

For solids, like for aqueous complexes, the logK-value for the formation reaction is used. Note that this is the inverse of the solubility product, i.e.: logKf = −logKsp

Owing to the Gibbs phase rule, the number of solids that may be present simultaneously in a chemical system is limited.

Unlike aqueous species, which have concentrations that may become gradually small, solid phases either are stable, or not. The concentration usually changes in a narrow range of conditions between the maximum possible amount, and zero.

If a solid is present in a system, its composition is constant, and its activity may be set equal to one (log activity = 0). Its concentration in mol/(L of solution) may be calculated and plotted. If the solid is not present, its concentration is zero, and its activity is undefined. In that case the programs calculate the saturation index (SI) instead of the activity:

SI = logKf° − log IAP

where Kf is the equilibrium constant for the formation of the solid, and IAP is the ion activity product for the solid (the actual product of activities as defined in the equilibrium constant of formation).