ABSTRACT
A particular salt may precipitate if its solubility product, Ksp, is exceeded. The stoichiometric solubility product of a salt is defined as the product of the molalities of the salt ions in a water solution saturated with salt. The stoichiometric solubility product of CaSO4 is, for example, defined as:
K (CaSO ) m msp 4 Ca SO4
of CaSO4 is defined
as the product of the activities of Ca++ and SO4 −
in a saturated solution in water:
K CaSO asp
( )4 = ++ −−a (17.2)
where
a m ; a m
The term γ stands for activity coefficient and is defined in Equation A.34 in Appendix A. By combining Equations 17.1 and 17.3, it can be seen that the stoichiometric and thermodynamic solubility products are related as follows (again exemplified through CaSO4):
K K
=
γ γ (17.4)
It can be seen from this equation that the stoichiometric and thermodynamic solubility products become equal if all activity coefficients are 1.0.
