Solubility Product and Saturation Index

Equilibrium constants for various kinds of reactions have been given various names. Reactions such as (3.2), with a solid phase on one side and its constituent ions on the other, is called a solubility product reaction, and the equilibrium constant for the reaction is called the solubility product, Ksp. In this case, we have found that Ksp 3.2) = 

Note that Q and K are identical in form. The difference is that the activity terms in 2 are not equilibrium activities, while those in K are. Similarly, if calcite is present, CaCo3(.v) = 1, and K = Ksp. If Q = Ksp, Ar/i = 0, and calcite is at equilibrium with its aqueous ions. When IAP Ksp, A, // 0, and calcite will precipitate. When 

IAP Ksp, Ar/x 0, and calcite will dissolve. The quantity (aCa2+ aco2-) in a real solution is called, appropriately enough, the Ion Activity Product (IAP) for calcite, and similarly for any other solubility product reaction. The AP/Ksp ratio is called 2, and the logarithm of the ratio is called the Saturation Index (SI), so that when SI 0 the mineral precipitates, and when SI 0 the mineral dissolves (Table 3.2). 

Speciation of sample 912-18 (at 25°C) in Merino (1975) using program solmin88 gives the following results for calcium and carbonate (see Table 3.4)  

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