Other indices do not account for two other critical parameters: the buffering capacity of the water, and the maximum quantity of precipitate that can form in bringing water to equilibrium. The PSI attempts to quantify the relationship between saturation state and scale formation by incorporating an estimate of buffering capacity of the water into the index.
Water high in calcium, but low in alkalinity and buffering capacity can have a high calcite saturation level. The high calcium level increases the ion activity product. A plot of ion activity product versus precipitate for the water would show a rapid decrease in pH as calcium precipitated due to the low buffering capacity. Even minuscule decreases in carbonate concentration in the water would drastically decrease the ion activity product due to the small quantity present prior to the initiation of precipitation.
Such water might have a high tendency to form scale due to the driving force, but scale formed might be of such a small quantity as to be unobservable. The water has the driving force but not the capacity and ability to maintain pH as precipitate matter forms.
The PSI index is calculated in a manner similar to the Ryznar stability index. Puckorius uses an equilibrium pH rather than the actual system pH to account for the buffering effects:
PSI = 2 (pHs) - pHeq
pHs is the pH at saturation in calcite or calcium carbonate
pHeq = 1.465 x log10[Alkalinity] + 4.54
and [Alkalinity] = [HCO3-] + 2 [CO32-] + [OH-]
See also: Calcium carbonate, Carbon dioxide, Chlorination, Dissolved oxygen, Langelier calculation, Langelier index, Larson-Skold index, Oddo-Tomson index, pH, Puckorius index, Ryznar index, Scaling Indices, Stiff-Davis index, Total dissolved solids, Water corrosivity