The Stiff-Davis index attempts to overcome the shortcomings of the Langelier Index with respect to high total dissolved solids' waters and the impact of "common ion" effects on the driving force for scale formation. Like the LSI, the Stiff-Davis index has its basis in the concept of saturation level. The solubility product used to predict the pH of saturation (pHs) for a water is empirically modified in the Stiff-Davis index. Stiff-Davis indices will predict that a water is less scale forming than the LSI calculated for the same water chemistry and conditions. The deviation between the indices increases with ionic strength. Interpretation of the index is by the same scale as for the Langelier Saturation index.
Stiff, Jr., H.A., Davis, L.E., A Method For Predicting The Tendency of Oil Field Water to Deposit Calcium Carbonate, Pet. Trans. AIME 195;213 (1952).
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