Overview Activity Chem Equilibrium Acids Carbonate System Mineral Phases

Langelier Saturation Index (LSI)

The Langelier Saturation Index (or Langelier Stability Index) is a measure of the saturation degree of calcium carbonate (CaCO₃) in water.

The LSI was proposed¹ by Wilfred Langelier in 1936 as the difference between the actual or measured pH and the “saturation pH” (abbreviated by pH_S):

(1)	LSI   =   pH – pHS

Perfect saturation is at LSI = 0. Based on the numerical value of LSI, aqion uses the following classification:

LSI ≈ 0	The water is in equilibrium with calcite.
LSI < -0.03	The water tends to be corrosive (calcite dissolves).
LSI >  0.03	The water tends to be scale forming (calcite precipitates).

The results are displayed in a special window as shown here.

There are various approximations to calculate pH_S, that enters 1. The software aqion, however, does not use approximate formulas as it is based on strict thermodynamics: law of mass action plus mass and charge balance (as provided by PhreeqC and its thdyn database).

Just as any other saturation index, LSI only indicates the presence of a driving force; it does not guarantee that calcite will actually dissolve or precipitate, but informs us that there is a tendency to dissolve or precipitate.

Remarks & Footnotes