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 pHS):

(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 pHS, 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

  1. WF Langelier: The Analytical Control of Anti-Corrosion Water Treatment. Jour. A.W.W.A., 28: 1500 (1936). 

[last modified: 2024-01-13]