The Langelier saturation index (sometimes Langelier stability index) is a calculated number used to predict the calcium carbonate stability of water. It indicates whether the water will precipitate, dissolve, or be in equilibrium with calcium carbonate. In 1936, Wilfred Langelier developed a method for predicting the pH at which water is saturated in calcium carbonate (called pH). The LSI is expressed as the difference between the actual system pH and the saturation pH:
:LSI = pH (measured) − pH
* For LSI > 0, water is supersaturated and tends to precipitate a scale layer of CaCO.
* For LSI = 0, water is saturated (in equilibrium) with CaCO. A scale layer of CaCO is neither precipitated nor dissolved.
* For LSI .
If the actual pH of the water is below the calculated saturation pH, the LSI is negative and the water has a very limited scaling potential. If the actual pH exceeds pHs, the LSI is positive, and being supersaturated with CaCO, the water tends to form scale. At increasing positive index values, the scaling potential increases.
In practice, water with an LSI between −0.5 and +0.5 will not display enhanced mineral dissolving or scale-forming properties. Water with an LSI below −0.5 tends to exhibit noticeably increased dissolving abilities while water with an LSI above +0.5 tends to exhibit noticeably increased scale-forming properties.
The LSI is temperature-sensitive. The LSI becomes more positive as the water temperature increases. This has particular implications in situations where well water is used. The temperature of the water when it first exits the well is often significantly lower than the temperature inside the building served by the well or at the laboratory where the LSI measurement is made. This increase in temperature can cause scaling, especially in cases such as water heaters. Conversely, systems that reduce water temperature will have less scaling.
:   Water analysis:
::        pH = 7.5
::        TDS = 320 mg/L
::        Calcium = 150 mg/L (or ppm) as CaCO
::        Alkalinity = 34 mg/L (or ppm) as CaCO
:    LSI formula:
::      LSI = pH − pH
::      pH = (9.3 + A + B) − (C + D) where:
::        A =  = 0.15
::        B = −13.12 × log(°C + 273) + 34.55 = 2.09 at 25 °C and 1.09 at 82 °C
::        C = log[Ca as CaCO] - 0.4 = 1.78         
:::           (Ca as CaCO is also called calcium hardness, and is calculated as 2.5[Ca])
::        D = log[alkalinity as CaCO] = 1.53