The Langelier Saturation Index (LSI)

The Langelier Saturation Index (LSI) is a long-established tool for assessing the tendency of water to precipitate or dissolve calcium carbonate (scale formation or corrosion). It was developed to help water chemists and engineers predict and control calcium carbonate stability in engineered water systems, including swimming pools, boilers, cooling towers and drinking-water supplies.

History

• Origin: The Langelier Saturation Index was introduced by Wilfred Langelier, a chemist at the United States Public Health Service, in a 1936 paper. Langelier sought a practical, quantitative way to express whether water was likely to deposit calcium carbonate or to remain corrosive.

• Context: In the early 20th century, scaling and corrosion posed major operational problems in municipal water systems and industrial equipment. Langelier’s work built on earlier understanding of carbonate chemistry and solubility equilibria to produce a simple index that could be applied using routine water analyses available at the time.

• Adoption and evolution: The LSI quickly gained acceptance because it provides a straightforward single-number indicator of water’s carbonate saturation relative to calcium carbonate. Over time, practitioners developed related indices (for example, the Ryznar Stability Index and Puckorius Scaling Index) that address different practical concerns or use different empirical adjustments. For pool water specifically, LSI remains a common baseline tool to inform calcium hardness, alkalinity and pH management to minimise scale and corrosion risk.

What the LSI indicates

• Positive LSI (> 0): Water is supersaturated with respect to calcium carbonate and has a tendency to precipitate scale (calcium carbonate deposition).

• Negative LSI (< 0): Water is undersaturated and tends to be corrosive or will dissolve calcium carbonate from surfaces.

• Near zero LSI (≈ 0): Water is approximately in equilibrium with calcium carbonate and is considered balanced regarding scale/corrosion tendency.

  • The Langelier Saturation Index (LSI) indicates whether water will precipitate, be in equilibrium with, or dissolve calcium carbonate. Calculate it using these steps:

    1. Gather required measurements

       • Water temperature (°C)

       • pH (measured)

       • Total alkalinity (as CaCO3, mg/L or ppm)

       • Calcium hardness (as CaCO3, mg/L or ppm)

       • Total dissolved solids (TDS) or electrical conductivity (optional; used for more precise pKs)

    2. Calculate the ionic strength correction factor (optional for high accuracy)

       • For most pool and spa water, this step may be simplified or omitted. If TDS (mg/L) is known, approximate ionic strength via TDS/1000. For typical pools use the simpler version below.

    3. Compute the pHs (the pH at which water is saturated with CaCO3)

       • Use the standard empirical formula: pHs = (9.3 + A + B) − (C + D) Where: A = (log10[TDS] − 1)/10 B = −13.12 × log10(temperature in °C + 273) + 34.55 (sometimes folded into pKs term) C = log10[calcium hardness as CaCO3 in mg/L] − 0.4 D = log10[total alkalinity as CaCO3 in mg/L]

       • For practical pool calculations, a commonly used simplified form is: pHs ≈ (9.3 + A + B) − (log10[Calcium as CaCO3] + log10[Alkalinity] − 0.4)

       • If you prefer an easier route, many operators use a standard table or calculator to obtain pHs from temperature, TDS, calcium and alkalinity.

       (Note: Different sources present slightly different coefficients; using a reputable pool chemistry reference or a digital calculator ensures accuracy.)

    4. Calculate LSI

       • LSI = pH (measured) − pHs

    5. Interpret the result

       • LSI < 0: Water is undersaturated with CaCO3 and tends to be corrosive (may dissolve calcium-bearing materials).

       • LSI = 0: Water is balanced; neither deposits nor dissolves CaCO3.

       • LSI > 0: Water is supersaturated and tends to precipitate calcium carbonate (scale formation).

    6. Practical target for pools and spas

       • Aim for LSI between −0.3 and +0.3 for most pools. Many commercial pools target slightly positive values (around +0.1 to +0.3) to reduce corrosivity while controlling scale.

    7. Adjusting water chemistry

       • To raise LSI (make water less corrosive/more scaling): increase pH, increase calcium hardness, increase alkalinity, or lower temperature/TDS.

       • To lower LSI (reduce scaling): lower pH, reduce calcium hardness (partial drain/refill), reduce alkalinity, or increase temperature/TDS depending on scenario.