The Langelier Saturation Index (LSI)
Langelier Saturation Index (LSI) or Saturation Index (SI)

The Saturation Index is an equilibrium model derived from the theoretical concept of saturation and provides an indicator of the degree of saturation of water with respect to calcium carbonate. It can be shown that the SI proximates the base 10 logarithm of the calcite saturation level. The Langelier saturation level approaches the concept of saturation using pH as a main variable. The SI can be interpreted as the pH change required to bring water to equilibrium.
Water with a Langelier saturation index of 1.0 is one pH unit above saturation. Reducing the pH by 1 unit will bring the water into equilibrium. This occurs because the portion of total alkalinity present decreases as the pH decreases, according to the equilibria describing the dissociation of carbonic acid:

• If SI is negative: No potential to scale, the water will dissolve CaCO3
• If SI is positive: Scale can form and CaCO3 precipitation may occur
• If SI is close to zero: Borderline scale potential. Water quality or changes in temperature, or evaporation could change the index.

The SI is probably the most widely used indicator of cooling water scale potential. It is purely an equilibrium index and deals only with the thermodynamic driving force for calcium carbonate scale formation and growth. It provides no indication of how much scale or calcium carbonate will actually precipitate to bring water to equilibrium.

It simply indicates the driving force for scale formation and growth in terms of pH as a master variable. In order to calculate the SI, it is necessary to know the alkalinity (mg/l as CaCO3), the calcium hardness (mg/l Ca2+ as CaCO3), the total dissolved solids (mg/l TDS), the actual pH, and the temperature of the water (°C). If TDS is unknown, but conductivityor resistivity is, one can estimate mg/L TDS using a conversion table, or by using the Dry Residue (mg/L DR, see Parameters file).

SI is defined as:

SI = pH - pHs

Where:

• pH is the measured water pH
• pHs is the pH at saturation in calcite or calcium carbonate (Hallopeau & Dubin method) and is defined as :
pHs = C - (log10 Alk + log10 CaO)

Where:

• C is a constant that depends on the mineralization of water and temperature [C = log K's - log K'2 + 9,2 ]
• Alk et CaO are the total alkalinity and calcium content, expressed as equivalent CaO (mg/L).

Remark:
Others index are the Ryznar Index (RI) and the Larson Index (LI) - Larson consider chlorides, sulphates and total alkalinity.
However, there is some controversy concerning the correlation of these indices, and particularly the LSI, with the corrosivity of waters. While some sectors of the water management industry squarely use the indices as a measure of the corrosivity of their waters, more alert specialists are very cautious as to how far one can extrapolate the indices to such usage. (use your browser)