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
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:
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
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:
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.