The Langelier Saturation
Index (LSI) |

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
Ca^{2+} as CaCO_{3}), 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:

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 :

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.

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