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Langelier Saturation Index

The Langelier Saturation Index (LSI) is a measure of whether water will precipitate or dissolve calcium carbonate scale and can be used to indicate water's corrosivity. LSI is calculated based on parameters like pH, alkalinity, calcium hardness and temperature. An LSI greater than 0 means water is scale-forming, less than 0 means it is corrosive, and equal to 0 means the water is in equilibrium.

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312 views2 pages

Langelier Saturation Index

The Langelier Saturation Index (LSI) is a measure of whether water will precipitate or dissolve calcium carbonate scale and can be used to indicate water's corrosivity. LSI is calculated based on parameters like pH, alkalinity, calcium hardness and temperature. An LSI greater than 0 means water is scale-forming, less than 0 means it is corrosive, and equal to 0 means the water is in equilibrium.

Uploaded by

Hamid Ullah
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© © All Rights Reserved
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Langelier Saturation Index (LSI) and its importance in water chemistry

Langelier Saturation index (LSI) is an important term in water chemistry. LSI is a measure of a
solution’s ability to dissolve or deposit calcium carbonate, is often used as an indicator of the
corrosivity of water. It is used for finding ability of water for corrosion or scaling in heat
exchanger where water is used as heat transfer media in either side of heat exchanger. The
scaling in pipe or heat exchanger tubes can cause insulation for heat transfer or metal loss in
case of corrosion tendency of water. LSI is an equilibrium model derived from the theoretical
concepts of saturation and helps in indicating the degree of saturation of water with respect to
calcium carbonate.

Langelier derived an equation for the pH at which water is saturated with calcium carbonate
(pHs). This equation is based on the equilibrium expressions of calcium carbonate solubility and
bicarbonate dissociation. To approximate actual conditions more closely, pHs calculations were
modified to include the effects of temperature and ionic strength

He Langelier Index is defined as the difference between actual pH (measured) and calculated
pHs. The magnitude and sign of the LSI value show water’s tendency to form or dissolve scale,
and thus to encourage corrosion. Although information obtained from the LI is not quantitative,
it can be useful in estimating water treatment requirements for heat exchanges in process
industry, low pressure boilers, cooling towers and its network, and water treatment plants, as
well as a general indicator of the corrosivity of water.

LSI = pH (measured) – pHs

 For LSI > 0, water is super saturated and tends to precipitate scale layer of CaCO 3.
 For LSI = 0, water is saturated (in equilibrium) with CaCO 3. A scale layer ofCaCO 3 is
neither precipitated nor dissolved.
 For LSI < 0, water is under saturated and tends to dissolve solid CaCO 3.

For calculating LSI, it is necessary to know the

 alkalinity (mg/l as CaCO3),


 calcium hardness (mg/lCa2+ as CaCO3),
 the total dissolved solids (mg/l TDS),
 the actual pH,
 The temperature of the water (oC).

LSI = pH – pHs
Where:

 pH is the measured water pH


 pHs is the pH at saturation in calcite or calcium carbonate and is defined as:
 pHs = (9.3 + A + B) – (C + D)
Where:
 A = (Log10 [TDS] – 1) / 10
 B = -13.12 x Log10 (oC + 273) + 34.55
 C = Log10 [Ca2+ as CaCO3] – 0.4
 D = Log10 [alkalinity as CaCO3]

To find the LSI online directly by putting the values kindly use the following link

https://www.energypurse.com/lsi-calculator/

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