Langelier Saturation Index (LSI) is a calculation that uses the data of multiple analyses to estimate how corrosive or scale-forming a water is. The chemistry of the water is analyzed, as well as the environmental conditions determined, to evaluate how likely the water would be to corrode or form a scale in pipes. LSI is essentially a measurement of the saturation of calcium carbonate in the water. If the water is over-saturated with calcium carbonate, minerals will precipitate out of the water, and the water will be scale-forming. If the water is under-saturated with calcium carbonate, it will dissolve minerals from the pipes, causing the water to be corrosive.
The tests performed to calculate the LSI of a water include pH, calcium hardness, total alkalinity, and total dissolved solids. The temperature of the in-service water is also used in the calculation.
The calculation is as follows:
where: pH is the measured pH of the water
pHs is the calculated saturation pH of the water
The saturation pH (pHs) is calculated as follows:
where: pK’2 is the negative log of the activity constant K’2
pCa2+ is the log of 1 / the concentration of Ca2+
pK’s is the negative log of the activity constant K’s
pCa2+ is the log of 1 / the concentration of Ca2+
Alk is the alkalinity concentration in moles / liters
ym is the activity coefficient of the monovalent ions
These variables are all determined from the analyses performed and then entered into the calculation to determine the LSI. LSI values range between +4 and -5, where positive values would indicate the water is likely to form a scale, and negative values would indicate the water is likely to contribute to corrosion of the pipes. An LSI value of 0 would indicate the water is neutral and unlikely to promote corrosion or formation of scale. An LSI of -0.5 would indicate the water is slightly corrosive, but would not be scale forming. An LSI of 0.5 would indicate the water would be slightly scale forming, but not likely corrosive. As LSI decreases, the potential for corrosion increases. As LSI increases, the potential for the formation of scale increases.
Langelier Saturation Index testing can be performed at the Materials Analytical Group laboratory. For more information contact Matt Anderson at 847.306.9240 or manderson@materialsanalyticalgroup.com. And for 15% off our services, let us know you have been reading our articles before receiving your invoice.
REFERENCES
Inspectapedia. (n.d.). Inspectapedia. Retrieved from Langelier Saturation Index: https://inspectapedia.com/water/Langlier-Saturation-Index.php
Labrador, G. o. (n.d.). Calculation of the Langelier Index. Retrieved from Deparment of Environment and Conservation: https://www.mae.gov.nl.ca/waterres/quality/drinkingwater/pdf/calculation_langelier_index.pdf
LennTech. (n.d.). Langelier Saturation Index Calculator. Retrieved from LennTech: https://www.lenntech.com/calculators/langelier/index/langelier.htm
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