The Langelier Saturation Index (L.S.I.) is a means of predicting the probable nature of a given water supply i.e. whether it will have predominantly scaling or corrosive tendencies when applied to an industrial system. A further refinement of the Langelier Index in the Ryznar Index. Both indices use pHs which is the pH at which a given water would be saturated with CaCO3.
The Langelier Index is an approximate indicator of the degree of saturation of calcium carbonate in water. It is calculated using the pH, alkalinity, calcium concentration, total dissolved solids, and water temperature of a water sample.
Langelier = pH – pHs
Where 0 is neutral, increasing negatively equals increasing corrosion tendency and increasing scaling tendency is seen by an increasingly positive number.
Ryznar = 2pHs – pH
where 6.0 is neutral above 6.0 = increasing corrosion tendency below 6.0 = increasing scaling tendency.
pH: this variable is the most likely to shift up and down. In the LSI calculation, it has no factor, just the pH value itself.
Temperature (ºF): Temperature affects the speed of chemical reactions in water. The lower the temperature, the easier it is for corrosive reactions to occur. The higher the temperature, the easier it is for calcium to come out of suspension.
Calcium Hardness: This is a measurement of how much calcium is dissolved in the water. Water that is over-saturated with calcium is likely to be more scale forming, but only if the pH and total alkalinity allow for it to come out of solution. Calcium hardness—like total alkalinity—serves as a buffer for pH.
Alkalinity: In the original Langelier Saturation Index formula total alkalinity is used.
Total Dissolved Solids (TDS): TDS is a measurement of everything that is dissolved in the water, floating around in suspension. It is measured in parts-per-million, and can include anything from calcium to metals and other chemicals.
pHs describes the value attained when all of the various forms of alkalinity are adjusted to the point where the water is just saturated in calcium carbonate.
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