THE LANGELIER SATURATION INDEX (LSI) aka THE LANGELIER INDEX (LI)

 

The Langelier Index is, simply, a measure of the departure of a water sample from equilibrium with solid CaCO3:

LI = pHmeasured - pHequilibrium            

pHeq = (9.3 + A + B) – (C + D)  

Where             1. A = (Log10[total dissolved solids] – 1)/10                corrects for ionic strength

                        2. B = 13.12 x log10(Degrees Kelvin) +  34.55              corrects for temperature 

                        3. C = Log10[Ca as CaCO3] – 0.4                       the Ca half of the solubility product       

                        4. D = Log10[alkalinity as CaCO3]                      the CO3 half of the solubility product 

 

Schock (1984) provided updated tables for calculating the A and  B parameters:

                                  

Schock, Michael R. "Temperature and ionic strength corrections to the Langelier Index—Revisited." Journal American Water Works Association 76, 8 (1984): 72-76.

The US Geological Survey has made an extensive study of groundwater compositions in which the Langelier Index and the CSMR were calculated (Belitz et al. 2016). They recommended the following levels of corrosion potential for the Langelier Index:

 

Langelier Index Scale Formation Potential
<-0.5 scale dissolving
- 0.5 to +0.5 neutral
> 0.5 scale depositing

Strictly speaking, these calculations only apply to CaCO3 scales, but the report treats this index as generally indicative of corrosion potential.

Belitz, K., Jurgens, B.C. and Johnson, T.D., 2016. Potential corrosivity of untreated groundwater in the United States (No. 2016-5092). US Geological Survey.

https://pubs.er.usgs.gov/publication/sir20165092

 

Distribution of values of the Langelier Index for a set of representative US water systems. Most systems fall in the "neutral" range

The Langelier Index is a perfectly good measure of the potential for corrosion (or for scaling) of calcite in distribution systems. It's usefulness as a general indicator of corrosion potential however is open to question. There is no mechnism connecting the behavior of calcite to that of other scale minerals other than possibly for carbonates of lead and copper. See The Langelier Index and LCR.