The Langelier Index (LI) is a system for estimating or predicting the amount or degree of problems with lime scale a particular water supply will cause. It is based on calculating the pH at which the water would reach the saturation point for calcium carbonate (pHs) using the data from a chemical analysis of the water.

Specifically, the LI is equal to the actual pH minus the calculated pHs. This produces a number, usually between -3 and +3. If the LI is positive, the water will deposit calcium carbonate; if the LI is negative, the water will dissolve calcium carbonate. To calculate the pHs, you need to know the total dissolved solids (TDS), the concentrations of calcium ion and total alkalinity, and the actual pH of the water. You also need to decide what temperature you’re interested in, and it is useful to have an electronic calculator that will give you the “logs” (Iogarigthms) of the concentrations of calcium and alkalinity. A short log table is given below, along with tables for special constants derived from the temperature and the TDS.

pHs = A + B – log(Ca+2) – log (alkalinity)

where A = constant derived from temperature
B = constant derived from TDS

Note that a negative LI is sometimes misused to predict the corrosiveness of a water to metal plumbing materials: The LI does have some influence on corrosion, but it is only one of many factors, and it is too simplistic an answer to that question.

Temp

Constant

TDS

Constant

Ca+2 or alk.

 

°C

A

mg/L

B

mg/L as CaCO3

log

0
4
8
12
16
20
25
30
40
50
60
70
80

2.60
2.50
2.40
2.30
2.20
2.10
2.00
1.90
1.70
1.55
1.40
1.25
1.15

0
100
200
400
800
1000

9.70
9.77
9.83
9.86
9.89
9.90

10
20
30
40
50
60
70
80
100
200
300
400
500
600
700
800
900
1000

1.00
1.30
1.48
1.60
1.70
1.78
1.84
1.90
2.00
2.30
2.48
2.60
2.70
2.78
2.84
2.90
2.95
3.00

Example LI Calculation

Given: pH = 8.40; TDS = 400 ppm; Ca+2 = 200 ppm as CaCO3 Alk. = 300 ppm as CaCO3. How bad will scaling be in an ice maker? (0°C)
LI = pH – [A + B – log Ca+2 – log alk.]
= 8.40 – [2.60 + 9.86 – 2.30 – 2.48]
= 8.40 – 7.68
= +0.72

Interpretation of LI Values

+3 = very severe scaling tendency
+2 = severe scaling tendency
+1 = moderate scaling tendency
0 = no scaling tendency
-1 = slightly corrosive to lime scale
-2 = moderately corrosive to scale
-3 = severely corrosive to lime scale

When softening is not an option, the scale-forming potential of the water can be limited by reducing the pH or the alkalinity, which are discussed above. But often the simplest and most cost-effective treatment to inhibit lime scale formation and deposition is Everpure’s proprietary InsuriceTM System, which is the combination of fine-filtration and low-level polyphosphate feed. (Regardless of the name, the Insurice approach also works very well for coffee brewing. The conditions inside steamers are too extreme, so they are treated by dealkalization.) Polyphosphate treatment is very important and needs its own discussion.