Saturation pH (pHs) for Langelier's Index

This is the engine underneath every LSI calculation, and the reason so many operators quietly distrust the number: it is four separate correction terms bolted together. A handles ionic strength through TDS, B handles temperature through the solubility product of calcite, C is the calcium hardness term and D the alkalinity term: both expressed as mg/L CaCO₃, never as the raw ion. Older handbooks hand you A, B, C and D from four printed lookup tables; this page uses the published closed-form fits instead, so temperature and TDS go in as real measurements rather than as a table row you rounded to the nearest 100. Work an example: TDS 400 mg/L, 25 °C, calcium hardness 240 mg/L as CaCO₃, alkalinity 180 mg/L as CaCO₃ gives A = 0.160, B = 2.085, C = 1.980, D = 2.255, so $\mathrm{pH}_s$ = 9.3 + 0.160 + 2.085 − 1.980 − 2.255 = 7.31.

Two traps catch people every time. First, C wants calcium hardness, not total hardness: feeding it a total-hardness number that includes magnesium inflates the calcium term and makes the water look far more scale-forming than it is. Second, B is strongly temperature-sensitive: the same water at 60 °C has B = 1.45 instead of 2.09, dropping $\mathrm{pH}_s$ by more than half a unit and pushing the LSI up by the same amount. That single term is why a boiler feedwater heater scales while the cold main it is fed from stays clean, and why any LSI you quote should carry the temperature you assumed.