Estimating pH from Hydroxide Alkalinity

Definition

A method to estimate high-range pH without a meter by converting hydroxide alkalinity to molar hydroxide concentration and computing pOH and pH.

Hydroxide alkalinity in ppm as CaCO3 is divided by the CaCO3 equivalent weight of 50,000 mg/eq to get eq/L, which equals mol/L because hydroxide is monovalent. Taking pOH as the negative log of the hydroxide molarity and subtracting from 14 gives pH, for example 300 ppm yielding pH 11.78. Dividing by molecular weight (100) instead of equivalent weight (50) is a common handbook error that introduces about a 0.3 pH unit mistake.

Related concepts

mg or L as Calcium Carbonate · Equivalent Weight · P or M Speciation Relationships

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Part of The Blueprint of Water · 📏 measurement