mg/L as Calcium Carbonate

Definition

The convention of expressing alkalinity and hardness as the mass of calcium carbonate that would carry the same neutralizing or reactive capacity, rather than the actual mass of the species present.

Different species vary in molecular weight and, more importantly, in how much acid they can neutralize, so reporting raw mass would compare apples to oranges. Converting everything to mg/L as CaCO3 places hardness and alkalinity on a single equivalent basis where they can be compared in softening calculations, scaling indices, and charge-balance relationships. CaCO3 was chosen as the reference because its numbers are convenient: 100 g/mol mass and 2 H+ neutralized per mole.

Related concepts

Hardness as CaCO3 · Milliequivalents per Liter · Equivalent Weight

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Part of The Blueprint of Water · 📏 measurement