Ion Concentration as CaCO₃ Equivalent
Worked example: 40 mg/L Ca2+ at EW 20.04 → 99.88 mg/L as CaCO3 — press Try an example to run it live, then adjust anything.
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The CaCO₃ yardstick →
Grade 11Grade 11 Chemistry
Everything as CaCO₃ →
UniversityProcess & Water Chemistry
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Ion Concentration as CaCO₃ Equivalent explained
A water analysis lists a dozen ions with a dozen different atomic weights, and you cannot add or subtract them directly — 20 mg/L of calcium and 20 mg/L of sodium are not the same amount of chemistry. Expressing everything "as CaCO₃" fixes that by converting each ion to the mass of calcium carbonate that carries the same number of charge equivalents. Calcium carbonate has a molar mass of 100.09 and a valence of 2, so its equivalent weight is 50.04 g/eq; divide that by the ion's own equivalent weight and you have the factor. Calcium (EW 20.04) gets 2.497, magnesium (12.15) gets 4.118, sodium (23.0) gets 2.18, bicarbonate (61.0) gets 0.82, and sulphate (48.03) gets 1.04. Forty mg/L of Ca²⁺ becomes 40 × 2.497 = 99.9 mg/L as CaCO₃.
Once every ion is on the CaCO₃ scale you can do the things that make an analysis useful: check that cations balance anions to within a few percent (the classic sanity test on any lab report), split total hardness into carbonate and non-carbonate fractions by comparing hardness to alkalinity, and size a softener or a dealkalizer directly from the numbers. The trap is the word "equivalent" — the divisor is the equivalent weight, molar mass divided by charge, not the molar mass. Using 40.08 for calcium instead of 20.04 halves every hardness figure you produce.
Ion Concentration as CaCO₃ Equivalent formula
- = Concentration as CaCO₃ (%)
- = Concentration as the ion (%)
- = Equivalent weight of the ion (g/mol)
Missing one of these? Work it out first, then come back
- Concentration as CaCO₃ — BOD Mass Loading, Glycol Dilution
- Concentration as the ion — BOD Mass Loading, Glycol Dilution
- Equivalent weight of the ion — Equivalent Weight from Molar Mass and Valence, Acid Feed to Reduce Alkalinity