Alkalinity Titration Calculator

Turn bench titration volumes into alkalinity as CaCO₃: total alkalinity plus the hydroxide / carbonate / bicarbonate split from the two endpoints.

Sample volume (mL)

Titrant normality (N)Standard acid, e.g. 0.02 N H₂SO₄.

Titrant to pH 8.3, P (mL)Phenolphthalein endpoint. Enter 0 if the sample never turns pink.

Titrant to pH 4.5, T (mL)Total titration to the methyl-orange endpoint.

Alkalinity (as CaCO₃)

Species (as CaCO₃)	mg/L

How the calculation works

APHA 2320 B — titration to pH 4.5
Alkalinity (mg/L as CaCO₃) = titrant (mL) × N × 50,000 ÷ sample (mL)
(50,000 mg/eq is the equivalent weight of CaCO₃.)

Speciation from the two endpoints (P = alkalinity to pH 8.3, T = total alkalinity, all as CaCO₃)
P = 0 → bicarbonate = T
2P < T → carbonate = 2P, bicarbonate = T − 2P
2P = T → carbonate = T
2P > T → hydroxide = 2P − T, carbonate = 2(T − P)
P = T → hydroxide = T

Notes & limitations

Method and speciation relationships per APHA Standard Methods for the Examination of Water and Wastewater, Method 2320 B (Alkalinity — Titration Method). Results are conventionally reported as mg/L as CaCO₃.

The pH 8.3 endpoint marks conversion of carbonate to bicarbonate; the pH 4.5 endpoint marks conversion of bicarbonate to carbonic acid. Use a calibrated pH meter rather than color indicators for the most defensible endpoints.
Interferences: soaps, oily matter, suspended solids, and precipitates can blur the endpoints; phosphates and silicates contribute to the titration. See Method 2320 for the full interference discussion.
The speciation table assumes the sample contains only hydroxide, carbonate, and bicarbonate contributors. If borates, silicates, or phosphates are significant, the split is approximate.
Titrant normality must be standardized and current — an old normality is the most common source of error.

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