Equivalent Weight of Soda Ash

Soda ash is sodium carbonate, and the carbonate ion accepts two protons on its way down to carbonic acid — CO₃²⁻ to HCO₃⁻ to H₂CO₃ — so the equivalent weight is half the formula mass: 105.988 / 2 = 52.994 g/eq. Each mg/L of pure soda ash therefore adds 50.04/52.99 = 0.94 mg/L of alkalinity as CaCO₃, a shade under one-to-one, which makes dosing arithmetic pleasantly close to "a pound in, a pound of alkalinity". It raises pH far more gently than caustic because carbonate is a weak base: the solution buffers itself instead of spiking at the injection point.

The name is a fossil of the industry that made it — the ash of soda-rich plants, then the Leblanc and Solvay processes that industrialised it, and the Wyoming trona beds that supply most of it now. In treatment work it raises alkalinity where the water also needs the buffering, feeds the lime–soda softening reaction as the carbonate source for non-carbonate hardness, and neutralises acid spills without the exotherm and overshoot of caustic. Its practical limit is solubility, about 215 g/L at 20 °C, well below caustic's — day tanks are made up more often and kept warm in winter.