Soda Ash Dose for Softening (as CaCO₃)

Lime removes hardness by supplying hydroxide that converts bicarbonate into carbonate, and the carbonate is what carries calcium out of solution. Where a water's hardness has no bicarbonate paired with it — calcium sitting with sulphate or chloride instead — more lime raises the pH and removes nothing at all. That unpaired portion is the noncarbonate hardness, it is simply total hardness minus alkalinity, and soda ash is what deals with it because soda ash brings its own carbonate. A water at 250 mg/L of hardness and 180 of alkalinity needs 70 mg/L as CaCO₃, which is about 74 mg/L of Na₂CO₃ as the product, before dividing by its roughly 98 % purity.

Two consequences follow, and both are about money and health rather than chemistry. Soda ash costs several times what lime costs, so the alkalinity figure is worth measuring properly rather than assuming — and when alkalinity equals or exceeds total hardness, the correct dose is zero, because every bit of the hardness is already carbonate hardness and lime alone will reach it. The surplus alkalinity in that case is sodium or potassium bicarbonate, which was never hardness. The second consequence is what the treatment does to the finished water: every equivalent of calcium taken out this way is replaced by two of sodium, so softening by this route does not lower total dissolved solids, it trades one cation for another. That matters to a system serving people on sodium-restricted diets and to anyone downstream running a boiler or a membrane. Finally, softening is never run to completion — the practical limits are around 30 to 40 mg/L as CaCO₃ for calcium and about 10 for magnesium, and most utilities deliberately stop between 80 and 120 mg/L of total hardness so the water leaves slightly scale-forming rather than aggressive to the mains.