Equivalent Weight of Caustic Soda

Caustic soda supplies one hydroxide per formula unit, so its equivalent weight and its formula mass are the same number: 22.990 + 15.999 + 1.008 = 39.997, near enough 40.00 that generations of technicians have never looked it up. Every 39.997 g adds one equivalent of alkalinity — 50.04 g as CaCO₃ — so a mg/L of pure NaOH raises alkalinity by 50.04/40.00 = 1.25 mg/L as CaCO₃, the quick mental conversion behind every caustic feed adjustment. Commercial 50 % membrane-grade solution weighs in at about 1.52 kg/L, which puts roughly 0.76 kg of NaOH, or 19 equivalents, in every litre pumped.

The number is friendly; the chemical is not. Hydroxide is the strongest base water allows, so caustic moves pH violently at the injection point — a poorly mixed feed can locally exceed pH 12 and precipitate hardness right at the quill, capping it with scale. Where the goal is alkalinity with less aggression, soda ash and bicarbonate trade a higher equivalent weight for gentler chemistry, which is exactly how those two entries below earn their place on the shelf.