Hypochlorite Product Mass from Available Chlorine

Chlorine chemicals are sold by product weight but dosed by chlorine content, and the gap between the two is large. Sodium hypochlorite bleach ships at 10–15% available chlorine (12.5% is the standard water-plant grade), calcium hypochlorite granules at 65–70%, and chlorine gas is the only 100% product. To deliver 10 lb of available chlorine from 12.5% bleach you need 10 × 100/12.5 = 80 lb of solution. From 65% cal-hypo you would need only 15.4 lb of granules. The phrase "available chlorine" is itself a convention — it measures oxidizing power relative to elemental Cl₂, which is why calcium hypochlorite can test above 100% on a molar basis and still be quoted at 65%.

Liquid hypochlorite decays, and this is the number one cause of failed disinfection in small systems. A 12.5% solution loses roughly half its strength in 30–60 days at room temperature, faster if it is warm, exposed to light, or contaminated with trace metals. A drum bought at 12.5% and used three months later may genuinely be 8%, so the pump set from the label under-doses by a third. Titrate incoming shipments, store cool and dark, rotate stock, and never dilute hypochlorite with hard water — the calcium precipitates and plugs the injection quill.