Calcium Hypochlorite
Properties
| State | Solid at room temperature |
| Color | White to light gray powder or granules |
| Solubility | Soluble in water (21 g/100 mL at 25 °C) |
| Melting Point | 100 °C (decomposes) |
About Calcium Hypochlorite
Calcium hypochlorite is essentially a solid form of bleach with much higher available chlorine — typically 65–70% by weight versus 5–8% in household sodium hypochlorite solutions — and that concentration is what makes it the disinfectant of choice for swimming pools and field water treatment. The chemistry runs on the hypochlorite ion's standard reduction potential, E° = +0.89 V at pH 7, which oxidizes the lipid bilayers and DNA bases of pathogens far faster than free Cl2 alone. In water, Ca(ClO)2 dissociates and equilibrates: ClO⁻ + H2O ⇌ HOCl + OH⁻, with pKa of HOCl at 7.53. HOCl is the actual germicidal species and is roughly 80 times more effective than the conjugate base ClO⁻, so pool operators target pH 7.2–7.6 to keep the bulk of the available chlorine in the protonated form. Industrially, it is manufactured by the Mathieson or sodium-process chlorination of slaked lime: 2 Ca(OH)2 + 2 Cl2 → Ca(ClO)2 + CaCl2 + 2 H2O. The compound is a strong oxidizer (UN 1748, Class 5.1, Packing Group II), and shipping incidents involving it — including several major warehouse fires — trace back to the same root cause: contact with organic material, oils, or moisture that triggers self-accelerating decomposition above about 60 °C.
Where you'll encounter it
Pool service technicians know calcium hypochlorite as 'cal hypo' or 'HTH', the granular or briquette product they shock pools with at about 1 lb per 10,000 gallons. Disaster relief teams carry 5-gallon buckets of the powder for emergency drinking water — in the field, 1 g of 65% material treats about 100 L to 5 ppm free chlorine, with a 30-minute contact time before drinking. Municipal systems in remote or developing-region installations prefer it over liquid bleach because the solid keeps its strength for years if stored dry and cool, while NaClO solutions lose 1% available chlorine per day at room temperature. Anyone who has unloaded Cal-hypo drums in summer warehouses has heard the safety brief about keeping it away from pool-cleaning algaecides — the cyanuric acid stabilizers and quaternary amines in those products are exactly the kind of organics that can ignite a hypochlorite pile.
Common Uses
- Granular shock chlorinator for swimming pools at 1 lb per 10,000 gallons
- Field water disinfection in disaster relief at 1 g of 65% per 100 L for 5 ppm free chlorine
- Stabilized solid disinfectant for remote municipal water systems where liquid bleach degrades
- Sanitizer for food-processing equipment and dairy lines at 100–200 ppm available chlorine
- Bleaching agent for cotton and pulp where solid handling beats liquid logistics
- Pre-cursor for chloropicrin and other chlorinated intermediates in soil fumigation
- Algaecide and biocide in industrial cooling towers
Safety Information
UN 1748, Class 5.1 oxidizer, Packing Group II. The headline hazards are oxidative ignition of organic material on contact (oils, sawdust, glycols, surfactants — even sweat-soaked rags) and chlorine-gas evolution on contact with acids: Ca(ClO)2 + 2 HCl → CaCl2 + Cl2↑ + H2O. OSHA PEL for Cl2 is 1 ppm ceiling; lethal at 1000 ppm for short exposures. Skin and eye contact gives caustic burns. Store in cool (<27 °C), dry, ventilated areas, isolated from acids, ammonium compounds, and reducers. Several major shipping fires — Cherbourg 1971, Mediterranean Sky 1985, the MOL Comfort and others — have been blamed on degraded or contaminated cal-hypo cargoes self-igniting.
This safety summary is for educational reference only and may not be complete. It is not a substitute for Safety Data Sheets (SDS), medical advice, or professional chemical safety guidance. Always consult appropriate SDS and qualified professionals before handling chemicals.