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Compounds and Cleaning · Compound · Pool shock · Ca(ClO)2

Calcium Hypochlorite

Calcium hypochlorite, sold cheaply as pool shock, is the most powerful water treatment in this book. As a dry granule it carries 65 to 70 percent available chlorine, so a single pound can disinfect on the order of 10,000 gallons. Kept dry and sealed it lasts for years.

UP TO 10 YEARSMULTI-USEHazardousWorks without power

Quick facts

Active chlorine
65 to 70%
Form
White granules
Hazard class
Corrosive oxidizer
Shelf life
Up to 10 years
Storage
Glass or HDPE, sealed
What to avoid
  • Moisture
  • acids
  • metal

See everything calcium hypochlorite must never touch

What it's good for

Drinking water

Kills bacteria and viruses

Surfaces

Mixes to bleach strength

Cisterns & tanks

Shock and store volumes

Tools & gear

General sanitation

Dissolved in water it releases the chlorine that kills microbes; that chlorine slowly escapes, so dry storage keeps it strong.

Good to know

What 'available chlorine' really means. The percent on the label is not how much chlorine is in the jar; it is a measure of oxidizing power, the yardstick all chlorine disinfectants share, which is why a little treats so much water. Not every pool product qualifies, though: stabilized tablets like trichlor and dichlor carry cyanuric acid to survive sunlight, the opposite of what drinking water needs. Reach only for plain calcium hypochlorite.

Handling and safety

Not for every microbe

Kills bacteria and viruses, but not Cryptosporidium; pre-filter cloudy water.

Never mix it

With acids or ammonia it makes toxic chlorine gas; add powder to water, never the reverse.

Keep it bone dry

It draws moisture and slowly weakens; a sharp chlorine smell means it is still good.

Buy it additive-free

For drinking water use pool shock with no algaecides or clarifiers.

Eight jobs it does

  • Make a chlorine stock for water
  • Treat clear water (1 tbsp stock/gal)
  • Surface disinfection at 1,000 ppm
  • Sanitize tanks and barrels
  • Shock chlorinate a cistern
  • Mold and mildew removal
  • Algae control in stock tanks
  • Sanitize canning surfaces

If it goes bad Decomposes with heat and moisture, releasing chlorine gas. Ventilate.

Field notes

Why metal is on the avoid list. Calcium hypochlorite, Ca(ClO)₂, oxidizes metal on contact. The reaction is most aggressive with aluminum, mild steel, and copper, and over time the corrosion weakens the metal until it fails. Store and handle it only with containers and tools that resist corrosion.

My pick: calcium hypochlorite on Amazon.

Recipes for calcium hypochlorite

Questions this answers

Tap a question for the short answer.

What is calcium hypochlorite used for?

It disinfects drinking water by killing bacteria and viruses, and a single pound can treat on the order of 10,000 gallons. It also mixes to bleach strength for surface disinfection at 1,000 ppm, sanitizes tanks, barrels, tools, and canning surfaces, shock chlorinates cisterns, removes mold and mildew, and controls algae in stock tanks.

How long does calcium hypochlorite last in storage?

Up to 10 years when kept dry and sealed. It draws moisture and slowly weakens, and a sharp chlorine smell means it is still good. Heat and moisture make it decompose and release chlorine gas.

How do I store calcium hypochlorite?

Keep it sealed in glass or HDPE and bone dry, away from moisture, acids, and metal. Never let it contact acids or ammonia, which produce toxic chlorine gas. If it decomposes from heat and moisture it releases chlorine gas, so ventilate.

Keep going

Read nextSurface disinfectant from calcium hypochlorite (pool shock)Used to sanitize hard surfaces, tools, containers, and cleanup areas in off-grid or emergency situations.Compounds and Cleaning · Formula

Related to this

Water · Formula Disinfect water barrels and cisterns with calcium hypochlorite Strength: ~200 ppm; Solution: 2 gal per 10 gal; Contact: 6-24 hours Hazardous Water · Formula Purify water with calcium hypochlorite (pool shock) Stock: ~1,000 ppm; Dose: 1 tbsp stock / gal; Wait: 30 minutes Hazardous Compounds and Cleaning · Reference Sourcing checklist: match the grade, read the label The in-store checklist for buying compounds: match the grade to the job, look for the right markings, and steer clear of the wrong product. Water · Reference Conversions and dosing: measure anything, even without tools 1 tsp = 5 mL. About 20 drops = 1 mL. A US nickel weighs 5 g. 1 ppm = 1 mg per liter.
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