Making chlorine solution for emergency drinking water

Chlorine is worth keeping at home for two jobs: making questionable water safe to drink when the tap supply fails, and keeping surfaces and hands clean when water is short. Boiling remains the most reliable way to disinfect water, since a rolling boil for one minute (three minutes above 6,500 feet) kills the germs that chlorine can miss, including the parasite Cryptosporidium. But boiling takes fuel, and chlorine takes none.

The problem is shelf life. Household bleach is a weak solution of sodium hypochlorite, and it breaks down steadily in the bottle, faster in a warm garage than in a cool closet. A jug bought for emergencies and forgotten for two or three years may be too weak to rely on. Write the purchase date on each bottle and replace it about once a year.

Using household bleach

Use only regular, unscented chlorine bleach whose label says it is for disinfection or sanitizing. Scented, “splash-less” and color-safe products are not suitable. The EPA’s emergency disinfection guidance gives these amounts for clear water:

Double the dose if the water is cloudy, colored or very cold. Stir, then wait 30 minutes. The water should smell slightly of chlorine. If it does not, add the same dose again and wait another 15 minutes.

Muddy water should be let stand until the sediment settles, then poured off through a clean cloth or coffee filter before any chlorine goes in. Dirt uses up chlorine and shelters germs from it.

The long-storage option: pool shock

Calcium hypochlorite is the same disinfecting chemistry in dry granular form, sold as pool shock. Kept sealed, cool and dry, it holds its strength for years, and a small bag goes a very long way.

Read the label before buying. Look for calcium hypochlorite as the only active ingredient, usually 65 to 73 percent. Avoid products that add algaecides, clarifiers or stabilizers, and never use chlorine tablets made for pool feeders, which are a different chemical.

To make a stock solution and use it, the EPA method is:

1. Add one heaping teaspoon of granules, about a quarter of an ounce, to 2 gallons of water. Always add the granules to the water, never water to the granules.
2. Stir until the particles have dissolved.
3. To disinfect, add one part of this solution to 100 parts of water. In practice that is 1 pint of solution to 12.5 gallons of water.
4. Stir and let the treated water stand for 30 minutes before drinking.

The stock solution is not drinking water. Keep it in a closed, opaque container, label it clearly, and make a fresh batch rather than storing it for months, because once dissolved it loses strength like bleach does. An earlier version of this page gave a stronger homemade mix and a formula from a military field manual; the EPA recipe above replaces it.

Handling the granules safely

Calcium hypochlorite is a powerful oxidizer. Stored carelessly, it can start a fire or release toxic gas.

• Keep it in its original container, tightly closed, somewhere cool and dry.
• Store it well away from gasoline, oil, paint, fertilizer, acids, ammonia cleaners and other pool chemicals.
• Use a clean, dry metal or plastic spoon, and never put a wet or dirty scoop back in the bag.
• Wear gloves and eye protection, mix outdoors or in a ventilated room, and keep children away.

Never mix bleach or pool shock with vinegar, ammonia or any other cleaner. Those mixtures give off chlorine or chloramine gas, which injures the lungs even in small amounts.