Thursday, March 17, 2016

Chemistry for Preppers: Make Your Own Bleach

While trying to find solidly prepper-oriented uses for having some knowledge of chemistry, I started thinking about priorities. This blog talks a lot about water and how to make it potable (fit to drink), so I started researching Sodium Hypochlorite (NaOCl, aka bleach).

I covered how to use bleach to disinfect water last year, and mentioned that one of the main problems with bleach is its relatively short shelf life. Store-bought bleach will decompose into mostly salt water in about 18 months of sitting on a shelf, so what would it take to replace it if TSHTF and there was none at the store?

Well, with a 12V DC power supply (car battery, solar panel, etc.), some table salt, and a little knowledge, you can make your own bleach.

Here's how it works
  • When you dissolve NaCl in water, the ions separate into Na+ and Cl-.
  • When you pass electricity through a solution of salt water it electrolyzes some of the water into Hydrogen gas (H2) and Hydroxide ions (OH-).
  • Electrolysis also causes some of the Cl- ions to gain electrons and form Chlorine gas (Cl2) which reacts with the Hydroxide ions to form hypochlorous acid (HClO).
  • These various parts recombine at the electrodes, forming Hydrogen gas (H2) at the anode and Sodium Hypochlorite (NaOCl) at the cathode..

The chemistry is fairly simple
  • At the anode: 2 H2O + 2 e- → H2 + 2 OH-
  • At the cathode: 2 Cl- → Cl2 + 2 e-
  • In the solution, the hydroxide ions (OH-) combine with the Cl2 gas to form HClO: 2 OH- + Cl2 → 2 HClO
  • When the Na+ ions still floating around in suspension knock the Hydrogen off it finally produces the bleach we want: 2 HClO + 2 Na- → H2 + 2 NaClO

What kind of equipment do you need? 
I've already mentioned a source of electricity (12V DC seems to be the most commonly used voltage in the texts I've read), but you'll also need the following:
  • 2 electrodes. The carbon rods from the insides of dead flashlight batteries will work, as will metal wire or rods. Be aware that metal will corrode rapidly in an electrified salt solution and may need to be cleaned or replaced often.
  • A container to hold the salt water, preferably something that is not metal. 12V DC isn't going to zap you, but shorting out a car battery can cause it to explode.
  • Some wire to connect the battery to the electrodes, I'd put a switch on one of the wires for safety
  • Some way to vent the H2 gas away from the anode. Hydrogen is very flammable, even at low concentrations, and should be treated with care.

Setting the system up
  1. Fill your container about 2/3 full of warm water.
  2. Add salt to the water until no more dissolves.
  3. Insert the electrodes into the salt solution.
  4. Connect the power supply to the electrodes.
  5. Turn on the power.
  6. Wait about 20 minutes and the process should be done.
The bleach produced by this method will be much more dilute than store-bought bleach -- about 0.5% instead of 5-8% -- so just use more of it to account for the dilution. Instead of 4 to 8 drops of commercial bleach (0.3 to 0.5 mL), you'll need to use 10-15 times as much, which works out to 3.0 to 7.5 mL. (A teaspoon holds 5 mL and that's about mid-range, so you can put away the eye-dropper and use a spoon.) Remember that you are looking for residual chlorine in the treated water after at least 20 minutes, and your nose will tell if it's there (see this link to my post from last year for details).

Another Option
If you have followed the chemistry but don't care to try to set up your own system, there is a group that makes a portable bleach generator. Theirs is a pass-through design and they suggest cycling the solution through at least 5 times to get the best results. They produce these for use in third-world countries, but will send you one (if they have them available -- most of them go overseas) for a $100 donation. This is on my wish list of things to get and try out, once I get the time and money to play with things.

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