Coyote Science

Coyote Science Activity 19

Bleachmaker
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Cheap Electrochemistry
By Bob Culbertson 2005 ©

Caution: A small amount of flammable gas is released during this experiment. Do it in a well ventilated area away from any fire or flame. If wax is used as a sealant melt it in a can in hot water, not directly over an open flame.

Using saltwater as a conductor of DC electricity causes the water and salt to separate (called "electrolysis") into a number of interesting compounds. At the negative side (cathode) hydrogen and sodium hydroxide are produced and chlorine gas forms at the positive side (anode). Sodium hypochlorite (NaOCl), or bleach, is formed by the reaction between sodium hydroxide and chlorine when the cathode and anode are separated. A small amount of bleach can be made for a few cents worth of common materials and a source of DC electricity.

The anode and cathode "electrodes" are made from pencil leads because the graphite and clay that they are made from doesn't contribute chemicals to the bleach. Stainless steel and many other metals should not be used. The pencil leads will eventually be destroyed by the chemicals they help make but usually takes a while to happen.

Seawater can be substituted for salt and water but filter it through a paper coffee filter or fine weave cloth first.

Materials:

2 Styrofoam plastic cups
1 cup water (about 200mL) or clear seawater
1 teaspoon plain salt (about 5 grams)
2 pencil leads about 7cm long (cut out of wooden pencils)
1 tube of waterproof glue or a metal can of melted wax (some glues can melt the Styrofoam cups!)
1 stiff piece of metal wire about 18cm (6inches) long (like part of a wire coathanger)

Misc:

Filters for seawater and the final product, several pieces of coffee filter paper or tightly woven cloth

Tools:

Some sort of DC electrical power source such as 2 flashlight batteries or solar cells (3volts at less than 1/2 watt)
2 30cm (12 inch) electrical wires with clips on both ends
1 small spoon
1 iron nail and a piece of sand paper to make a bleach "Tester"

How to do it:

Push the stiff metal wire through both sides of one of the cups about 1 cm (1/2 inch) above the bottom then push one of pencil leads through those same holes so that the ends stick out of the cup. Make another set of holes about 2cm (5/8 inch) above the first set and put a pencil lead through them too. Seal the holes in the cup with glue or by dipping it in melted wax. Repeat as many times as necessary but don't get wax inside the cup. Carefully scrape away the wax from the ends of the pencil leads.

Mix the salt and water together (or filtered seawater) and pour into the cup with the pencil leads. Connect the positive terminal of the DC power source to the bottom pencil lead and the negative terminal to the top one with the electrical wires with clips on the end. Bubbles will start forming and there may be the faint odor of chlorine. Bleach is now being made.

Using 3 VDC takes a long time to make bleach, perhaps 24 hours, so it should be tested to see when it is ready. Take the bleach "Tester" (iron nail and sandpaper) and rub the nail with the sandpaper until about 2cm of it is shiny. Put the nail into a cup and cover it with some of your bleach. If the bleach is ready the nail should become very rusty in a just a few minutes; if not then the bleach making process must continue some more.

To use the bleach:

Filter it, then mix with 2 liters of clean water. Wash some fruit with it and compare the washed fruit with unwashed and see which becomes moldy first.

References:

Chemistry, by Glenn H. Miller, 1969, Harper & Row, Publishers, New York, Evanston and London, Library of Congress Catalog Card Number: 69-10550

Clorox Company at www.clorox.com

posted by Bob C.  # 11:54 AM