25.3.

Figure 25-4. Reclaiming Carbon Rods

In the simplest possible terms, we shall pass an electric current through salt water. All that is absolutely necessary is to connect two wires to a battery and dip them into a glass of salt water. Copper wires would rapidly corrode, however, and so we shall use carbon electrodes, just as we did in the aluminum-alkali battery (Chapter 21). Convenient carbon electrodes may be scavenged from non-alkaline flashlight batteries, even "dead" ones. Such "heavy duty" batteries have an outer zinc case and a carbon rod down the center; alkaline have the reverse geometry, with a zinc rod down the center. Wearing safety glasses and gloves, use a pair of pliers to dismantle two standard (Leclanche) flashlight batteries. Peel off the steel jacket, exposing the zinc can underneath. The button end of the battery is usually sealed with a plastic cap; pry this cap off as shown in Figure 25-4(L) to reveal the carbon rod beneath. Carefully twist and pull the carbon rod from the battery. The black stuff in the zinc can is a paste of manganese dioxide and ammonium chloride insulated from the zinc by a paper sleeve. Save the carbon rods and dispose of the rest in the trash.

Figure 25-5. Constructing the Chloralkali Cell

The body of the cell will be constructed from a 2-liter plastic soft-drink bottle. Use a knife to cut the top from the bottle at the shoulder. Poke two small holes on opposite sides of the bottle near the bottom and slide a carbon rod into each hole, leaving half an inch of each rod outside the bottle. Apply glue[1] to make a water-tight seal between the rods and the bottle, as shown in Figure 25-5(L), and allow the glue to dry or set. You will also need two smaller plastic bottles of such a size that they fit inside the 2-liter bottle, side-by-side. Use a knife to cut the bottoms from these bottles and trim them so that they fit completely inside the 2-liter bottle, one over each of the carbon rods.

Figure 25-6. Filling the Chloralkali Cell

Fill the 2-liter bottle to the brim with water and add salt (sodium chloride), stirring until no more will dissolve. With their caps removed, push the two smaller bottles into the larger one, bottoms first, so that each one sits over a carbon rod. The mouths of these bottles should be flush with the surface of the saturated salt solution. Screw on the caps, trapping as little air as possible in the small bottles, and lift them gently, as shown in Figure 25-6(R); the water level in the 2-liter bottle will fall but the smaller bottles will remain filled with water.

Figure 25-7. Operating the Chloralkali Cell

Use a wire to connect each of the carbon rods to a terminal of a lantern battery. Very soon bubbles will begin to form on the surface of the carbon electrodes and will rise to be collected in the two smaller bottles, as shown in Figure 25-7. One of these gases is hydrogen, the other chlorine. If you have been paying attention, you should be able to predict which is which by carefully considering Figure 25-3 and noting which electrode is connected to the positive and which to the negative terminal of the battery. Just picture electrons coming out of the negative terminal and flowing into the positive one. Hydrogen will collect more quickly than chlorine because some of the chlorine will dissolve in the increasingly alkaline solution to form sodium hypochlorite, the active ingredient in ordinary laundry bleach. As the gases displace the water in the smaller bottles, the water level in the larger one will rise; lifting the smaller bottles from time to time will prevent the larger one from overflowing. Nevertheless, it is probably a good idea to keep your chloralkali generator in a sink, tub, or bucket while it is in operation.

When one bottle is full of hydrogen gas, it is time to test your products. Carefully lift the smaller bottles from the larger one and place their open bottoms on a counter top or table which will not be harmed by bleach. The bottle which was full of gas will contain hydrogen. Light a splint or fireplace match and hold it against the bottom of the hydrogen bottle as you turn it over. The hydrogen will light with a satisfying pop. Be careful to point the bottle away from arms, faces, and other flammable materials.

The other bottle contains chlorine, the toxic gas, used on the battlefields of World War I. It is also the gas used to disinfect drinking water and swimming-pool water. The smell of laundry bleach is actually the smell of chlorine, which emanates from any hypochlorite solution. There is no need to be paranoid about the few hundred mL of gas you have generated, but neither should you be complacent. This element, liberated from ordinary salt, deserves your profound respect. Insert a wet strip of colored paper into your bottle of chlorine and you will see that it immediately bleaches the dye. When you are finished, release your chlorine either outdoors or into a fume hood.

The solution in your chloralkali cell consists of un-reacted sodium chloride, sodium hydroxide, and sodium hypochlorite. The mercury cathode of the Castner-Kellner cell was designed to produce sodium hydroxide in a container separate from the chlorine and salt, preventing the formation of sodium hypochlorite. Without such an arrangement you will be unable to test the pH of your solution using pH test paper because the sodium hypochlorite will bleach the blue color which would otherwise indicate alkali. Instead of testing the pH, you can test the bleaching power of your solution by dipping a strip of colored construction paper into it.

ImportantQuality Assurance
 

In order to pass, your hydrogen must be flammable and your chlorine must smell like chlorine. This is the strong power of all powers, for it overcomes everything fine and penetrates everything solid.

Notes

[1]

Goop™ is an excellent choice. There are also epoxies designed specifically for use on plastics.