Several types of electrolysers have been used for the production of hypochlorites and chlorine but only two are suitable for water-works purposes: in one, the cathodic and anodic products recombine in the main body of the electrolyte; in the other, the diaphragm process, they are separated as removed and the final products are chlorine gas and a solution containing caustic soda and some undecomposed salt.
Until a few years ago the non-diaphragm process was the only one used for water treatment and it will consequently be discussed first.
_Non-diaphragm Process._ The theoretical voltage required for the decomposition of sodium chloride is 2.3 but when the products recombine in the electrolyte, side reactions occur which increase the minimum voltage to 3.54. On this basis one kilowatt hour gives 272 ampere hours and as one ampere hour is theoretically capable of producing 1.33 grams of chlorine, 1.21 kilowatt hours are necessary for the production of 1 pound of chlorine by the decomposition of 1.65 pounds of salt.
Charles Watt (1851) discovered this process and was the first to recognize the necessary conditions which are (1) insoluble electrodes, (2) low temperature of electrolyte, and (3) rapid circulation of electrolyte from the cathode to the anode. The control of the temperature is very important, for as it increases, side reactions occur with the formation of chlorates, and the efficiency is decreased.
The non-diaphragm cells used in Europe (Haas and Oettel, Kellner, Hermite, Vogelsand, and Mather and Platt) have been described by Kershaw.[2] In the Haas and Oettel electrolyser the electrodes are composed of carbon but in the other types at least one electrode is made from platinum or a platinum alloy. The Dayton electrolyser, which is the cell most familiar in North America, is shown in Fig. 9.
[Illustration: FIG. 9.--Dayton Electrolytic Cell.]
The outer cell is made of soapstone and is approximately 2-1/2 feet long and 2 feet wide. The main electrodes consist of four pieces of Atcheson graphite connected together by screws and metal strips to which is attached a clamp for connecting electrical terminals. Circulation of the brine is produced by glass baffle plates and secondary electrodes placed one inch apart between the main electrodes. The cell is intended to be used at 110-volts pressure but by wiring two cells in series a 220-volt circuit may be employed. An inlet and outlet are provided at each end of the tank to permit the direction of the flow to be periodically reversed for the purpose of removing the lime deposit from the graphite plates.