Electro‐Chemical Properties of Chlorinated Water

Abstract

THE majority of cases operation of a water plant which depends upon chlorination for bacterial control relies upon the validity of the ortho-tolidine test. This chemical test is the best that is available at the present time but its shortcomings are well known and are the main source of inadequate water sterilization. The errors inherent to the ortho-tolidine test are three fold: 1. As the reagent is made up with hydrochloric acid, small traces of oxidizing compounds, chiefly higher oxides of manganese, will oxidize the hydrochloric acid to chlorine and produce false residuals. This difficulty cannot be overcome by the use of a different acid, as most waters contain a sufficient amount of chloride ion to give a false residual. 2. If the water contains nitrite, a colored compound presumably due to diazotization, is developed when ortho-tolidine is added. This compound approximates in hue the color developed by chlorine. 3. Any chloramine formed in the water, not only the simple chloramines but also chloramines arising from the action of chlorine upon so-called albuminoid amino groups, is split into chlorine and ammonia compounds when the pH of the water is greatly lowered, as is the case when the acid ortho-tolidine reagent is added. As some of these more complex chloramines have little or no bactericidal power, a residual of higher bactericidal effect is indicated by the ortho-tolidine test in these cases. Rideal and Evans in 1921 (1) suggested that the bactericidal power of chlorine compounds may be subject to similar variations as their oxidizing power, and suggested a device utilizing this principle. Some years ago one of us (2) drew the attention of workers in the