Rideal[6] was the first to note the strong germicidal power of chloramine and attributed the persistent germicidal activity of hypochlorites in sewage to the formation of chloramine and chloramine derivatives.
Further evidence against the nascent oxygen theory of chlorine disinfection is to be found in the fact that such active oxidising agents as sodium, potassium, and hydrogen peroxides have a much lower germicidal activity than chlorine when compared on the basis of their oxygen equivalents. [Table III] shows chlorine to be approximately five times as active as potassium permanganate when compared on this basis.
TABLE III.[C]—COMPARISON OF BLEACH AND
POTASSIUM PERMANGANATE
| Contact Period. | Bleach Available Chlorine 0.35 p.p.m. | Potassium Permanganate. | ||||
| Oxygen Equivalent. Parts Per Million. | ||||||
| 0.08 | 0.133 | 0.266 | 0.400 | |||
| Nil | 140 | ... | ... | ... | ||
| 30 | mins | 90 | 122 | 115 | 110 | |
| 1 | hour | 68 | 115 | 100 | 80 | |
| 1 | 1⁄2 | hours | 63 | 108 | 95 | 75 |
| 4 | hours | 50 | 95 | 80 | 50 | |
| [C] Resultsare B. coli per 10 c.cms. | ||||||
The germicidal activity of oxidising agents has been shown by Novey and others to be somewhat proportional to the energy liberated during the reaction but even when this factor is taken into consideration chlorine compounds are more active than other oxidising agents. Hypochlorous acid is far superior to hydrogen peroxide as a germicidal agent and is as active as ozone, which liberates a greater amount of energy.
2HClO = 2HCl + O2 + 18,770 calories
2H2O2 = 2H2O + O2 + 46,120 calories
2O3 = 3O2 + 60,000 calories.
Again, solutions of chlorine gas and hypochlorites having the same oxidising activity, as determined by titration with thiosulphate after the addition of potassium iodide and acid, i.e. contain equal amounts of available chlorine, show approximately the same germicidal activity in water. On the addition of ammonia, the hypochlorite solutions retain their ability to liberate iodine from potassium iodide (Wagner test) but the property of oxidising such dyestuffs as indigo is destroyed and the germicidal activity is increased. Ammonia, when added to solutions of chlorine gas, diminishes the property of liberating iodine from potassium iodide, the bleaching effect on dyestuffs, and the germicidal action. It is often assumed that chlorine forms hypochlorous acid on solution in water Cl2 + H2O = HClO + HCl but the results obtained on the addition of ammonia indicate that either very little hypochlorous acid is formed or that ammonia and hypochlorous acid do not form chloramine in the presence of hydrochloric acid.