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.
2H_{ 2}O_{ 2} = 2H_{ 2}O + O_{ 2} + 46,120 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 Cl_{ 2} + H_{ 2}O = 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.
When chlorine gas was treated with a 0.5 per cent solution of ammonia in the proportion of 1 molecule of chlorine to 1.90-1.95 molecules of ammonia, Noyes and Lyon[8] found that nitrogen and nitrogen-trichloride were formed in equimolar quantities.
12NH_{ 3} + 6Cl_{ 2} = N_{ 2} + NCl_{ 3} + 9NH_{ 4}Cl.
Bray and Dowell[9] showed that this reaction depended upon the hydrogen ion concentration and proceeded in accordance with the following equations:
(i). Acid solution 4NH_{ 3} + 3Cl_{ 2} = NCl_{ 3} + 3NH_{ 4}Cl
(ii). Alkaline solution 8NH_{ 3} + 3Cl_{ 2} = N_{ 2} + 6NH_{ 4}Cl.
In (i) with a ratio of chlorine to ammonia of 3:1 by weight, one-half of the chlorine is lost as ammonium chloride and one-half forms nitrogen trichloride, concerning which comparatively little is known; in (ii) the whole of the chlorine forms ammonium chloride, which has no germicidal value.