The first to notice the effect of ammonia on the germicidal value of hypochlorites was S. Rideal[2] who noted that during the chlorination of sewage, the first rapid consumption of chlorine was succeeded by a slower action which continued for days in some instances, and was accompanied by a germicidal action after free chlorine or hypochlorite had disappeared. Rideal stated that: "It became evident that chlorine, in supplement to its oxidising action, which had been exhausted, was acting by substitution for hydrogen in ammonia and organic compounds, yielding products more or less germicidal." On investigating the effect of ammonia on hypochlorite it was found that the addition of an equivalent of ammonia to electrolytic hypochlorite increased the carbolic acid coefficient of 2.18, for one per cent available chlorine, to 6.36 (nearly three times the value). Further experimental work showed that the increase was due to the formation of chloramine.
The author, in 1915, during a series of experiments on the relative germicidal action of hypochlorites, attempted to prepare the ammonium salt by double decomposition of bleach and ammonium oxalate solutions.
Ca(OCl)_{ 2} + (NH_{ 4})_{ 2}C_{ 2}O_{ 4} = CaC_{ 2}O_{ 4} + 2NH_{ 4}OCl.
The velocity of the germicidal action of the solution was found to be about ten times greater than the germicidal velocities of other hypochlorites of equal concentrations, (Race[3]), and from a consideration of the chemical formula of ammonium hypochlorite it appeared probable that it would be very unstable and decompose into chloramine, which Rideal had previously shown to have an abnormal germicidal action, and water. NH_{ 4}OCl = NH_{ 2}CL + H_{ 2}O. After these results have been confirmed, the effect of adding ammonia to bleach solution was tried and it was found that 0.20 p.p.m. of available chlorine and 0.10 p.p.m. of ammonia produced equally good results as 0.60 p.p.m. of chlorine only. Similar results were obtained on the addition of ammonia to electrolytic hypochlorite.
Experiments made with a view to determining the most efficient ratios of ammonia gave very surprising results: chlorine to ammonia ratios (by weight) between 8:1 and 1:2 gave approximately the same germicidal velocity.[3] The action of the ammonia on the oxidising power of bleach, as measured by the indigo test, was also found to be disproportionate to the amount added.