The subject was investigated by Dakin and Dunham,[7] who first tried chloramine-T (sodium toluene-_p_-sulphochloramide). It was found that heavily contaminated waters, and particularly those containing much carbonates, required a comparatively high concentration of the disinfectant: 40 parts per million of chloramine-T were necessary in some cases and such an amount was distinctly unpalatable. By adding tartaric acid or citric acid the effective concentration could be reduced to 4 p.p.m. but the mixture could not be made into a tablet without decomposition and a two-tablet system was deemed undesirable. Toluene sulphodichloramines were next tried. Excellent bacteriological results were obtained but the manufacture of tablets again presented difficulties. When the necessary quantity of dichloramine was mixed with what were assumed to be inert salts--sodium chloride for example--the normal slow rate of decomposition was accelerated. The dichloramine, in tablet form, was also found to be too insoluble to effect prompt sterilisation. The most suitable substance found by Dakin and Dunham was "halazone" or _p_-sulphodichloraminobenzoic acid (Cl_{2}N·O_{2}S·C_{6}H_{4}·COOH). This compound is easily prepared from cheap readily available materials and was found to be effective and reasonably stable. The starting point in the preparation of halazone is _p_-toluenesulphonic chloride, a cheap waste product in the manufacture of saccharine. By the action of ammonia, _p_-toluene sulphonamide is produced and is subsequently oxidised by bichromate and sulphuric acid to _p_-sulphonamidobenzoic acid. This acid, on chlorination at low temperatures, yields _p_-sulphondichloraminobenzoic acid (halazone). The reactions may be expressed as follows: CH_{3} COOH COOH / \ / \ / \ | | --> | | --> | | \ / \ / \ / SO_{2}·NH_{3} SO_{2}·NH_{2} SO_{2}·NCl_{2} Halazone is a white crystalline solid, sparingly soluble in water and chloroform, and insoluble in petroleum. It readily dissolves in glacial acetic acid from which it crystallizes in prisms (M.P. 213° C.). The purity of the compound can be ascertained by dissolving in glacial acetic acid, adding potassium iodide, and titrating with thiosulphate; 0.1 gram should require 14.8 to 14.9 c.cms. of N/10 sodium thiosulphate. Each chlorine atom in halazone is equivalent to 1 molecule of hypochlorous acid and the "available" chlorine content is consequently 52.5 per cent or double the actual chlorine content. >SO_{2}·NCl_{2} + 4HI = >SO_{2}·NH_{2} + 2HCl + 2I_{2}. From the bacteriological results given by Dakin and Dunham it would appear that 3 parts per million of halazone (1.5 p.p.m. available chlorine) are sufficient to sterilise heavily polluted waters in thirty minutes and that this concentration can be relied upon to remove pathogenic organisms.