| NH2Cl + 2HI | = | I2 + NH4Cl. |
| NCl3 + 6HI | = | 3I2 + NH4Cl + 2HCl. |
Halazone
For the sterilisation of small individual quantities of water such as are required by cavalry and other mobile troops bleach and acid sulphate tablets have been usually employed. Such tablets have given fairly satisfactory results but certain difficulties inherent to these chemicals have made it desirable to seek other methods.
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 (Cl2N·O2S·C6H4·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:
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.).