Direct iodometric determination of halogen (1 +) compounds using arsenite

Summary

The iodometric determination of active halogen compounds (O-Hal, N-Hal) comprises two reactions, the transformation of the oxidation capacity into iodine (reaction with iodide), and its volumetric analysis (titration with thiosulphate or arsenite). In bicarbonate-alkaline medium, necessary for the titration with arsenite, some compounds give too low values (up to 50%) — compared with those obtained under acid conditions with thiosulphate — and show large scattering. This can be attributed to disproportionation products (e. g. chlorite, bromite, iodate) of the hypohalogenic acids present as intermediary reaction products (or as the actual analyte), which are formed under the weakly alkaline conditions (pH ≈ 8.2) and would need at least weakly acid conditions to be completely converted into iodine. By maintaining an at least 30-fold molar excess of iodide throughout the reaction (slow addition of the dissolved analyte to the iodide-buffer solution) the reaction can be directed quantitatively to the formation of I(0). This procedure works with all active chlorine compounds and weakly or slowly hydrolyzing N-bromo and N-iodo compounds. With strongly or fast hydrolyzing N-halogen compounds, the iodide surplus has to be increased considerably, while the method fails with hypobromic acid. Compared with the already known methods using arsenite, the direct iodometric determination is characterized by lower requirements for apparatus and work. As to the thiosulphate titration it is an advantage that only halogen (1 +) is determined and not impurities with halogen compounds of a higher valency state formed by disproportionation.