W. Gottardi et V. Bock, DIRECT IODOMETRIC DETERMINATION OF HALOGEN (1+) COMPOUNDS USING ARSENITE, Fresenius' journal of analytical chemistry, 347(10-11), 1993, pp. 400-408
The iodometric determination of active halogen compounds (0-Hal, N-Hal
) comprises two reactions, the transformation of the oxidation capacit
y into iodine (reaction with iodide), and its volumetric analysis (tit
ration with thiosulphate or arsenite). In bicarbonate-alkaline medium,
necessary for the titration with arsenite, some compounds give too lo
w values (up to 50%) - compared with those obtained under acid conditi
ons with thiosulphate - and show large scattering. This can be attribu
ted to disproportionation products (e. g. chlorite, bromite, iodate) o
f the hypohalogenic acids present as intermediary reaction products (o
r as the actual analyte), which are formed under the weakly alkaline c
onditions (pH almost-equal-to 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 r
eaction can be directed quantitatively to the formation of I(0). This
procedure works with all active chlorine compounds and weakly or slowl
y hydrolyzing N-bromo and N-iodo compounds. With strongly or fast hydr
olyzing N-halogen compounds, the iodide surplus has to be increased co
nsiderably, while the method fails with hypobromic acid. Compared with
the already known methods using arsenite, the direct iodometric deter
mination is characterized by lower requirements for apparatus and work
. As to the thiosulphate titration it is an advantage that only haloge
n (1+) is determined and not impurities with halogen compounds of a hi
gher valency state formed by disproportionation.