Determination of iron(II) with bipyridylglyoxal dithiosemicarbazone

Bipyridylglyoxal dithiosemicarbazone reacts with iron(II) or (III). The Fe(III) complex is yellow (lambda(max) 400 nm). Fe(II) forms a red-violet 1:2 complex at pH 2.5 (lambda(max) 550 nm) and a green-blue 1:1 complex at pH 5-10 (lambda(max) 590-610 nm). Both ferrous complexes can be oxidized to the ferric complex; this reaction is reversible. The quantitative application of the ferrous complex has been studied.