Successive potentiometric titration of iron(II) and iron(III) with cobalt(II) in the presence of 1,10-phenanthroline

A novel potentiometric titration of iron(II) and iron(III) is proposed. The method is based on the effect of 1,10-phenanthroline (phen) on the redox reactions of chromium(VI) and iron(III) with cobalt(II). In the presence of phen the conditional redox potential of the Co(III)/Co(II) system falls below those of the Cr(VI)/Cr(III) and Fe(III)/Fe(II) systems at pH around 1. Therefore, chromium(VI) and iron(III) can be titrated with cobalt(II) in the presence of phen, alternatively. Firstly, to a sample solution containing iron(II) and iron(III), a known amount of chromium(VI) is added in excess at pH around 1, so that iron(II) can be oxidized to iron(III) in the absence of phen. After the pretreatment, the sample solution containing the excess of chromium(VI) and iron(III) (total iron) is titrated with a standard cobalt(II) solution in the presence of phen. The concentrations of iron(TI) and iron(II) can be calculated from the first and second potential breaks at the equivalence points for the titration. The relative standard deviations of this method were 0.66 and 0.81% for four determinations of 5x10(-4) mol dm(-3) iron(II) and 1x10(-3) mol dm(-3) iron(III), respectively. The proposed method was successfully applied to the determination of total iron in standard iron-ore samples (JSS No. 801-4, 814-1 and 850-4) and to the successive determination of iron(II) and iron(III) in a Mosan iron-ore sample (JSS No. 812-3).