Iron(II) as a new electron transfer reagent in buffer medium and in presence of oxalate for the selective determination of Chromium(VI) and some related analytical applications of the reagent in the medium

Iron(II) in buffer medium and in presence of oxalate has been proposed as a new electron transfer reagent or reducing agent for the selective determination of chromium(VI) in presence of several other metal ions such as Mo(VI), U(VI), V(IV) etc. The method consists in taking a chromium(VI) solution [ containing 3-11mg of Cr] into a buffer medium of pH ~ 4.05.2 containing about 0.040.10M potassium oxalate and then titrating it against iron(II) solution. By reversing the above redox process i.e. by taking iron(II) in buffer-oxalate medium [ pH 1.5-2.5 and oxalate concentration 0.040.10M ] and titrating it against chromium(VI) solution taken in the burette, a new method for the selective determination of iron(II) [containing 9-30mg of Fe] has also been developed. The endpoint in both the methods can be detected either potentiometrically or using a thiazine dye as a redox indicator. In combinat ion with one of our methods reported earlier using iron(II) as a reducing agent in phosphoric acid medium, the method now proposed for the determination of chromium(VI) allows the quantitative analysis of metal ions present in synthetic mixtures such as : (1). Cr(VI) Mo(VI) ;(2). Cr(VI) -U(VI); and (3). Cr(VI)V(V). Similarly, in combination with the stannous chloride method for the estimation of iron(III) available in literature, the method new now developed for the determination of iron(II) permits the quantitative analysis of Fe(II) and Fe(III) present in synthetic mixtures and for analysis of iron ore for iron content. The formal redox potentials of Cr(VI)/Cr(III) and those of Fe(III)/Fe(II) couples have been measured in the buffer media of varying pH and oxalate ion concentrations under the optimum titration conditions. Based on these potentials data, an explanation for the conditions needed in the determination of chromium(VI) with iron(II) has been offered. The transition potentials of the indicators used have been measured. Based on these potentials data the suitability of the indicators in the present redox processes has been explained. The advantages of the present reductant over the other conventional reductants have been brought out. The reaction conditions under which Cr(VI) is reduced to Cr(III) by oxalate were investigated with a view that these findings may help to mitigate the ground water pollution due to Cr(VI) which is more toxic than Cr(III).