Potentiometric determination of lead(II) based on its reduction with iron(II) in triethanolamine medium

Abstract

In the method proposed here lead(II) is reduced to the metal in alkaline triethanolamine medium in the presence of small amounts of bismuth(III) by iron(II) solution and the excess of Fe(II) is back-titrated potentiometrically with K2Cr20 v solution. The addition of bismuth is to accelerate the reaction and make it quantitative. A two-fold excess of Zn(II), Fe(III), AI(III), As(III) and Sn(IV) does not interfere with the determination, whereas Co(II), Mn(II), Cd(II), Sb(III), TI(II!), Se(IV) and Te(IV) cause interferences. The maximum error does not exceed + 1.2, in case of amounts of 5 20 mg of Pb per aliquot. Procedure. Into the titration flask the solution of lead(II) salt containing 5.0 to 20.0 mg of Pb is transferred. Bismuth(III) solution equivalent to 0.05 0.20 mg of Bi is introduced. TEA and NaOH solutions sufficient to yield overall molarities of 0.10 0.15 M and 0.15 0.20 M, respectively, in a volume of 50 ml are added. The contents are diluted to 45 ml, the vessel is stoppered and a flow of nitrogen is put on. After 5 min, 5 ml of 0.05 N FeSO 4 solution is introduced and after 2 3 min, the excess of FeSO 4 is titrated with 0.025 N dichromate solution recording the potentials in the usual way. A break of ca. 300 mV per 0.05 ml of 0.025 M dichromate is observed at the end point. In the vicinity of the end point the potentials stabilize rather slowly, hence a gap of 2 3 min should be maintained between the consecutive additions of titrant. Since TEA samples invariably contain some oxidants, a blank using the same amount of FeSO4 solution, under the identical conditions, has to be carried out.