Back titration with mercuric nitrate in alkaline medium

Abstract

R~ILL~ ~ and PORT~B~IELD 11 have shown that mercuric nitrate can be used for the coulometric back titration of excess EDTA in the determination of many metals, l~ecently KHALIFA et al. a manifested that the Hg/HgY 3- system could be successfully applied for the same purpose. This method has been applied for the accurate determination of twenty metal ions involving di-, tri-, and tetravalent cations and the analysis of binary and tertiary mixtures involving them 3,5,7. The present work is undertaken to ascertain the feasibility of applying tile same method for the quantitative estimation of scandium and palladium as well as the analysis of their binary mixtures with some other metals. Xylenol orange gives a distinct end point in the direct titration of scandium with EDTA, though the same indicator is applied in titrations of many other metals with EDTA. 9 The titration involving Erioehrome Black T as an indicator should be carried out in the presence of malie acid at a temperature higher than 70~ This method recommending cyanide and 2,3-dimercaptopropanol as a masking agent for Zn, Cd, Ni, CO, tIglI and Ca ~a could not be applied because of the difficulty in obtaining dimereaptopropanol (we did not succeed in using cyanide alone as a masking agent). The method using PAN as an indicator is suitable for determining 3 to 10 mg of scandium but not for micro amountsL Palladium can be, determined by back titration of excess EDTA with zinc solution using Eriochrome Blaeke T 1~ at p~ 10 ~- 1 with dilute potassium hydroxide. Buffers are not recommended because of the tendency of palladium to form strong complexes with buffer materials. In concentrated solution the yellow eolour of palladium -- EDTA tends to obscure the indicator. Palladium can be determined in the dilute solution by back titrating excess EDTA with bismuth using pyrocateehol as an indicatorS, 13.