A rapid and simple general complexometric method was presented for the determination of lead, cadmium and thallium or mercury or arsenic(V) in laboratory synthesized mixtures similar to those of some ores, minerals and alloys of such metals. The precision and accuracy attainable in successive titrations of Pb2+, Cd2+ and Tl3+. Or Hg2+ or AsO43- (As5+) with 0.05 and/or 0.01 mol l(-1) solutions of disodium ethylenediaminetetraacetate (Na(2)EDTA) and standard Pb(NO3)(2) of the same concentration using Bromo-Cresol Orange (BCO) as a new metallochromic indicator with visual endpoint indication were studied. For the analysis of a three component mixtures of the aforementioned ions, Tl3+ was at first directly titrated with Na,EDTA at pH 0.5-1 (HNO3) using BCO as indicator. At the thallium endpoint an excess of Na(2)EDTA was added and the pH was adjusted at pH similar to 4.8 using hexamine-HNO3 buffer (solution A). The excess EDTA was back-titrated with standard solution of Pb(NO3)(2). 1,10-Phenanthroline (1,10-phen) was added to release the EDTA combined with Cd2+ while thiosemicarbazide (TSC) was used to liberate the EDTA from the mercury-EDTA chelate. To determine AsO43- ion in such type of mixtures the pH of (solution A) was raised to a value of 10 using ammonia buffer. Excess standard Mg2+ solution was added and the formed precipitate of MgNH4AsO4 was separated, dissolved and its magnesium content equivalent to AsO43- was determined complexometrically using Eriochrome BIack-T (EBT) indicator. The interference caused by different anions, cations and organic acids was investigated. A comparison of the indicators BCO and Xylenol Orange (XO) for successive titration of the studied metal ions was carried out. The proposed successive titration method was applied successfully to some real samples of ores, minerals and alloys of the studied metal ions and the results were satisfactory and agreed with those obtained by AAS. (C) 1997 Elsevier Science B.V.
