Complexometric determination of some toxic mixtures of ions using bromo-cresol orange with visual endpoint indication

摘要

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 Pb~(2+), Cd~(2+) and T1~(3+) or Hg~(2+) or AsO_4~(3-) (As~(5+) ) with 0.05 and/or 0.01 mol 1~(-1) solutions of disodium ethylenediaminetetraacetate (Na_2EDTA) and standard Pb(NO_3), 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, T1~(3+) was at first directly titrated with Na_2EDTA atpH 0.5-1 (HNO_3) using BCO as indicator. At the thallium endpoint an excess of Na_2EDTA was added and the pH was adjusted at pH ~ 4.8 using hexamine-HNO, buffer (solution A). The excess EDTA was back-titrated with standard solution of Pb(NO_3)_2. 1,10-Phenanthroline (1,10-phen) was added to release the EDTA combined with Cd~(2+) , while thiosemicarbazide (TSC) was used to liberate the EDTA from the mercury-EDTA chelate. To determine AsO_4~(3-) ion in such type of mixtures the pH of (solution A) was raised to a value of 10 using ammonia buffer. Excess standard Mg~(2+) solution was added and the formed precipitate of MgNH_4AsO_4 was separated, dissolved and its magnesium content equivalent to AsO_4~(3-) was determined complexometrically using EriochromeBlack-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 proposedsuccessive 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.