Complexometric determination of gallium, indium and thallium with ethylenediaminetetracetic acid and Semimethylthymol Blue by visual end-point indication

The precision and accuracy achieved in consecutive back-titrations of Ga3+, In3+ and Tl3+ with solutions of disodium ethylenediaminetetraacetate (Na(2)EDTA) and standard Th(NO3)(4) of the same concentration, using Semimethylthymol Blue (SMTB) as a metallochromic indicator sith visual end-point indication, were studied. For one aliquot Ga3+, In3+ and Tl3+ were determined by back-titration of excess of Na(2)EDTA with standard Th4+ solution at pH 2 (HClO4-NaClO4) using SMTB as indicator. At the end-point L-ascorbic acid was added to reduce Tl3+ to Tl+ and the liberated Na(2)EDTA equivalent to Tl3+ was titrated versus the same Th4+ standard solution. In another aliquot containing Ga3+, In3+ and Tl3+, the latter two ions were quantitatively reduced to the metallic state using magnesium powder and concentrated hydrochloric acid and Ga3+ was determined by indirect titration of Na(2)EDTA against Th4+ solution after adjusting the pH to 2 using SMTB as indicator. Indium was determined by difference. The interference of various anions, cations and organic acids in the determination of Ga3+, In3+ and Tl3+ was investigated. A comparison of the indicators SMTB and Methylthymol Blue for consecutive back-titrations of the metal ions studied using different statistical tests was carried out. The proposed consecutive back-titration method was applied successfully to some real samples of ores, minerals and alloys of the studied metals and the results were satisfactory and agreed with those obtained by atomic absorption spectrometry.