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

Abstract

The precision and accuracy achieved in consecutive back-titrations of Ga³⁺, In³⁺ and Tl³⁺ with solutions of disodium ethylenediaminetetraacetate (Na₂EDTA) and standard Th(NO₃)₄ of the same concentration, using Semimethylthymol Blue (SMTB) as a metallochromic indicator with visual end-point indication, were studied. For one aliquot Ga³⁺, In³⁺ and Tl³⁺ were determined by back-titration of excess of Na₂EDTA 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 Na₂EDTA 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 Na₂EDTA 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.