Determination of trace amounts of gallium, indium and thallium by successive titrations using semi-xylenol orange with spectrophotometric and/or visual end-point indication

Abstract

The precision and accuracy attainable in successive titrations of Ga³⁺, In³⁺ and Tl³⁺ with a 0.001 mol dm^–3 solution of disodium ethylenediaminetetraacetate (Na₂EDTA) using Semi-xylenol Orange (SXO) as a metallochromic indicator with spectrophotometric and/or visual end-point indication were studied. Indium(III) was initially titrated directly at pH 5.4–5.8 (490 nm) by using hexamethylenetetramine (hexamine) buffer in the presence of ascorbic acid to reduce Tl³⁺ to Tl⁺. At the indium end-point, an excess of Na₂EDTA was added, and Ga³⁺ was determined by back-titration with a standard PbCl₂ solution. Gallium(III), In³⁺ and Tl³⁺ in another aliquot were determined by indirect titration of an excess of Na₂EDTA with a standard PbCl₂ solution at pH 5.4–5.8 (490 nm). The interference of various anions, cations and organic acids in the determination of Ga³⁺, In³⁺ and Tl³⁺ was studied. A comparison of the indicators SXO and Xylenol Orange for successive titrations of the metal ions studied was carried out.