Tiron as a masking reagent for aluminium in the determination of fluoride with an ion-selective electrode: equilibrium studies and application

Abstract

Equilibria between aluminium fluoro complexes and Tiron (sodium 1,2-dihydroxybenzene-3,5-disulphonate; Na₂H₂L) were studied. Formation of the mixed ligand complexes of AlLF_j(j= 1 or 2) defines the interference by aluminium in the determination of fluoride with ion-selective electrodes using Tiron as a masking reagent. AILF_j+ H₂L^2–⇌ AIL₂+jF^–+ 2H⁺ AILF_j+ 2H₂L^2–⇌ AIL₃+jF^–+ 4H⁺ The second and even the third Tiron can react with these mixed ligand complexes to liberate fluoride together with two to four protons. Hence higher recoveries of fluoride are obtained at higher pH with a higher concentration of Tiron. At pH 6, 0.1 mol dm^–3 Tiron solution has a tolerance limit of around 10^–3 mol dm^–3 of aluminium, and is superior to the corresponding solution of citrate or CDTA in releasing fluoride from aluminium. Tiron was applied to the analysis of crude cryolite and fluorspar after decomposition of the samples with aluminium chloride solution.