Determination of fluoride and oxalate using the indicator reaction of Zr(IV) with methylthymol blue adsorbed on silica gel

Solid-phase spectrophotometric and visual test-methods of fluoride and oxalate determination are proposed. The methods are based on the competitive reactions of ZrOCl 2 with methylthymol blue immobilized on silica gel and fluoride or oxalate in solution. Absorbance of the solid-phase reagent at 590 nm decreases with the growth of fluoride and oxalate contents in solution. The developed methods demonstrate high selectivity. The interference of Bi(III) and SO 4 2−, PO 4 3− is eliminated by the addition of 0.01 mol L −1 solution of ascorbic acid and 0.01 mol L −1 of BaCl 2, respectively. To eliminate the fluoride interference with oxalate determination 1 × 10 −3 mol L −1 solution of Ca(NO 3) 2 at pH 1.5 was added. The anions of the organic acids were destructed prior to F − determination by ultrasonic exposition (44 kHz, intensity of ≤10 W cm −2 for 3 min). The proposed methods were applied to the analysis of mineral water, toothpaste and biological fluids.