Kinetic Photometric Determination of Oxalate by Its Activation Effect on Catalytic Oxidation of Iodide and Formation of Iodine-Starch Complex

Abstract

A method for the kinetic photometric determination of oxalate ion in aqueous solutions, in-cluding extracts from foodstuffs, has been suggested. The initial rate method has been used: the straight parts of kinetic curves (approximately 5 min) are processed by the least square method to get the slope coefficients used as analytical signals. The procedure is based upon the oxalate ion action as an activator in the iron(II)-catalyzed reaction of iodide oxidation by potassium bromate. Unlike the previously used light absorbance in the UV region, the suggested procedure controls the rate of the iodine-starch complex formation at 590 nm. The greatest initial rate for the reaction is at pH 5.05 of the acetate buffer solution and the optimal concentration of soluble starch 0.30 mg/mL. The calibration graph is linear in the oxalate concentration range (0.1–10) mg/mL. In this interval precision error varies between 0.67 % and 4.71 %, accuracy error is within (0.22–9.30) %. The procedure has been applied to food analysis by the standard addition method. Aqueous extract has been obtained from 2.5 g of finely chopped sample of raw beet, stirred with 100 mL of water for 1 h, centrifuged, filtered, and quantitatively transferred to 250-mL volumetric flask. Determination of oxalate in 1-mL aliquot has required the same volume of colored beet extract for a reference solution, but otherwise the procedure stands valid. The oxalate content has been found to equal (379 ± 2) mg/g of the raw beet (P = 0,95; n = 4), with 0.6 % precision error and 3.0 % accuracy error.