電流滴定法による亜セレン酸とセレン酸の定量

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  • Differential Determination of Selenite and Selenate by Amperometric Titration -Studies on Analytical Methods by Ampero metric Titration Using a Rotating Platinum Electrode. XLIX.-

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An amperometric titration for the determination of selenate and both of selenite and selenate in the mixture was investigated. In order to detect the end point, the oxidation current of thiosulfate ion(titrant) at a rotating platinum electrode(2000 rpm) was measured at +O.75 V vs. SCE. Selenate was reduced with hydrochloric acid to selenite, which could b e titrated with sodium thiosulfate in the presence of hydrochloric acid. Therefore, two titrations before and after the reduction gave the amount of selenite and the sum of selenite and selenate, respectively.<BR>(1) Determ ination of sodium selenate: Ten cm3 of sample solution containing 2-47 mg of sodium selenate was introduced into the titration cell, and the solution was boiled for 6-4 min after the addition of 10 cm3 of concentrated hydrochloric acid. The solution w as cooled to room temperature, and 10 cm3 of 10 mol⋅dm-3 hydrochloric acid and water we re added to make the total volume up to 50 cm3. The resultant solution was titrated w ith 0.05 cm3 portions of 10-1 mol⋅dm-3 sodium thiosulfate standard solution at intervals of 5 s (O.05 cm3/5s). Sodium selenate was determined with a relative error and a coefficient of variation less than 0.1% at its amounts from 2 mg to 47 mg.<BR>(2) Differential determination of selenite and selenate in mixtures: A mixture containing 5 cm3 each of 10-2 mol⋅dm-3 selenious acid and 10-2 mol⋅dm-3 sodium selenate was analyzed by two aliquots. One aliquot was used to titrate selenious acid with 10-' moldm-3 sodium thiosulfate standard solution(0.05 cm3/5s) at room temperature after the addition of 20 cm3of 10 mol⋅dm-3 hydrochloric acid and water to make the volume up to 50 cm3. Another aliquot was used to titrate the sum of selenious acid and sodium selenate in the same manner as described in (1). The amount of sodium selenate was estimated from the difference of the two titrations. Mixtures containing selenious acid and sodium selenate, from 1: O.5to 1: 40, were analyzed with a relative error and a coefficient of variation less than 0.3%. The effect of concomitant compounds was examined.

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