電位差滴定によるヒ素(III)の定量 : ヨウ素酸カリウムを用いる電位差滴定(第3報)

概要

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Potentiometric redox titration of arsenic(III) in an aqueous solution of hydrochloric acid was studied by the use of a standard potassium iodate solution. The titration curve showed two steps, the first one corresponded to the reaction 5As3++2I5+→5As5++2I0 and the second to the reaction 2As3++I5+→2As5++I+ or 4I0+I5+→5I+. With an increase in the concentration of hydrochloric acid the redox potential in the first stage became higher and the first inflection on the curve was diminished but the second inflection became obvious especially in 36 M acid. On the contrary, with a decrease of the concentration the first inflection became sharp especially in 0.151.5M acid but the second became flat. (Fig. 1) Arsenic could be determined accurately by using one of the two inflections in appropriate concentration of the acid. The reproducibilities were higher than 99.8% for 0.0010.1 M solution of arsenic trioxide. The optimum conditions for the titration were as follows: concentration of hydrochloric acid, 0.151.5M for the first inflection and 35 M for the second inflection; temperature of the solution, 5°C; titration rate at the end point, 0.4 ml per minute.
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