PREPARATION AND STABILITY OF STANDARD ARSENIC(III) SOLUTIONS

The effect of dissolution solvents on the assay of diarsenic trioxide and the stability of arsenic(III) nas evaluated by high precision coul ometric titration with electrogenerated iodine. The assays obtained on a standard reference material were ca. 0.1% lower than the certified value, when it was dissolved in 2 mol dm(-3) sodium hydroxide solution s. However, when 2 mol dm(-3) sodium carbonate solutions were used for the dissolution, the assays were in good agreement with the certified value. It is apparent that the decreases in assay are mainly due to t he oxidation of arsenic(III) to arsenic(V) by the high basicity and di ssolved oxygen. Consequently, it is recommended to dissolve diarsenic trioxide in sodium carbonate solutions. About 20 and 4% of arsenic(III ) were lost from 1 and 1000 mu g cm(-3) arsenic(III) solutions, respec tively, in 2 mol dm(-3) sodium hydroxide stored in high-density polyet hylene and Teflon containers for 2 months. Arsenic(III) in a sodium ca rbonate (2 mol dm(-3)) or sodium hydroxide (ca. pH 12.5) solution rema ined unchanged at least for 2 or 1 month in both containers, respectiv ely. The rate of the air oxidation of arsenic(III) was strongly depend ent on the pH of solutions.