Arsenic Trioxide

Arsenic Trioxide
Chemical Formula As2O3
Formula Weight 197.84
Equivalent Weight 49.46 (Molar = 4 Normal)
CAS No. 1327-53-3

Arsenic Trioxide, As2O3, is a very good reductometric primary standard but it dissolves slowly in water. When dissolved in an alkali such as Sodium Hydroxide, readily soluble Sodium Arsenite, NaAsO2, is produced. When this solution is neutralized with an acid, it is then commonly called Arsenious Acid, H3AsO3.

Arsenic Trioxide and its solutions can be used to standardize oxidizing titrants such as Ceric Sulfate, Iodine, and Potassium Permanganate. Ferroin Indicator can be used in the Ceric Sulfate titration, and Starch Indicator can be used in the Iodine titrations. Potassium Permanganate is self-indicating, changing from purple to colorless at the endpoint. Typical reactions are:

H3AsO3 + 2 Ce4+ + H2O → H3AsO4 + 2 Ce3+ + 2 H+

H3AsO3 + I2 + H2O → H3AsO4 + 2 H+ + 2 I-

5 H3AsO3 + 2 MnO4- + 6 H+ → 5 H3AsO4 + 2 Mn2+ + 3 H2O