Phenylarsine Oxide (PAO) solutions are used as alternatives to Sodium Thiosulfate solutions and occasionally as a replacement for Sodium Arsenite in some analyses for Chlorine. While PAO solutions are more stable and less prone to bacterial growth than comparable Normalities of Sodium Thiosulfate, PAO is not very soluble at a neutral pH and therefore its maximum usable concentration is limited.
Phenylarsine Oxide solutions are most often used for Chlorine and Iodine determinations, either directly or indirectly. The endpoint can be determined amperometrically as with most Chlorine titrators, or by using Starch Indicator in the presence of Iodide. A typical reaction is:
C6H5AsO + I2 + 2 H2O → C6H5AsO(OH)2 + 2 HI
The Arsenic is oxidized from the trivalent to the pentavalent state, as usual, and the Iodine may be formed by the addition of Potassium Iodide to a sample containing Chlorine.
The quality and availability of Phenylarsine Oxide starting material has deteriorated in recent years and the price has gone up significantly. RICCA CHEMICAL COMPANY has developed proprietary purification and manufacturing procedures that allow us to continue to make high quality PAO solutions. The higher price coupled with safety and environmental concerns associated with Arsenic compounds make Sodium Thiosulfate solutions more attractive alternatives for many applications, in spite of their need for restandardization and susceptibility to bacterial growth.