INDIRECT KINETIC DETERMINATION OF AS(III) BASED ON ITS ACCELERATING EFFECT ON THE OS(VIII)-CATALYZED REACTION BETWEEN IODIDE AND BROMATE IN A MICELLAR MEDIUM

An indirect kinetic-photometric method for the determination of As(III) over the range 7-300 ng ml-1 is proposed. It is based on the accelerating effect of this ion on the Os(VIII)-catalysed reaction between iodide and bromate, which is monitored via the triiodide formed. Arsenic(III) acts as an accelerator only in a very rapid initial step of the reaction and the rate processes occurring in the system after this step do not depend directly on the As(III) concentration. Therefore, the contribution of this ion to the reaction must be measured after its accelerating effect has cancelled. Dodecyltrimethyl-ammonium bromide (DTAB) micelles accelerate the reaction, and hence increase the sensitivity and selectivity of the determination of As(III) compared to reactions taking place in an aqueous medium. Some observations on the mechanism via which DTAB acts on the reaction are reported. The role of micellar media in catalytic kinetic methods is discussed.