Electrocatalytic Oxidation of As(III): II . Kinetic Studies at Pt Electrodes

The empirical rate law for the electrocatalyzed oxidation of As(III) at a Pt RDE in acidic media was determined from potentiostatic measurements to be k = k 0 t / t 0 − b exp − E / E 0 where t 0 = 1.0 s and E 0 = 0.161 V vs. SCE. For an electrode surface covered with less than the equivalent of a monolayer of PtO ( i.e. , Q OX / ; Q H < ca . 2 ), b = 0.30 , whereas for an extended oxide coverage ( i.e. , Q OX / Q H > 2 ), b = 0.5 . The coefficient k 0 is independent of time ( t ) and electrode potential ( E ) , but decreases as a function of the log of As(III) flux, i.e. , ω 1 / 2 C As ( III ) b . It is PtOH and not PtO that is the active agent for oxygen‐atom transfer in the electrocatalytic mechanism. The finite rate of oxidation of As(III) observed for Q OX / Q H > > 2 is concluded to result from ·OH generated as the first step in the mechanism for continued oxide growth. It is concluded also that the generation of ·OH at a highly oxidized Pt surface occurs at a rate independent of applied potential.