Chromium speciation in seawater: The probable role of hydrogen peroxide

The rates of oxidation of Cr(III) to Cr(VI) with H 2 O 2 were measured in NaClO 4 solutions as a function of pH, temperature, ionic strength, and H 2 O 2 concentration. The rate of oxidation of Cr(III) was found to be first order with respect to 1/[H + ] or [OH − ] (pH 7–9) and [H 2 O 2 ] (112–2,621 µ M). The rate equation is given by where the value of k 2 = k 1 /[H 2 O 2 ][OH − ] = k 1 [H + ]/ [H 2 O 2 ] K w * ( k 1 is the pseudo‐first‐order rate constant and K w * is the stoichiometric dissociation constant for the ionization of water). The values of k 2 were independent of ionic strength and fit the equation with SD = 0.06 in log k 2 . The energy of activation for the reaction is 9.9±0.8 kcal mol −1 . The rate constant was used to estimate the half‐time of Cr(III) in surface seawater. At a concentration of 0.1 µ M H 2 O 2 , the lifetime of Cr(III) is controlled by H 2 O 2 .