Kinetics of reactions and yield of hydrogen ions from pulse radiolysis of aqueous solutions of potassium chromate

Pulse Radiolysis and Transient Absorption of Aqueous Cr(VI) Solutions up to 325 °C

“…According to Cr­(VI) speciation calculations, the pH 7 results would correspond to a solution of 95% HCrO 4 – , which makes this value identical to within the uncertainty with the value obtained in this work. The pH 13 result, corresponding to a solution of 100% CrO 4 2– , gives a rate faster than that found in this and other studies and is likely in error. − …”

“…Based on their rate coefficients and calculated reaction radii, these reactions with the hydrated electron are likely to be diffusion controlled and occur via long-range electron transfer processes . The rate coefficients for HCrO 4 – and CrO 4 2– reacting with e aq – have been reported previously using electron pulse radiolysis at 25 °C; however, it is difficult to compare directly with these results as most studies do not report the uncertainties, the exact solution conditions, and/or whether or not they corrected for ionic strength. − Baxendale et al and Peled and Czapski both found that k (CrO 4 2– + e aq – ) = 1.8 × 10 10 M –1 s –1 , which is slightly faster than the value measured in this work. , More recently, Lai and Freeman measured the Arrhenius behavior of CrO 4 2– + e aq – up to 80 °C and found that k (CrO 4 2– + e aq – ) = 1.7 × 10 10 M –1 s –1 at 25 °C, with an activation energy of E A = 16 kJ/mol and pre-exponential factor of A = 10 13 M –1 s –1 . Again, the reported rate is faster than that found in this work; however, the Li 2 CrO 4 solutions employed by Lai and Freeman would have been ∼pH 8, affording ∼6% speciation to HCrO 4 – , which would increase their measured rate relative to a solution of pure CrO 4 2– .…”

“…The reactive species from water radiolysis can undergo subsequent reactions with many of the aqueous solution matrix chemicals present to generate secondary radiolysis products, or they can react with any nearby solutes, such as aqueous chromium ions. Chromium ions are present in dilute aqueous solutions of Cr­(VI) in the form of chromic acid: − where the ionization constants and their temperature and ionic strength dependencies can be derived from fits to thermodynamic data. ,,− The polynuclear dimeric dichromate ion, Cr 2 O 7 2– , also exists at higher concentrations than those studied in this work, and it becomes the dominant form of Cr­(VI) below pH 5 at [Cr­(VI)] > 7 mM. , Some reactions between the radiolysis products of water and these chromium ions have been studied in the past, − ,− but the pH and exact Cr speciation in these solutions are rarely reported, and the temperature dependance of the reported rate coefficients are largely unknown. In addition, most of the literature does not specify the uncertainties or any ionic strength corrections that may have been employed, making comparison of these values difficult.…”