The quantitative analysis of chromous ions by channel how double electrode(CFDE) technique

The purpose of this study is to prove that the Channel Flow Double Electrod e (CFDE) technique can be used to detect chromous ions, with a view to impr oving the material balance during a Fe-Cr repassivation sequence. The chrom ous ions are produced under flowing conditions by potentiostatic activation of a chromium single-crystal electrode and are then oxidised downstream on a gold electrode. The experimental collection efficiency N-exp is the rati o of the collecting current for the oxidation of Cr2+ to the steady state c urrent of the dissolution of chromium metal. The latter current is calculat ed using Faraday's law, from the amount of material dissolved after a long term activation. The value of N-exp is very close to the theoretical collec tion efficiency N-0 and it seems to be independent of the flow velocity in the range 40-120 cm s(-1). This result is discussed and compared with the l iterature. Moreover as chromous ions are thermodynamically unstable in acid ic media, an estimation of the limiting rate constant, k, of Cr2+ oxidation reaction is calculated to be less than 0.7 s(-1). (C) 1999 Elsevier Scienc e Ltd. All rights reserved.