Raman spectroscopic analysis of the speciation of dilute chromate solutions

Abstract The relative concentrations of various Cr VI species are important to several mechanisms for corrosion inhibition by chromate conversion coatings and chromate containing primers. The pH dependent conversion of CrO 4 −2 to Cr 2 O 7 −2 is well known, but the existence and importance of bichromate, HCrO 4 − , is more controversial. Past reports have described the pH and concentration dependence of the CrO 4 −2 /HCrO 4 − /Cr 2 O 7 −2 distribution, while recent reports have concluded that HCrO 4 − does not exist in aqueous media. The current work shows that HCrO 4 − does indeed exist at a pH below 6 and at low total Cr VI concentration. The Raman signature of the Cr–O–Cr bond in dichromate decreases relative to the Cr–O symmetric stretch as [Cr VI ] decreases. The results are quantitatively consistent with previous reports on the CrO 4 −2 /HCrO 4 − /Cr 2 O 7 −2 equilibrium, and establish that HCrO 4 − is the dominant Cr VI species for low [Cr VI ]. In the low concentration and slightly acidic conditions likely to be present during corrosion inhibition by Cr VI in the field, HCrO 4 − is the most prevalent form of Cr VI in water.