Speciation of aqueous chromium(VI) solutions with the aid of Q-mode factoranalysis followed by oblique projection

The identities of the species of chromium(VI) that are present in aqueous s olution, their spectra and their equilibria, continue to be a subject of di scussion in the literature. In this paper, the composition of the Cr(VI) eq uilibria was estimated from the UV-vis spectra of dilute potassium dichroma te solutions, without any prior knowledge of the quantities of pure compone nts, with the aid of Imbrie Q-mode factor analysis (Q-mode FA) followed by Varimax rotation and Imbrie oblique projection. Combining these results wit h the k-matrix method, it was possible to obtain the spectra of the individ ual Cr(VI) species. Sets of 3.3 x 10(-4) and 3.3 x 10(-5) mol 1(-1) Cr(VI) solutions were studied. In the pH range from 1 to 12, two factors were iden tified, which were related to the two species, chromate ion (CrO42-) and bi chromate ion (HCrO4-). When the analysis was extended to concentrated acid media, another factor appeared, which was related to chromic acid (H2CrO4). No evidence for the dichromate ion (Cr2O72-) was seen at the Cr(VI) concen trations used. The spectra of the pure components were obtained and pK valu es for the first and second chromic acid dissociations were estimated as -0 .54 and 5.8, respectively. (C) 2000 Elsevier Science B.V. All rights reserv ed.