Raman spectroscopic investigations of the CrO/Cr₂O equilibrium in aqueous solution

Abstract

With the intention of doing a study of the CrO:-/Cr,O:-equilibrium in aqueous solution, we have carefully re-examined the Raman spectra of CrO:-, Cr,O:-and CICrO;. Diluting stepwise an aqueous solution of 00:-ion we have showed the non-existence of the HCrO; species which was previously considered to exist in such systems. By studying the effect of pH on the Raman spectra we calculated the constant for the equilibrium: 2 Cr0:-+ 2H'S Cr20:-+H20 to be log (jZ2 = 13.77 k 0.02 (20 "C and KNO, 0.8 M). Using this value of the constant, we were able to establish the distribution diagram of the Cr0:-and Cr20:-species versus pH.

Raman spectroscopic investigation of chromium(VI) equilibria — another look

“…The intense peak at ca 844 cm 1 , observed in the case of strongly alkaline solutions, becomes progressively less intense and ultimately vanishes upon acidification of the solution. This peak is clearly to be associated with CrO 4 2 vibrations, and the observed wavenumber of the maximum agrees closely with that reported in earlier studies. 4,5 At a shift of ca 899 cm 1 , a second intense band is seen in the spectra of solutions of low pH c ; its intensity progressively decreases with increasing pH c .…”

“…The accumulated evidence supporting the existence of HCrO 4 in the simultaneous equilibria CrO 4 2 C H C HCrO 4 1 2CrO 4 2 C 2H C Cr 2 O 7 2 C H 2 O 2 in aqueous media of pH ³ 3-10 now seems to be fairly conclusive. 1 It was the interpretation of Raman data by Michel and co-workers 4 (1) and (2) were considered, rather than only (2).…”

“…1 It was the interpretation of Raman data by Michel and co-workers 4 (1) and (2) were considered, rather than only (2). However, Brito et al 3 reported that the results of these calculations were 'not sufficiently good due to the paucity of the experimental data.'…”

Ab initio thermochemistry of some geochemically relevant molecules in the system Cr-O-H-Cl

“…Ϫ , the existence of the aqueous species was denied in a "seemingly quite convincing way," (Cruywagen et al, 1998) by Poulopoulo et al (1997 (but see also Michel and Machiroux, 1983;Michel and Cahay, 1986). Their observations promoted new spectrophotometric (Cruywagen et al, 1998), Raman spectroscopic (Heyns et al, 1999), and infrared and X-ray (Hoffmann et al, 2001) investigations, which all confirmed the results of the potentiometric experiment of Palmer et al (1987), consistent with the data of Dellien et al (1976) (G°f ,298.15,aq HCrO 4 Ϫ ϭ Ϫ764.8 kJ/mol).…”

Aplicação de métodos quimiométricos na especiação de Cr(VI) em solução aquosa

“…Por outro lado, ainda existe controvérsia na literatura sobre as espécies de Cr(VI) presentes em solução aquosa, os seus espectros e os valores das constantes de equilíbrio envolvidas. Por exemplo, nos anos 80, Michel et al [9][10] , analisando espectros Raman, indicavam que CrO 4 2-e Cr 2 O 7 2-eram as únicas espécies de Cr(VI) presentes em solução aquosa, negando a existência de espécies protonadas. Estudos recentes, como o de Poulopoulou et al 11 , analisando espectros no UV/visível, continuam questionando a presença da espécie HCrO 4 -em soluções aquosas.…”