¹⁷O NMR Study of Chromium(VI) Ions in Water

Abstract

A mathematical model for oxygen exchange out of labeled (e.g., 17O) Na2CrO4 into solvent H2O according to the dimerization reaction 2CrO4 2- + 2H+ ⇌ (1/K a(HCrO 4 − )) 2HCrO4 - ⇌ (α/β) Cr2O7 2- + H2O is described. Two rates of isotopic change are identified: (1) a rapid change intimately associated with the attainment of chemical equilibrium, but not identical with it (rate constants ν1 and ν2, respectively), in which the fractional label in monomeric HCrO4 - + CrO4 2- (p) decreases more rapidly than that in the dimer Cr2O7 2- (q), i.e. ṗ > q̇, followed by (2) a slower decrease in both (q̇ > ṗ) until isotopic equilibrium with the solvent is established, p = q = r (rate constant ν3). Visible−UV and 17O NMR spectra have been used to characterize the elusive HCrO4 - ion. Vibrational fine structure is seen in the visible spectrum (C 3 v symmetry), and integration of the monomeric (HCrO4 - + CrO4 2-) 17O absorption agrees with only four O atoms in HCrO4 -. A pH-dependent shift to higher frequency (δCrO 4 2− = 812 ppm; δHCrO 4 − = 860 ppm) is used to give pK a(HCrO 4 − ) = 5.80 (I = 1.0 M, 25 °C), and comparisons with the integrated Cr2O7 2- signal (δCr 2 O 7 2− = 1115 ppm) give K d's for dimerization (= α/β) of 81 M-1 (I = 1.0 M) and 132 M-1 (I = 6.0 M), at 25 °C. Dimerization and hydrolytic rate constants (α and β) have been obtained under the conditions of the 17O exchange experiments; they are shown to contain spontaneous, buffer, and [H+] and [OH-] contributions in agreement with earlier investigations. 17O exchange out of enriched Na2CrO4 has been followed in aqueous solutions over the concentration range 0.01−2.0 M and over the pH range 6.38−13.0 at 25 °C and constant ionic strength (I = 1.0, 6.0). The dimerization reaction (eq 1) contributes only at the highest Cr(VI) concentration (2.0 M), and then only slightly (i.e., ∼10% at pH 7.3). The rate data have been interpreted in terms of the direct exchange paths Cr17O4 2- + H2O → (k 1) Cr17O3O2- + H2 17O; HCr17O4 - + H2O → (k 2) HCr17O3O- + H2 17O; H2Cr17O4 + H2O → (k 3) H2Cr17O3O + H2 17O; Cr2 17O7 2- + H2O → (k 4) Cr2 17O6O2- + H2 17O, with rate constants k 1 = 7.2 × 10-8 M-1 s-1, k 2 = 7.6 × 10-4 M-1 s-1, k 3 = 1.7 × 105 M-1 s-1 (I = 1.0 M, 25 °C), and k 4 = 4.1 × 10-3 M-1 s-1 (I = 6.0 M, 25 °C). The monomeric paths (but not the rate constants) agree with those found in an earlier study, but that involving Cr2O7 2- is new. Rate constants for 17O exchange are compared with those of other substitution reactions of HCrO4 - and Cr2O7 2-.

Kinetics and Mechanism of the Reduction of Chromium(VI) and Chromium(V) byD-Glucitol andD-Mannitol

“…In the [H ] range under study, Cr VI may exist as HCrO 4 À [44], and this species is proposed as the reactive form of Cr VI , in agreement with the first-order dependence of the reaction rate on [Cr VI ]. It is known that the chromic oxidations of alcohols and glycols are preceded by the formation of a chromate ester [41] [42].…”

Redox and complexation chemistry of the CrVI/CrV-d-glucaric acid system

“…Oxidation of Glucar by Cr VI /Cr V /Cr IV In the range of substrates and acid concentrations used in the kinetic measurements, the oxidation of Glucar by Cr VI In the [H + ] range under study, Cr VI exists 50 predominantly as HCrO 4…”

Kinetics and mechanism of the reduction of CrVI to CrIII by D-ribose and 2-deoxy-D-ribose

“…In the [H + ] range studied, Cr VI exists mainly as HCrO 4 - (33). Thus, the first step of the mechanism proposed in Scheme 1(a) may be interpreted as the formation of a monochelate with Rib acting as a bidentate ligand bound to Cr VI at C1-O and C2-O to yield the anionic species A -.…”