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 2CrO42- + 2H+ ⇌ (1/Ka(HCrO4−)) 2HCrO4- ⇌ (α/β) Cr2O72- + 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- + CrO42- (p) decreases more rapidly than that in the dimer Cr2O72- (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 (C3v symmetry), and integration of the monomeric (HCrO4- + CrO42-) 17O absorption agrees with only four O atoms in HCrO4-. A pH-dependent shift to higher frequency (δCrO42− = 812 ppm; δHCrO4− = 860 ppm) is used to give pKa(HCrO4−) = 5.80 (I = 1.0 M, 25 °C), and comparisons with the integrated Cr2O72- signal (δCr2O72− = 1115 ppm) give Kd'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 Cr17O42- + H2O → (k1) Cr17O3O2- + H217O; HCr17O4- + H2O → (k2) HCr17O3O- + H217O; H2Cr17O4 + H2O → (k3) H2Cr17O3O + H217O; Cr217O72- + H2O → (k4) Cr217O6O2- + H217O, with rate constants k1 = 7.2 × 10-8 M-1 s-1, k2 = 7.6 × 10-4 M-1 s-1, k3 = 1.7 × 105 M-1 s-1 (I = 1.0 M, 25 °C), and k4 = 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 Cr2O72- is new. Rate constants for 17O exchange are compared with those of other substitution reactions of HCrO4- and Cr2O72-.