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-.

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The present work deals with the carbonation process in cement based materials such as concrete. In order to clarify the evolution of the two main phases involved in the process, portlandite and calcium carbonate as a function of depth, spatially resolved neutron diffraction experiments have been performed at SALSA diffractometer at ILL in carbonated cement paste samples. Specimens submitted to different carbonation processes, both natural and accelerated, have been analyzed with this non destructive technique. The evolution of the main diffraction peaks of portlandite and calcite has been followed by means of neutron diffraction patterns measured at different depths. The results indicate that, in specimens subjected to CO 2 atmospheres for 24 and 48 hours, the amount of calcite increases from the centre of the specimen to the surface. In both type of specimens calcite is formed at all depths analyzed, with higher quantities for the ones submitted to the longest carbonation period. Regarding the evolution of portlandite in these specimens, it almost completely disappeared, with only a low amount of the phase constant throughout the sample. In specimens subjected to air in a closed chamber for 21 months, higher amounts of portlandite were observed throughout the sample and little increase of calcite in the outer part, pointing out a much less severe reaction. The absorption effects are characterized by measuring in perpendicular directions and an absorption coefficient is calculated for portlandite.