EQUILIBRIA OF CHROMATE(VI) SPECIES IN ACID MEDIUM AND AB INITIO STUDIES OF THESE SPECIES

摘要

The study of the H+-CrO42- system in 3.0 M KCl as ionic medium at 25 degrees C by means of emf (glass electrode) and direct calorimetric measurements, in the total Cr-VI concentration (B), average number of H+ bounds per central group CrO42- (Z), and pH ranges: 25 less than or equal to B less than or equal to 100 mM, 0 less than or equal to Z less than or equal to 1.16 and 1 less than or equal to pH less than or equal to 8, respectively, indicates the formation of the following complexes H-p(CrO4)(q)((p-2q)), stability constants (log beta(pq)(+/-3 sigma)) and partial molar enthalpies (Delta H-pq(+/-3 sigma) kcal.mol(-1)) : HCrO4-, 5.888(4), -0.6(1); Cr2O72-, 13.900(3), -5.7(1); H2CrO4, 7.004(7), 1.8(2) and HCr2O7-, 15.007(5), -5.0(1), respectively, according to the general reaction: pH(+) + qCrO(4)(2-) reversible arrow H-p(CrO4)(q)((p-2q)). The results previously obtained by Raman spectroscopy for this system are better adjusted when the HCrO; species is included. The energies and optimized structures from ab initio calculations for the CrO42-, HCrO4-, Cr2O72-, H2CrO4 and HCr2O7- species have also been obtained. For the HCrO4- species the values of bond lengths and-angles theoretically calculated are in good agreement with the experimental data of the anion in the crystal structure of the compound (PPh4[(CrO3)-O-VI(OH)]. Finally, correlations between thermodynamic experimental and structural theoretical parameters are discussed. (C) 1997 Elsevier Science Ltd. [References: 69]