DETERMINATION OF THE 2ND HYDRATION SHELL OF CR3(-SOLUTIONS BY EXTENDED X-RAY-ABSORPTION FINE-STRUCTURE() AND ZN2+ IN AQUEOUS)

The geometric structures of Cr(NO3)(3) and Zn(NO3)(2) aqueous solution s in a wide range of concentrations 2.7-0.005 m have been determined b y means of the extended X-ray absorption fine structure (EXAFS) techni que. X-ray absorption spectra at the K-edges of Zn and Cr have been me asured in the transmission and fluorescence modes at the Synchrotron R adiation Source (U.K.). The analysis of all the experimental data (13 solutions) is compatible with a unique structural model, which basical ly agrees with the concentric shell model of Frank and Evans for ionic hydration. Therefore, it is shown that the EXFAS technique allows the determination of a second hydration shell in a wide range of concentr ations, from almost saturated to highly dilute solutions. M-O distance s (Cr-O-I = 2.00 Angstrom, Zn-O-I = 2.05 Angstrom) and the coordinatio n number (6 for both cations) for the first hydration shell are not af fected by concentration. A second hydration shell is detected in both cases, although for chromium solutions, this contribution to the EXAFS spectra is more important than for zinc. The distance is around 4.0 A ngstrom, but in Cr3+ solutions a slight increase in the Cr-O-II distan ce is observed with dilution (4.02 Angstrom for 0.01 m, and 3.95 Angst rom for 2.6 m). The Zn-O-II distance shows no systematic trend, the av erage distance being 4.1 Angstrom. The coordination number for this sh ell is 13.3 +/- 1 for Cr3+ solutions and 11.6 +/- 1.6 for Zn2+ solutio ns. The most concentrated Zn2+ solution (2.7 m) presents a singular be havior, since its coordination number decreases to 6.8 +/- 1.5. The da ta analysis procedure is thoroughly described, and the possibilities o f an alternative hypothesis for the second contribution to the EXAFS s pectrum, such as multiple scattering effects, are carefully discussed.