Determination of structures of cupric-chloro complexes in hydrochloric acid solutions by UV-Vis and X-ray absorption spectroscopy

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Abstract

Ultrahigh-purity metals are indispensable to understanding the nature of materials, but the purity of contemporary metals is insufficient for determining their intrinsic properties. The fundamentals of the anion-exchange reaction must be clear in order to increase the refining efficiency of anion-exchange separation for purification of metals. The thermodynamic analysis of the anion-exchange reaction needs to take account of the distributions of metal-chloro complexes in the solution phase in addition to those between the resin and the solution phase. The structures of metal-chloro complexes in the solution phase must be determined as the first step in determining what species is adsorbed on the resin. Copper is one of the base metals most commonly used in modern society, and the anion-exchange behavior of cupric species is representative. The distribution and the molar attenuation coefficients of cupric-chloro complexes in hydrochloric acid solutions were obtained employing factor analysis followed by fitting a thermodynamic model to ultraviolet-visible absorption spectra. The cumulative formation constants were determined as follows: log 10β1= 0.599 , log 10β2= 0.343 , log 10β3= − 1.88 , and log 10β4= − 5.25 , and the Setchénow coefficient for the neutral species of [CuIICl2]0 is 0.188. Using the distribution assessed by the above thermodynamic parameters, the X-ray absorption spectra (XAS), consisting of X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) of individual species, were assessed by factor analysis. EXAFS theoretical models were fitted to experimental spectra of individual species to determine their structures and coordination geometries for the first time, although it is generally very difficult to obtain the spectrum of the individual species in a matrix containing other species simultaneously. The structures of five cupric-chloro complexes were determined to be distorted octahedrons of [CuII(H2O)4eq(H2O)2ax]2+,[CuII(H2O)4eq(H2O)axClax]+, and [CuIICl2ax(H2O)4eq]0, a planar triangle of [CuIICl3]−, and a tetrahedron of [CuIICl4]2−. The obtained spectra can be used as standards. The XANES spectra of cupric-chloro complexes were interpreted qualitatively. The Cl ligand in [CuII(H2O)4eq(H2O)axClax]+ is attracted to the center atom of CuII by electrostatic force, as the H2O ligands are. Contrastingly, the bonding system of the Cl ligands in the latter three species involves covalency.

Original languageEnglish
Pages (from-to)61-74
Number of pages14
JournalStructural Chemistry
Volume30
Issue number1
DOIs
Publication statusPublished - 2019 Feb 1

Keywords

  • Cupric-chloro complexes
  • Extended X-ray absorption fine structure
  • Factor analysis
  • Hydrochloric acid solution
  • Multivariate curve resolution-alternating least squares
  • Ultraviolet-visible absorption spectrum
  • X-ray absorption near edge structure
  • X-ray absorption spectrum

ASJC Scopus subject areas

  • Condensed Matter Physics
  • Physical and Theoretical Chemistry

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