Structural properties of the inner coordination sphere of indium chloride complexes in organic and aqueous solutions

• Published in: Dalton transactions : an international journal of inorganic chemistry Vol. 43; no. 4; pp. 1630 - 1635
• Main Authors: Narita, Hirokazu, Tanaka, Mikiya, Shiwaku, Hideaki, Okamoto, Yoshihiro, Suzuki, Shinichi, Ikeda-Ohno, Atsushi, Yaita, Tsuyoshi
• Format: Journal Article
• Language: English
• Published: England 28.01.2014
• Subjects: Amines; Anions; Aqueous solutions; Chlorides; Concentration (composition); Indium; Niobium; Solvent extraction
• ISSN: 1477-9226; 1477-9234
• DOI: 10.1039/c3dt52474d

The nature of the inner coordination sphere of In(3+) present in both the organic and aqueous solutions during the solvent extraction of In(3+) from an aqueous HCl solution with tri-n-octyl amine (TOA) was investigated by In K-edge XAFS. This information was then used to clarify the details of the extraction properties of indium chloride anion complexes with TOA. In aqueous HCl solution (0.1-10 M), In(3+) exists as octahedral anion complexes, [InCln(H2O)6-n](3-n) (n ≥ 4); the [InCl6](3-) complex is dominant at 10 M HCl. The extraction of In(3+) from HCl solution with TOA was performed using two kinds of diluents: nitrobenzene (NB) or n-dodecane (DD), which contained 20 vol% of 2-ethylhexanol as an additive. The stoichiometric composition of the extracted complexes, which is estimated from the distribution ratios of In(3+), is affected by the diluents and the HCl concentration of the aqueous phase; the apparent values of TOA/In(3+) in the extracted complex are 3 for DD-1 M HCl (diluent-aqueous phase) and DD-5 M HCl, 2 for NB-1 M HCl and NB-5 M HCl, and 1 for NB-10 M HCl. The EXAFS analysis of these extracted complexes shows that the In(3+) has ∼4 Cl(-) at ∼2.336 Å and no H2O in the inner coordination sphere; additionally, the shape of the XANES suggests that their coordination geometry is tetrahedral. Therefore, the same tetrahedral [InCl4](-) complex is formed during the extraction in spite of the variation in the stoichiometric composition (TOA/In(3+) = 1-3) of the extracted complexes.