Solvation of extracted substituted ammonium salts in nitrobenzene-benzene mixtures

Abstract

The extraction of iron(III) from aqueous hydrochloric acid by tri-n-octylamine hydrochloride (TOAHCl) in nitrobenzene-benzene mixtures ranging from pure benzene to pure nitrobenzene has been studied as a function of amine salt concentration, solvent composition, and temperature. A less extensive extraction study of iron(III) by tetra-n-heptylammonium chloride (THACl) in nitrobenzene-benzene mixtures was also performed. In addition, a brief study was made of the degree of aggregation of both amine salts in benzene. The results of the extraction study using TOAHCl are interpreted in terms of a phenomenological thermodynamic model which includes the solvation of the primary extracted species by nitrobenzene and by additional TOAHCl molecules in the organic phase. This extraction system was found to be metal concentration independent. It is postulated that the organic phase contains the following metal species: TOAHFeCl4•TOAHCl, TOAHFeCl4•(TOAHCl)2, (TOAH+ + FeCl4) solvated, and TOAHFeC14•nøN02, where n was found to be ca. 3.0. The thermodynamic quantities, ΔG° , ΔH° , and Δ5° , were calculated from the various equilibrium constants and are discussed in terms of molecular interactions. The extraction study carried out using THACl was found not to fit the extraction model proposed for TOAHCl and provided little information because of gross non-ideality of the organic solutions of this salt. On a qualitative basis, the results suggest that the nitrobenzene solvation observed in the TOAHCl system may be due to cation solvation only. The aggregation studies demonstrated that TOAHCl exists as an equilibrium monomer and dimer mixture in dilute solutions (≤ 0.05 molar) in benzene, while the THACl is very highly aggregated even in dilute benzene solutions (0.01M).