Organophosphorus extractants:

In the present day extraction chemistry, an extractants such as ethers, alcohols and ketones have been replaced by organophosphorus extractants but the model extraction equilibria study is generally presented by taking the former class of compounds. The main reason is that some of these examples give a better insight of the different equilibria steps which may be possibly involved in the extraction of metal ion by solvation. A large number of metal ions are extracted in ethers from aqueous mineral acid media. The well-known example generally cited in this class is the extraction of Fe(III) in diethyl ether from HCl medium. It is one of the previous systems known in solvent extraction chemistry. An extraction of Fe (III) in ethyl ether from strong hydrochloric acid solution was reported by Rothe way back in 1892. Since then several efforts have been made to identify the extracting Fe(III) species. Some spectroscopic studies have also been carried out to identify the extracting species. The extracting species is postulated as solvated chloroferric acid i.e. HFeCl₄.2 (C₂H₅)₂O. However, the situation is not so simple because of the involvement of different types of equilibria.

The extraction of Fe(III) increases with the increasing HCl concentration, reaches to a maximum value (∼99%) at 6M HCl and then starts decreasing with the increasing acid concentration. A decrease in extraction above 6M HCl is attributed to the high solubility of ethyl ether in concentrated HCl. This decrease is not observed up to a much higher acid concentration (∼ 8M) in the case of diisopropyl ether which has lesser solubility in HCl. If β, β ′ - dichlorodiethyl ether is used, no decrease in extraction is observed even at 12M HCl. There are some other issues included in the organic phase as regards the existence of ion association complex. At extremely low iron concentrations, dissociation has been experiential. Instead, at higher iron concentrations polymerization has been postulated.