It is well known that TBP has the ability to extract Fe(III) in acidic medium of hydrochloride. In this medium, Fe(III) occurs in the form of HFeCl4 which combines with TBP by an association reaction. The formation of the association induces the transfer of Fe(III) from aqueous solution into the organic solution. In the sulfuric acid medium, Fe(III) hardly occurs in a similar form, therefore it is barely extracted by TBP. As there is TOA presenting in the organic phase, the sulfuric acid is firstly extracted by TOA. The removal of sulfuric acid leads to the increase of pH which causes the hydrolysis of Fe(III). According to the equilibrium constants of Fe(III) species
listed in Table 2, it is derived that a reasonable dominating species of Fe(III) is Fe(OH)2+ or Fe(OH)2 + because the pH of the aqueous solution is in the range of 1–2. Iron(III) may be extracted into the
organic phase in these two species.