The anion exchange of metal complexes-IV. The iron(III)-chloride system

The anion exchange method described in previous communications was applied to the iron(III)-chloride complex system. The distribution of iron was determined by the batch method. The mononuclearity of the complexes was confirmed for the range 10^-3-4 × 10^-2 M iron. Distribution curves were obtained for the range 1·0-12 M chloride in lithium chloride and hydrochloric acid solutions. Up to 9 M chloride there is no hydrogen-ion effect, but for more concentrated solutions of hydrochloric acid the distribution is lower than for lithium chloride solutions. Both curves show a maximum. A corrected distribution curve was constructed for the lithium chloride experiments, showing the complexes FeCl₂⁺, FeCl₃ and FeCl₄^-, with the successive stepwise formation constants log k₂ ≥ -0.7, log k₃ = -1·40 and log k₄ = - 1·92. In addition to these species, some undissociated HFeCl₄ is also formed in concentrated hydrochloric acid. Spectrophotometric measurements in concentrated chloride solutions confirm these results qualitatively.