CHARACTERIZATION OF THE COMPLEXES OF IRON(III) WITH L-TARTRATE IN THE SOLVENT-EXTRACTION SYSTEM - FORMATION OF IRON(III)-CLUSTER COMPLEXES

The complexes of iron(III) with L-tartaric acid (hereafter abbreviated as iron(III)-tartrate complexes) formed in aqueous solutions at various pH values are extracted with (methyl)trioctylammonium chloride (TOMAC ; R3R'NCI) in toluene. The complex species before and after extraction have been characterized by various techniques. A magnetic susceptibility titration experiment has shown that iron(III)-tartrate complexes in aqueous solution vary in structure accoriding to the conditions of pH and R=[L-tartaric acid]/[iron(III) ion] and are extracted with TOMAC into toluene from aqueous solution without great changes in structure. The complex species at R=1 (pH 6 approximately 12), R=3 (pH 3 approximately 12) and R=10 (pH 3 approximately 8) are of a type of iron(III) cluster complexes. It has been revealed that in aqueous solutions at R=3 approximately 10 the ratio of tartrate anion to iron(III) ion in the complexes are less than 2 and that this ratio decreases with an increase in pH. The estimated compositions of extracted complexes at R=3 (tart=tartrate anion) are as follows: [(R3R' N)1.5Fe(tart)1.3(OH)1.9] (pH 6.1) [(R3R' N)1.2Fe(tart)0.6(OH)3.0] (pH 10.1) [(R3R' N)0.9Fe(tart)0.3(OH)3.3] (pH 12.0) It is reasonably considered that the complex species at R=3 (pH greater than or similar to 10) bear a structural resemblance to iron(III) hydroxide clusters whose surfaces are modified with tartrate anions. Those negatively-charged carboxyl groups projecting to the outside can make ion-pairs with TOMAC cations and the complex species are extracted in toluene in such a form as reversed micelle. These complexes may be less than several tens angstrom in diameter.