Stability constants of the iron(II1) compIexes of salicylic acid, sulfosalicylic acid, and 4-aminosalicyIic acid, together with several of the acidity constants of these, have been determined at 25 "C and in solutions with 0.5 M background electrolyte. For the spectrophotometric measurement of the stability constants improved methods for determining the extinction coefficients of the first and second complexes are described. Protonated species appear only with the aminosalicylate complexes, and the site of this protonation is discussed. Previous values of these equilibrium constants have been collected and critically assessed.Canadian Journal o f Chemistry, 48, 2574Chemistry, 48, (1970 Introduction A spectrophotometric procedure for the estimation offormation constants of metal complexes initially suggested by Vareille (1) has been extended (2, 3), particularly by increasing the number of inde~endent estimates that could be extracted from measurements on a suitable group of solutions. In this paper are communicated some additional refinements to the experimental procedure and to the treatment of the experimental data, together with the results of studies on complexes formed by iron(II1) with salicylates.These particular complexes were selected for study for several reasons. The substantially different magnitudes of the successive stability constants make it possible to study spectrophotometrically the individual steps in the formation of these complexes. One of the advantages of the procedure is its ability to test the dependence of the measured values of formation constants on hydrogen-ion concentration. Thus the method had been used (3) to confirm the formation of a protonated complex MHL in the reaction between iron(II1) and Tiron (disulfonated catechol) (4). It was of interest to see whether, under comparable conditions of low pH, a similar reaction occurred with sulfosalicvlic acid. The iron(II1) complexes of this reagent have already been investigated by several workers (5-lo), and apart from some early observations ( 9 , no 'This work was begun in the Department of Chemistry, University of Toronto, and completed and prepared for publication in the Department of Medical Chemistry, Australian National University, Canberra. 2Present address: Department of Chemistry, Tel-Aviv University, Tel-Aviv, Tsrael. 3Present address: Department of Chemistry, Queen's University, Kingston, Ontario. dependence of the formation constants on the acidity of the solution had been reported. However, the experimental methods or conditions applying to these previous investigations may not have been chosen with the objective of proving or disproving the formation of a protonated species. Mattoo (7) specifically commented on the absence of a protonated 1 :I complex when subjecting Vareille's experimental results to his own method of calculation. Salicylic acid and some other salicylates have been reported in one study to form protonated iron(II1) complexes (1 l), though other workers (9, 10, 12-14) have not reported on such a ...

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