The spectrophotometric and potentiometric study of complex formation between gadolinium ions and 5-sulphosalicylic acid (SSA) has been reported in an earlier paper.1) It was observed that at pH lower than 7, the reaction between the metal ion and the ligand results in the formation of a protonated complex of the type MHL in equilibrium with the usual ML complex; between pH 7 and 9, however, ML2 species are predominantly present in solution. We have now studied the reaction of SSA with other rare earths. The data of stability constants obtained will be discussed in this paper in terms of the bonding in these complexes. Experimentaland at constant ionic strength of 0.1M.The technique used for the potentiometric and the spectrophotometric measurements was similar to that described earlier.1) The spectrophotometric measurements were made at pH 6.0 which was much below the pH of hydrolysis in each ease. The spectra of all the rare earth complexes with this ligand were similar; two maxima were observed.The wavelengths corresponding to them are the concentration chosen, the metal perchlorate had negligible absorption. The ligand, however, had finite absorption and during the evaluation of the stability constants the values of the optical density were corrected for this absorption by Beer's law.The results obtained for all the rare earth complexes were remarkably similar. In the range of pH investigated (5 to 10), both 1:1 and 1:2 complexes are formed with this ligand. Below pH 7, the reaction is of the type The equilibrium constants k and k' have been evaluated in each case by exactly the same manner as described 1) N. V. Thakur, S. M. Jogdeo and C. R. Kanekar, J. Inorg. Nucl. Chem., 28, 2297 (1966).

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