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The formation constants of the first two chelate species formed by rare earth and yttrium ions with propionate, methoxyacetate, and thioglycolate ligands have been determined by a potentiometric method at 20.0°and an ionic strength of 0.10. The supporting electrolyte was sodium perchlorate. The data indicate that methoxyacetate and thioglycolate (a-mercaptoacetate) ligands behave essentially as monodentate ligands, as do acetate and propionate. It is apparent that the affinity of the simple carboxylic acid ligands for rare earth ions decreases in the order acetate > propionate > isobutyrate. It is also clear that the order of the electron-donating ability of the functional groups studied is OH >> OCH3 > SH. The ionization constants of propionic, methoxyacetic, and thioglycolic acids were determined at 20.0°and an ionic strength of 0.10.of thioglycolic acid at 30°and an ionic strength of 0.10, corresponding to an ionization constant of 2.82 X 10~4 compared to our 20°value of 3.89 X 10"~4, it is consistent that the rare earth complex stability constants observed at 30°are larger than those observed at 20°at a comparable ionic strength.

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