Stability constants of chromium(III) oxalate complexes in 1 m NaClO4 at 60degrees C

Stability constants of inert chromium(III) oxalate complexes in 1 m NaClO4 medium at 60 degrees C were determined by direct chemical analysis of the s um [C2O42-] + [HC2O4-] + [H2C2O4]. i.e. the concentration of ligand not bou nd to the metal. Solutions containing different total molalities of metal, ligand and perchloric acid were equilibrated for several days at 60 degrees C, then cooled to room temperature and analysed by precipitating the oxala te ion with Hg-2(2+) ion generated coulometrically. The end-point was asses sed potentiometrically with the Hg-2(2+)/Hg half-cell. At room temperature the chromium(III)-oxalate complexes are sufficiently inert, only not bound to metal oxalate species react with Hg-2(2+) ion. The data, combined with t he acidity constants of oxalic acid, are consistent with Cr3+ + C2O42- reversible arrow CrC2O4+, log beta(1) = 7.40+/-0.05 Cr3+ + 2C(2)O(4)(2-) reversible arrow Cr(C2O4)(2)(-), log beta(2) = 13.54+/ -0.05 Cr3+ + 3C(2)O(4)(2-) reversible arrow Cr(C2O4)(3)(3-), log beta(3) = 18.07/-0.05 Molar absorptivities of the various complex species, determined from the be ta(n) values and absorbancy measurements (visible), match well literature d ata. The coincidence may be useful for validating the present model.