Rare Earth Chelates of 2-(p-Sulphophenylazo )-1 : 8-DihydroxyNaphthalene-3: 6-Disulphonate (SPADNS)

Rare earths react with SPADNS to form violet coloured water soluble complexes at pH 6·0. The colour formation does not depend upon reaction time and is instantaneous. The absorbance values are not changed by keeping the mixtures at room temperature even after 24 hr. Spectral studies show the formation of only one 1 : 1 complex in all the cases with the \(\lambda\)max of the various chelates lying between 550 mμ to 565 mμ at pH 6·0. The values of the apparent conditional stability constants have been calculated by three different methods and the results are fairly in good agreement with each other. The values of log K for the various lanthanoid chelates of SPANDS lie between 4·4 to 5·8 (pH 6·0; 25°), Position of the chelate ring has also been suggested on the basis of the charge on the complexes determined by electrophoresis experiments.