Composition of the scandium and rare earth complexes of arsenazo

A spectrophotometric method for the determination of scandium with arsenazo was recently reported by Onishi and Banks. However, the composition of the complex formed was not studied. There is disagreement in the literature pertaining to the composition of the rare earth complexes of arsenazo. Therefore, the present work was undertaken. The empirical formula of the scandium (III) complex in solution was established by three spectrophotometric techniques : mole ratio method, continuous variation method, and slope ratio method. It is indicated that at pH 6.1(0.1M in acetate) the complex is formed by one mole of scandium reacting with one mole of arsenazo (R), giving the empirical formula as ScR. The results of the continuous variation method indicate that at pH 7.2 (0.1M in triethanolamine) arsenazo forms 1 to 1 complexes with the rare earths. Lanthanum (III), cerium (III), and ytterbium (III) were used.