The interference of lanthanum in the spectrophotometric determination of yttrium in the form of β-type complex with chlorophosphonazo III (CPA III) is proved to be a cocoloration effect. It is due to the formation of a mixed polynuclear complex between CPA III and lanthahum-yttrium with a cyclic structure. The composition of the complex has been established to be La:CPA III:Y = 1:2:1 by the method of molar ratio. The optimal conditions for the formation of polynuclear complex and influence factors have also been studied. A "replacement type" reaction is proposed to account for the cocoloration effect.

Key words: COLOR REACTION, PHOSPHOROTHIOIC ACID P, BENZENE AZO COMPOUNDS, ORGANO CHLORIDE COMPOUNDS, RARE ELEMENT, LANTHANUM, POLYNUCLEAR COMPLEX, YTTRIUM, NAPHTHALENEDISULFONIC ACID P, CHLOROPHOSPHONAZO