Abstract: The bimetallic ternary complex formation in the aqueous solution of La³⁺ and Cu²⁺ with alizarin fluorine blue (AFB) was studied by spectrophotometry.The ternary complex LaCu₃(AFB)₃ has maximum light absorption at 550nm.The colour reaction selectivity is fairly good and the method can be applied to the direct photometric determination of copper in lanthanum or some pure metals.Anew mathematical approach to the treatment of spectrophotometric data for differentiation between mononuclear and polynuclear in solution is presented.The improvable fixed logarithm method is for complexes of the type M_mB_n(m=n) or M_mB_n(m=n).We report that La(Ⅲ) combines with AFBat pH 4.5 to form a single complex,withm=n=2,La₂(AFB)₂.