Equilibrium Study on the Characteristic Color-Changing Reaction of Lanthanum Complex of 3-[[Bis(carboxymethyl)amino]methyl]-1,2-dihydroxyanthraquinone (Alizarin Complexon) with Fluoride Ion

Abstract

Complexation equilibria between lanthanum (M), 3-[[bis(carboxymethyl)amino]methyl]-1,2-dihydroxyanthraquinone (Alizarin Complexon, H4L) and fluoride were studied by potentiometry with fluoride ion selective electrodes and by spectrophotometry. M2HL2 and a further protonated species react with fluoride to form M2HL2F and M2H2L2F, whereas M2L2 does not. The unexpectedly large stability constant of 104.12 for the reaction, M2HL2+F−\ightleftharpoonsM2HL2F, may be ascribed to the incorporation of fluoride ion in a specially organized core of M2HL2.