Complexation reactions of the rare earth ions with thoron and xylenol orange

Expecting the existence of various mixed protonated hydroxo species, the previously studied chelation equilibria of Y(III), La(III), Ce(III), Pr(III), Nd(III), Sm(III), Eu(III), Gd(III), Tb(III), Dy(III), Ho(III), Er(III), Tm(III), Yb(III), and Lu(III) with 1-(o-arsonophenylazo)-2-naphthol-3, 6-disulphonic acid (thoron, apans) and 3,3'-bis[N,N'-di(carboxymethyl) aminomethyl]-o-cresolsulphonphthalein (xylenol orange, dcac) have been reinvestigated. The most probable complexation routes are : MH2A → MH2A(OH) → MH2A(OH)2 [light rare earth (RE)-apans system]; MH2A → MH2OH → MH2A(OH)2 → MHA(OH)2 (middle and heavy RE-apans system); NH3A → NH2A → MHA → MHA(OH) → MA(OH) (RE-dcac system). The equilibrium constants of the reactions, involving the formation of these species, have been evaluated under the conditions : (i) taking into account the presence of metal hydroxo species, (ii) neglecting the metal hydrolysis, and (iii) assuming 100% metal-ligand association. (author)