pH dependence of the reactions of arsenazo III with the lanthanides

Spectrophotometric techniques, together with computer simulations, were used to show that the 1 : 1 arsenazo III (3,6-bis[(2-arsonophenyl)-azo]-4,5-dihydroxy-2,7-naphthalenedisulphonic acid)-lanthanide(III) complex has four protonation states which respectively dominate at pH 3.3, 6.0, 8.3 and >10. Formation and molar absorptivity constants were calculated for the various complexes. Acid dissociation constants for free arsenazo III were also calculated from Spectrophotometric data.