pH-Potentiometric study on rare earths and calcium binary and ternary complexes with DL-malic acid and L-hydroxyproline

The stability constants of M-L binary system and M-L-L′ (M=La³⁺-Yb³⁺, Y³⁺ and Ca²⁺; L=DL-malic acid, L′=L-hydroxyproline) ternary system were determined by pH-potentiometric method under the simulating physiological condition (37 °C, I=0.15 mol/L NaCl). The complex species M_pL_qL′_rH_s (abbr as pqrs) in the systems were ascertained by program COM-PLEX. The results show that there are three species (1101, 1100 and 1200) in M-L binary system and one species (1010) in M-L′ binary system. In addition to the above four species, a new species, 1112 was found in the M-L-L′ ternary system, which is the only species of mixed ligand. Rare earth ions form more stable complexes than calcium ion does, and the stability differences between their complexes in the ternary system are less than that in the binary system. The distributions of all the species in La-L-L′ ternary system vs pH are discussed.