Calculations of chemical equilibria in Tb(NO3)3-H2O, Tb(NO3)3-heparin-H2O, and CaCl2-Tb(NO3)3-heparin-H2O systems in physiological saline solution

Abstract

Chemical equilibria in the heterogeneous system Tb(NO3)3-H2O, physiological saline solutions containing terbium nitrate, and unfractionated heparin ((H4L) Tb(NO3)3-H4L-H2O-NaCl), and solutions containing calcium chloride, terbium nitrate, and unfractionated heparin (CaCl2-Tb(NO3)3-H4L-H2O-NaCl) were studied by mathematical modeling and pH titration. A physicochemical model was designed for two-phase equilibria in the system Tb(NO3)3-H2O, which consists of an aqueous solution and a solid phase of precipitated terbium hydroxide. Formation constants were calculated for terbium hydroxide ions Tb(OH)i(3−i)+ (i = 1, 2, 3) in an aqueous phase, and a correlation was found between the amount of precipitated Tb(OH)3i and pH. The four-component solution Tb(NO3)3-H4L-H2O-NaCl in the range 2.3 ≤ pH ≤ 10.4 is homogeneous; as a result of its investigation, the formation constants were ascertained for significant terbium complexes with heparin: TbL, TbHL24, and Tb(OH)2L3−. Chemical equilibria in the five-component solution CaCl2-Tb(NO3)3-H4L-H2O-NaCl were modeled proceeding from the models developed for equilibria in the four-component solution subsystems Tb(NO3)3-H4L-H2O-NaCl and CaCl2-H4L-H2O-NaCl. The modeling showed that the Tb3+ ion is an efficient competitive complex former to the Ca2+ ion, which forms complexes with heparin, and decreases tenfold the concentration of the major complex NaCaL at 6.8 ≤ pH ≤ 7.4 (the pH range of blood plasma stability).