Electrokinetic characterization and colloid stability of calcium oxalate monohydrate dispersions in the presence of certain inhibitors

Abstract

The microelectrophoretic behavior and colloid stability of dilute dispersions of calcium oxalate monohydrate (COM) in solution have been studied. The COM crystals were precipitated at different experimental conditions of the nucleation process. In a first step, we have investigated the influence of predominant cations or anions on the sign of the COM surface charge. In a second step, the effect of addition of several inhibitors (D-glutamic acid and sodium heparin) during the nucleation process was also investigated. The i.e.p. of the COM particles depends on the excess of calcium or oxalate ions during the nucleation process. The electrokinetic behavior of the COM particles precipitated in the presence of D-glutamic acid show a similar behavior to that of the standard COM particles, as regards the pH and pCa. The ζ-potentials of the COM particles precipitated in the presence of sodium heparin are negative for the entire pH range due to the specific adsorption of this mucopolysaccharide. When the heparin concentration in solution is higher than 3 · 10−3 g/l the COM surface is completely covered by this macromolecule and the precipitated samples display the same electrokinetic behavior. — The colloid stability of a calcium oxalate sol has been studied using the empirical relationship between the ζ-potential and the coagulation behavior provided by Eilers and Korff. The EK index shows that from a colloidal point of view the COM dispersions are very unstable when the COM crystals are preciptated in the presence of 10 p.p.m. of D-glutamic acid. However, the COM crystals precipitated in the presence of 100 p.p.m. of D-glutamic acid present the greater EK index values, which indicates the inhibitory character of D-glutamic acid at this concentration. For the COM dispersions prepared from crystals precipitated in the presence of low and high contents of sodium heparin, the EK index reveals that the COM dispersions become more stable as the heparin concentration in solution increases, which indicates the possible inhibitory character of the heparin.Key words Calcium oxalate monohydrate electrophoretic mobility colloid stability inhibitors