The microelectrophoretic mobility and colloid stability of calcium oxalate monohydrate dispersions in aqueous media

Abstract

The microelectrophoretic behavior and colloid stability of dilute dispersions of calcium oxalate monohydrate (COM) in electrolyte solutions have been studied. The effects of pH and potential-determining ions (Ca 2+ and C 2O 4 2−) on microelectrophoretic mobility of COM dispersions at two different particle concentrations (0.4 and 0.05 g/liter) have been calculated together with the effect on the electrokinetic behavior of certain experimental conditions, such as the storage time of the COM solution, temperature, and particle concentration. The pH at which the isoelectric condition was established was found to depend upon the particle concentration of the COM dispersions. Change in mobility on the addition of four monovalent anions was investigated. These anions behaved as indifferent ions. The addition of divalent cations, however, caused an increase in mobility, which suggests that Co 2+, Ba 2+, and Mg 2+ are specifically adsorbed onto the COM surface. When an Al 3+ ion is used in electrophoretic measurements a strong increase in mobility is obtained. This change, however, is due to the effect of Al 3+ on calcium activity. A divalent anion such as HPO 4 2− has a strong tendency to reduce the mobility. Temperature plays an important role in the electrophoretic behavior of the COM particles. The pH (i.e.p.) had a linear dependence on temperature. Mobility measurements versus silicate concentration were made in order to check any effect that silicate ions might have on the electrokinetic behavior of COM particles. Evidence has been obtained which indicates that silicate species interact specifically with the COM surface, so that the negative mobility found with very dilute dispersions is a consequence of the special affinity of silicate ions toward the positively charged COM particles. Specific adsorption potential of SiO 3 2− seems to confirm this assumption. The colloid stability of a calcium oxalate sol has been studied using the empirical relationship between the ψ-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 pCa varies from 4.0 to 5.3. It also suggests that the minute crystallites observed in slightly basic urine tend to be aggregated. With regard to cations of different valencies, the highest EK index values correspond to the cobalt ion, which recommends its use as an inhibitor of calcium oxalate agglomeration.