STUDIES ON URINARY CALCIUM ION ACTIVITY IN UROLITHIASIS

1. Determination of Calcium Ion Activity with an Ion-Selective Electrode

Abstract

Calcium ion activity is physicochemical value corresponding to effective calcium ion concentration, which directly relates to the saturation level of calcium complex in solution. Therefore, urinary calcium ion activity seems to be an important factor for the formation of urinary calculus. A method, using an ion-selective electrode, for the direct measurement of urinary calcium ion activity is described here. Calcium ion activity values of standard solutions were calculated by Debye-Hückel equation, and used to calibrate the electrode.
There was a good agreement between the calculated and measured values of calcium ion activities in a series of CaCl₂-NaCl solutions. pH (5-8) and temperature (10-40°C) had no influence on the determination in CaCl₂-NaCl solutions.
Reproducibility of calcium ion activity determination in urine was satisfactory. In urine, however, the measured activities were increased in proportion to the temperature, and decreased by alkalinization, which induced precipitation of calcium salt.
Urinary calcium ion activity (25°C) of calcium stone formers was significantly higher than that of healthy controls. There was a significant correlation between urinary calcium ion activities and total calcium concentrations. The ratio of calcium ion activity to total calcium concentration showed no difference between controls and stone formers, but it was significantly lower in stone formers with crystalluria.