SELECTIVE TRANSPORT OF CALCIUM-ION FROM A MIXED CATION SOLUTION THROUGH AN HDEHP N-DODECANE SUPPORTED LIQUID MEMBRANE

The separation of calcium ion from a mixed cation solution is studied. A lipophilic ester of phosphoric acid, bis-(2-ethylhexyl) hydrogen phosphate, is used as the carrier in a supported liquid membrane with n-dodecane as the solvent. With this system, a feed solution containing equal concentrations (100 ppm) of seven cations (LiCl, MgCl2, NaCl, CaCl2, SrCl2, KCl, BaCl2), the order of transport is Ca2+ > Sr2+ > Ba2+ > Mg2+ with Li+, Na+ and K+ showing no appreciable transport. The flux and selectivity for calcium has been studied as a function of the feed composition, the strip phase acidity, the concentration of carrier in the membrane, the absence of well-mixed aqueous phases, and the porosity and pore size of the support material. Experiments with synthetic feed concentrations characteristic of a brine solution show that only calcium ion is transported through the membrane. Experiments with synthetic feed concentrations characteristic of extracellular fluid show considerable selectivity for calcium ion. These results suggest that in selected situations, this membrane system may have application as a calcium ion detecting electrode.