Calcium selective test strip for water and milk

We have developed a selective and reversible test strip based on an ion-exchange mechanism to determine calcium. The optical test strip contains a polymeric film of plasticised PVC that contains all of the reagents necessary to produce a response to calcium, among them the new ionophore, 4,13-bis[(N-adamantylcarbamoyl)propionyl]-1,7,10,16-tetraoxa-4,13-diazacyclooctadecane. The measurement of the absorbance at 655 nm in a standard photometer makes it possible to determine calcium activities. The composition of the membrane and reaction conditions have been adjusted to obtain adequate selectivity. The test strip responded linearly to calcium between 0.050 and 135 mM in activities. The reproducibility intermembrane at a medium level of the range was 6.2%, as RSD, of log a(Mg(2+)), and 3.4% as RSD intramembrane. The procedure was applied to the determination of calcium ion in different types of waters (tap, well, spring and mineral) and milks (whole, skimmed, skimmed with calcium added, special types) validating the results against a reference procedure.