Optical test strip for calcium determination based on a neutral ionophore Original Research Article

An optical strip test to determine calcium is proposed. The membrane consists of a polyester sheet and an active layer composed of a cation-selective neutral ionophore, such as 18-crown-6-ether, a proton selective neutral chromoionophore, such as lipophilised Nile Blue, and potassium tetrakis(4-chlorophenyl)borate as a lipophilic salt. The membrane responds reversibly to calcium ions by an ion-exchange mechanism, changing its colour from blue to red. All experimental variables that influence test strip response, especially in terms of selectivity and response time, have been studied. The system responded linearly in activities in the range 0.046–134 mM. The detection limit is 0.046 mM, the reproducibility intermembrane at a medium level of the range was 8.5%, as relative standard deviation (R.S.D.) of logaCa2+ and 2.7% as intramembrane. Calcium was determined by using the test strip proposed in different types of waters (tap, mineral and spring) validating results against a reference procedure. This proposed method for calcium determination in waters is quick (less than 5 min), inexpensive, selective and sensitive and uses only conventional instrumentation.