Determination of calcium by a single-use optical sensor

A new strip test to determine calcium is proposed. The test uses an irreversible and disposable optical sensor which is a polyester sheet containing all the reagents necessary to produce a selective response to calcium and has on it a circular film 6 mm in diameter and 25 mum in thickness. This sensing zone was formed using PVC and Chlorophosphonazo III as a chromogenic reagent and tributylphosphate (TBP) as a plasticizer. When the sensor was introduced for 15 min into a water, a change of colour from violet to blue was observed. The absorbance at 659 nm was used as the analytical signal. The sensor responded linearly in the range of 0.8-3.1 mg l(-1). The reproducibility of the preparation of the membrane was 4.1% (as relative standard deviation, R.S.D.) and of the measurement of calcium at 1.7 mg l(-1); the concentration level was 6.3% as R.S.D. The procedure was applied to the determination of calcium in different types of water (natural, tap, mineral, sea, and river), checking results against AAS procedure. A study of the interference produced by other species commonly present in water demonstrated the selectivity of the method, even when magnesium is present. (C) 2000 Elsevier Science B.V. All rights reserved.