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 μm 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.