Characterisation of a transparent optical test strip for quantification of water hardness Original Research Article

An optical and reversible test strip that uses an ion-exchange mechanism which responds equally to calcium and magnesium and makes it possible to determine water hardness is described. The transparent test strip, made of a polyester sheet, has a circular polymeric film of plasticised poly(vinyl chloride) (PVC) that contains all of the reagents necessary to produce an equal response to calcium and magnesium, namely, a cation-selective neutral ionophore, such as 4,13-[bis(N-adamantylcarbamoyl)acetyl]-1,7,10,16-tetraoxa-4,13-diazacyclooctadecane, a chromoionophore, such as lipophilised Nile Blue, and potassium tetrakis (4-chlorophenyl)borate as a lipophilic salt, which it is evaluated by absorbance measurement at 655 nm in a standard photometer. All experimental variables that influence test strip response, especially in terms of selectivity and response time, have been studied. The sensor responded linearly to hardness up to 14,800 mg l−1, in activities, expressed as CaCO3. The detection limit is 1.9 mg l−1 as CaCO3, the reproducibility intermembrane at a medium level of the range was 7.0%, as R.S.D., of log aCa2+/Mg2+ and 2.6% as intramembrane. The procedure was applied to the determination of hardness in different types of waters (tap, well, mineral and spring) validating results against a reference procedure. This proposed method is quick, inexpensive, selective and sensitive and uses only conventional instrumentation.