Calcium(II)-selective potentiometric sensor based on p-isopropylcalix[6]arene in PVC matrix

A new PVC-based membrane using p-isopropylcalix[6]arene (I) as an ionophore has been developed as a calcium-ion-selective sensor. Among various membranes prepared with and without plasticizer, the best performance was shown by a membrane with a composition (mg) I : NaTPB : PVC (2 : 2 : 120). This sensor exhibits a good potentiometric response to Ca2+ ion over a wide concentration range (3.9 x 10(-6) to 1 x 10(-1) M) with a near Nernstian slope (30 mV per decade of activity) and response time of 15 s. The sensor can be used for a period of 3 months without any drift in potential. The selectivity coefficient values are in the order of 1.0 x 10(-3) for mono-, bi-, and trivalent cations, which indicates a good selectivity for Ca2+ ion. The useful pH range for the electrode was found to be 2.5-6.0, and it works well in mixtures with non-aqueous content up to 25% (v/v). The sensor can also be used successfully as an indicator electrode in the potentiometric titration of Ca2+ against EDTA.