Plastic membrane electrodes for the determination of flavoxate hydrochloride and cyclopentolate hydrochloride

Four novel ion-exchangers (Fx-Rt (I), Fx-TPB (II), Cp3-PMA (III) and Cp3-PTA (IV)) of antispasmodic and anticholinergic drugs, flavoxate hydrochloride (FxCl), 2-piperidinoethyl-3-methyl-4-oxo-2-phenyl-4h-1-benzopyran-8-carboxylate hydrochloride, cyclopentolate hydrochloride (CpCl) and (2-(dimethylamino)ethyl (RS)–(1-hydroxycyclopentyl)phenylacetate) hydrochloride were synthesized and incorporated into poly(vinyl chloride)-based membrane electrodes for the quantification of FxCl and CpCl in different pharmaceutical preparations. The influence of membrane composition on the potentiometric response of the membrane electrodes was found to substantially improve the performance characteristics. The best performance was reported with membranes having compositions (w/w) of Fx-Rt (2%):PVC (49%):DOP (49%), Fx-TPB (7%):PVC (46.5%):DOP (46.5%), Cp3-PMA (8%):PVC (46%):DOP (46%) and Cp3-PTA (9%):PVC (45.5%):DOP (45.5%). The proposed sensors exhibited Nernstian responses in the concentration ranges of 1.39 × 10−6–5.00 × 10−4, 9.90 × 10−7–3.75 × 10−5, 1.39 × 10−5–2.53 × 10−3 and 3.21 × 10−6–8.62 × 10−4 M, with detection limits of 5.50 × 10−7, 9.8 × 10−7, 9.8 × 10−6 and 2.95 × 10−6 M for the (I), (II), (III) and (IV) electrodes, respectively. The membrane electrodes performed satisfactorily over pH ranges of 2.0–5.5, 2.0–5.5, 2.0–5.0 and 2.0–7.5, with fast response times of 20, 30, 15 and 20 s for the (I), (II), (III) and (IV) electrodes, respectively. The practical utility of the sensors was demonstrated by the determination of FxCl and CpCl in pure solutions and pharmaceutical preparations using standard additions and potentiometric titration.