Application of poly(vinyl chloride) pilocarpine membrane electrodes in ophthalmic products

Abstract

The construction and evaluation of two types of electrodes sensitive to pilocarpine, without an inner reference solution, based on pilocarpine–tetrakis(4-chlorophenyl) borate (Type I) and pilocarpine–tetrakis[3,5-bis(trifluoromethyl)-phenyl] borate (Type II) are described. The ion exchangers were dissolved in 2-nitrophenyl octyl ether and the resulting sensor solution was immobilized in poly(vinyl chloride)(PVC). Under similar experimental conditions, the calibration graphs for electrodes of Types I and II were linear in the concentration ranges between 3.0 × 10^–5 and 6.0 × 10^–5 to 0.1 mol dm^–3(I= 0.1 mol dm^–3 and pH = 6.3), respectively. The slopes of the calibration lines were close to the theoretical value and indistinguishable from each other. The reproducibilities of the potential for the two types of electrodes were similar, and approximately ±2 mV throughout a working day. The response time was better than 10 s for pilocarpine concentrations between 10^–4 and 10^–2 mol dm^–3 for both types of electrodes. Both types of electrodes were in operation for a period exceeding 7 months. The electrodes exhibited useful analytical characteristics for the determination of pilocarpine in pharmaceutical products. Direct potentiometric calculations in solutions with a concentration of pilocarpine hydrochloride of about 48 µg cm^–3 showed mean recoveries between 106 and 98% for determinations carried out with Type I electrodes and between 93 and 101% with Type II electrodes.