A novel naphazoline-selective membrane sensor and its pharmaceutical applications

New naphazoline (NPZ) ion selective membrane electrodes of both conventional and coated graphite types were prepared based on the ion-pair of naphazoline tetraphenylborate (NPZ–TPB). The conventional type electrode was fully characterized in terms of membrane composition, life span, pH, ionic strength and temperature. The electrode exhibited a Nernstian response for naphazoline in the concentration range 1.0 × 10−5 to 5.0 × 10−2 and 5.0 × 10−6 to 5.0 × 10−2 M with a slope of 58.4 ± 1.1 and 57.0 ± 0.1 mV/concentration decade and a limit of detection of 4.0 × 10−6 and 2.5 × 10−6 M for conventional and coated graphite types, respectively. It could be used in the pH range 3.0–8.0. It was applied to potentiometric determination of naphazoline in pure state and pharmaceutical preparation under batch conditions. The selectivity of the electrode toward a large number of inorganic cations, sugars and amino acids was tested. The solubility product of the ion-pair and the formation constant of the precipitation reaction leading to the ion-pair formation were determined conductometrically.