A Porphyrin Based Potentiometric Sensor for Zn2+ Determination

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Abstract
PVC based membranes of disodium salt of porphyrin 3,7,12,17-tetramethyl-8, 13-divinyl 2,18-porphine dipropionic acid (I) as ionophore with sodium tetra phenyl borate (NaTPB) as anion excluder and dibutyl phthalate (DBP), dioctyl phthalate (DOP), dibutyl butyl phosphonate (DBBP), tris(2- ethyl hexyl)phosphate (TEP), tri-n-butylphosphate (TBP) and 1- chloronaphthalene (CN) as plasticizing solvent mediators were prepared and constructed for determination of Zn(II). The PVC based membrane of (I) with DBBP as plasticizer and having anion excluder, NaTPB in the ratio PVC: I: NaTPB: DBBP (150: 10: 2: 200) gave the best results in terms of working concentration range (1.3×10-5-1.0 ×10-1M) with a Nernstian slope (30.0 mV/decade of activity). The useful pH range of the sensor is 3.0 –7.4, beyond which a drift in potential was observed. The response time of the sensor is 10s and the lifetime was about 2 months during which it could be used without any measurable divergence. It had good stability and reproducibility. The membrane worked satisfactorily in non-aqueous medium up to 40% (v/v) non-aqueous content. The selectivity coefficient values indicate that the electrode is highly selective for Zn2+ over a number of other cations except Na+ and Cd2+. Although Na+ and Cd2+ are likely to cause some interference, they would not interfere if present at the concentrations < 1 ×10-5 and < 5 ×10-5 M, respectively. The electrode has been used as an indicator electrode to determine the end point in the potentiometric titration of Zn2+ with EDTA.