Chemical Sensors Based on Pyridyl Ionophores for Selective Determination of Some Transition Metal Ions

Abstract

In the present study some pyridyl based ionophores were chosen for construction of ion selective electrodes and one pyridyl based ionophore was used for membrane transport studies. The linked 2,2’-dipyridylamine derivatives (mdpa, pdpa and tpydpa) and 4’,4’’-(1,4-phenylene)bis(2,2’:6’,2’’-terpyridine) (ditpy) have been explored as neutral carrier ionophores for preparing poly (vinyl chloride) based electrodes selective to Ag(I) ions. Based on the stability constants determined by sandwich membrane method and the energy minimization studies using CAChe software (in case of 2, 2’- dipyridylamine derivatives), the ionophores were found selective for Ag (I) ions. Different compositions of the membrane were studied. The best performance in case of ‘tpydpa’ was found with the electrode composition (w/w): ionophore (3%); PVC (33%); o-NPOE (64%). This electrode exhibits Nernstian response with a slope of 59 mV/decade of activity in the concentration range 5.5x10-6M - 1.0x10-1M of Ag (I). The electrode shows satisfactory performance over a pH range of 3.0–9.0, with a fast response time of 14s. The best performance in case of ‘ditpy’ was found with the electrode composition (w/w): ionophore (5%); PVC (31%); NaTPB (3%); o-NPOE (61%). The electrode exhibits Nernstian response with a slope of 60 mV/decade of activity in the concentration range 5.5x10-5M - 1.0x10-1M of Ag (I). The electrode shows satisfactory performance over a pH range of 4.0–9.0, with a fast response time of 15s. In both cases response of the electrodes were highly selective to Ag+ ions over a number of uni-, di- and trivalent metal ions. Also, the electrodes have been used successfully as indicator electrodes in potentiometric titrations of Ag (I) ions and for determination of silver content in waste water. Ionophore N',N'',N''' tris(2-pyridyloxymethyl) ethane (TPOME) has been used as an ionophore in a PVC based membrane for construction of lead ion selective electrode. The membrane electrode with a composition 33: 5: 62: 3 (PVC: TPOME: o-NPOE: NaTPB) exhibits Nernstian response towards Pb(II) ions with a slope 30mV/decade, over a concentration range of 1x10-5M to 1x10-1M. The potential response remains almost unchanged over pH range 3.7-6.4. The electrode shows fast response time of 15±2 seconds and a lifetime of four months. It shows good selectivity for Pb(II) ions over other mono-, di- and trivalent cations. The electrode response is satisfactory in mixed solvent media up to 30% (v/v) non-aqueous contents. Selectivity of the ionophore for Pb(II) ions was determined by spectrophotometeric method. The electrode can also be used as an indicator electrode in the potentiometric titration of Pb(II) ions with standard chromate solution and its determination in real life samples. N,N,N′,N′-tetrakis(2- pyridylmethyl) ethylenediamine (I) as an ionophore has been used to develope as zinc ion-selective electrode. The best performance was observed with an electrode composition (w/w); PVC (30%), 2-nitro phenyloctylether (60%), ionophore (I) (5%) and sodium tetrakis(4-chlorophenyl) borate as lipophilic salt (NaTPB, 2%). The electrode shows a linear dynamic response in the concentration range 5.0x10-6 M to 1.0x10-1 M with a Nernstian slope of 30 mV/decade and a detection limit as 1x10-6 M. It has a response time of 20 seconds and can be used for at least 4 months without any significant divergence in potentials. The proposed sensor shows good selectivity with respect to alkali, alkaline earth and transition metal ions. It can be used in the pH range 3.0-8.0 and electrode has been used as an indicator electrode in potentiometric titrations of zinc(II) ions with Na2S. The electrode also has been applied successfully for the determination of zinc ions in real life samples. Transport experiments across polymer inclusion membrane (PIMs) containing BIP as a neutral carrier are presented. The physical immobilization of 2,6- Bis(benzimidazolylpyridine) (BIP) as an ion carrier and o-nitrophenyloctyl ether (o- NPOE) as plasticizer and cellulose triacetate(CTA) as the polymeric support was used to prepare polymer inclusion membranes. Transport experiments were carried out in a permeation cell in which membrane film was tightly clamped between two compartments. Different receiving phases were used for the study while the solutions of metal nitrates were used as source phases. The carrier concentration in the PIM was varied in the range 4 to 7 mg. The best results were obtained with membrane composition cotaining 5 mg carrier and HNO3(0.001M) as the receiving phase. Samples of aqueous and source receiving phases were removed periodically via a sampling port with a syringe and analyzed to determine Zn2+ ion concentration by using atomic absorption spectroscopy method. The surface characterization of the membranes in all cases has been carried by scanning electron microscopy.