DEVELOPMENT OF SOME PVC BASED ION SELECTIVE ELECTRODES AND THEIR APPLICATIONS

Abstract

Analytical chemists are putting up untiring efforts with the development of instruments and techniques, which provide fast, accurate, low cost and selective method of analysis. Thus, ion selective electrodes (ISEs) have been the subjects of widespread interests for analytical chemists as these devices fulfil these requirements. Moreover, the ability to make direct or indirect measurement in complex samples without concerning about the colour and/or turbidity of the sample and the fact that such measurements require relatively inexpensive equipments make ISE-based techniques more attractive and applicable in various fields such as environmental, agricultural, clinical etc. ISEs consist of a solid or liquid membrane, which when placed between two solutions of different concentrations allows the transport of specific ions. As a result of restricted migration of ions a potential difference across the membrane is developed, which is used for the determination of ionic concentration. The key components of the membranes are lipophilic complexing agents capable of reversibly binding ions. They are called ionophores or ion-carrier, which favour selective transport and this selective binding transduces analyte ionic activity into potential read out. Various materials tried or being tried as ionophore includes macrocycles, crown ethers, calixarenes and inorganic and organic ion exchangers etc. as they respond selectively to specific ions. Widespread interest in developing ion selective electrodes began in 1966 when Frant and Ross prepared a highly selective electrode for fluoride ion based on the crystal of LaF3. Since then a number of electrodes have been reported and are now commercially available for alkali and alkaline earth metals. However, success with regard to heavy metals and anions is still limited and continuous efforts are, therefore, being made to develop electrodes for these ions. With the availability of selective materials, possibilities of developing sensors for heavy (i) metals and anions have opened up. Efforts in this direction were initiated using porphyrins, calixarenes, crown ethers, Schiff bases and dendrimers embedded membranes to develop electrodes for some metal ions and anions. The results of electrochemical studies of the membrane sensors based on these ionophores are briefly summarized as under. Estimation of arsenite and zinc assumes importance because of their toxic and carcinogenic nature. Avery few electrodes have been reported for arsenite and in case ofzinc though many zinc selective electrodes have been developed but most of them have not found very successful. It was, therefore, thought desirable to prepare new electrodes for these ions, which show better selectivity. Porphyrins, 5,10,15,20-tetrakis(4-methoxy phenyl)porphyrinatocobalt(II) (I) and disodium salt of 3,7,12,17-tetramethyl-8,13-divinyl- 2,18-porphine dipropionic acid (II), have been evaluated as suitable ionophores for the development of As02" and Zn2+-selective electrodes respectively. As a result a number of heterogeneous membranes were prepared using polyvinyl chloride) (PVC) as binder and the optimum composition of the membrane was determined by trial. The composition, which gave the membrane of adequate mechanical stability and good electroanalytical performance, was taken as optimum composition. The effect of different plasticizers and anion excluder on the response of these membranes was also studied. It was found that the membrane of 5,10,15,20-tetrakis(4-methoxyphenyl)porphyrinatocobalt(II) (I) having composition 5:150:150 (I:PVC:DBBP) (w/w; mg) gave the best performance. The electrode of this membrane generates linear potential response in the concentration range of 7.9 x10" - 1.0 x 10"1 Mwith anon-Nernstian slope of29.0 mV/decade ofactivity and a response time of 60 s. It showed good selectivity over other anions except I03", which exhibits slight interference. Similarly, PVC based membranes of disodium salt of 3,7,12,17-tetramethy1-8,13-divinyl- 2,18-porphine dipropionic acid (II) having composition 10:150:200:2 (ii) (II:PVC:DBBP:NaTPB) (w/w; mg) were found to respond to Zn2+ selectively in the concentration range of1.3 x 10"5 - 1.0 x 10'1 Mwith a slope of30.0 mV/ decade of activity. The values of selectivity coefficient indicate that the electrode is selective over a number of cations except Na+ and Cd2+. The electrode was successfully employed as indicator electrode to determine the endpoint in the potentiometric titration of Zn2+with EDTA. Calixarenes are another class of electroactive compounds, which have been explored for developing some new electrodes. Neutral ionophore containing hydrogen bond donating moieties or immobilized Lewis acidic binding sites can be used to develop ISEs for phosphate. Thus, a modified calix[6]arene, 5,1l,17,23,29,35-hexa-tert-butyl-37,38,39, 40,41,42-hexakis(carbamoylmethoxy)calix[6]arene (III), have been used to design a HPO42" - selective electrodes as it combines hydrogen bond donor/acceptor functions, electrostatic and dipole-dipole interactions. Membranes based on (III) with different plasticizers and without plasticizer were prepared and studied to know the best composition, which impart good stability as well as electroanalytical properties. It was found that the membranes of (III) having composition 5.7:150 (IILPVC) (w/w; mg) exhibited linear response in the concentration range of 1.8 x 10"5 - 1.0 x 10"' M with a fast response time of 20 s. The electrode is sufficiently selective and the order of selectivity indicates that sulfate and hydrogen carbonate are well discriminated, while C104" and SCN" are less. The electrode was used for the determination of HPO42" in real samples. Another calix[6]arene, 5,ll,17,23,29,35-hexakis(l,l,3,3-tetramethylbutyl-37,38,39,40,41,42-hexakis(carboxy methoxy)calix[6]arene (IV), have been investigated as selective material to develop Sr2+- selective electrodes. Optimum performance was observed with (IV) having composition 6:150:170:3 (IV:PVC:DBA:NaTPB) (w/w; mg). The electrode exhibited Nernstian response in the concentration range of 1.9 x 10"5 - 1.0 x 10"' M with a response time of 15 s. The (iii) electrode is enough selective over other alkaline earth metal ions and used to determine the end point in the potentiometric titrations ofcarbonate ions with Sr +. Besides porphyrins and calixarenes, Schiff base, dendrimer and crown ether have also been used to prepare ISEs, which selectively respond to zinc, mercury and lithium ions, respectively. N,N'-bis(acetylacetone)ethylenediimine (V) has been synthesized and screened for some heavy metal ions (Zn2+, Cu2+, Ni2+, Cd2+, Hg2+ and Pd2+). Primary experiments revealed that N,N'-bis(acetylacetone)ethylenediimine (V) responds selectively for Zn2+ and thus, used for the fabrication of PVC based membrane electrode for Zn2+. The electrode having composition 5:100:200:6 (V:PVC:NPOE:NaTPB) (w/w; mg) exhibited a near Nernstian response (32.0 mV/decade of activity) in the concentration range of 1.0 x10"5 - 1.0 x 10"' M. Asecond generation triethylene glycolmonomethylether end-grafted carbosiloxane dendrimer (VI) has been synthesized by the reaction of Si(OCH2CH2CH2SiMe(OCH2 CH2CH2SiMe2Cl)2)4 with triethylene glycolmonomethylether. The product has been characterized by IR, 'H, 13C{'H} and 29Si{'H} NMR spectroscopy and explored as ionophore for developing a Li+-selective electrode. The electrode worked linearly over the concentration range of2.5 x 10"5 - 1.0 x 10"' Mwith a slope of52.0 mV/decade ofactivity. The electrode was used in real sample analysis. In addition to this, a double armed crown ether, N,N'- dibenzyl-l,4,10,13-tetraoxa-7,16-diazacyclooctadecane (VII) has been investigated as suitable sensing material for Hg2+ ions. The electrode having an optimum composition 12:150:100:2 (VII:PVC:DBBP:NaTPB) (w/w; mg) exhibited Nernstian response in the concentration range of 3.1 x 10"5 - 1.0 x 10"' M, a fast response time (15 s), and adequate lifetime (4 months). The electrode is sufficiently selective for Hg2+ in presence of normal interferents like cadmium, sodium and iron. (iv) The results of potentiometric studies of proposed electrodes show that these are better than the reported electrodes in some respects and thus, the present work adds to our knowledge in the field of chemical sensors.