SYNTHESIS AND ELECTROANALYTICAL STUDIES OF SOME CHELATING IONOPHORES

Abstract

Rapid industrialization and urbanization has no doubt improved the quality oflife. However its adverse effects are severe pollution of air, water and soil due to mobilization of several harmful metals and organic pollutants into the environment. Environmental pollution by toxic metals is well recognized and their toxicity leads to the accumulation of toxins in ourtissues and organs. Presence of metals trace concentration in animal life is useful but at higher concentration toxic effects appears and life is subjected to many nutritional deficiencies, neurological disorders and can even lead to autoimmune disorders, cancer and other debilitating chronic diseases. It is normally difficult for anyone to avoid exposure to harmful metals as they are widely prevalent in environment due to their presence in almost all products of modern consumerism viz. construction materials, cosmetics, medicines, processed foods, variety offuel and various agricultural products. In view of toxicity of these metals and their understood occurrence in the environment, it is important to monitor these pollutants. A number of analytical techniques such as Atomic Absorption Spectrometry, Inductive Coupled Plasma-Mass Spectrometry, Ion Chromatography, Flame Photometry, Cyclic voltametry, isotopic dilution, radiometric activation analysis are available for quantitative analysis of metals present in the environmental samples. These techniques provide accurate results but their maintenance and operational cost is high and requires adequate expertise and large scale infrastructure back up. Thus, the analysis is generally limited to laboratory level only. A reliable, low cost, quick and portable analytical technique is often required especially for the analysis of large number of samples and such requirements are often met with ion selective electrodes (ISEs) to a significant extent. The technique is generally non destructive, adaptable to small sample volume with possible applications in real-time analysis. Moreover, analysis of coloured and viscous samples can also be carried out easily. Further the technique involves a number of advantages such as simple set up, low cost and very convenient to use for online measurement. Due to these advantages the determination of concentration of a particular ion using an ISE has taken a leading place among all electrochemical methods of analysis. ISEs find application in a variety of fields like clinical, environmental, industrial, agricultural and process monitoring, as well as detectors in HPLC and capillary electrophoresis. As a result, a number of good ion sensors have been developed and marketed which are now convenient tools for analysis. These ion sensors have been used for the quantification of metals in food products, biological fluids, soil, effluents and wastewater. Membrane ion selective electrodes consist of a semi-permeable membrane that separates two solutions of different concentrations of an appropriate electrolyte and responds selectively to particular ion, even in the presence of other ions. The membrane constituent is normally an active ion-exchanger ingredient generally called an ionophore or electroactive material. In spite of great utility of ion sensors in analysis, it has not been possible to prepare them for many ions of importance, mainly due to the non availability of good ionophores. It is obvious that the development of good ion sensors requires an ionophore which has high affinity for a particular cation/anion and poor for others but such materials are not available in abundance. Due to importance of ion sensors various types of materials such as solid electrolytes, inorganic and organic ion exchangers, insoluble salts of multivalent metals, metal chelates, macrocycles, calixarenes, Schiff base, hydrogen bonding receptors, other neutral carriers have been used as ionophores for the development of ion selective electrodes. In recent past, intensive studies on the design and synthesis of highly selective and sensitive ion-carriers as sensory molecules and their applications as ion-selective electrodes for the routine assessments of various ions have ii been reported. However, many ofthese electrodes have not been very successful as they exhibit a significant interference to other ions, poor sensitivity and selectivity, high response time and function over a limited pH range. Therefore, it is desirable to further explore different materials for the preparation of membranes which may act as selective sensor for target ions. The development ofpotentiometric membrane-based ion sensor with analytically useful selectivity remains a formidable challenge. Work for developing new ISEs ofhigh selectivity is always a useful goal. We have, therefore, explored some newly synthesized and previously reported compounds as ionophores and used their membranes as ISEs for some metals. For the sake of convenience, simplicity and clarity, the work embodied in the thesis has been organized as following six chapters. First Chapter; General Introduction: It presents an up-to-date review of the literature on sensors of alkaline, alkali earth, transition, rare earth metal ions and anions. The problem of present research activities have also been posed in the context of the cited work. Second Chapter; Theory andMethodology ofIon Selective Electrodes: This encompasses classification of ISEs, theory of membrane electrodes, its potentials and terms used in the study of ion selective electrode. The concept of the selectivity of ion selective membranes and methods of its determination has also been discussed. Third Chapter; Schiffbaseas copper ionselective electrodes: It is reported that Schiff base have been widely used as chelating ligands in the coordination chemistry of transition metals and some of them show selective affinity for iii metals. It is, therefore, possible to use Schiff bases as selective ionophores in the preparation of membrane electrodes of better characteristics. To test this possibility in a realistic fashion, we investigated them as potential sensory component for the development of membrane ion-selective electrodes. In this chapter, the synthesis and characterization of 2-[{(2-hydroxyphenyl)imino}methyl]-phenol (Li) and 2-[{(3- hydroxyphenyl)imino}methyl]-phenol (L2), and their analytical application as ionophores for the fabrication of Cu2+ ion selective electrodes are described. Polymeric membranes electrodes (PME) of Li and L2 have been prepared and investigated as Cu -selective sensors. Poly (vinyl chloride) (PVC) was used as matrix for the sensor's design and influence of the other membrane components i.e. ionophore, plasticizer and the lipophilic additive that are known to have a dramatic effect on sensor's performance have also been evaluated. Effect of various plasticizers viz., dibutyl phthalate (DBP), dibutyl sebacate (DBS), benzyl acetate (BA), o-nitrophenyloctylether (o-NPOE) was studied in detail and improved performance was observed in several instances by the addition of oleic acid (OA) as anion excluder. Optimum performance for the electrodes based on Li was observed with membrane having ingredients in composition Li:DBS:OA:PVC = 6:54:10:30 (%, w/w). This sensor worked satisfactorily in the concentration range 3.2 x 10"8 to 1.0 x 10"2mol L"1 with aNernstian slope of 29.5 mV decade"1 of acu2+ and having 01 1 the detection limit as 2.0 x 10" mol L" (1.27 ng mL" ). Optimization of membrane composition of the electrodes based on L2 showed that the membrane having composition L2:DBP:OA:PVC = 6:54:10:30 (%, w/w) exhibit the Nernstian slope (29.6 ± 0.5 mVdecade" of aa, ) and showed linear potential response in the concentration range of 2.5 x 10"7- 1.9 x 10"2 mol L"1 with limit of detection 1.2 x 10"7 mol L"1. Wide pH range (3.0 •*- 8.5), fast response time (5 s), good performance in presence of small amounts of (up to 20 % v/v) non-aqueous contents and adequate shelf life (3 months) indicate the iv utility of the proposed sensors. The selectivity coefficients for these electrodes were determined by matched potential method (MPM) and the results indicate their selective response for Cu +ions over various interfering ion. However, it was found that the sensor based on ionophore (Li) has good selectivity over the sensor based on ionophore (L2). These electrodes could be successfully used for the determination of copper in edible oils, tomato plant material; river water and as an indicator electrode in potentiometric titration of copper ion with EDTA. Fourth Chapter; Gd3+ ion-selective coated graphite electrode based on N5 donor chelating ligand: This chapter incorporates investigation on the use of a N5 donor chelating ligand as ionophore in the preparation of membrane electrode selective to Gd3+. Chelating ligand 2,6-Bis-[l-{N-cyanoethyl,N-(2-methylpridyl)}aminoethyl]pyridine (L3) having five nitrogen donor sites for complexation with cations has been synthesized. Stability studies of the several metal-ligand complexes showed that L3 has high affinity towards Gd +ion and therefore, it was used as a potential ionophore for the preparation ofcoated graphite Gd ion selective electrode. Among several electrodes prepared, the one having membrane of composition L3:NaTPB:PVC:o-NPOE = 8:4:30:58 (%, w/w) gave best performance over the widest working concentration range of 2.8 x 10"7- 5.0 x 10"2mol L" 1showing Nernstian behaviour (19.6 ±0.3 mV decade"1) with lower limit ofdetection as o 1 6.3 x 10" mol L" . Furthermore, it showed a fast response time (12 s) and can be used for 2.5 months without significant divergence in its characteristics. Further, the electrode can tolerate the concentration of different surfactants upto 1.0 x lO^mol L"1 and can be used successfully in ethanol 30 % (v/v) and methanol and acetonitrile 10 % (v/v) water mixture. The useful pH range of this electrode for the detection of Gd3+ in a solution is 2.0 to 8.0. The potentiometric selectivity coefficients of the electrode were evaluated by IUPAC recommended fixed interference method (FIM) and matched potential method (MPM) and it was found that the electrode is sufficiently selective for Gd3+ over many cations and could be used for the determination of Gd ions in waste water and rock samples. It could also be used successfully as an indicator electrode in the potentiometric titration of GdCl3 with EDTA. Fifth chapter; Nation-selective coated graphite electrodes based on lariat ethers: This chapter describes the synthesis of novel lariat ethers viz 1,5- di(cyanoethane)-2,4: 7,8:13,14-tribenzo-l,5-diaza-9,12-dioxacyclopentadeca-2,7,13- triene (L4) and 1,5-di (cyanoethane)-2,3, 4-pyridine-7,8:13,14-dibenzo-l,3, 5-triaza-9, 12-dioxa cyclopentadeca-2,7,13-triene (L5) and their application as potential ionophores for the fabrication of Nd selective and sensitive coated graphite electrodes. Complexation study of the two ionophores towards various metal ions in acetonitrile solutions revealed the formation of 1:1 metal-ionophore complexes. A number of PVC based coated graphite electrodes (CGEs) of L4 with different plasticizers and lipophilic additives were prepared and explored as Nd3+ selective electrodes. The results showed that optimized membrane composition for the electrode based on L4 was found to be; L4:o-NPOE:NaTPB:PVC = 5:57:3:35 (%, w/w). This electrode exhibited good performance over the wide working concentration range 8.4 x 10"8 - 3.1 x 10"2 mol L"1 with Nernstian slope (19.8 ± 0.4 mVdecade"1 ofam3+) and low detection limit (3.8 x 10"8 mol L"). However, the electrode based on L5 having optimum membrane composition of its ingredients L5:o-NPOE:NaTPB:PVC = 5:53:3:39 (%, w/w) performed best over the widest working concentration range of 4.6 x 10"8-5.0 x 10"2mol L"1 with Nernstian slope VI (19.7 ± 0.5 mVdecade"1 of aNd3+) and low detection limit (1.6 x 10"8 mol L"1). The potentiometric response of these electrodes was excellent in the range of pH 3.5 to 7.6 and they showed fast response time of 14 s and 10 s, for L4 and L5 based CGEs, respectively with a shelf life of three months without significant divergence in the performance characteristics. They could also tolerate up to 20%(v/v) methanol, ethanol and acetonitrile, if present in the test solution. The presence of cationic and anionic surfactants at 1.0 x 10"* mol L"1 or above caused significant interference in the performance of the electrodes. Further, the values of selectivity coefficients determined by FIM indicate that the proposed electrodes are efficiently selective over a number of monovalent (Ag+, Na+, K+, Li+), divalent (Hg2+, Fe2+, Co2+, Ni2+, Cd2+, Mg2+, Ca2+, Zn2+, Cu2+) and trivalent ions (Gd3+, Pr3+, Ce3+, La3+, Tb3+, Dy3+ Sm3+ and Yb3+). However, higher concentration of Co2+, La3+, Pr3+ and Yb3+ caused some interference. These electrodes were used successfully as indicator electrodes in the potentiometric titration of Nd against EDTA and also direct determination ofNd3+ ions from binary mixtures and water samples. Sixth chapter; Ccf+ ion-selective electrodes based on N4, N2S2 and N2O2 type chelating ligands: This chapter describes the synthesis, characterisation and analytical application of three chelating compounds N!,N2 -dicyanoethyl-N',N2-bis(pyridine-2-ylmethyl)benzene- 1,2-diamine [L6], N1,N2-dicyanoethyl-N1,N2-bis(thiophen-2-ylmethyl) benzene-1, 2- diamine [L7] and N1,N2-dicyanoethyl-N1,N2-bis(furan-2-ylmethyl)benzene-l,2-diamine [L8]. Membranes having constituents PVC, plasticizers, lipophilic anionic additives and the ionophores were coated on the surface of spectroscopic graphite rods and their potential response investigations carried out. The electrodes having membranes of vii different compositions were investigated and it was found that the among all the electrodes of L^, the one having membrane composition in the ratio L^.o- NPOE:NaTPB:PVC as 4:51:2.5:42.5 (%, w/w) performed best i.e. worked over a wide working concentration range 2.5x10" -1.0x10" mol L" with Nernstian slope 29.5±0.07 1 R 1 mV decade" and low detection limit 7.8x10" mol L" . This electrode generated constant potential in the pH range 2.0 to 8.0 with a fast response time (12 s) and shelf life of 4 weeks. The CGE based on L7 having membrane composition L7:o-NPOE:NaTPB:PVC = 3:52.5:1.5:43 (%, w/w) performed best i.e. widest linear working concentration range 1.7xl0"8 - l.OxlO"1 mol L"1 with Nernstian slope 29.6±0.08 mV decade"1 and lower detection limit as 7.0x10"9 mol L . The useful pH range ofthis electrode was observed to be 2.6 to 8.0. This electrode showed a fast response time of 7 s towards Cd2+ ion and the potentiometric characteristics of the electrode do not change significantly over a period of 6 weeks. A similar optimization of membrane composition of the electrodes based on Lg showedthat the electrode with composition L8:o-NPOE:NaTPB:PVC = 7:49:3.5:40.5 (%, w/w) showed best performance in terms of wide working concentration range (4.6x10"7 - 5.0xl0"2 mol L"1), Nernstian slope (29.8± 0.05 mV decade"1) and low detection limit Q 1 (8.4x10" mol L"). The response of the electrode is fast (17 s) and can work satisfactorily in the pH range 3.0 to 7.5 with a shelf life of 4 weeks. Selectivity coefficients of the electrodes were evaluated by FIM and the results showed that these electrodes have high selectivity towards Cd2+ over a large number of cations and could tolerate upto 20 % (v/v) non-aqueous impurity. In view of high sensitivity and selectivity, the utilityof these electrodes was tested in the potentiometric determination of Cd2+ in medicinal plants, soil and industrial waste water. They could also be used successfully as an indicator electrode inthe potentiometric titration of Cd2+ against EDTA.

Thus, the present investigations on the membranes of Schiffs bases, chelating ligands and lariat ethers have resulted in the development of selective and sensitive sensors for Cu2+, Gd3+, Nd3+ and Cd2+ ions. The investigated sensors ofmetal ions have been found to be better than reported sensors with respect to various response characteristics and resulted in the availability of some improved and novel ion sensors which can be used successfully for analytical purposes. Thus, the present work adds to our knowledge in the field of chemical sensors