ELECTROANALYTICAL STUDIES ON MEMBRANE SENSORS FOR ION DETERMINATION

Abstract

Electrochemical sensors are the most rapidly growing class of chemical sensors. Ion-selective electrode (ISE) is an electrochemical sensor based on a thin selective membrane as recognition element. The membrane allows only the selective permeation of certain ionic species due to the incorporation of special ingredients, called "ionophore". ISE is an electrochemical half-cell that forms a complete electrochemical cell in conjugation with an external reference electrode and the measured potential difference is linearly dependent on the logarithm ofthe activity of a given ion in solution. Various techniques are available for measuring cations and anions in aqueous solutions viz. UV-Vis spectroscopy, atomic absorption spectroscopy, inductively coupled plasma atomic emission spectroscopy, inductively coupled plasma mass spectrometry, isotopic dilution, radiometric neutron activation methods, fluorescence, chemiluminescence phosphorescence techniques, high performance liquid chromatography, ion chromatography, voltammetry etc. Although some of these methods are precise and selective for the determination of a number of cations and anions in the solution, their applications are limited by various factors such as, cost, instrumentation complexityand consumption of time. Thus, there is a substantial need for the development of a selective, portable, inexpensive technique for the determination of various metal ions and anions. Due to a number of advantages such as simple set up, low cost, fast response time, wide measuring concentration range and analytically relevant selectivity, the determination of concentration of a particular ion using an ISE has taken a leading place (i) 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. The development of potentiometric membrane-based ion sensor with analytically useful selectivity remains a formidable challenge. In pursuance of this objective, we have explored some newly synthesized and commercially available ionophores and their potentiometric characterization for quantification of some metal ions and certain anions. For the sake of convenience, simplicity and clarity, the work embodied in the thesis has been organized as follows: The First Chapter of the thesis 'General Introduction' 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. The Second Chapter of the thesis 'Theory and Methodology' encompasses classification of ISEs, theory of membrane, its potentials and terms used in the study of ion selective elictrode. The description of the selectivity of sensor membrane and method of its determination has also been described. The Third Chapter of the thesis 'Membranes of Macrocyclic and Schiff Base Complexes as Ion Selective Sensors'1 deals with synthesis and characterization of macrocyclic and Schiff base complexes and their analytical application in the preparation of anion-selective electrodes. In order to improve the performance characteristics of PVC based membranes the optimization of membrane composition was carried out by the addition of various plasticizers and ionic additives and their potential responses were investigated. It was found that the membrane of 2,3,10,11-tetraphenyl-1,4,9,12- (ii) tetraazacyclohexadeca-l,3,9,ll-tetraene zinc(II)acetate (Zn-M) used for the fabrication of Br~ selective sensor. Membrane having composition Zn-M/HTAB/NPOE/PVC in as 5/3/297/150 (w/w; mg) exhibited the best result. It exhibited a working concentration range of 2.2 x 10"6 to 1.0 x 10"1 M with a slope of 59.2 mV decade"1 activity and a detection limit of 1.4 x 10"6 M. The response time of electrode was 20 s and it can be used in the pH range of 3.5 - 9.5. This bromide sensor was successfully used as an indicator electrode in the potentiometric titration of bromide ions with silver ion and also in the determination of bromide content in tap water and wine samples. Similarly membranes of A^,A^'-bis(salicylidene)-l,4-diaminobutane cadmium(II) perchlorate (Cd-Si) and JV,iV'-bis(salicylidene)-3,4-diaminotoluene cadmium(II) perchlorate (Cd-S2) complexes were examined as I~ selective sensors. It was found that among two cadmium chelates of Schiff bases, the sensor based on Cd-Si performed better than that of Cd-S2 and the best performance was shown by the membrane of composition of Cd-Si/HTAB/DBP/PVC as 7/3/210/115 (w/w; mg). It worked in the concentration range of 5.3 x 10"7 to 1.0 * 10"2Mwith Nernstian slope of59.2 mV decade"1 activity. This membrane also showed low detection limit of 1.9 x 10"7 Mwith fast response time of 11 s and was independent of pH range of 2.5 - 9.0. The sensor was used as an indicator electrode in the potentiometric titration of iodide ions against silver ions. It was also applied successfully for the determination of iodide content in environmental and medicinal samples. The Fourth Chapter, 'Membranes of Chelating Ligands as Ion Selective Sensors' includes a variety of sulfur, nitrogen and oxygen containing chelating ligands as electroactive components in the formation of membrane sensors. A highly selective PVCbased membrane sensor using a-furildioxime (FD) as neutral carrier has been prepared and (iii) explored as Ca -selective electrode. The membrane having composition FD/KTpClPB/DBP/PVC as 4/2/220/105 (w/w; mg) showed wide concentration range of 2.6 x 10"7 to 1.0 x 10"1 Mfor Ca2+ with a Nernstian slope of 29.5 mV decade"1 activity. The response time of the sensor was 10 s and it worked well in a wide pH range of 3.5 - 9.0. The sensor was also used as an indicator electrode in the potentiometric titration of Ca(II) ions against ethylenediaminetetraacetic acid (EDTA), as well as for the analysis of Ca ions in soil drainage water samples. 5-Amino-3-methylisothiazole (AMI) was used as an ionophore for trace level determination of Co2+ ions. Membrane having a composition ofAMI/OA/o-NPOE/PVC as 10/15/300/150 (w/w; mg) exhibited the best results. The sensor exhibited a Nernstian slope of 29.5 mV decade"1 activity in a linear range of 6.3 x 10"7 to 1.0 x 10"1 Mfor Co2+ ions. The detection limit of this electrode was 3.9 x 10"7 M. It has a fast response time of 12 s. The proposed electrode revealed a good selectivity for Co2+ ions over a wide variety of other tested cations and could be used in the pH range of 3.3 - 9.0. The electrode was successfully applied as an indicator electrode for the potentiometric titration of cobalt ions with EDTA as well as for the direct determination ofCo2+ ions inwater samples. Membranes of 3-(2-pyridinyl)-2H-pyrido[l,2,-a]-l,3,5-triazine-2,4(3H)-dithione (PPTD) and acetoacetanilide (AA) were investigated as Cu2+ selective sensors. The sensor based on PPTD ionophore performed better than that of sensor based on AA ionophore with composition PPTD/OA/NPOE/PVC as 5/10/240/120 in (w/w; mg) worked satisfactorily in concentration range of 5.0 x 10"8 to 1.0 x 10"2M (detection limit 4.0 x 10"8 M) with a Nernstian slope of29.5 mV decade"1 activity. Wide pH range (3.0 - 9.5) and fast response time (12 s), indicate the vital utility of the proposed sensor. The electrode was used for the determination of copper in different milk powder and water samples and as indicator electrode in potentiometric titration of copper ion with EDTA. (iv) 5 I Rubeanic acid (RA) was used as an ion carrier for the fabrication of La selective sensor. Membrane having a composition as RA/OA/o-NPOE/PVC as 5/18/208/180 (w/w; mg) exhibited the best results. This sensor exhibited a Nernstian response for La over a wide concentration range of 3.2 x 10"8 - 1.0 x 10"2M with a detection limit of 2.5 x 10"8 Mand slope of 20 ± 0.2 mV decade"1 activity. It showed a response time of 15 s and can be used in a pH range of 3.5 - 9.5. It was successfullyused as an indicator electrode in the potentiometric titration of lanthanum ions against EDTA. Membranes of l-phenyl-3-(2-thiazolyl)-2-thiourea (PTT) and l-phenyl-3-(2- thiazolyl)-2-urea (PTU)were explored as ytterbium selective sensors. The sensorbased on PTT having composition PTT/NaTPB/DOS/PVC as 3.5/1.5/160/80 (w/w; mg) was found to perform best and worked satisfactorily in the concentration range of 1.2 x 10" to 1.0 x 10"2 M(detection limit 5.5 x 10"8 M) with a Nernstian slope of 19.7 mV decade"1 activity. This sensor showedwide pH range (3.0 - 8.0) with fast response time (10 s). The proposed electrode comparatively showed good selectivity for Yb +ion and can be used for its determination in binary mixtures and sulfite determination in white and red wine samples. The Fifth Chapter, 'Membranes of Tripodal Ligands and Complexes as Ion Selective Sensors' include synthesis and characterization of tripodal ligands and complexes for the construction of ion-selective sensors. PVC based membranes of potassium [hydrotris-N-(2,6-xylylthioimidazolyl)borate] [KTt2'6"xylyl] and potassium [hydrotris(3,5-phenylmethylpyrazolyl)borate] [KTpphMe] were investigated as cadmium ion selective sensors. [KTt2'6"xylyl] was found to be more selective and sensitive ion carrier for Cd(II) membrane sensor than [KTpph,Me]. A membrane composed of [KTt2'6"xyly1]/ NaTPB/DBP/PVC with 6/3/160/90 (w/w; mg) worked well over a very wide concentration (v) range (8.5 x 10"8 to 1.0 x 10"2 M). This sensor exhibited a very low detection limit of o 1 4.4 x 10" M with a Nernstian slope of 29.4 mV decade" activity between pH values 3.0 - 7.0 with a very fast response time of 8 s. The sensor displayed good discrimination towards Cd ions with regard to most of the common cations. The proposed sensor based on KTt2'6"xylyl ionophore was also used for the direct determination of cadmium ions in different types of water samples and human urine samples. Potassium[hydrotris(N-tert-butyl-2-thioimidazolyl)borate [KTtt_Bu] and potassium [hydrotris(3,5-tert-butylisopropylpyrazolyl)borate] [KTpt"Bu'I"Pr] were synthesized and explored as ionophores for the preparation of PVC membrane sensor for Zn2+ ions. Sensor having [KTtt_Bu] ionophore with composition [KTtt"Bu]/NaTPB/DBP/PVC amounts (w/w; mg) of 15/4/275/150 performed better than that based on [KTpt"Bu'i"Pr]. This sensor exhibited a Nernstian slope of 29.4 mV decade"1 activity for Zn2+ ions over a wide concentration range (1.4 x 10"7 to 1.0 x 10_1 M) with limit of detection 9.5 x 10"8 M. It has a relatively fast response time (12 s) and could be used over wide a working pH range of 3.5 - 7.8. The sensor was found to be useful in the estimation of zinc at trace levels in industrial waste water, river water and urine samples. Tripodal complex, [hydrotris(N-tert-butyl-2-thioimidazolyl)borate] zinc(II) perchlorate [Zn(Ttl"Bu)(C104)] was used as a carrier for thiocyanate selective sensor. Best response was observed with the membrane composed of [Zn(Ttt"Bu)(C104)]/ HTAB/DOP/PVC ratio of 5/2/190/100 (w/w; mg). This proposed sensor exhibited significantly enhanced selectivity toward SCN- ions over the concentration range of 6.3 x 10"7 to 1.0 x 10"2M with a lower detection limit of 3.2 x 10"7 Mand a Nernstian slope of 59.4 mV decade-1 activity. This sensor exhibited fast response (14 s) and applicability over a wide pH range (3.5 - 9.0). The proposed electrode showed fairly good

discrimination of thiocyanate from several inorganic and organic anions. It was successfully applied for direct determination of SCN- within saliva, urine, serum and river water samples. Similarly, tripodal cadmium complex of [hydrotris(3,5-phenylmethylpyrazolyl) borate] cadmium(II)perchlorate [Cd(Tpph,Me)(C104)] and macrocyclic ligand 5,7:12,14:19,21:26,28-Bzo4-[28]-5,13,19,27-tetraene-8,ll,22,25-N4-l,4,15,18-04 [Bzo[28]tetraeneN404] was synthesized, characterized and used as ionophores in the preparation of dihydrogen phosphate ion-selective sensors. Spectroscopic investigations indicate high affinity of these receptors for H2P04" ion. Sensor having membranes of [Cd(TpPh'Me)(C104)], with composition [Cd(TpPh'Me)(C104)]/HTAB/NPOE/PVC as 6/3/180/100 (w/w; mg) performed well. This sensor worked over a wide concentration range of 2.1 x 10"7 to 1.0 * 10"2 Mwith a Nernstian compliance (59.0 mV decade-1 activity). The proposed phosphate sensor showed fast response time of 14 s and its potentials remain constant within a pH range of approximately 3.0 - 7.0. The sensor was used successfully for the determination of phosphate in soil water samples.