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|Title:||STUDIES ON SOME POTENTIOMETRIC SENSORS FOR ION DETERMINATION|
|Keywords:||CHEMISTRY;POTENTIOMETRIC SENSORS;ION DETERMINATION;FLAME PHOTOMETRY|
|Abstract:||The industrial revolution represents the starting point for large-scale mobilization of metals from the earth's crust to the global environment. They are present in virtually every area of modern consumerism from construction materials to cosmetics, medicines to processed foods, fuel sources to agents of destruction, appliances to personal care products. It is very difficult for anyone to avoid exposure to any of the many harmful metals that are so prevalent in our environment. Environmental pollution by toxic metals is well recognized and their toxicity leads to the accumulation of toxins in our tissues and organs causing nutritional deficiencies, neurological disorders and can even lead to autoimmune disorders, cancer and other debilitating chronic diseases. Recently, routine determination of metal ions is carried out using various analytical techniques such as Atomic Absorption Spectroscopy, Inductively Coupled Plasma-Mass Spectrometry, Ion Chromatography, Flame photometry, Cyclic Voltammetry etc. These techniques have proved to be advantageous but suffer from high capital cost, require adequate expertise and the fact that they are laboratory based. So there is vital need for the development of selective, portable, inexpensive diagnostic techniques for the determination of various metal ions. The past decade has revealed a significant transformation in the application of electrochemical techniques for the analysis of various analytes. The role of potentiometric sensors in analytical chemistry is to develop new sensors and to improve the merits of existing ones in the rapidly growing avenue of research now-a-days. (i) Potentiometric detection based on Ion-Selective Electrodes (ISEs) offer several advantages, such as speed, ease of preparation and procedures, simple instrumentation, relatively fast response, wide dynamic range, reasonable selectivity and low cost. Besides, they are also suitable for on-site analysis. The development of ISEs has been quickly followed by application not only in inorganic analysis but also in the pharmaceutical field. Research in drug type of substances is an expeditiously expanding domain in chemical sensors and used extensively for quality control of various drugs and have become a useful tool for solving analytical problems connected with complex pharmaceutical formulations and biological matrices. 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 been reported. However, these electrodes have not been found very successful as they exhibit a significant interference from 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. For pursuing this objective, we have explored some newly synthesized and commercially available ionophores and their potentiometric characterization for quantification of some metal ions and certain drugs. 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" gives an insight into the definitionand importance of ion selective electrodes in analytical chemistry. (ii) It also presents an up-to-date review of all the literature on previously reported sensors of transition metals, rare earths, drugs and miscellaneous ones. The problem of present research activities has also been posed in the context of the cited work. The Second Chapter, 'Theory of Membrane Potential' mainly presents the principle, classification and delves into the terms used in the study of ion-selective electrode. The description of the selectivity and methods of its determination has been described. It is reported that Schiff bases have been widely used as chelating ligands in the coordination chemistry of transition metals. To test this possibility in a realistic fashion, we investigated these materials as potential sensory elements in ion-selective electrodes. The Third Chapter of the thesis 'Membranes of Schiff s bases as ion selective sensors' deals with the synthesis and characterization of some Schiff bases and their analytical applications. Poly(vinyl chloride) (PVC) was used as matrix for the sensor's design and influence ofthe other membrane parameters i.e. plasticizer and the lipophilic additive that are known to have a dramatic effect on sensor's performance have also been evaluated. Membrane of N,N'-bis (salicylidene)-3,4-diaminotoluene (Si) was effectively used in the fabrication of Co2+ selective sensor. The performance characteristics of PVC based membranes of (Si) have been evaluated in absence and presence of various plasticizers viz. dioctylphthalate (DOP), benzyl acetate (BA), o-nitrophenyloctyl ether (o-NPOE), dibutylsebacate (DBS), dibutylphthalate (DBP). It was found that the performance of all plasticized membranes of (Si) is better than its membrane without plasticizer. Of all the plasticized membranes, membrane with DOP plasticizer performs best. (iii) Therefore, the effect of amount of various ingredients on the membrane with DOP plasticizer was also studied and the membrane with composition (Si):NaTPB:DOP: PVC (w/w, %) in ratio of 1.22:0.68:57.06:41.04 exhibited a working concentration range of 7.9 x 10"8 - 1.0 x 10"1 Mwith a slope of30.0 mV decade"1 activity and a detection limit of5.0 o x 10" M. This membrane also showed lowest response time of 12 s and is independent of pH in the range 2.0-9.0. The selectivity studies of this sensor show that the proposed sensor possesses excellent selectivity for Co2+ over a large number of mono-, bi- and trivalent cations. This selectivity allowed its use as an indicator electrode in the potentiometric titration of Co2+ with EDTA and also for the determination of Co2+ in beer and water samples. Similarstudies were also carried out with PVC based membranes of N,N'-bis [2- (salicylideneamino) ethyl] ethane-1,2-diamine (S2) with and without plasticizers. A large number of membranes of different compositions were prepared and investigated. It was found that the membrane with PVC:o-NPOE:OA:(S2) (w/w, %) in ratio of 35.20:58.90: 4.70:1.19 performs best and used for fabrication of Ce3+ selective sensor. It exhibited a working concentration range of 1.4 x 10"7 -1.0 x 10"2 Mwith a slope of 20.0 mV decade"1 and a detection limit of 8.9 x 10"8 M. This membrane also showed lowest response time of 10 s and is independent of pH range 3.0-8.0. Further, selectivity studies indicated that this sensor is sufficiently selective towards Ce3+ over a large number of ions. The sensor can be used as an indicator electrode in the quantification of oxalate, fluoride and carbonate by potentiometric titration against Ce3+ solution and also for the determination ofCe3+in various binary mixtures. (iv) As the values of formation constants of ionophores viz. N-(acetoacetanilide) -1,2- diaminoethane (S3) and N,N'-bis (acetoacetanilide)-triethylenetetraamine (S4) towards Cr3+ ion is high, their membranes were explored as Cr3+ selective sensors. Among several membranes prepared with and without plasticizer, the membranes having compositions (S3):NaTPB:PVC:DBP (w/w, %) in ratio of 1.27:0.73:36.73:61.27 and of (S4):NaTPB: PVC:o-NPOE (w/w, %) in ratio of 1.27:0.73:36.73:61.27 performed best and exhibited working concentration ranges of 8.9xl0"8-1.0 x 10"1, 8.3 x 10"7-1.0x 10"1 Mwith slopes of 19.8 and 19.2 mV decade"1 activity and detection limits of 5.6 x 10"8, 6.3 x 10"7 M Cr3+, respectively. The selectivity studies for the two sensors under consideration showed that the sensor based on ionophore (S3) is more selective for Cr3+ over a number of ions as compared to the sensor based on ionophore (S4). As the sensor based on (S3) showed better selectivity than (S4), it was used for further electroanalytical studies. The sensor showed lowest response time of 10 s and is independent of pH range 2.0-7.0. It was used as an indicator electrode in the potentiometric titration of Cr with EDTA and in direct determination of Cr3* in different water and food samples. Similar studies were performed with PVC based membranes ofN,N'-[bis(pyridin- 2-yl)formylidene]butane-l,4-diamine (S5) and N-(2-pyridinylmethylene)-l,2-benzenediamine (Se) as Cd2+ selective sensors. The optimization of membranes compositions were carried out and the results showed that membranes having compositions (S5):KTpClPB:PVC:o-NPOE (w/w, %) in ratio of 2.15:1.07:32.26:64.52 and of (S6): KTpClPB:PVC:DOP (w/w, %) in ratio of 2.15:1.07:32.26:64.52 performed best and exhibited working concentration ranges of7.9 x 10"8 - 1.0 x 10"1 M, 1.3 x 10"6- 1.0 x 10"1 Mwith slopes of 30.0 and 28.0 mV decade"1 activity and detection limits of 5.0 x 10"8, (v) 1.0 x 10"6 MCd2+, respectively. The selectivity coefficients oftwo ionophores have been measured and it was found that the sensor based on ionophore (S5) has good selectivity over ionophore (S6). Thus, it was explored for further analytical investigations. The sensor showed lowest response time of 10 s and is independent of pH range 2.0-6.0. It was used as an indicator electrode in the potentiometric titration of Cd2+ with EDTA and in direct determination ofCd2+ indifferent water and soil samples. It should be emphasized that sulphur containing ligands have encouraging prospects in selective recognition of ions. The Fourth Chapter, 'Membranes of sulphur containing ligands as ion selective sensors' underline the prospects of ligands such as 2-amino-6- purinethiol (L), 5-amino-l,3,4-thiadiazole-2-thiol (I2), S-2-benzothiazolyl-2-amino-a- (methoxyimino)-4-thiazolethiol acetate (I3) in the construction of ISEs. Membranes of (Ii) and (I2) were explored in the construction of Hg2+ selective sensors. A number of PVC based membranes were prepared with and without plasticizer for (L) and (I2). The results show that the membranes having compositions (Ii):NaTPB:PVC:DOP (w/w, %) in ratio of 3.17:1.58:31.83:63.42 and of (I2):NaTPB:PVC:DOS (w/w, %) in ratio 3.17:1.58:31.83:63.42 performed well and exhibited working concentration ranges of 7.0 x 10"8-1.0 x 10"1, 1.5 x 10"6-1.0 x 10"1 Mwith slopes of 30.0 and 27.7 mV decade"1 activity and detection limits of 4.4 x 10"8, 9.7 x 10"7 M Hg2+, respectively. The electroanalytical features of (L) depict good performance as compared to (I2) as proved by selectivity studies. The sensor based on (Ii) showed lowest response time of 10 s and pH studies confirmed that Hg2+ can be found in two different ions form, Hg2+ and [Hg(OH)]+ in pure water as a function of pH. (vi) The excellent characteristics of the sensor based on (I\) enhance its practical utility for assay of environmental, biological samples and potentiometric titration with EDTA. Membrane of S-2-benzothiazolyl-2-amino-a-(methoxyimino)-4-thiazolethiol acetate (L$) has been investigated as a selective sensor for Tb3+ ions. The sensor with membrane composition of (I3):KTpClPB:BA:PVC (w/w, %) in ratio of 3.17:1.58: 63.53:31.72 exhibited a working concentration range of 1.5 x 10"7 -1.0 x 10"2 Mwith a slope of 19.5 mV decade"1 activity and a detection limit of 9.3 x 10"8 M. The sensor showed lowest response time of 10 s and is independent of pH range 2.0-8.0. The response of the sensor was highly selective to Tb3+ over a large number of anions. As a result of good selectivity, the sensor could be successfully used for determination of fluoride ion in mouth wash samples and for the recovery of Tb +ion in water samples. It can also be used in the potentiometric titration of Tb3+ with EDTA and direct monitoring of Tb3+ in various binary mixtures. Beside these the detailed studies of all aforementioned ionophores with respect to lifetime, performance in non-aqueous media have been reported in these Chapters. Some of the membrane sensors developed was checked for their performance in the presence of surfactants as well. For pharmaceutical analysis the best sensors are based on ion-pair complexes. The Fifth Chapter, 'Membranes of drugs as ion selective sensors' describes polymeric membrane sensors for selective monitoring of the drugs ions. Membranes of anthelmintic drug tetramisole hydrochloride (TmCl) based on tetramisole-tetraphenylborate (Tm- TPB), chlorophenylborate (Tm-CIPB), phosphotungstate (Tm3-PT) as ion pair complexes (vii) exhibited working concentration ranges of 7.4 x 10"7- 1.0 x 10"2 M, 1.7 x 10"6-1.0 x 10"2 M, 5.6 x 10"6- 1.0 x 10"2 MwithNemstian slopes of59.0, 58.0, 57.0 mV decade"1 activity and detection limits of 5.1 x 10"7, 1.0 x 10"6, 3.1 x 10"6 M Tm+, respectively. The solubility product of the ion-pair and the formation constant of the precipitation reaction leading to the ion-pair formation were determined conductometrically. The electrodes exhibit good selectivity for TmCl with respect to a large number of excipients like inorganic cations, organic cations, amino acids and sugars. The electrodes were proved to be useful in the determination of TmCl in pure solutions and in pharmaceutical preparations such as Dicaris tablets and Vermisol syrup. Statistical evaluation by student's Mest and F-test showed insignificant systematic error between developed ion-selective sensors and previously reported methods. Similarly membranes of benign prostatic hyperplasia drug alfuzosin hydrochloride (AzCl) based on alfuzosin-tetraphenylborate (Az-TPB), chlorophenylborate (Az-CIPB), phosphotungstate (Az3-PT) as ion associations exhibited working concentration ranges of 8.3 x 10"6- 1.0 x 10"2 M, 3.8 x 10"6-1.0 x 10"2 M, 7.5 x 10"7- 1.0 x 10"2 M with Nemstian slopes of 57.0, 56.0, 58.5 mV decade"1 activity and detection limits of 5.1 x 10"6, 2.5 x 10"6, 4.8 x 10"7M Az+, respectively. Conductometric studies were performed for the determinationof the solubility product of the ion-pair and their formation constant. The selectivity coefficients of the electrodes reflect a very high selectivity for the Az+ over a large number of excipients. These electrodes were used for the determination of alfuzosin in its pharmaceutical preparations (Alfoo, Alfusin, Fual tablets) and in different biological fluids such as serum and urine without any preliminary treatment. (viii) The validity of this method is adjudged by comparing the results with previously reported methods by student's /-test and F-test. The results showed that the calculated values did not exceed the theoretical values, confirming the accuracy of the obtained results. Membranes of antiepileptic drugs such as lamotrigine (LTG), felbamate (FBM), primidone (PRM) doped with drugs-tetraphenylborate (TPB) and drugsphosphotungstate (PT) as ion pair complexes were explored. They exhibited working concentration ranges of 5.2 x 10"7-1.0 x 10"3 M, 2.6 x 10"7-1.0 x 10"3 M, 1.8 x 10"7-1.0 x 10"3 Mwith slopes of 56.4, 58.0, 60.0 mV decade"1 activity for LTG-TPB, FBM- TPB, PRM-TPB, respectively while LTG-PT, FBM-PT, PRM-PT exhibited working concentration ranges of 1.5 x 10"6-1.0 x 10"3 M, 5.8 x 10"7-1.0 x 10"3 M, 6.6 x 10"7-1.0 x 10"3 Mwith slopes of 52.3, 62.3, 54.2 mV decade"1 activity, respectively. These sensors exhibited excellent selectivity for drugs cations over a large number of excipients. The proposed electrodes were successfully applied in the assay of pharmaceutical formulations (Lametec, Mysoline and Felbatol tablets) and biological fluids (plasma and urine) with high accuracy and percentage recovery. The results have been also evaluated statistically using the Student's Mest and F-test indicating insignificant differences between the applied and references methods. Beside these, drugs selective electrodes were fully characterized in terms of composition, life span, usable pH range and used in the determination of aforementioned drugs in pharmaceutical preparations in four batches ofdifferent expiry dates. Thus, the present investigations on the membranes of some Schiffs bases, sulphur containing ligands and drugs have resulted in the development of selective and sensitive sensors for Co2+, Ce3+, Cr3+, Cd2+, Hg2+, Tb3+ ions and drugs viz. tetramisole, alfuzosin, lamotrigine, felbamate and primidone. The investigated sensors of metal ions have been found to be better than reported sensors with respect to various response characteristics while the drug sensors provide novelty. The present studies on potentiometric sensors have 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 ofchemical sensors. Most of the work carried out has been published or communicated in international journals of repute.|
|Research Supervisor/ Guide:||Singh, A. K.|
Gupta, V. K.
|Appears in Collections:||DOCTORAL THESES (chemistry)|
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