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dc.contributor.authorMangla, Rajni-
dc.guideGupta, V. K.-
dc.description.abstractChemical sensors (ion-selective electrodes (ISEs), ion sensors or sensors) are being widely used for the determination of various species as they enjoy a number of advantages not available in many other methods of ion analysis. The most attractive features of this technique are the speed with which samples can be analyzed, portability of the device, sample non-destruction, their cost effectiveness and also large measuring range, often spanning across as many as six decades of ion concentration. Moreover, their fabrication in the laboratory is quite easy and may become commercially available soon after their development. In view of the importance of chemical Sensors in diverse fields, extensive efforts have been initiated during the last two decades to develop sensors for various ions. The potentiometric sensor consists of a membrane, most frequently solid or liquid, which when placed across the two appropriate solutions, allows the transport of particular cationic or anionic species. As a result of this restricted transport, a potential difference across the membrane is developed which reflects the composition of the exterior phase and can be related to the activities of ions in the exterior solutions. Different type of materials tried for the purpose of development of membranes for the potentiometric sensors include solid electrolytes, inorganic and organic ionexchangers, crown ethers, metal chelates, cryptands, calixarenes, macrocycles etc. A survey of literature revealed that inspite of large work done in the area of (i) the sensors, suitable sensors for a number of toxic metals have not been developed so far. Thus, there is a need for developing better sensors for such metals, which has become possible with the availability of highly selective materials and their judicious choice. Therefore, efforts have been made by me to develop sensors for some toxic metals and the results obtained are summarized as under: The estimation of cadmium, magnesium, zinc, cobalt, molybdenum, lead and nickel is important as they occur in various effluents and are toxic above certain concentration levels. Macrocycles, which also include crown ethers, porphyrins, calixarenes, tetraazaannulenes, etc. are the compounds of recent interest for developing highly selective sensors for a number of ions. These compounds provide different donor atoms, ring size and ligand geometry to host ions and have been explored by me for developing sensors for cadmium, magnesium, zinc, cobalt, molybdenum, lead and nickel. The optimum composition of the membranes was determined by repeated trials. The effect of addition of different plasticizers viz. dioctyl phthalate (DOP), dibutyl phthalate (DBP), dibutyl butylphosphonate (DBBP), tri-n-butyl phosphate (TBP), tris-2-(ethylhexyl)phosphate (TEP), chloronaphthalene (CN) was studied. It was found in most of the cases that the addition of plasticizers improved the response characteristics of the membranes but in some cases deteriorated some of the response characteristics. Concentration of the equilibrating solution and time of contact were also optimized so that the (iii membrane which imparts good stability as well as better electroanalytical properties is obtained. The membrane exhibiting best performance with regard to working concentration range, slope and response time was chosen for all further studies. PVC based membranes of crown ethers monoaza-18-crown-6 (I), dicyclohexano-18-crown-6 (II) and benzo-15-crown-5 (III) were tried for developing sensors for cadmium and magnesium. The membrane of I having I:NaTPB:PVC:DOP in the ratio 1:1:10:10 (w/w) exhibited the best response for cadmium with the linear potential response in the concentration range l.OxlO"5- l.OxlO"1 Mwith a detection limit of 1.12 mg/L and a Nemstian slope of 29.0 ± 1.0 mV/decade of activity between pH range 5.0-7.7. The sensor exhibited a fast response time of <10s, it was inert towards acids, salt solutions and non-aqueous media (up to 35% (v/v)) and was used over a period of 3 months with good reproducibility. It was selective over a number of mono-, bi- and trivalent cations. For the crown, dicyclohexano-18-crown-6 (II), membrane having the composition 20:4:150:150 (II:NaTPB:DBP:PVC) (w/w) showed best properties with a Nernstian response of 29.0 ±1.0 mV/decade of activity and a working concentration range of 2.1xl0"5-1.0xl0"1 M. The working pH range of the electrode is 1.9-7.0. It displayed a fast response time of 17s and gave reproducible results for about 6 months. The proposed electrode revealed high selectivity for cadmium over alkali, alkaline earth and some transition and heavy metal ions. It also worked satisfactorily in mixtures having 10% (v/v) non- (iii) aqueous content. The practical utility of both the above described cadmium sensors has been demonstrated by using them as indicator electrodes in the potentiometric titration of Cd2+ with EDTA and also for its determination in wastewater. Benzo-15-crown-5 (III) was used as an electroactive material for developing a magnesium-selective electrode. The membrane with the composition 10:3:150:150 of III: NaTPB: DOP: PVC (w/w) exhibited the best properties with a working concentration range of l.OxlO^-l.OxlO"1 M and a slope of 31.0 ± 1.0 mV/decade of activity. The working pH range was 2.2-9.8 and 20% (v/v) non-aqueous content was tolerable. It displayed a fast response time of 15s and a lifetime of 4 months. It has also been successfully used as an indicator electrode in the potentiometric titration of Mg2+ with EDTA. Porphyrins can easily form complexes with metal ions. Therefore, these are tried for fabricating zinc, cobalt and molybdate selective sensors. Dimethyl- 8,13-divinyl-3,7,12,17-tetramethyl-21 H,23 H-porphine-2,18-dipropionate (protoporphyrin IX dimethyl ester) (IV) was used for developing Zn2+-selective sensor and 5,10,15,20-tetrakis(4-methoxyphenyl)porphyrinatocobalt (V) was used to develop Co2+ and Mo042"-selective sensors. The membrane having dimethyl-8,13-divinyl-3,7,12,17-tetramethyl-21 H,23 H-porphine-2,18-dipropionate (IV) as active material along with sodium tetraphenyl borate (NaTPB) as anion excluder and dioctyl phthalate (DOP) as solvent mediator in the ratio 15:100:2:200 (IV:DOP:NaTPB:PVC) (w/w) exhibited a working concentration range of LSxlO'M.OxlO*1 M, with aslope of 29.0 ±1.0 mV/decade of activity (iv) and a fast response time (10s). The working pH range of the sensor is 2.1-4.0 and could be successfully used in partially non-aqueous medium (up to 40% v/v). It exhibited good selectivity over anumber of cations and had a lifetime of more than 5.months. It could be used as an indicator electrode for the end point determination in the potentiometric titration of Zn2+ against EDTA. The membranes of 5,10,15,20-tetrakis(4-methoxyphenyl)porphyrinatocobalt (V) having the composition V, NaTPB and PVC in the ratio 2:1:100 showed the best response for Co2+ ions with aNernstian response of 30.5 ±1.0 mV/decade of activity. Addition of plasticizers did not improve the response characteristics. Working concentration range for the electrode is l.QxlO^-l.OxlO'1 Min the pH range 1.9-5.8. It displayed aresponse time of 20s, life span of about 4months and 15% (v/v) non-aqueous content was tolerable without showing any considerable divergence in working concentration range or slope. The proposed electrode revealed good selectivity for Co2+ over other cations and has been successfully used as an indicator electrode for the potentiometric titration of Co2+ against EDTA as well as for its determination in battery wastes. The results obtained agreed well with those obtained from AAS and ICP-AES. The ionophore Vwas also used for fabricating asensor that can sense molybdenum as molybdate (MoO/% The membrane having V:DBP:PVC in the ratio 12:100:60 (w/w) exhibited aworking concentration range of 5.0xl0"5-1.0xl0 Mwith anear-Nernstian slope of 32.0 ±1.0 mV/decade of activity of Mo042". The electrode exhibited wide PH range of 5.4-10.5 and good reproducibility (v) with a useful lifetime of 4 months. The membrane worked satisfactorily in partially non-aqueous medium up to a maximum 25% (v/v) non-aqueous content and showed moderate selectivity. Membranes of 4-terf-butylcalix[4]arene (VI) have been explored for developing a Pb2+ sensor. The best performance was shown by the PVC based membranes of 4-terf-butylcalix[4]arene (VI) having the composition 2:1:100:200 (VI:NaTPB:DBP:PVC). The sensor showed a wide working concentration range of I.lxl0"5-l.0xl0'1 Mwith a Nernstian slope (30.0 ± 1.0 mV/decade of activity), a fast response time (lis) and a long lifetime (6 months). The working pH range of the sensor was 2.1-4.0 and it showed excellent selectivity for Pb2+ over other mono-, bi- and trivalent cations which are reported to cause interference in the working of other sensors. Its performance in partially non-aqueous medium was also satisfactory. The sensor has been successfully used as an indicator electrode in the potentiometric titration of Pb(II) against EDTA and for its determination in a battery waste. Membranes having 5,7,12,14-tetramethyldibenzotetraazaannulene (Me4Bzo2TAA) (VII) as electroactive material, sodium tetraphenyl borate (NaTPB) as an anion excluder in PVC matrix in the percentage ratio (w/w) 2:1:97 (VII:NaTPB:PVC) exhibited a linear response to Ni2+ in the concentration range 7.9xl0"6-1.0xl0"1 Mwith a slope of 30.0 ± 1.0 mV/decade of activity and a response time as fast as 15s. Addition of plasticizers in some cases improved the response time but the slopes were affected adversely. The (vi) sensor worked well in the pH range 2.7-7.6 and could be satisfactorily used in 30% (v/v) non-aqueous content. The sensor was selective for nickel over a large number of cations with slight interference from sodium and copper only. The electrode can be used over a period of 6 months and has been successfully used as an indicator electrode in the potentiometric titration of Ni2+ against EDTA as well as for its determination in some Indian brand chocolates. Results were found comparable with those obtained from AAS and ICP-AES.en_US
dc.subjectTOXIC METALSen_US
dc.typeDoctoral Thesisen_US
Appears in Collections:DOCTORAL THESES (chemistry)

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