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|Title:||ELECTROANALYTICAL STUDIES ON ION-SELECTIVE SOLID MEMBRANE ELECTRODES FOR SOME HEAVY METALS|
|Authors:||Singh, Lok Pratap|
|Keywords:||CHEMISTRY;ELECTROANALYTICAL STUDIES;ION-SELECTIVE SOLID MEMBRANE ELECTRODES;HEAVY METALS|
|Abstract:||The utility of ion-selective electrodes (ISEs) is being increasingly realized in viewof the rapid growth ofindustry andtechnology all overthe world. As suchtheyhave been the subject of widespread interest for analytical chemists as they permit rapid, accurate and low-cost analysis. Moreover, analysis by these electrodes could be nondestructive and adaptable to small sample volumes. ISEs find applications in various industrial and biologically important fields viz., in ion-monitoring and in the analysis of sea water, soils, nuclear fuels, industrial effluents and pharmaceutical compounds. In many frequently performed analysis, such as determination of nitrate in drinking water or sodium ions in boiler feed-waters, the use of ISEs has made feasible measurements which are otherwise difficult. In view of the importance of ISEs in diverse fields, extensive efforts have been initiated during the last two decades to develop electrodes for various ions. These electrodes consist of solid or liquid membrane which when placed across the two appropriate solutions, allows the transport of a particular cationic or anionic species. As a result of restricted transport of the ions, a potential difference across the membrane is developed which is used for the determination of ionic concentration. As such those materials which favour selective transport are used as electroactive components in the membranes. Various materials tried for this purpose include solid electrolytes, inorganic and organic ion-exchangers, metal chelates, macrocycles, crown ethers, cryptands, calixarenes etc. As a result of intensive efforts in this field, ISEs are now commercially available for hydrogen, ammonium, alkali, alkaline earth and few heavy metal cations and halides, CN", NO3, S2", S042", POj" etc. among anions. Literature survey reveals that suitable ISEsfor a number ofheavy metals have not yet been developed and efforts are, therefore, needed in this direction. With the availability of highly selective materials, the possibility of developing selective elec trode systems for these metals hasopened up. Therefore, efforts were initiated using the membranes ofselective inorganic and organic ion-exchangers, crown ethers and - i - macrocycles to develop electrodes for heavy metals. The results of these investigations are briefly summarized as under: The estimation ofcadmium, copper, chromiumand mercury is importantas they occur in various effluents and are toxic above certain concentration level. A number of ISEs are reported in literature for the estimation of these metals but they have limitations mainly with regard to selectivity and working concentration range. It was therefore, thought desirable to prepare new electrodes for these metals which may show better selectivity. The literature survey revealed that inorganic ion-exchangers, cerium(IV) vanadate, cerium(IV) phosphomolybdate and basic lead sulphate show highly selective exchange for Cd2+, Cu2+ and CrO2" while cerium(IV) selenite and antimony(ll I) arsenate exhibit selective exchangefor Hg2+ ions. Thus, it wasexpected that membrane of these ion-exchangers may act as good ISEs for the ions for which the exchangers are selective. As a result, heterogeneous membranes of these exchangers were prepared using polystyrene as binder. The optimum composition of the membrane (polystyrene-exchanger ratio) was determined by trial. The composition which gave the membranes of adequate mechanical stability and good electroanalytical performance was taken as optimum composition. Generally, 10 - 20% polystryrene content was found optimum for preparing membranes of various exchangers. These were prepared at 80 - 95 °C under a pressure of 440-475 a\m. (6500-7000 psi) in a metallurgical specimen mount press. After the preparation of membranes the functional properties viz., water content, swelling, porosity and electrolyte absorption of the membranes were determined and found to be small indicating that transport through the membranes occur mainly through the exchange sites. Thus, the heterogeneous membranes of cerium(IV) vanadate, cerium(IV) phosphomolybdate and basic lead sulphate were used for developing selective electrodesfor Cd2+, Cu2+ and CrO2" ions and ofcerium(lV) seleniteand antimony(lll) arsenate for Hg2+ ions. The electrodes show linear potential response generally in the concentration range - 10"5 - 10"1M with Nernstian/near-Nernstian slope and a response time of less than one minute. All these membrane electrodes also work satisfactorily in partially non-aqueous medium and have sufficiently long life time which varies from two months to one year for different electrodes. Electrodes were sufficiently selective and used to determine the end-point in the potentiometric titrations. CrO2" -selective electrode showed enough selectivity and was used for the estimation of chromium (as chromate) in the waste from electroplating industry. The results obtained were in good agreement with those obtained by atomic absorption spectrophotometer. These electrodes are comparable to the existing electrodes in terms of working concentration range, Nernstian slope, pH range and life time but are superior in respect of fast response time and selectivity over certain cations. Thus, the above results show that the membranes of inorganic ion-exchangers have provided selective electrodes for Cd2+,Cu +, Cr04" and Hg2+ ions. Organic ion-exchangers are the another class of electroactive compounds which have been explored for developing some new electrodes. Ion-exchangers, 2,4-dihydroxypropiophenoneoxime-formaldehyde (2,4-DPPO-F) and 2-hydroxyacetophenoneoxime-thiourea-trioxane (HAOTT) exhibit highly selective exchange for UO§+ ions while 1-(2'-aminoaryl)-4,4,6-trimethyl-1,4,5,6-tetrahydro-6- hydroxypyrimidine-2-thiol (HPT) shows selective extraction for Hg2+ ions. Therefore, they appear to be suitable electroactive materials for preparing membranes which are expected to respond selectively to U02+ and Hg2+ ions. The homogeneous membranes of these exchangers were not mechanically stable and therefore, heterogeneous membranes were prepared using PVC as binder. The binder and the exchanger were mixed in different ratios and preliminary investigations with the membranes were carried out to know the best composition which impart good stability as well as better electroanalytical properties. In addition to these, the membranes were also prepared by adding plasticizers, dibutyl phthalate (DBP) and dioctyl phthalate (DOP). It was generally found for all the exchangers that PVC based membrane containing plasticizer performed better as compared to membranes without - iii - plasticizer. Thus, the PVC based membranes (without plasticizer) of 2,4-DPPO-F and HAOTT' work in the concentration range 5.0x10"5 -1.0x10*1 and 5.0x10"4 -1.0x10"1M for U02+ ions with a slope of 34.0 and 36.0 mV per decade of concentration, respectively, while on the addition of plasticizer the working concentration range is ehanced (1.0x10"5 - 1.0x10"1M) and slope also improved. The electrodes having membranes (without plasticizer) exhibit a steady response of -120s while the elec trodes with plasticizers show a fast response time of ~30s. These electrodes have beenfound to be sufficiently selectiveto UO2/ ionsovera largenumberofmonovalent, bivalent and trivalent cations. They also perform satisfactorily in partially non-aqueous medium and have a life time of about three months. Similarly, Hg2+-selective electrodes were prepared using HPT as electroactive material. By changing substituent R(-H, -CH3, -OCH3) three HPTs (HPTa, HPTb and HPTc) were synthesized and their PVC based membranes (with and without plasticizers) were fabricated. Electrodes with membranes of HPTs (without plasticizer) show linear potential response to Hg2+ ions in the concentration range -5.0x10"4 - 1.0x10'1M with a near- Nernstian slope of - 25.5mV per decade of concentration while the electrodes having plasticizer in their membranes exhibit linearity in the concentration range - 1.0x10'5 - 1.0x10"1Mwith a Nernstian slopeof -28.5mVperdecade of concentration. Electrodes without plasticizer exhibit a response time of - 75 - 90s while electrodes with DBP and DOP added to their membranes, show a fast response time of - 20 - 30s. These electrodes work well in partially non-aqueous medium, have useful lifetime of three months andexhibit good selectivity for Hg2+ over bivalent andtrivalent cations. Among monovalents, Ag+ and NHJ cause maximum interference and their concentration which can be tolerated over the whole working concentration range has been determined. Recent studies have shown that in addition to ion-exchangers, solid electrolytes, insoluble salts of multivalent metals etc., crown ethers and macrocycles can also be potential source for use as electroactive component in the membranes of ISEs because oftheir unique property ofselective complexation with metals. Wehave therefore, used crown ethers and macrocycles to develop electrodes for UO2*, Cu + and Ni2+ ions. The crown ether, dicyclohexano-18-crown-6 is known to extract U02+ ions selectively. Therefore, we have explored its membrane as ISEs for U02+ ions. As such, PVC based membrane containing 10% dicyclohexano-18-crown-6 shows linear potential response to U02+ ions in the concentration range 5.6x10"4 -1.0x10"1M with a slope of36.0 mV per decade ofconcentration. Further, the electrodes prepared with the same crown ether after the addition of plasticizer DBPand DOP also respond to U02+ ions but are better in all respects compared to the electrode prepared using membrane without plasticizer. The addition of plasticizer enhances the working concentration range, brings the slope nearer to Nernstian value and reduces the response time. All the electrodes were satisfactory in partially non-aqueous medium and showed high selectivity for U02+ ions over a large numberofcations. In addition to crown ether, two macrocycles, 5,7,7,12,14,14,-hexamethyl-1,4,8,11- tetraazacyclotetradeca-4,11 -dienediperchlorate and 3,5,7,7,10,12,14,14-octamethyl- 1,4,8,11-tetraazacyclotetradeca-4,11-diene diperchlorate have also been used as electroactive materials to prepare Cu2+ and Nonselective electrodes. Theelectrodes for Cu2+ and Ni2+ ions were prepared by using PVC based membranes of the two macrocycles. They show linear potential response to Cu2+ and Ni2+ ions in the concentration range-1.0x10"4-1.0x10"1Mwith a near-Nernstian slopeof- 25.0mV per decade of concentration. The addition of plasticizer, DBPand DOP to the membranes enhances the working concentration range of the electrodes to - 1.0x10"5 -1.0x10"1M with a slopeof-28.0mVperdecade ofconcentration. Theelectrodeshave been found to be sufficiently selective compared to a large number of monovalent, bivalent and trivalent cations. They could also be used to determine the end-point in the potentiornetric titrations of Cu2+ and Ni2+ ions. Lastly, polystyrene based membranes of these two macrocycles were also prepared and the electrodes based on these membranes were investigated as Ni2+-selective electrodes. They show linear response to Ni2* ions in the concentration range -5.0x10"6 - 1.0 x 10"1M with a Nernstian slope of - 28.5 mV per decade of concentration. As compared to PVC based membrane electrodes ofthe two macrocycles for Ni2+ ions, polystyrene based membrane electrodes are better in terms of wider working concentration range but have high response time. These electrodes are comparable to the electrodes reported in literature in terms of working concentration range, slope, response and life time but are superior with regard to selectivity over certain cations particularly, Ag+ and Hg2+ for Cu2+-selctive electrodes and Co2+ and Cr3* for Ni2+-selective electrodes.|
|Research Supervisor/ Guide:||Gupta, V. K.|
Jain, A. K.
|Appears in Collections:||DOCTORAL THESES (chemistry)|
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