STUDIES ON THE CATALYTIC AND ELECTROANALYTICAL APPLICATIONS OF SOME CHELATING ION EXCHANGE RESINS AND CROWN ETHERS

Abstract

Complexation of crown ethers with alkali metal ions results in enhanced ion separation and increased effective anion nucleophilicity. Such changes have been advantageously utilized to alter rates and stereochemical course of many reactions. Besides this such compounds provide a highly suitable material for the development of membrane sensors for metal ions. Investigations reported in this dissertation deal with these two aspects i.e., the measurement of catalytic activity and ion activity using some macrocyclic crown ethers. In order to investigate the best performance of a membrane sensor, it is necessary to optimize the membrane ingredients i.e. PVC, crown ethers and plasticizer, so that the membrane sensor demonstrates optimal activity in terms of selectivity, working concentration range and slope. The time and concentration of the equilibrating solution are also ascertained so that the membrane develops reproducible, noisefree and constant potentials. These investigations were performed with the membranes incorporating all the three ionophores mentioned above. The membrane sensor having 18-crown-6 as electroactive phase can estimate Ag+in the concentration range 5.40 to 10.80xl03mg dm"3 between pH 3.0 to 9.0 with a slope of 52 mV decade"1 of Ag+ concentration. The response time of the sensor is <30 s, standard deviation of twenty identical readings being + 1.0 mV. A membrane can be used continuously for a period of three months. It exhibits fairly good selectivity for Ag + in comparison to other cations.. In addition to this, the electrode assembly can also be used in partially non-aqueous medium and it can also tolerate the presence of surfactant at a level of < 5.0x10"5 mol dm3 concentration. The sensor CO has successfully been used as an indicator electrode in the potentiometric precipitation titration involving silver ions. Another compound 15-crown-5 has been used to estimate Cd2+ in a wide concentration range (0.28 to 11.24xl03 mg dm3) with a near- Nernstian slope of 27 mV per decade of Cd2+ concentration. The static response time of the assembly is < 40 s. Membrane has been used over a period of three months. Potentials remain constant between pH 2.0 to 6.0. Various alkali, alkaline and transition metal ions donot disturb the functioning of the electrode unless present in very high concentrations.. Electrode can also be used safely in partially non-aqueous medium. Small amount of the surfactant donot disturb the working of the membrane sensor. In addition, the assembly can be used as an indicator electrode in the potentiometric titration of Cd2+ with EDTA. Dibenzo-18-crown-6 is found to be a suitable matrix, in a PVC based membrane, for the estimation of Sr2+. The electrode can estimate Sr2+ in the concentration range of 0.28 to 8.76x103 mg dm3. The slope of the plot is super-Nernstian with a value of 28 mV decade"1 of Sr2+ concentration. Stable potentials are recorded in < 25 s and are reproducible within an error of + 0.5 mV. A membrane has been quite frequently tried for a period of three months without observing any disturbance. The working pH range of the proposed assembly is 2.0 to 6.0. Selectivity coefficient values for various interfering ions are quite low and these may not interfere with the working of the electrode assembly unless their concentration exceeds many fold with the concentration of Sr2+ . The proposed electrode assembly can safely be used in a partially non-aqueous medium with 30% non-aqueous content and it can also tolerate the presence of cationic surfactant at low concentration. In addition, the electrode has also been used in potentiometric titration involving Sr2+ ions. (ii) Organic resins incorporating oxime and hydroxyoxime functional groups form an important class of polymeric ligands, because of their strong chelating properties. These polymeric ligands i.e. chelating ionexchange resins are found to possess specific selectivity for some metal ions and play an important role in separation processes. As such, these compounds provide a suitable matrix for the development of membrane sensors. Among these compounds, 2-hydroxynaphthaldoxime formaldehyde (HDF) possesses good ion-exchange characteristics and chemical stability. This product exhibits the following selectivity sequence for bivalent cations: Pb2+ > Zn2+ > Cu2+ > Ni2+ > Co2+ > Pd2+ As such it was considered worthwhile to try the membranes of the resin, 2-hydroxynaphthaldoxime formaldehyde for the estimation of the metal ions mentioned in the series given above. Although the material displays selectivity in the sequence mentioned above, the electroanalytical selectivity of the membrane changes due to the application of a neutral binding material, incorporated during the fabrication of the membrane. During some preliminary studies the proposed chelating ion-exchange resin (HDF) membrane is found to be selective for Zn2+, Co2+ and Cu2+ and meet all the stiff requirements demanded of solid state membrane. As such it was thought desirable to explore the possibility of using this membrane as a sensor for Zn2+, Co2+, or Cu2+. Both polystyrene as well as PVC based membranes of the compound (HDF) were prepared and tried for the estimation of metal ions mentioned above. The relevant functional properties like water content, porosity, swelling, electrolyte absorption and conductance could be determined only in the case of polystyrene based membrane of the ion-exchange resin (HDF). The values of water content, porosity and swelling are negligibly small, thus magnifying the importance of exchange sites in the exchanger material. The electrolyte uptake of the polystyrene based membrane is also consistent with other characteristics. The conductance values of polystyrene as well as PVC based membranes in different cationic forms show no apparent gradation either with size, swelling or charge on the ion. The polystyrene based membrane having HDF as electroactive phase can estimate Zn2+ in a concentration range of 32.70 to 6.54xl03mg. dm"3 with a slope of 27 mV per decade of concentration. The response time is forty seconds and the standard deviation of twenty identical measurements is + 1.5 mV The electrode assembly can be used for a period of two months without any disturbance. The change of anion of salt solution does not affect the working of the electrode assembly. Potentials stay constant in the range of pH 3.0 to 7.0 which can be taken as the working pH range of this assembly. The low values of selectivity coeffic ient of other metal ions indicate no disturbance by the presence of interfering ions. The electrode assembly can also be used in partially non aqueous medium and can tolerate surfactant ions upto 5.0x105 mol dm3 concentration. The system is a good end point indicator for the potentiometric titration involving Zn2+ with EDTA. The same membrane (Polystyrene based HDF) has also been tried for the estimation of Co2+. Membrane sensor exhibits a linear response, estimating cobalt ions in the range of 10.49 to 5.90xI03 mg dm-' concentration with a non-Nernstian slope of 22.50 mV decade"1 [Co2+]. The static response time is fifty seconds and the standard deviation of twenty identical measurements is + 2.0 mV. The electrode assembly could only be used for a period of two months. The change of anion in salt solution IV does not affect the working of the electrode assembly. The working pH range of the membrane electrode is 2.0 to 6.0. It also works well in a partially non-aqueous medium having a maximum of 30% non-aqueous content and can also tolerate small amount of surfactant in test solution. The selectivity pattern indicates a significant interference by monovalent cations where as bi and polyvalent cations donot seem to disturb the functioning of the electrode assembly if these are present in low concentrations. The system can be used as an indicator electrode in potentiometric titration of Co2+ with EDTA. Besides this, considering the selectivity pattern of the compound, the polystyrene based HDF membrane has also been tried for the estimation of Cu2+. Membrane sensor can estimate copper in the range 31.80 to 6.36 xlO3 mg dm3 concentration with a slope of 25 mV decade"1 [Cu2+]. Static response time of the membrane sensor is forty seconds over the entire concentration range and standard deviation of twenty identical measurements is + 1.0 mV. The electrode assembly could be used for a period of four months. The change of anion does not affect the functioning of the membrane sensor and the valid pH range is 2.0 to 5.0. The sensor works well in partially non-aqueous content and also in the presence of small amount of cationic surfactant. The selectivity pattern indicates significant interference of monovalent cations if present in large concentration while bi and trivalent cations donot disturb the functioning of the electrode assembly. The sensor under investigation, can also be used as an indicator electrode in potentiometric titrations involving copper ions. A perusal of the above mentioned data reveals that the sensor exhibits a better performance for the estimation of copper in comparison to zinc and cobalt ions, however it is also possible to estimate Zn2+ as well as Co2+ with this electrode system with some restrictions. (v) Poly (vinyl chloride) based membranes of the same resin 2-hydroxynaphthaldoxime formaldehyde were also fabricated and tried for the estimation of three cations (Zn2+, Co2+ and Cu2+) in order to have a comparative assessment with those prepared with polystyrene as binder. Electroanalytical performance of the PVC based membrane of the same resin is summarized in following paragraphs: Ihe membrane sensor can estimate Zn2+ in a wide range of concentration viz; 6.54 to 6.54xl03 mg dm"3 with a slope of 30 mV decade [Zn2+]. Response time of the sensor is thirty seconds and the standard deviation of twenty identical readings is +1.0 mV. The membrane sensor can be used for a period of three months. The change of anion does not affect the functioning of the electrode assembly. Potentials stay constant in the pH range 3.0 to 7.0. The electrode assembly can be used in partially non-aqueous medium as well and can tolerate surfactant ions upto a concentration of 5.0xl05 mol dm3. Some monovalent cations may cause disturbance in the working of the electrode assembly. Polyvalent cations would normally not interfere unless present in very high concentrations. The electrode assembly can be used as an indicator electrode in potentiometric titration involving Zn2+ ions. The electrode system has also been used for the estimation of cobalt ions in a wide range of (4.43 to 5.90x103 mg dm"3 ) concentration with a slope of 28.50 mV per decade [Co2+]. The static response time, in which membrane generates stable potentials, is fifty seconds at all dilutions and have a standard deviation of +1.5 mV. The membrane sensor can be used for two months. The anions do not affect the working of the electrode assembly. Potentials stay constant in the range of pH 3.0 to 6.0. The functioning of the electrode assembly is normal in solutions having a m aximum of 15% (v/v) non-aqueous content and can tolerate detergent (vi) ions upto 5.0x10"5 mol dm'3 concentration. The electrode system is susceptible to interference by monovalent ions while other polyvalent cations donot seem to disturb the functioning of membrane sensor. Electrode assembly can be used as an indicator electrode in potentio metric titration involving Co2+ ions. Beside this, the electroanalytical behaviour of the same membrane sensor has also been observed for copper ions. It responds to copper ions in the range 3.18 to 6.36x103 mg dm'3 concentration. The static response time of the sensor is < 10 s and the standard deviation of twenty identical measurements is +1.0 mV. The change of anion does not affect the working of the electrode assembly. The functional pH range for the membrane sensor is 1.8 to 5.3. It can also be used in partially non-aqueous medium having a maximum of 30% (v/v) non-aqueous content and can tolerate surfactant ions upto a concentration of 5.0x105 mol dm3. As usual, here also the monovalent ions register some interference while the polyvalent cations would not cause any disturbance at all. The assembly has been successfully used as an end point indicator in the potentiometric titration of copper ions with EDTA. A comparative evaluation of the PVC based membrane of the resin, 2-hydroxynaphthaldoxime formaldehyde, as a sensor for the estimation of zinc. Cobalt or copper ions reveals the best response and selectivity for copper in comparison to zinc and cobalt ions. It is also observed that the electroanalytical performance of both the PVC as well as polystyrene based membrane is much suitable for copper amongst the three cations under investigation. Out of the two types of membranes of HDF resin (using different binding material) a much better performance is recorded in the one fabricated with PVC as binder material. (VII Crown ethers have been used as complexing agents and phase transfer reagents to facilitate a large number of reactions. They function by selectively capturing metal ions in their cavities by electrostatic forces. Crown ployethers and their ability to complex metal salts aroused research interest due to the possibility of increased salt solubility and increased anion reactivity in organic solvents. Potassium permanganate is insoluble in all kinds of organic solvents, but in the presence of crown ethers it becomes soluble in a non-polar solvent such as benzene. In the organic phase the resulting product exists as an ion pair and has been, effective in various oxidation processes. Present work deals with the oxidation of some dicarboxylic acidsby permanganate ion in acidic medium which has been studied spectrophotometrically under pseudo-first-order conditions both in presence and in absence of the catalysts (crown ethers). Higher values of pseudo-first-order rate constants have been recorded in the presence of a crown ether. The effect of the concentration of acid, permanganate ion and temperature has been studied. The various activation parameters (energy of activation, enthalpy of activation, free energy of activation and entropy of activation) for the oxidation of (oxalic, malonic, succinic and adipic acid), with permanganate ion in acidic medium have also been determined. A comparative evaluation of these parameters both in presence and absence of catalysts (18-crown-6 & 15-crown-5) suggest that crown ethers alter the reaction rate and a combination of various factors (strength, structure and size of the dicarboxylic acids, size of crown ethers) affect the reactivity to a great extent.