ELECTROANALYTICAL STUDIES WITH MEMBRANE SENSORS BASED ON SOME MACROCYCLIC COMPOUNDS

Abstract

The development of high performance ion-selective electrodes (ISEs) is a fast growing area. These devices find applications in the analysis of raw materials, in quality control of the products, monitoring of environment and numerous other situations due to fast speed, sensitivity, cost, reliability and non-consumption of sample in the process. ISEs are specially useful for field applications and also in biological studies where a large number of samples need a rapid, cheap and reliable method of analysis. Besides this, the device can be used irrespective of colour, viscosity, etc., of the test solution and almost no sample preparation is necessary. Despite the major advancement made in this field, efforts have continuously been directed to explore the more selective ligands and there have been relatively few notable advances. The best ion selective electrode is undoubtedly the pH glass electrode, which has been in general use for over sixty years. This is followed by the fluoride electrode which is by far the most successful ISE for anion. Thereafter a range of cationselective electrodes based on neutral carriers, polymeric resins and inorganic gels have been reported, but very few have been commercialised so far. The development of highly selective membrane sensors based on neutral carriers has been a major breakthrough in the field of ion-selective electrodes. These compounds have been used as electroactive phase in membrane sensor for a variety of substances ranging from inorganic to organic species. Many reports have appeared on the neutral carrier based membranes after the successful application of valinomycin as potassium-selective electrode. Among the various neutral carriers, macrocyclic compounds are the matrix of choice and crown ethers are the most sought after substances for the fabrication of ISEs. Crown ethers, in addition to their use as potential extractants for various species, have also been widely exploited as suitable ionophores in ISEs. Crown ethers (0 generally function on the basis of 'hole size concept' i.e., a strong complexation is expected between crown and metal when both are of matching size. Based on this concept, some plasticized PVC membranes of crown ethers viz. benzo-15-crown-5, 15-crown-5, 12-crown-4 and dibenzo-24-crown-8 have been investigated for the estimation of Cd2+, Pb2+, Cu2+ and Cs+, respectively. Before going into the detailed investigations with a membrane sensor, the amount of membrane ingredients was optimized first so that it demonstrates best performance in terms of detection limit and slope. Also the time and concentration of equilibrating solution were also ascertained so that the membrane develops reproducible, noise free and stable potentials. These investigations were performed with all the membranes included in this dissertation. 1 + The membrane sensor based on benzo-15-crown-5 can estimate Cd in the concentration range 3.16xl0"5 to l.OOxlO'M between pH 3.8 to 7.0 with a slope of 20 mV per decade of Cd2+ concentration. The response time of the sensor is <30 s, standard deviation of 20 identical measurements being ±1.0 mV. A membrane can be used continuously for a period of two months. Selectivity of a sensor is an important characteristics which clearly establishes the superiority of the sensor for Cd in comparison to other ions. In addition to this , the electrode can also be used in partially non-aqueous solution and can tolerate the presence of cationic and anionic surfactant at a level of =sl.0xl0~5M concentration. The electrode has successfully been used as an indicator electrode in the potentiometric titration involving cadmium ions. An electrode based on 15-crown-5 has been used to estimate Pb2+ in a wide concentration range (2.51xlO"5-1.00xlO"1M) with a Nernstian slope of 30 mV per decade of Pb + concentration. The static response time of the assembly is 30 s. The membrane has been used over a period of six months. Potentials remain constant between pH 3 to 6. Various alkali, alkaline and transition metal ions do not disturb the functioning of the electrode unless present in very high concentrations. Electrode also can be used safely in partially non-aqueous medium. More than 1.0xl0"5M concentration of surfactants register some interference in the functioning of the electrode assembly. (i>) In addition, the electrode has been used as an indicator in the potentiometric titration of Pb2+ with EDTA and also in the analysis of Pb2+ in a local river, Hindon. The results obtained with the help of this membrane sensor correlate well with those obtained by other sophisticated instruments like AAS and ICP, thereby reflecting the utility of the electrode, under investigation. 12-Crown-4 is found to be a suitable material, in a PVC based membrane, for the estimation of Cu2+. The electrode can estimate Cu2+ in the concentration range 1.78xl0"5 to l.OOxlO'M. The slope of the plot is super-Nernstian with a value of 50 mV per decade of Cu2+ concentration. Stable potentials are recorded in <30 s and are reproducible within an error of ±1.0 mV. A membrane can be used for more than six months between the pH range 3 to 6. Selectivity coefficients for various interfering ions are quite low, when calculated through the fixed interference method using the expression without superscript. The electrode can also be used in a solvent having 25% non-aqueous content and it can tolerate the presence of cationic surfactant at low concentrations. In addition, the electrode has also been used in the potentiometric titration involving copper(II) ions. The last membrane sensor incorporating a crown ether is based on dibenzo-24- crown-8 and the same has been tried for the estimation of Cs+ in a wide concentration range O.SxIO'M.OxIO'M) between pH 3 to 9. The slope of the concentration vs potential plot is near-Nernstian in nature (50.7 mV per decade of Cs+ concentration). The response time of the electrode is quite fast (<10 s). Potentials are reproducible within an error of ±0.2 mV. The membrane has been used over a period of six months without observing any appreciable drift in potentials. Selectivity coefficient values for this electrode system are quite low for various interfering ions in comparison to determinand ion, Cs+. The electrode can safely be operated in presence of 25% non aqueous medium and can be used as an indicator electrode in the potentiometric titration of Cs+ with phosphotungstic acid. It is generally believed that the replacement of oxygen by nitrogen and sulfur in crown ethers called aza- and thia-crown ethers, respectively, do not change the cavity (iii) dimension of the macrocycle (however conformations may change), but due to inherently different donor properties of oxygen, nitrogen and sulfur, the aza- and thia-crowns favour complexation with transition metal ions in comparison to alkali or alkaline earth metal ions. It is also quite well known that the substrates used as electroactive phase for the fabrication of sensors for polyvalent ions suffer with sensitivity and selectivity problems and with the availability of a variety of new suitable compounds, this aspect calls for more exhaustive investigations. Membrane sensors reported for uranyl ions suffer with strong interference from iron and these electrodes can only be operative at and around pH 3.0. In these investigations the compounds monoaza crowns, such as monoaza-12-crown-4 and monoaza- 15-crown-5 have been used to develop electrodes for the estimation of La and U02 , respectively. An electrode based on monoaza-12-crown-4 can estimate La down to 3.16x10 M |La3+]. The slope of potential vs concentration plot is super-Nernstian in nature (50.7 mV per decade of La3+ concentration). The response time of the electrode is -50 s and reproducible to ±0.4 mV. A membrane has been used for a period of five months without observing any noticeable change. The electrode is tolerant of pH change in the range 3 to 7. It functions well in presence of a large number of metal ions unless these are present in very high concentrations. The electrode can also be used in partially non-aqueous medium and in presence of surfactants at lower concentration. The system is good end point indicator for the potentiometric titration involving La with EDTA. The plasticized PVC membrane of monoaza-15-crown-5 candetermine U02 between the concentration range 1.26X10"4 to l.OOxlO'M [UO2/]. In this case also, a super- Nernstian slope (50.9 mV per decade of [U02+]) is observed. The response time is reasonably fast (-20 s) for this electrode and potentials are reproducible to ±1.2 mV. A membrane can continuously be used for a period of two months after which a slow divergence, in potentials, is recorded. (iv) As mentioned above the pH of solutions is very important in the case of U02 responsive electrode. As such, very careful observations were made to known the working pH range of the assembly. It was observed that potentials remain constant between pH 3.0 to 3.8 and therefore same is taken as the working pH range of the system. A large number of cations do not interfere in the working of the assembly and low values of selectivity coefficient are obtained for various metal ions in comparison to U02+. A compound containing sulfur as donor group generally form strong complex with mercury(II) ions and also the cavity size of Hg matches to that of pentathia-15- crown-5. As such, it was thought desirable to use the latter as an ionophore in Hg2+-selective electrode. The electrode can determine Hg in a wide concentration range (2.51x10"' to l.OOxlO'M), with a slope of 32.1 mV per decade of Hg2+ concentration, the response time is *30 s and the standard deviation of 20 identical measurements is ±1.0 mV. In this case also the membrane could be used for a period of forty days without observing any appreciable drift in potentials. Potentials of 1.0x10" M Hg + solution remain constant between pH 2.7 to 5.0 and the same is taken as the working pH range of the assembly. Potentiometric selectivity coefficients for a large number of interfering ions are generally low, except Cd2+ which interferes strongly with the working of this assembly. The electrode can successfully be operated in 25% non-aqueous medium. The assembly can also tolerate the presence of s2.0xlO"5M cationic surfactant. Besides above, electroanalytical studies with membranes of 2.2.2-cryptand have also been undertaken. 2.2.2-cryptand is a macrobicyclic compound where the addition of a third donor strand to crown makes it a three dimensional network with a hydrophilic cavity to hold the cation within itself. Although cryptands are generally better extractant than crowns, the membrane properties of these compounds have not been explored extensively. As such efforts have been made to use 2.2.2-cryptand as an •y i ionophore, in plasticized PVC membrane, to estimate Zn and for a comparative assessment of its utility the same has also been tried for the estimation of Cd . (v) The electrode can estimate Zn2+ in a concentration range 3.16xl0"5 to l.OOxlO'M with a slope (potential vs concentration) of 22 mV per decade of Zn2+ concentration. The electrode response is very fast (<10 s), stable (5 min) and reproducible within an error of ±0.5 mV. The membrane has been used over a period of two months and no 3 2+ drift in potentials was observed during this period. Potentials of 1.0x10" M Zn solution remain constant in a wide pH range (2.8 to 7.0). A sharp change in potentials at low pH may be the result of protonation of cryptand in the membrane phase thus losing its capability to hold metal ions within its cavity. Selectivity of the 2 + electrode for Zn in presence of other interfering ions has also been tested. It was observed that monovalent and polyvalent ions record almost no interference in the working of this assembly, unless present in very high concentrations. Electrode can tolerate the presence of 25% (v/v) organic content and 1.0xl0"5M concentration of cationic and anionic surfactant, however, a higher amount of these species register some interference in the functioning of the assembly. Besides this, the membrane can 2+ also be used as an indicator electrode in the potentiometric titration of Zn with EDTA. The membrane based on 2.2.2-cryptand has also been tried for the estimation of Cd2+. It can estimate Cd2+ in the concentration range 3.55xl0"5 to l.OOxlO^M at pH 3.7 to 7.1 with a slope of 28.5 mV per decade of Cd2+ concentration. The response time in this case is -30 s and potentials are reproducible within an error of ±1.2 mV. The electrode can be used in 35% non-aqueous medium and has also been used in the potentiometric titration involving Cd2+. However the selectivity feature of a membrane electrode which dictates the commercial potential of a sensor, clearly establishes the superiority of this membrane electrode (based on 2.2.2-cryptand) for the estimation of Zn + as compared to Cd2+.