Studies And Application Of Metal Ion-Selective Electrodes

The metal elements usually exist widely in nature with natural concentrations. But as mining, smelting, processing and commercial manufacturing activities increased, lots of metals such as lead, mercury, chromium, cobalt, etc. are discharged into the environment. Metals are the essential life elements of growth and development of aquatic biology and human. However, they will result in strong toxicity to the body when their concentration exceeds a certain limit. Therefore, the determination of metals in environmental samples is very important in the environmental analysis. At present, the main conventional methods for determination are flame and graphite furnace atomic absorption spectrometry, inductive coupled plasma atomic emission spectrometry, inductive coupled plasma mass spectrometry, etc. But these methods involve multiple sample manipulations and costly expenses for most analytical laboratories. Thus, it is necessary to develop a convenient method to determine metal ions. Ion selective electrodes (ISEs) that offer several advantages, such as adequate selectivity, wide linear range, low detection limit, fast response time, especially easy preparation and low cost, have inevitably been utilized for preparing sensors for several ionic species and the quantity of available electrodes has grown significantly over recent years. Nowdays, studies on the carrier of the ion selective electrode is a very active direction on the studies of the ion selective electrodes. In this thesis, a series of complexes are synthesized and used as carriers of the ion selective electrodes. The performance of the ion selective electrode is characterized by electrochemical methods and the proposed electrodes are applied to the detection of metal ions in real samples.1. Studies on the N-S Schiffbase ligand application on Ni (II) ion selective electrodeNi (II) ion-selective electrode is prepared by incorporating a new N-S Schiff base ligand, glyoxal-bis(S-benzyldithiocarbazate)(GBSB) as neutral carrier into the PVC matrix. The proposed electrode exhibits an excellent near-Nernstian response for Ni2+ion ranging from 2.8×10-7 to 1.0×10-1 mol/L with a detection limit of 1.2×10-7mol/Land a slope of 31.9±0.3 mV/dec in pH 4.0 nitrate solution at 25℃. It has an appropriate response time and suitable reproducibility, and can be used at least three months. The operational pH range of the proposed electrode is 4.0-7.5. In preliminary experiments, the complexation of GBSB with some cations was investigated conductometrically in a DMSO solution in order to obtain a clue about the stability and selectivity of the resulting complexes. The change in conductance of Ni2+ was larger than that for other cations. And the response mechanism is discussed in view of the alternating current (AC) impedance technique. In addition, the electrode is successfully used as an indicator electrode in potentiometric titration of Ni2+ion and in the direct determination of Ni2+ion in milk power and chocolate samples.2. Studies on a new aryl amide bifunctional bridging ligand application on Cr (III) ion selective electrodeA novel Cr(III) ion-selective electrode is constructed by incorporating a new aryl amide bifunctional bridging ligand,2,2'-bis{[(2"-benzylaminoformyl)phenxoyl]methyl}-diethylether (BBPMD) as a neutral carrier into the PVC matrix. The proposed electrode, with optimum membrane composition, exhibits an excellent near-Nernstian response for Cr3+ion ranging from 2.8×10-6 to 1.0×10-1 mol/L with a detection limit of 8.6×10-7mol/Land a slope of 19.5±0.2 mV/dec in pH 3.0 nitrate solution at 25℃. It has an appropriate response time, suitable reproducibility, and good selectivity towards Cr3+ion. The operational pH range of the proposed electrode is 2.5-6.5. The coordination number and association constant of coordination reaction of the carrier towards different metal ions were measured by means of UV-vis spectra titration. The absorbance increases gradually and the peak has a considerable red shift. The A. C. impedance technique was used researching the electrochemical characteristics on the electrode surfance. The excellent analytical features of the proposed electrode could lead to its successful application as an indicator electrode in potentiometric titration of Cr3+ion and in the direct determination of Cr3+ion in tea leaves and coffee samples.3. Studies on macrocyclic tetrabasic ligand application on barium (II) ion selective electrodeIn this study, a novel barium (II) ion-selective polymeric membrane electrode based on macrocyclic tetrabasic ligand (H4L) that derived from the reaction of 4,6-diacetylresorcinol and ethylenediamine in the molar ratio 1:1 as a neutral carrier was described. The electrode containing 2.04 wt% ionophore,66.40 wt% o-NPOE,31.23 wt% PVC and 0.33 wt% HTAB displayed excellent potential response characteristics to Ba2+ion ranging from 3.6×10-6 to 1.0×10-1 mol/L, with a detection limit of 1.9×10"6 mol/L and a slope of 29.7±0.2 mV/dec in pH 4.0 nitrate solution at 25℃. The response time was about 10 s for Ba2+ion over the entire concentration range and the potentials stayed constant for more than 2 min. And the proposed electrode could be used two months and didn't show visible loss of response characteristics. The potential remains constant at pH range 2.5-7.5. The selectivity coefficients of Ba2+ion selective electrode were determined by the separate solution method (SSM). The electrode is selective towards Ba2+ion over a number of other cations except for Ni2+ion, as the selectivity coefficient value is slightly higher. To know the exact concentration of Ni2+ion that can be tolerated in the determination of Ba2+ion, some mixed run studies were carried out and the proposed electrode could tolerate Ni2+ion at the concentration≤5.0x10-5mol/L. The complexation of H4L with some metal ions was investigated conductometrically in a DMSO solution in order to obtain a clue about the stability and selectivity of the resulting complexes. Then, the response mechanism was discussed in view of UV-visible spectroscopy and the A.C. impedance technique. Finally, the proposed electrode was successfully applied to the detection of Ba2+ions in real samples as well as sulphate (Ⅱ) ions in water. The statistical comparison for the results of the proposed method and the reference one was carried out with regard to precision and accuracy by using F-test and t-test.4. Studies on the double-cavity dioxotetraamine ligand application on Al(III) ion selective electrodeThis work describes the fabrication of a selective polymeric membrane potentiometric sensor for Al3+ion based on N, N'-propanediamide bis (2-salicylideneimine) (NPBS) as a neutral carrier. The proposed sensor, with optimum membrane composition, exhibited an excellent near-Nernstian response for Al3+ion ranging from 7.9×10-7 to 1.0×10'1 mol/L with a detection limit of 4.6×10-7 mol/Land a slope of 19.4±0.3 mV/dec in pH 3.0 nitrate solution at 25℃. It showed a relatively fast response time (8 s), suitable reproducibility and stability, and good selectivity towards Al3+ion. The operational pH range of the proposed sensor was 2.5-4.0. The excellent selective response of the proposed electrode towards Al3+ions mainly resulted from the coordinate interaction between the carriers and Al3+ions. To confirm the interaction between the carriers and Al3+ions, the UV-visible spectroscopic experiments were carried out. The coordination number and association constant of coordination reaction of the carrier towards different metal ions were measured by means of UV-visible spectra titration. And the alternating current (A. C.) impedance technique was used researching the electrochemical characteristics on the electrode surfance. In addition, the proposed sensor was successfully used as an indicator electrode in potentiometric titration of Al3+ ion and in the direct determination of Al3+ion in real samples.