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Electrochemical Determination of Perchlorate Ion By Polymeric Membrane and Coated Graphite Electrodes Based on Zinc Complexes of Macrocyclic Ligands
The quick determination of trace quantities of ionic species, by simple methods, is of special interest in analytical chemistry. Ion selective electrodes based on solvent polymeric membranes with incorporation of ion carriers are shown to be very useful tools for chemical, clinical and environmental analyses as well as in process monitoring [1-3]. In addition to the cation-selective electrodes, recently the design and synthesis of sensory molecules for anion-selective electrodes has become a challenging subject [4]. For a truly anion-selective electrode, a strong interaction between the ionophore and the anion is required in order to complex anion in a selective fashion. Potentiometric response of the membranes doped with these complexes is believed to be based on the coordination of analyte anion with carrier molecule.
The electrode characteristics and selectivities of PVC-based perchlorate selective coated graphite electrode (CGE) and polymeric membrane electrode (PME) incorporating the synthesized zinc complexes of 6,7:13,14-Dibenzo-2,4,9,11-tetramethyl-1,5,8,12-tetraazacyclotetradecane-1,4,6,8,11,13-hexaene (I1) and 6,7:13,14-Dibenzo-2,4,9,11-tetramethyl-1,5,8,12-tetramethylacrylate-1,5,8,12-tetraazacyclotetradecane-6,13-diene (I2) are reported here. Several membranes having different compositions of PVC, plasticizers, ionic additives and ionophores were fabricated and the best response was observed for the membrane having composition I2: PVC: BA: HTAB in the ratio of 7: 32: 59: 2 (w/w; mg). The PME shows good performance characteristics in terms of detection limit of 5.4 × 10−7 mol L−1, working concentration range of 8.3 × 10−7 to 1.0 × 10−2 mol L−1 and slope 58.7 ± 0.3 mV decade−1 of activity. Thus the CGE with same membrane composition was prepared and its potentiometric response characteristics were compared. The electrode exhibits Nernstian slope for perchlorate ions over wide concentration ranges i.e. 8.3 × 10-7 to 1.0 × 10-2 mol L-1 (with PME) and 1.0 × 10-7 to 1.0 × 10-2 mol L-1(with CGE) and response time of 12 s and 9 s for PME and CGE respectively. The proposed electrodes generated constant potentials in the pH range of 3.0-8.0 for PME and 2.5-9.0 for CGE. The high selectivity of CGE for perchlorate ions permits its use in the determination of perchlorate ions in various samples such as water and human urine samples.
Fig. 1. (a) Structure of zinc complex of 6,7:13,14-Dibenzo-2,4,9,11-tetramethyl-1,5,8,12-tetraazacyclotetradecane-1,4,6,8,11,13-hexaene (I1) (b) Structure of zinc complex of 6,7:13,14-Dibenzo-2,4,9,11-tetramethyl-1,5,8,12-tetramethylacrylate-1,5,8,12-tetraaza-cyclotetradecane-6,13-diene (I2).
Table 1. Response characteristics of the perchlorate ion-selective electrodes based on PME and CGE
Properties |
Potentiometric Response |
|
PME |
CGE |
|
Optimized membrane composition (w/w; mg) |
(I2: PVC: BA : HTAB) ≡ (7: 32: 59: 2) |
(I2: PVC: BA : HTAB) ≡ (7: 32: 59: 2) |
Working concentration range ( mol L-1) |
8.3 ´ 10-7 - 1.0 ´ 10-2 |
1.0 ´ 10-7– 1.0 ´ 10-2 |
Detection limit ( mol L-1) |
5.4 ´ 10-7 |
8.4 ´ 10-8 |
Slope (mV decade-1 of activity) |
58.7 ± 0.3 |
59.3 ± 0.2 |
Response time (s) |
12 |
9 |
Life span |
30 days |
65 days |
pH range |
3.0 - 8.0 |
2.5 - 9.0 |
