INTRODUCTION:
Amongst the various analytical techniques available, use of ion selective membrane electrode is well established routine analytical technique. Advances in analytical industry in terms of better selectivity, fast response and long life has prompted a new search for better ionophores possessing electrical neutrality, lipophilic character, capability to select and reversibly bind metal ions to induce a selective permeation of metal ions through the membrane electrode1-3. In the past extensive methods have been made to improve the selectivity of Ag+ ion selective electrode. Thus, many cyclic and cyclic macromolecule have been suggested as electroactive ionophores e.g. non-macrocyclic compounds4-9, macrocyclic molecule compounds10, polyamine11 and metalloporphyrin12. The present paper deals with schiff base 5-MeOSalps based ISE for determination of Ag+. The Ag+ selectivity may be due to the electrostatic interaction between the metal ion, the aza crown cavity and the Schiff base. This sensor has been proved to be superior compared to some of those electrodes reported earlier in terms of a wide concentration range, better response time and longer life span.
Because Macromolecule ligands form selective and stable complexes with the metal ion of compatible dimension13 and can potentially be applied to their selective separation and determination14, continuous interest has been focused on the design and synthesis of new functionalized macro molecules for specific applications15.
EXPERIMENTAL:
REAGENTS AND CHEMICALS:
All reagents used in this were of analytical reagent grade. Most of the salts were purchased from E. Merck and were used as received. High molecular PVC powder and diamines like o-phenylenediamine, 2,6-diaminopyridine was purchased from Sigma Aldrich. Glyoxyl (40% aqueous solution), benzyl, semi-carbazide hydrochloride were purchased from CDH. Various plasticizers like benzyl acetate (BA), tetrakis- (4-chloro phenyl borate) (KTK) were taken from Fluka and 2-nitrophenyloctylether (o-NPOE) and sodium tetra phenyl borate (NaTPB) used were from Merck. Triply distilled water was used throughout. Ethanol was refluxed and distilled over lime before use.
SYNTHESIS OF ELECTROACTIVE MATERIAL:16
A solution of 5-methoxysalicylaldehyde (2.45 g, 16.1mmol.) in ethanol (10ml) was added to a solution of 2-aminophenyldisulphide (2.00 g, 8.05 mmol) in ethanol (50 ml) with stirring. The mixture was refluxed for 2 hrs. and the yellow precipitates of 5-MeOSalps was filtered off and recrystallised from chloroform/ethanol (94%).
Fig. 1 Structure of N.N’[1,1’dithiobis(phenyl)]bis(5’-methoxysalicyl-aldimine) (5-MeOSalps)
ELECTRODE PREPARATION:17
The solution of PVC membrane was prepared by thorough mixing of ionophore (Schiff base legend) (1% wt.), DBP (dibutylphthalate) as plasticizer (66% wt.) and PVC (33% wt.) and was dissolved in THF. The resulting solution was poured into a glass mould and THF was allowed to evaporate off at room temperature for 24 hrs. A flexible membrane of the thickness of 0.2-0.4 mm was obtained. The disc of 6mm diameter was cut and pasted onto the glass tube with PVC glue and conditioned with AgNO3 solution of 0.01M for 2-3 days. The ratio of member constituents, time of contact and concentration of equilibrating solution were properly adjusted to obtain the best potential response.
POTENTIOMETRIC RESPONSE:
All the potentiometric measurements were performed at constant temperature of 25 ± 0.5°C by digital pH meter (Elico L1-10 India). Saturated calomel electrode was used as a reference electrode. The potential across the membrane was measured by setting up the following cell assembly.
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External 0.1M AgNO3 PVC Test Internal
SCE Membrane Solution SCE
The concentration of the test solution was varied in the range of 1.0 × 10-8 to 1.0 ×10-1 M.
TREATMENT OF THE ELECTROPLATING WASTE:18
The electroplating waste (40 ml) was first filtered and 5 ml conc. H2SO4 was added. The final solution was made upto 50 ml maintaining the pH between 3-4 and solution was suitably diluted.
RESULTS AND DISCUSSION:
5-MeOSalps was employed as Ag+ ion selective ionophore in the preparation of silver ion selective electrode. The potential responses of various ion selective electrodes are shown in Fig.1. The measurements were performed in the concentration range of 1.0 ×10-6 to 1.0 × 10-1 M AgNO3. The different electrodes show linear response in concentration range from 1.0 × 10-1 to1.0 × 10-5 M. The fig.1 reveals that only Ag+ ions shows nearly nernstian slope of 59 mV per decade. The results thus indicate that silver ions are more easily attracted to the PVC 5-MeOSalps membrane, resulting in a nernstian potential response over a wide range.
Fig.1 Potential response of various ion selective electrode based on the ligands 5-MeOSalps. 1: refers to Ag+ 2 : refers to Hg2+ 3: refers to Na+, K+, Rb+, Cs+ , Mg2+, Ca2+, Ba2+, Cu2+, Co2+, Ni2+, Zn2+, Pb2+, Cd2+.
The sensitivity and selectivity obtained for a given ionophore depends significantly on the membrane ingredients, nature of the solvent and the additive used19-22. Therefore the influence of membrane composition on the potential response of Ag+ was investigated. Plasticizers are frequently used to enhance the response characteristic e.g. working concentration range, slope etc. The effect of addition of the plasticizer to membrane was, therefore, studied and significantly improvement with ligands to working concentration range and slope has been observed. The effect of plasticizers (DBP, BA, and NPOE), additive (NaTPB and KTK) and amount of ion carriers on the performance of the membrane was studied. Among the solvent mediator used DBP shows the best sensitivity with membrane no.11 i.e. use of 33% PVC, 5% 5-MeOSalps, 60% DBP, 2% NaTBP result in the nernstian slope of 58.2 ± 0.3 mV per decade over a wide dynamic range. It may be noted that the presence of lipophilic negatively charged additives not only diminishes the ohmic resistance23 but also enhances the response. Moreover the additives may catalyze the exchange kinetics at the sample membrane interface. Table-1 shows that the presence of 2% NaTPB as additive increases the slope of sensor to 58.2 mV per decade.
The calibration parameter, response time and life time of different membrane have been determined. Electrode presented nernstian slope of 58.2 ± 0.3mV/decade (Fig. 2) for Ag+ ions over a wide concentration range of 1.0 ×10-1 to 8.0 ×10-6 M with detection limit, as determined from the intersection of the two extrapolated segment of the calibration graph 5.0 ×10-6 M (0.54 µgmL-1). The concentration of the internal solution (AgNO3) of the electrode was changed from (1 × 10-3 to1 × 10-5 M) and it is found that variation in the internal solution concentration does not cause any significant change in the potential response of Ag+.(Fig. 3). A 1.0 ×10-4 M AgNO3 solution gives smooth functioning of electrode system. The sensor has a short response time of 5 s. The potential remain constant for more than 5 min. and after that it shows slow divergence (Fig. 4).
TABLE -1 Optimization of Membrane Composition