1. INTRODUCTION:
Cadmium is a soft, malleable, ductile, bluish-white metal belongs to d-block of the periodic table. It is similar in many respects to zinc but forms complex compounds.[3] Unlike other metals, cadmium is resistant to corrosion and as a result it is used as a protective layer when deposited on other metals. Cadmium occurs as a minor component in most zinc ores and therefore is a byproduct of zinc production. It was used for a long time as a pigment and for corrosion resistant plating on steel while cadmium compounds were used to stabilize plastic [1,2]. The occurrence of cadmium in environment is widespread as it is used for many industrial and agricultural purposes like metal plating, cadmium-nickel batteries, mining, pigments, stabilizers, alloys and phosphate fertilizers [3]. Thus Cadmium is an environmental hazard. The highest concentration of cadmium has been found to be absorbed in the kidneys of humans, and up to about 30 mg of cadmium is commonly inhaled throughout childhood and adolescence. Cadmium is biopersistant and once absorbed by an organism, remains resident for many years. Inhalation accounts for 15-20 % of absorption through the respiratory system; 2-7 % of ingested cadmium is absorbed in the gastrointestinal system [4].
The common methods applied for the determination of heavy metals are separation of the species by chromatography, followed by detection and determination of the element with a specific detector such as atomic absorption spectrometry or mass spectrometry [5,6], but all these methods are time consuming, involving sample manipulations, relatively expensive and required large infrastructure back up.
Selective analytical methodologies which are environmental friendly, cost effective and easily operated have therefore been proposed as alternative to standard methods. Potentiometric sensors are specially studded for these purpose because these they offer advantages such as low cost, simple handling, high selectivity and high sensitivity [7].
Few ionophores such as polytungustoantimonate [8], benzo–15crown-5 [9], dibenzo-24-crown-8 [10], monoaza–18-crown–6 [11], 3,4:12,13-dibenzo-2,5,11,14-tetraoxo-1,6,10,1tetraazacyclooctadecane [12], tetraazacyclo-hexadecane [13], dicyclohexano-24-crown–8 [14], N-(2,6-nylylthioimdazolyl)borate [15], dicyclohexano-18-crown-6 [16], N-N1-[bis(pyridin-2-yl)formylidene]butane 1,4–diamines [17] are available in the literature. However, some scope in the improvements of ISE higher detection limits is still quite possible, which will generate a fresh interest in the design and synthesis of new cation receptors with much greater selectivity then previously reported [18]. The work in this paper is related with the development of cadmium(II) selective sensor using5, 11, 17, 23-Tetrabutyl-25, 27-bis(7’-methoxycumarine-3’-methyloxy)-26,28-dihydroxycalix[4]arene as neutral carrier.
2. EXPERIMENTAL:
2.1. Reagents and Equipments
All reagent were of analytical grade and used without any further purification. High molecular weight Poly(Vinyl Chloride) (PVC), 1-chloronapthalene (CN), oleic acid (OA), dibutylbutyl phosphonate (DBBP), dioctylphthalate (DOP), dibutylphthalate (DBP), tris(2-ethylhexyl)phosphate (TEP), o-nitrophenyl octyl ether (o-NPOE), potassium tetrakis(4-chlorophenyl borate (KTpClPB) and tetrahydrofuran (THF) were purchased from Merck.
K2CO3, MgSO4 and CH2Cl2 were purchased from Aldrich. All metal nitrates were also purchased from Merck. Doubled-distilled water was used to prepare the metal nitrate solutions.
Saturated silver electrodes (SSE) were used as reference electrodes, a digital potentiometer was used for potential measurements at 25 ± 0.10C.
2.2. Synthesis of ionophore
The ionophore 5, 11, 17, 23-Tetra-tert-butyl-25, 27-bis (5’-methoxycumarine-2’-methyloxy)-26,28-dihydroxycalix[4]arene was synthesized according to procedure already published [19] and the brief synthesis is given as below:
A solution of 4-bromomethyl-7-methoxycoumarin (1.33 mmol) and Tetra-tert-butyl calix[4]arene (1.325 mmol) in methylene chloride (8 mL) was reflux for 3 hrs in presence of K2CO3. The reaction was then allowed to stir at room temperature for 4 hours. The reaction mixture was then diluted with methylene chloride, washed with water, dried with MgSO4 and concentrated in vacuo to give a white solid (fig.-1) (0.53 mg, 68% yield).
Fig. 1. 5, 11, 17, 23-Tetra-tert-butyl-25, 27-bis(7’-methoxycumarine-3’-methyloxy)-26,28-dihydroxycalix[4]arene
2.3. Development of PVC membrane
A heterogeneous membrane of this compound was fabricated using PVC as binder material by the method of Craggs et al [20]. The PVC-based membranes have been fabricated by dissolving a mixture of 33% PVC, 54% plasticizer (DBBP, DBP, TEP, DOP o-NPOE, CN, OA respectively), 8% KTpClPB, and 5% ionophore in THF (15 ml). The components were added in terms of weight percentage. The resulting solution was stirred well and poured in a glass casting ring on a smooth tile. The solvent was allowed to evaporate at room temperature for 24 hours in order to obtain the uniform membrane. A membrane sheet about 0.5 mm of thickness and 5 mm diameter was cut away from inner edge and glued it to one end of a glass tube with the help of araldite and m-seal to avoid leakage. The Emf measurements were carried out with the cell assembly given below:
|
Internal reference Silver electrode |
Internal reference solution (0.01 M Cd(II)) |
Cd(II)ion Selective Membrane |
Test solution of Cd(II) ion |
External reference Silver electrode |
3. RESULTS AND DISCUSSION:
Formation constant of the ion-ionophore complex within in the membrane phase is a very important parameter that dictates the practical selectivity of the sensor. In this study the formation constant is calculated using equation (1).
Where, LT is the total concentration of ionophore in the membrane segment, RT is the concentration of lipophilic ionic site additives, n is the ion-ionophore complex stoichiometry and T is absolute temperature. The ion I carries a charge of zI. Ther determined formation constant for the examined different complexes are presented in Table 1.
Sensitivity and selectivity of any electrode were significantly affected by the nature of plasticizer, composition of the membrane, internal solution, etc. [21]. Effect of composition of membrane on the response characteristics of the electrode like slope of the calibration curve, measurement range and detection limit were studied.
Table.2: Formation constant of metal ion-ionophore complex
|
Metal ions |
Formation constants |
|
Cd(II) Cr3+ La3+ Eu3+ Sm3+ Gd3+ Sb3+ Cu2+ Mn2+ Co2+ La3+ Ni2+ Pb2+ Hg2+ Ag+ Fe3+ Al3+ Na+ |
4.6 4.2 3.6 3.2 2.4 3.0 2.9 2.0 1.8 2.5 2.6 2.5 2.8 1.7 1.9 2.1 2.5 1.3 |
The six membranes of different composition were fabricated by using different type of plasticizers i.e. DOP, DBP, DBBP, TEP, CN and OA keeping PVC, ionophore and NaTPB in fixed composition. However, obtained membrane (No.1) using o-NPOE as plasticizer gave the satisfactorily results [Table-2]. Varying the proportion of the sensor to mediator showed that master membranes with as little as 5% sensor and 54% of solvent mediator functioned well [22].
The potential response of electrochemical cells with 10-2 M Cd(II) as internal solution was determined in the range of 5 x 1 0-10 M to 1.0 x 1 0-1 M Cd(II) solution and the calibration curve depicted in fig. 2. It has been clearly shown that membrane no. 1 containing plasticizer o-NPOE, shown linear response in the conc. range 1 x 1 0-9 M to 1.0 x 1 0-1 M with the slope of 23.0 ± 0.3 mV/decade of activity.
The effect of pH on the response characteristics can be explained by coordination competition between ionophore and hydroxide ion. The influence of pH of the test solution of 1.0 × 10-2 M Cd(II) ion on the potentiometric response of the membrane electrode was examined in the range of 1-10. The proposed electrode work satisfactorily in the pH range of 2.0 – 8.6 (Fig. 3.)
Fig.2: Variation of membrane potential of PVC based membranes of ionophore with varying concentrations of Cd(II) ions, plasticizer with o-NPOE (1); DBP (2); DOP (3); DBBP (4); TEP (5); CN (6); OA (7).
Fig. 3. Effect of pH on cell potential of membrane No. 1 for 1.0 × 10-2 M Cd (II) solution.
