INTRODUCTION:

Calcium is essential for living organisms, particularly in cell physiology, where movement of the calcium ion Ca²⁺ into and out of the cytoplasm functions as a signal for many cellular processes. As a major material used in mineralization of bones and shells, calcium is the most abundant metal by mass in many animals. Calcium salts are colorless from any contribution of the calcium, and ionic solutions of calcium (Ca²⁺) are colorless as well. Many calcium salts are not soluble in water. When in solution, the calcium ion to the human taste varies remarkably, being reported as mildly salty, sour, "mineral like" or even "soothing." It is apparent that many animals can taste, or develop a taste, for calcium, and use this sense to detect the mineral in salt licks or other sources¹. In human nutrition, soluble calcium salts may be added to tart juices without much effect to the average palate. Calcium is an important component of a healthy diet and a mineral necessary for life. The National Osteoporosis Foundation says, "Calcium plays an important role in building stronger, denser bones early in life and keeping bones strong and healthy later in life." Approximately ninety-nine percent of the body's calcium is stored in the bones and teeth².

The rest of the calcium in the body has other important uses, such as some exocytosis, especially neurotransmitter release, and muscle contraction. In the electrical conduction system of the heart, calcium replaces sodium as the mineral that depolarizes the cell, proliferating the action potential. In cardiac muscle, sodium influx commences an action potential, but during potassium efflux, the cardiac myocyte experiences calcium influx, prolonging the action potential and creating a plateau phase of dynamic equilibrium. Long-term calcium deficiency can lead to rickets and poor blood clotting and in case of a menopausal woman, it can lead to osteoporosis, in which the bone deteriorates and there is an increased risk of fractures. While a lifelong deficit can affect bone and tooth formation, over-retention can cause hypercalcemia (elevated levels of calcium in the blood), impaired kidney function and decreased absorption of other minerals. High calcium intakes or high calcium absorption were previously thought to contribute to the development of kidney stones. However, a high calcium intake has been associated with a lower risk for kidney stones in more recent research^3-5. Vitamin D is needed to absorb calcium. Calcium metal is hazardous because of its sometimes violent reactions with water and acids. Calcium metal is found in some drain cleaners, where it functions to generate heat and calcium hydroxide that saponifies the fats and liquefies the proteins (e.g., hair) that block drains. When swallowed calcium metal has the same effect on the mouth, esophagus and stomach, and can be fatal.

EXPERIMENTAL:

Reagents and Chemicals:

All reagents used were of analytical grade. Distilled deionised water was used throughout. Methyl-2-pyridylketone (MPK), Thiosemicarbazide (TSC), high relative molecular weight PVC, sodium tetraphenyl borate (NaTPB), Nitrobenzene (NB) and tetrahydrofurane (THF) were purchased from Aldrich and used as received.

Synthesis of ionophore: 20 mL ethanolic solution of Methyl-2-pyridylketone (MPK) was taken in 100 mL round bottom flask. 20 mL ethanolic solution of thiosemicarbazide was added drop wise in the flask and condenser was fitted. Reaction mixture was refluxed at 70 ºC for 10 hrs. After refluxing it was cooled at room temperature. After that on cooling the reaction mixture in refrigerator overnight a white was formed. It was further washed with water and dried over P₄O₁₀ white compound (Figure 1) formed.

Figure 1 Scheme of Synthesis

Yield: 52 %, Melting point 187 ºC

CHN Analysis: Anal. Calculated for C₈H₁₀N₄S: C, 41.35; H, 3.56; N, 26.2; S, 14.21 Found: C, 42.14; H, 3.26; N, 25.4; S, 14.10

IR Characterization: IR (KBr) (v/cm^-1): (C=N), 1586, 1461 (phenyl). No peaks due to carbonyl group suggest that condensation take place.

Mass spectra: m/z at 225 (M+). Other fragments are at m/z 78, 105, 150 and 193.

Electrode preparation: The procedure to prepare^6-7 the PVC membrane was thoroughly mixing 30 mg powdered PVC, and 60 mg plasticizer NB in 10 mL of THF. To this mixture 4 mg NaTPB and 6 g MPKTSC was added and the solution was mixed thoroughly. The resulting mixture was transferred into a glass dish. The THF content of the mixture was evaporated slowly, until an oily concentrated mixture was obtained. A Pyrex tube was dipped into the mixture for about 10 sec.; so that a non-transparent membrane of about 0.3 mm thickness was formed. 9-10 tubes were then pulled out of the solution and kept at room temperature for 10 hrs. The tube was filled with an internal filling solution containing calcium. The electrode was finally conditioned for 48 hrs. by soaking in calcium solution.

EMF Measurements:

All EMF measurements were carried out with the following assembly:

External Reference Electrode (SCE) | internal solution of calcium | PVC membrane | sample solution | Internal Reference Electrode (SCE)

RESULTS AND DISCUSSION:

The MPKTSC ligand has the relatively low solubility in water due to the existence of pyridine group in structure, indicating its sufficient lipophilic character, which prevents its leaching into the solution surrounding the membrane electrode and also suggesting the presence of nitrogen donating atoms which proved that ligand could be better potential ion carrier for bivalent metal ions in PVC membrane electrodes. It has been observed from figure 2, that the ionophore as neutral carrier was found to be highly responsive to Ca²⁺with respect to several other metal ions.

Figure 2 Potential responses of different ions based on MPKTSC

Effect of Membrane Composition on Potential Response:

It is well known that the membrane composition and especially in some cases, the nature of the additive have a significant influence on the sensitivity and selectivity for a certain ionophore. The performance characteristics of several membranes, having ingredients of different proportions, are listed in Table 1. It has been observed that the membrane no. 5 with the composition PVC : NB : MPKTSC : NaTPB in the ratio of 30: 60: 6: 4 exhibits a Nerstian slope over a broad Ca ion concentration range^8-9.

Table 1 Optimization of Membrane Ingredients

Membrane no.	Composition %				Slope (mV/decade)
Membrane no.	PVC	Plasticizer NB	Ionophore MPKTSC	Additive NaTPB	Slope (mV/decade)
1	30	64	5	-	8.9 ± 0.3
2	30	63	6	5	9.7 ± 0.4
3	30	62	5	4	12.6 ± 0.3
4	30	61	5	5	15.5 ± 0.2
5	30	60	6	4	19.3 ± 0.3
6	30	64	-	-	9.1 ± 0.3

pH Effect and Response Time:

In order to study the pH effect on the sensor performance, the potential at pH values from 1.0 to 13.0 at a specific Ca²⁺concentration. Corresponding results are shown in Figure 3. The potential remained constant from pH 2.5 to 9.5, beyond some drift was observed. Drift observed may be attributed to the formation of Ca²⁺hydroxyl complexes in the solution. At lower pH, the potentials increased, indicating that the membrane sensor responded to protonium ions, as a result some nitrogen atoms get protonation of the ionophore. In analytical applications dynamic response time is an essential parameter. In this study the average time required for sensor to reach ±1 mV potential of the final equilibrium value was measured. The potential versus time plot is shown in Figure 4, where plasticized membrane electrode reached equilibrium responses in a short time (<10 sec.).

Figure 3 pH effect of the test solution on the potential response of Ca²⁺ ion-selective electrode

Figure 4 Dynamic response time of Ca²⁺ion-selective electrode for step changes in the Ca²⁺ concentration

Selectivity of Ca²⁺Electrode:

Selectivity is perhaps the single most important characteristic of any electrode, which defines the nature of the device and the extent to which it may be employed in the determination of particular ion in the presence of other interfering ions. Potentiometric selectivity coefficients of calcium membrane electrodes were evaluated by Matched Potential Method^10-11 (MPM). MPM is recommended by IUPAC to overcome the difficulties associated with methods based on the Nicolsky-Eisenman equation. According to this method, primary ion (A) of a specified activity is added to a reference solution is measured. In a separate experiment, interfering ions (B) are successively added to an identical reference (containing primary ion) solution until the measured potential matches to that obtained only with primary ions.

Analytical application:

Potentiometric titrations were performed by using the proposed electrode as an indicator electrode for titration of 20 mL of 10^-4 M Ca²⁺ ions with 10^-2 M EDTA. Titration curves are shown in Figure 5, the sensor can monitor the amount of calcium ions.

Figure 5 Potentiometric titration curves of 20 mL of 10^-4 M Ca²⁺ ions with 10^-2 M EDTA

CONCLUSIONS :

On the basis of the present results discussed in this paper, Methyl-2-pyridylketone-thiosemicarbazide (MPKTSC) can be used as an electroactive ionophore to produce Ca²⁺ISEs with excellent electrode characteristics like linear response, selectivity and selectivity with a number of common interfering ions. This may be due to the good complexing ability of ligand for Ca²⁺ions and consequently the potential response of the electrode.

REFERENCES:

1. P. Buhlmann, E. Pretsch and E. Bakker, Carrier-based ion-selective Electrodes and Bulk Optodes. 2. Ionophores for Potentiometric and Optical Sensors, Chem. Rev. 98, 1998, 1593.

2. Kakabadse, G.J. (Organizer), The Use of Ion-Selective Electrodes in Chemistry, Biology and Medicine. University of Manchester Institute of Science and Tech., 1973, 10-12.

3. M. Akhnod, M. B. Najafi, J. Tashkourian, Anewcerium (III)-selective membrane electrode based on 2-aminobenzothiazole, Sens. Actuators B 99, 2004, 410-415.

4. G. J. Moody, R. B. Oke, J. D. R. Thomas, A Calcium-Sensitive Electrode based on a Liquid ion Exchanger in a Poly(vinyl chloride) Matrix, Analyst 95, 1970, 910-914.

5. E. Lindner, K. Toth, E. Pungor, Dynamic Characteristics of ion-selective electrodes, CRC Press, Boca Raton, FL, 1988.

6. A. Craggs, G. J. Moody, and J.D.R. Thomas, PVC Matrix Membrane Ion-Selective Electrodes, Journal of Chemical Education, Vol. 51,1974, 541-544.

7. E. Bakker, P. Buhlmann, E. Pretsch, Carrier-based ion selective electrodes and bulk optodes. Part I. General Characteristics, Chem. Rev. 97, 1997, 3083-3132.

8. A. R. Fakhari, M. R. Ganjali, M. Shamsipur, PVC-based bexathia-18-crown-6-tetrone sensor for mercury (II) ions, Anal. Chem. 69, 1997, 3693-3696.

9. M. Javanbakht, M. R. Ganjali, H. Eshghi, H. Sharghi, M. Shamsipur, Mercury(II) ion-selective electrode based on dibenzodiazathia-18-crown-6-dione, Electroanalysis 11 (1999), 81-84.

10. R. P. Buck, E. Lindner, IUPAC Recommendations for Nomenclature of ion selective electrodes, Pure Appl. Chemistry 66, 1994, 2527-2536.

11. Y. Umezawa, K. Umezawa, H. Sato, Recommended methods for reporting K (pot, a, b) values (IUPAC Technical Report), Pure Appl. Chemistry 67, 1995, 507-518.

Received on 06.06.2011 Modified on 14.06.2011

Asian J. Research Chem. 4(8): August, 2011; Page 1269-1271