INTRODUCTION:

Vitamin C (Ascorbic Acid) is a water soluble antioxidant. It was first isolated in 1928 by the Hungarian biochemist and Noble prize winner Szent Gyorgyi. It is an unstable easily oxidized acid and can be destroyed by oxygen, alkali and at high temperature.

Unlike animals humans cannot synthesize vitamin C, rendering its ingestion from oxygeneous supplement or necessary diet , human inability to synthesize ascorbic acid is the absence of the active enzyme. 1 – Gulonolactone oxidizes from the liver (Burns, 1959). Body requires vitamin C for normal physiological functions. It helps in the metabolisms of tyrosine, folic acid and tryptophan. ¹ It helps to lower blood cholesterol and contributes to the synthesis of the amino acid carnitine and catecholamine that regulate nervous system. It is needed for tissue growth and wound healing. It helps in the formation of neurotransmitters and increases the absorption of iron in the gut. Being an oxidant, it protects the body from the harmful effects of free radicals and pollutants. Deficiency of this vitamin causes defective formation of the collagen fibers of connective tissue due to which the process of healing wounds retards and also cause disease scurvy. ²

Vitamin C is essential for the process from bone formation to scar tissue repair^.3 Literature survey reveals that no work has been reported on complex tendencies of vitamin C with transition metal ions in aquo-organic medium. Therefore in order to understand the complex formation tendencies of vitamin C in aquo-organic medium at constant temperature 298 K and ionic strength 0.1M. NaClO₄ is studied.

Vitamin C (Ascorbic Acid)

MATERIAL AND METHODS:

All the chemicals were used are A.R. grade. Ligand sample of vitamin C (Ascorbic Acid) was obtained in pure form.The solvents MeOH and EtOH were used are A.R. and purified by distillation method. Metal ions were used in their nitrate form (S.D.fine chem.). 0.2N NaOH and 0.2N NaClO₄ solution were also prepared in double distilled water.

Table No.1 The dissociation constant of ascorbic acid in MeOH-water medium and EtOH medium at 298 K and at μ = 0.1 M NaClO₄

Ascorbic acid	pK₁	pK₂
36% MeOH-water	4.8	12.37
30% EtOH- water	4.72	12.21

Table No.2 The metal ligand stability constant of Ascorbic acid with transition metal ions in 36% MeOH-water medium at 298 K and at μ = 0.1 M NaClO₄

Metal	V(II)	Cr(II)	Fe(III)	Co(II)	Ni(II)	Cu(II)	Zn(II)
log K₁	9.9	11.55	11.67	12.18	11.83	9.82	9.4
log K₂	8.87	9.47	8.45	9.35	9.37	8.02	7.98

Table No.3 The metal ligand stability constant of Ascorbic acid with transition metal ions in 30% (v/v)EtOH-water medium at 298 K and at μ = 0.1 M NaClO₄

Metal	V(II)	Cr(II)	Fe(III)	Co(II)	Ni(II)	Cu(II)	Zn(II)
log K₁	12.17	12.1	12.45	10.55	7.61	6.7	8.52
log K₂	10.36	9.67	10.5	8.33	5.19	5.46	6.79

The sodium hydroxide was standardized against oxalic acid solution and it was used for further potentiometric titrations. The ionic strength (μ) was maintained at 0.1M by using NaClO₄(B.D.H) as supporting electrolyte. All solutions were prepared in carbon dioxide free double distilled water. The concentration of metal ions solutions was estimated by standard procedures^4-6 . The micro burette with graduation of 0.02 ml was used and it is calibrated by the method of vogel ⁷. All glasswares used in the experiments are of Borosil quality. An Elico model LI – 120 digital pH meter in conjuction with an Elico combined glass electrode consisting of glass and reference electrodes entity of the type CK-61/ CN-91/ CM-51 were used for pH measurement. The precaussions suggested by Bates ⁸, Albert and Sergeant ⁹ were adopted for smooth handling of the electrode.

The experimental procedure for binary metal complexes involves following titrations

1) Free HClO₄

2) Free HClO₄+ Ascorbic acid

3) Free HClO₄+ Ascorbic acid + metal ion solution.

These solutions were titrated against standard solution of sodium hydroxide. The concentration of ascorbic acid and metal ions were .01M and the concentration of HClO4 acid was 0.2N. The ionic strength of the solutions was maintained constant i.e. 0.1 M by adding appropriate amount of 2M NaClO₄. The titrations were carried out at 298 K in an inert atmosphere by bubbling free nitrogen gas through an assembly containing electrode to expel out carbon dioxide gas . The reading on pH meter in 36 % (v/v) MeOH -water and 30 % (v/v) EtOH-water medium were recorded at every addition of sodium hydroxide in order to evaluate dissociation constant and stability constants. The method of determination of the formation functions was described by Irving and Rossotti¹⁰, and Hearon and Gilbert¹¹ which was used earlier by Calvin and Melchior ¹²and now known as Calvin- Bjerrum titration technique.

RESULTS AND DISCUSSION:

The proton ligand and metal ligand stability constants of ascorbic acid in 36 % (v/v) methanol-water and 30% (v/v) ethanol- water medium at 298 K and at μ = 0.1 M NaClO₄ are depicted in table no1, 2, and 3.

In ascorbic acid two pK values are obtained. These are due to dissociation of two enolic groups in ascorbic acid .pK₁is due to dissociation of -OH(3) and pK₂ is due to dissociation of -OH(4). pK₂ value is higher than pK₁. Metal ligand stability constant i.e. logK₁ are higher values than logK₂ for the transition metals.

Since Methanol and Ethanol are protic solvents therefore ascorbic acid gets easily solvated in them.

The pK values of ascorbic acid are higher in 36% MeOH-water medium than in 30% EtOH-water medium. This indicates that in ethanol more acidic complexes are formed as compared to methanol. The effect of solvent depends on its dielectric constant and its ability to solvate and stabilize ligands in acid base equilibria. Solvent having highest polarity or dielectric constant stabilizes the ionized species, higher will be the pKa value and more stable will be in solution.

The metal ligand formation curve data for ascorbic acid in 36% (v/v) methanol-water and 30% (v/v) ethanol-water medium with transition metals that metal ions forms 1:1 and 1:2 complexes¹² with vitamin C (Ascorbic acid). The order of metal ligand stability constants is as follows.

36% (v/v) MeOH-Water medium

Co(II) > Ni(II) > Fe(III) > Cr(II) > V(II) > Cu(II) > Zn(II)

30% (v/v)EtOH-Water medium

Fe(III) > V(II) > Cr(II) > Co(II) > Zn(II) > Ni(II) > Cu(II)

CONCLUSION:

In the present work of pH metric study was performed to determine stability constants and to asses binary species for ascorbic acid with transition metals in 36% MeOH-water and 30% EtOH-water medium pH range 1.98 to 11.9 The following conclusions have been drawn.

1) Ascorbic acid forms complexes with transition metal ions in the pH range 2.0 to 12.0

2) The pK values in 36% MeOH-water and 30% EtOH-water medium are evaluated.

3) The logK values in 36% MeOH-water and 30% EtOH-water medium is evaluated

REFERENCES:

1. Iqubal Khalid and et al. Pakistan Journal of Nutrition. 3 (1); 2004: 5 – 13.

2. Howald H .and et al. Ascorbic acid and athletic performance Ann N. Y. Acad Sci. 258; 1975: 458- 464,

3. Groff J. L. and et al. The water soluble vitamins in advanced Nutrition and Human Melabolism Minnepolis West publishing Company. 1995: 223 – 237

4. Dogan Alven and et al., Ind J. of Chem., July, 42(A); 2003: 1632-1635.

5. Jabalpurwala K.E and et al. Inorg. Nucl. Chem; 26; 1964:1027.

6. Schwarzenbach G. E and et al. Complexometric titrations, Menthuen and Cop Ltd; London; 1957, 69, 79-82.

7. Vogel A.I and et al. A Textbook of Practical organic Chemistry, Pergamom Green and Co. ltd. London; 1956.

8. Bates R.G and et al. Determination of pH Theory and Practice, A Wiley Interscience Publication. New York; 1973.

9. Albert A. and et al. Determination of Ionisation Constants, Chapman and Hall Ltd; 2^nd Edⁿ, London; 1971, 10.

10. Irving H. and et al. J. Am. Chem. Soc; 1953:3397.

11. Hearon J.Z and et al. J. Am. Chem. Soc; 77; 1955: 2594.

12. Calvin M. and et al. J. Am. Chem. Soc; 70;1948:3270

Received on 17.07.2012 Modified on 29.07.2012

Asian J. Research Chem. 5(8): August, 2012; Page 963-965