ANALYTICAL SCIENCES DECEMBER 1996, VOL of Ascorbic Complex

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1 ANALYTICAL SCIENCES DECEMBER 1996, VOL Spectrophotometric an Iron(II)-Pyridine Determination -Picolinic Acid of Ascorbic Complex Acid Using Satya P. ARYAt and Meenakshi MAHAJAN Department of Chemistry, Kurukshetra University, Kurukshetra , Haryana, India An extractive spectrophotometric method for the determination of micro amounts of ascorbic acid has been developed by exploiting its reducing nature. Iron(III) is converted to iron(ii) by the addition of ascorbic acid, which forms an extractable yellow-colored complex with picolinic acid and pyridine. The absorbance is measured at 400 nm. Beer's law is obeyed in the concentration range µg ml-'. This method has been satisfactorily applied to the analysis of a variety of samples, including pharmaceuticals, food products and biological fluids. Keywords Ascorbic acid determination, extraction, chloroform, Iron(II), picolinic acid The determination of the ascorbic acid contents in different pharmaceutical products, fruits, vegetables and biological fluids is of significance, since it plays an important role in the maintenance of the good health of human beings. Also, the degradation of marketed products, such as fruit juices and soft drinks, is affected by the ascorbic acid concentration. The analysis of such samples for ascorbic acid contents is therefore of special interest. Many analytical methods have been reported in the literature for the determination of ascorbic acid. These include titrimetric1-3, fluorometric4, high-performance liquid chromatographic5-g, amperometric9,lo, 1' 12, enzymatic13,14 and spectrophotometric15-19 methods. Titrimetric analyses suffer the disadvantage of using a large amount of sample and being interfered due to colored substances. Not only are the fluorometric and polarographic methods time consuming, but the latter suffers interference due to electrochemical impurities present in food samples. Chromatography requires expensive equipment and demands additional operator's attention, which prevents applications in small industrial units and laboratories. Also, the use of an enzymatic technique is generally limited by the observed low sensitivities. However, spectrophotometric methods are commonly used for routine analyses. It was therefore proposed to develop a spectrophotometric procedure for ascorbic acid determination in such samples. The method is based on the complexation of reduced iron(ii) with picolinic acid in the presence of pyridine, followed by its extraction into chloroform, which results in a yellow-colored extract, whose absorbance is measured at 400 nm. Experimental Instrument A Shimadzu spectrophotometer (Model-UV ) with 1-cm matched glass cells was used for an absorbance measurement. Reagents and solutions All of the reagents were of analytical grade; redistilled water was used for preparing the solutions. Iron(III) solution. A stock solution of iron(iii) (1 mg ml1) was prepared by dissolving a calculated amount of ferric ammonium sulfate (s.d. fine-chem) in 100-ml distilled water containing 1 ml of concentrated hydrochloric acid. A lower concentration 100 µg m1-1 was obtained by suitable dilution of the stock solution. Ascorbic acid solution. A fresh solution of L-ascorbic acid (s.d, fine-chem) (200 µg ml-1) was used. Picolinic acid A 0.2%(w/v) aqueous solution of picolinic acid (Koch-light Laboratories Ltd.) was used. Pyridine (Eastman Organic Chemicals) and trichloroacetic acid (TCA) (s.d. fine-chem) were used as such. Procedure To 1 ml of iron(iii) solution (100 µg ml1) taken in a separatory funnel, an aliquot of ascorbic acid was added, followed by the addition of 0.7 ml of picolinic acid and 1 ml of pyridine (in each extraction). The aqueous phase volume was made to 10 ml and the complex was extracted twice with 10 ml of chloroform shaking each time for 0.5 min. The combined extract was taken in a 100 ml beaker, to which g of anhydrous sodium sulfate was added to remove any traces of water. The complex was transferred to a 25-ml volumetric flask, and the volume was made up to the mark with chloroform.

2 942 ANALYTICAL SCIENCES DECEMBER 1996, VOL. 12 The absorbance of the complex was measured at 400 nm against a reagent blank. The contents of ascorbic acid were determined from a working curve prepared by taking ascorbic acid ( µg ml-1) and using the conditions of the procedure. Determination of L-ascorbic acid in pharmaceutical preparations Tablets/capsules. An accurately weighed amount of the powder obtained from 10 tablets/capsules equivalent to 50 mg ascorbic acid was dissolved in water. The solution was filtered, the filtrate transferred to a 50-m1 volumetric flask, and volume made up with distilled water. An analyte solution (100 sg ml-1) was prepared by appropriate dilution. A known volume of the prepared solution was analyzed for ascorbic acid contents by the recommended procedure. Ampoule. After the ampoule contents were transferred quantitatively to a suitable size standard flask, and diluted with water, the general procedure was adopted for its assay. Analysis of vegetables, fruits and fruit juices To determine the ascorbic acid in an onion/ apple sample, a known mass was homogenized and extracted separately with 25 ml of 20% trichloroacetic acid (TCA). The extract was centrifuged and filtered into a 100-ml volumetric flask, and the volume was made with water. A suitable aliquot was used for an analysis. To determine the ascorbic acid in citrus fruits, each fruit was squeezed, and a portion of it was mixed in the ratio 4:1 with 20% trichloroacetic acid. It was then filtered into a suitable sized flask and brought up to the mark. A portion of this juice solution was analyzed. Analysis of milk The milk sample (50 ml) was mixed with an equal volume of 20% TCA solution. The mixture was centrifuged after 5 min, filtered and the volume was made to 100 ml. In an analysis of a milk sample, after the ph of an iron(iii) solution containing an aliquot of the prepared solution and picolinic acid was adjusted to , the rest of the procedure was followed. Determination of L-ascorbic acid in urine A fresh urine sample was mixed with a 20% TCA solution (in ratio of 4 :1) and filtered. A 5-10 ml portion of the filtrate was analyzed concerning its ascorbic acid content. Results and Discussion Iron(III) is easily reduced to iron(ii) by L-ascorbic acid. The thus formed iron(ii) was found to give a colored complex with picolinic acid; in the presence of pyridine it becomes conveniently extracted into chloroform. Detailed studies including the spectral characteristics, effect of different parameters and common Fig. 1 Absorption spectra of iron(ii)-picolinic acid-pyridine complex in chloroform. (A) absorbance of the complex measured against reagent blank. (B) reagent blank measured against chloroform. ingredients, Beer's law and its applicability to different samples, were carried out, as described below. Spectral characteristics The absorption spectrum of the yellow-colored complex against a reagent blank in chloroform was studied (Fig. 1), which exhibits a broad absorption band at nm. Since the reagent blank absorbs little in this region, absorbance measurements were carried out at 400 nm. Effect of reaction variables A variation in different parameters which influence the absorbance of the complex was carried out (Table 1). In a study of these parameters, 10 ml of the aqueous phase containing 100 µg of iron(iii) was extracted twice each time with 10 ml of chloroform. The conditions used therein are indicated at the bottom of Table 1. Effect of picolinic acid concentration The absorbance of the complex increases along with an increase in the picolinic acid concentration up to 0.7 ml of a 0.2% solution, and thereafter remains constant up to 5 ml. Therefore, subsequent studies were made with 0.7 ml of picolinic acid. Effect of pyridine The addition of pyridine is essential for obtaining the complex extracted into chloroform, which renders it more hydrophobic. The presence of pyridine not only helps to extract the complex, but also to maintain the

3 ANALYTICAL SCIENCES DECEMBER 1996, VOL Table 1 Effect of the reaction variables desired ph of extraction. However, a ph adjustment is not required by using its optimum concentration. The amount of pyridine also affects the absorbance. The maximum absorbance of the complex was found in the range ml of pyridine. Hence, 1 ml of pyridine will suffice to give the optimum absorbance. The appearance of turbidity upon adding 2.0 ml or more of pyridine does not influence the absorbance up to 3.0 ml. Waiting time after adding ascorbic acid The addition of ascorbic acid reduces the iron(iii) solution instantaneously, and, hence, no waiting is required for carrying out the reduction. Effect of the equilibration time The variation in the equilibration time also influences the extraction of the complex. The complex can be quickly extracted; 30 s time of equilibration was found to give maximum absorbance. Thereafter, the absorbance remains constant for up to 2 min. Choice of solvent The extraction behavior of the complex was studied in different solvents. Although the complex is not extracted with solvents such as carbon tetrachloride, benzene, isopentyl alcohol, 1-butanol, cyclohexane, methyl isobutylketone (MIBK), isopentyl acetate and ethyl acetate, it is extracted into chloroform and dichloromethane. Chloroform was chosen as an extractant because of higher absorbance. Beer's law and statistical data The absorbance of the complex varies linearly over the µg ml1 concentration range of ascorbic acid. A least-square analysis of Beer's plot gave the following linear regression equation (n=8): A400 = C, where A4oo is the absorbance at 400 nm and C is the total concentration in µg. This linear equation has a slope of with an intercept of The complex is quite stable, since the absorbance of the complex remains unchanged for 24 h. The reproducibility of the method was tested with 6 replicate determinations of 100 µg ascorbic acid, which gave a standard deviation of and a relative standard deviation of 0.7%. Interference studies The tolerance limits of diverse substances commonly encountered in vitamin C formulations and samples were investigated. The results are listed in Table 2, where the different substances are classified under such headings as vitamins and amino acids, sugars, organic compounds, inorganic cations and anions; the basis of their tolerance was a ±1.2% deviation in absorbance from the average value. All of the tested vitamins and amino acids, except for cysteine, are tolerated, but to varying degrees (Table 2). No interference was observed from sugars, inorganic cations (Ca2+, K+, Nat, Mgt) and anions (CL, S042- ). However, anions such as 5032-, P043- and were found to seriously interfere with the determination at mg levels. Some of the organic compounds, such as formaldehyde, methanol, acetone, benzoic acid, tartaric acid, oxalic acid, salicylic acid and citric acid, were also found to be tolerated. Analysis of pharmaceutical and natural samples The proposed procedure was applied to the determination of the ascorbic acid contents in various pharmaceutical preparations (Table 3), and the results compared with the I.P. method.22 The analyses by both methods were found to be in good agreement. Also, satisfactory recoveries were obtained in each of the products tested. The scope of applicability of the proposed method is not restricted to the pharmaceutical formulations alone, since it can be used to analyze several natural samples, such as fruits, vegetables, milk and biological fluids. The data pertaining to the analysis of such products are summarized in Table 4. The values were obtained within the reported range of ascorbic acid. The satisfactory recovery of ascorbic acid in food products is suggestive of the noninterference of the complex matrix present in such samples. The method offers many advantages, including its simplicity and rapidity. The instantaneous color

4 944 ANALYTICAL SCIENCES DECEMBER 1996, VOL. 12 Table 2 Tolerance limits of diverse substances Table acid 3 Determination of ascorbic acid in various pharmaceutical formulations and recovery of the added ascorbic Table 4 samples Determination of L-ascorbic acid in food and urine development and stability of the colored complex makes the assay of several products much easier and convenient. The analysis of pharmaceuticals, food and biological samples is indicative of its wide applicability. The method is so rapid that it takes hardly 5 min for a single determination. Also, the extraction of the complex into the nonaqueous phase helps in preventing the interference of colored substances. Moreover, no pretreatment is required, like that needed in some of the reported methods.2o,21 Apart from its simplicity, it is quite sensitive and selective, since most of the common ingredients of vitamin C samples were found to be tolerated. Our sincere thanks are due to the Chairman, Department of Chemistry, Kurukshetra University, Kurukshetra for laboratory facilities and UGC for financial support to Meenakshi Mahajan.

5 ANALYTICAL SCIENCES DECEMBER 1996, VOL References 1. D. E. Hugles, J. Pharm. Sci., 72,126 (1983). 2. J. Rohle and F. Voight, Nahrung, 28, 579 (1984). 3. W. Zhou, M. Ji and R. Dong, Shipin Kexue, 90, 56 (1987). 4. H. Horwitz (ed.), "Official Methods of Analysis of Analytical Chemists", 13th ed., p. 746, Association of Official Analytical Chemists, Washington, D.C., X. Chen and M. Sato, Anal. Sci.,11, 749 (1995). 6. M. Yoshida, T. Nishimune and K. Sureki, Korean J. Pharmacol., 28, 53 (1992). 7. H. Iwase, J. Chromatogr., 606, 277 (1992). 8. A. Bognar, Dtsch. Lebensm Rundsch, 84, 73 (1988). 9. U. Shukla and B. B. Prasad, Chem. Anal., 33, 639 (1988). 10. T. B. Singh and B. B. Prasad, Chem. Anal., 26, 541 (1981). 11. R. Barbera, R. Farre, M. J. Legarda and R. Pintor, Alimentaria, 247, 89 (1993). 12. R. M. Issa, F. M. Abdel-Gawad, F. A. Aly and A. M. El- Shinawi, J. Drug. Res., 15, 133 (1984). 13. L. Casella, M. Gullatti, A. Marchesini and M. Petrarulo, J. Food. Sci., 54, 374 (1989). 14. A. Lechien, P. Valenta, H. W. Naurnberg and G. J. Partriarche, Fresenius' Z. Anal. Chem., 311,105 (1982). 15. Y. S. Fung and S. F. Luk, Analyst [London], 110, 201 (1985). 16. S. A. Al-Tamrah, Anal. Chim. Acta, 209, 309 (1988). 17. M. Hashmi (ed.), "Assay of Vitamins in Pharmaceutical Preparations", p. 286, Wiley Interscience, New York, H. Abdelmageed, P. Y. Khashaba, H. F. Askal, G. A. Saleh and I. H. Refaat, Talanta, 42, 573 (1995). 19. N. K. Pandey, Anal. Chem., 54, 793 (1982). 20. R. C. Williams, D. R. Baker and J. A. Schmidt, J. Chromatogr. Sci.,11,118 (1973). 21. M. I. Karayannis and D. I. Farasoglou, Analyst [London], 112, 767 (1987). 22. "Indian Phamacopoeia", p. 49, Photolithio Press, Faridabad, W. H. Sebrell and R. S. Harris "The Vitamins", p. 261, Academic Press, New York, (Received July 8, 1996) (Accepted September 9, 1996)