In the Laboratory edited by
Advanced Chemistry Classroom and Laboratory
Joseph J. BelBruno Dartmouth College Hanover, NH 03755
Microscale Determination of Vitamin C by Weight Titrimetry
W
Gaston A. East* and Erica C. Nascimento Instituto de Química, Universidade de Brasilia, Caixa Postal 4478, CEP 70.919-900 Brasilia – DF, Brazil; *
[email protected] Ascorbic acid (AA), or vitamin C (VitC), is an important vitamin that occurs naturally in many foodstuffs (fruits, vegetables, dairy products, meats, etc.) or is added to them as an antioxidant. Many pharmaceuticals used for the treatment and prevention of colds, as well as multivitamin preparations, contain variable amounts of the vitamin in a variety of matrices. Consequently, many techniques, including titrimetry, spectrophotometry, chromatography, and electrochemistry, have been used for the determination of AA. Current titrimetric methods for the assay of AA involve a redox procedure using an oxidizing agent as titrant. However, the end-point location remains a problem when the solution is colored. Herein we describe an electroanalytical method for the microscale determination of VitC that combines simplicity and low cost with high precision and accuracy, and yet is useful in colored sample solutions. The technique involves weight titration of AA using (diacetoxyiodo)benzene (IBDA) as titrant in semi-aqueous medium and differential electrolytic potentiometry (DEP) for the end-point location. The results compare well with those obtained using 2,6-dichlorophenolindophenol (DCIP) as titrant. Determination of AA by the microscale method described herein would be an interesting laboratory exercise in instrumental analysis courses, since the work gives students experience with an electroanalytical technique using a nontraditional oxidimetric titrant that may be readily prepared in the lab. Simultaneously, the student applies an analytical procedure to real-life samples of biochemical interest. The experiment has been successfully introduced in the Instrumental Analysis course at the Chemistry Institute of the University of Brasilia. The class is made up of senior undergraduate students majoring in chemistry, who enjoy the opportunity of analyzing a familiar compound. Overview of the Experiment The advantages of weight titrimetry over volumetric titrations have been pointed out (1–4). The advent of electronic analytical balances allows gravimetric titrations to be performed more readily and more rapidly than volumetric titrations. Furthermore, the titrations require only a few milliliters of the titrant and are therefore suitable for microscale determinations. Precision of 0.01% can be achieved (5). IBDA is an organic oxidimetric titrant used in nonaqueous or semi-aqueous media. It is a strong oxidizing agent belonging to the [bis(acyloxy)iodo]arenes family (6, 7 ). DEP is an electroanalytical technique for end-point detection; it has much higher sensitivity than zero-current potentiometry (8, 9). The method measures the potential set up by a minute current between two identical electrodes 100
immersed in a stirred solution containing the analyte. The nature of the electroactive species in the solution defines the shape of the titration curve obtained (Fig. 1) (9). These curves take the shape of the first differential of the zero-current potentiometric curve (9, 10), making it easier to locate the end-point. In addition, the instrumentation needed for DEP is inexpensively available nowadays owing to the proliferation of solid-state, low-cost pH meters and low-price operational amplifier–based low-current sources (11, 12).
Reaction The procedure involves the redox reaction of ascorbic acid with IBDA as follows: O
O
I(CH3COO)2
H CHOH CH2OH OH
HO
(diacetoxyiodo)benzene
ascorbic acid
O
O
+
I
H CHOH
+ + 2CH3COO− + 2H
+
CH2OH O
O
iodobenzene
dehydroascorbic acid
Experimental Procedure A brief outline of the procedure is given below. Full details are available online.W The sample solution is prepared by dissolving a precisely weighed tablet of VitC, ground to a fine powder, in water,
c
a ∆E b Mass of Titrant
Figure 1. DEP curves obtained in redox titrations (9). (a) The oxidant is not reversibly electrolyzed. (b) Both oxidant and reductant are reversibly electrolyzed. (c) The reductant is not reversibly electrolyzed.
Journal of Chemical Education • Vol. 79 No. 1 January 2002 • JChemEd.chem.wisc.edu
In the Laboratory
filtering the insoluble matter, and collecting the filtrate in a volumetric flask. Assuming that the IBDA standard solution is available, the experiment starts with the electrochemical conditioning of the indicating electrodes in a 2 M H2SO4 solution. Next, ca. 0.5 g of KBr, 1 mL of 2 M HCl, 5 mL of water, and a measured amount of the sample are placed into the electrolytic cell. Figure 2 shows the experimental setup, in which the two platinum electrodes are connected to a constant-current source and to a millivoltmeter, respectively. A typical value of the polarizing current is 1.5 µA. A pH meter is used to monitor the differential potential and the solution is stirred throughout the experiment by means of a magnetic stirrer. A 3-mL buret-syringe is filled with 1.5–2 mL of a 0.005 M IBDA standard solution and weighed to the nearest ± 0.1 mg on an electronic balance. An aliquot of the titrant is added to the cell from the syringe, which is then weighed to find the mass of titrant delivered. A stabilization period of ca. 3 min is allowed before recording the differential potential, ∆E. The titration proceeds as before until the differential potential reaches a low value, after peaking at the end-point. The endpoint of the titration is located from a plot of ∆E versus mass of IBDA (curve C in Fig. 1). Students are required to determine the amount of AA in a pharmaceutical tablet and compare the results with the value shown in the description of the product as provided by the manufacturer. Hazards
the differential potential drops suddenly to a minimum value. This allows the speed of the titration to be greatly increased, making the method as swift as direct titration utilizing a visual indicator. Table 1 shows the results for the gravimetric titration of VitC in some pharmaceutical preparations. The average value of five titrations is reported for each proprietary drug listed. The highly accurate results obtained for the determination of VitC, in spite of the fact that some of the resultant solutions were colored, demonstrate the application of the proposed method. For comparison, results for the determination of AA following the AOAC official titration method with 2,6-dichlorophenolindophenol (14) are included in Table 1. Results obtained by the two methods agreed closely. These results were obtained by a single student working a full semester. In addition, two classes of 15 (class A) and 12 (class B) students were assigned the determination of AA in 500-mg VitC tablets from Schering-Plough (Lot No. 6PKEB17). The students worked in pairs. The average value for Class A was 494.8 ± 1.6 (SD) (n = 8), and for class B the average value was 495.3 ± 1.8 (SD) (n = 6). Conclusion This experiment offers the student the opportunity of working with a simple, accurate, rapid, safe, and economical nonconventional microanalytical method for the determination of vitamin C in pharmaceutical preparations. In addition, the students may prepare, purify, and check the purity of the titrant, thereby introducing organic synthesis into the
Glacial acetic acid solutions are corrosive; severe burns to the eyes and skin may occur upon contact. Vapors are irritating to the eyes and the respiratory tract; delayed pulmonary edema may result from very high exposures. The use of goggles and rubber gloves is therefore recommended to handle IBDA solutions. Avoid contact with skin and eyes. Emergency measures for skin and/or eye contact include the flushing of IBDA-acetic acid solution with water for at least 15 minutes.
constant current source pH/mVmeter
platinum electodes
Results and Discussion Ascorbic acid, being a mild reducing agent, is quantitatively oxidized by IBDA. Titration of VitC employing DEP for end-point detection produces a very sharp end-point, allowing minute quantities of the analyte to be accurately titrated. Potential breaks at the end-point of up to 550 mV/ 0.05 mL of 0.005 M IBDA have been found in this work. Under the same working conditions, on the other hand, zerocurrent potentiometry gave a potential break at the end-point of only 340 mV/0.05 mL of 0.05 M IBDA; that is, we obtained potential breaks about 200 mV higher than this, despite using just one-tenth of the concentration of the titrant. These results corroborate the superior sensitivity of the DEP technique over classical (zero-current) potentiometry. Consequently, DEP is a very suitable method for end-point detection in microtitrations. The sharpness of the break allows the end-point to be located with high precision. If high precision is not required, there is no need to plot the DEP curve and the end-point may be estimated as the point where
magnetic stirrer
Figure 2. Scheme for DEP titrations.
Table 1. Weight-Titrimetric Determination of Ascorbic Acid in Pharmaceuticals AA Found/mga
Label Value/ mg
Trials (No.)
Sample
Supplier
Cebion
Merck Rio de Janeiro
1000
1010.1 ± 3.8 1958.4 ± 4.1
5
Cewin
Sanofi Winthrop 1500 Rio de Janeiro
1492.7 ± 2.4 1487.7 ± 1.8
5
1008.0 ± 5.3 1003.3 ± 2.0
5
Walgreens Walgreen Co. Finest Deerfield, IL aValue
1000
IBDA/DEP Method
2,6-DCIP Method
± 95% confidence interval.
JChemEd.chem.wisc.edu • Vol. 79 No. 1 January 2002 • Journal of Chemical Education
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In the Laboratory
project. Except for the pH/millivoltmeter, all the equipment necessary to conduct the experiment can be homemade at low cost. The method has been shown to be accurate and precise, giving clear and well-defined end-points. WSupplemental
Material
Detailed lab documentation and notes for the instructor are available in this issue of JCE Online. Literature Cited 1. Harris, D.C. Quantitative Chemical Analysis; Freeman: New York, 1996; p 29. 2. Skoog, D. A.; West, D. M.; Holler, F. J. Fundamentals of Analytical Chemistry, 6th ed.; Saunders: Orlando, FL, 1992; p 113.
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3. Siemer, D. D.; Reeder, S. D.; Wade, M. A. J. Chem. Educ. 1988, 65, 467. 4. Guenther, W. B. J. Chem. Educ. 1988, 65, 1094. 5. Bishop, E. Anal. Chim. Acta 1959, 20, 315. 6. Pillai, V. N. S.; Nair, C. G. R. Talanta 1975, 22, 57. 7. Kokkinides, G.; Papadopoulou, M.; Varvoglis, A. Electrochim. Acta 1989, 34, 133. 8. Bishop, E. Mikrochim. Acta 1956, 619. 9. Bishop, E. Analyst 1958, 83, 212. 10. Bishop, E. Analyst 1960, 85, 422. 11. Hartshorn, L. G.; Bishop, E. Analyst 1971, 96, 885–886. 12. Braun, R. D. J. Chem. Educ. 1996, 73, 858. 13. Pausaker, K. H. J. Chem. Soc. 1953, 10. 14. Official Methods of Analysis of AOAC International, Vol. 2, 16th ed.; Cunniff, P., Ed.; AOAC International: Arlington, VA, 1995; Chapter 45.
Journal of Chemical Education • Vol. 79 No. 1 January 2002 • JChemEd.chem.wisc.edu
