Daniel G. Marsh, Deborah 1. Jacobs, and Hans Veening Bucknell University Lewisburg, Pennsylvania 17837
Analysis of Commercial Vitamin C Tablets by lodometric and Coulometric Titrimetry
Since the publication of Professor Pauling's well-known book "Vitamin C and the Common C o l d (I), there has been a marked increase in the use of vitamin C as a preventive medicine. In view of the controversy concerning the relative merits of kitamin C as a common cold and flu ~reventive,i t seemed that the determination of ascorbic acid as a lahoratory exercise in the elementary analytical chemistry course would be an interesting undergraduate experiment. This experiment achieves three objectives: (1)the student carries out a classical titrimetric oxidationreduction procedure; (2) the determination provides experience for the student with an instrumental electro-analytical technique; (3) the student learns how to apply routine analytical procedures to a problem which is both timelv and relevant. ~ i G i m e t r i ctechniques are generally the most frequently cited analytical methods for the determination of ascorbic acid. The 1J. S. Pharmacooeia recommends the titration of ascorbic acid with staidard iodine solution ( 2 ) . The methods used bv the Food and Drue Administration include a direct titration with 2,6-d~chlor~indo~henol as well as a fluorimetric ~rocedure(3).A continuous determination of ascorbic acidby pbotobleaching of methylene blue has also been r e ~ o r t e drecently (4). Other techniques which have been used for the determination of ascorbic acid in multivitamin samples include gas chromatography i.5) and polamgraphy (6). The experiment described here was included as part of the laboratory work in the elementary analytical course (96 students) a t Bucknell during the spring of 1972. Ascorbic acid was determined in commercial vitamin C tablets by utilizing its quantitative oxidation with bromine, a reaction first reported by Schulek, Kovkcs, and Rozsa (7),and later suggested as a possible coulometric determination by Skoog and West (8). It was the latter citation which prompted us to formulate this undergraduate experiment. The experiment consists of two parts. First, the student determines the ascorbic acid content of vitamin C tablets iodometricallv utilizine a back titration witb standard thiosulfate. Yhiswell l&wn method has been outlined by Kolthoff. Sandell. Meehan, and Bruckenstein (9). The en-. tire class performed this part of the experiment. The second part utilizes an instrumental technique. Ascorbic acid is titrated coulometrically by direct oxidation to dehydroascorbic acid using electrolytically generated bromine. The end ooint is detected amnerometricallv. Because of equipmeni limitations, this portion of the experiment was limited to about 35 specially selected students. lodometric Titration of Ascorbic Acid (Koppeschaar Method) The procedure .given by Skoog and West (10) was followed for the iodometric titration. The eauioment and . . skills necessary to carry out this portion of the experiment are tvoical of classical oxidation-reduction titrimetry. The oxidiiion-reduction reaction of bromate with exceis bromide in acid solution is used to generate an excess of bromine which oxidizes the ascorbic acid in the unknown sample. This reaction is fast and stoichiometric (7, 8). 626 /Journal of Chemical Education
rk!!, 1.5V
Batt.
Switch SPST
E 25K ohm
DPDT
Switch
Pt Indicator Electrodes
Figure 1. Detector circuit for amperometric end point detection. DPDT = double pole double throw. SPST = single pale single throw.
The excess bromine is then converted to iodine by reaction witb potassium iodide. Finally, the iodine (triiodide) produced by this reaction is titrated with standard thiosulate using starch indicator. The unknown samples issued to the students consisted of commercially available vitamin C tablets with ascorbic acid levels ranging from 100-500 mg per tablet. The unknown tablet is weighed to the nearest *0.1 mg and transferred to a 250.0-ml volumetric flask, dissolved in approximately 100 ml of distilled water (dissolution takes about one-half hour), diluted to volume and mixed. The resulting solution will be somewhat cloudy due to the presence of inert substances in the tablet. The solution must he kept out of the light as much as possible and must be titrated within 2 hr or significant deterioration will result. Coulometric Titration of Ascorbic Acid at Constant Current The determination of ascorbic acid is carried out in this part of the experiment by using a slightly modified version of the coulometric procedure reported by Evans (11). The apparatus includes a constant current amperostat of the type described by Vincent and Ward (12). A milliammeter was used to monitor the curr?nt through the generator electrodes. The titration flask and the generating and indicator electrodes were identical to those previously described (11). The amperometric end point detection sysPresented at the 165th National Meeting of the American Chemical Society, Dallas, Texas, April, 1973.
tem consisted of two platinum electrodes, a 1.5-Vdry cell, a 25-Kohm single turn potentiometer and a sensitive microammeter. These comDonents were assembled in a small case. In addition, a standardized millivoltmeter (Beckman Zeromatic) was used to monitor a n a p d i e d DOtential of 0.25 V across the indicator electr&ies. The amperometric detector circuit is shown in Figure 1 and the coulometric assembly is shown photographically in Figure 2. The electrolysis is carried owt a t 20 mA using platinum electrodes in an aqueous, acidic medium containing dissolved potassium bromide and the dissolved unknown sample. The mixture is stirred magnetically and the bromine generated a t the anode instantly reacts with the dissolved ascorbic acid from the unknown sample. ~
~~
OH HO-CH,-CH-C.
I
H I/\/O
.C
+
Br;
-
Figure 2. Caulometric apparatus used for ascorbic acid titratians.
ascorbic acid
dehydraascorbic acid Reagents and Procedure
1) Electrolytic Soluent. The solvent consists of 1:I glacial acetic aeid-water and dissolved KBr (0.15 F ) . 2) Unknown Solution. The same unknown solution used in the first part of the experiment can he used here. 3) Coulometric Titmtion. A blank titration is initially carried out in order to remove oxidizable impurities, and known quantities of ascorbic acid (from 2-20 mg) are then successively added and titrated using the procedure of Evans (11). Each electrolysis is terminated at a steady indicator current of 20 @amp.Finally, the titration is repeated for each of three identical 10.0 ml unknown samples. The students prepare a calihration curve of mg ascorbic acid versus titration time. A linear plot which passes through the origin is obtained and is utilized to calculate the number of mg of ascorbic acid (or percentage by weight) in the titrated sample and in the unknown. A typical calihration plot is shown in Figure 3. The average value of three titrations is reported. As part of the laboratory report, the answers to several questions on eoulometry and ampemmetry were assigned.
Titration rime (min)
Figure 3. Coulometric calibration electrodes current: 20.0 mA.
for ascorbic acid. Generating
Table 1. lodometric Analysis of Commercial Vitamin C Tablets and Pure Ascorbic Acid Samples
me Ascorbic A G in~sample (Manu- mg Ascorbic Number of facturer's Acid Found Trials Label)
Results and Discussion
Results for the iodometric and coulometric methods are given in Tables 1 and 2. It is seen that this experiment gives very satisfactory results when performed by students. A careful analysis of the deviations of the measurement yields an uncertainty of f2% depending on the accuracy of the student's standardization of the thiosulfate solution. It was thought that the presence of particulate matter &om the tablets in the unknown solutions could indicate incomplete extraction of ascorbic acid from the tablets. Several control experiments in which the students pulverized and ground the tablets prior to the analysis indicated that this was not the case. The presence of the undissolved material did require extra care, however, when pipetting the solutions. The advantage of coupling the rather involved iodometric procedure with the faster and simpler cou~ometric analysis illustrates the obvious advantage of the latter: The coulometric method also illustrates the very useful amperometric end point detection technique, as well a s the use of a reagent, such as electrolytically generated bromine which is difficult to use a s a standard buret titrant. It is essential t h a t the instructor explain the theory
curve
Sam& Rexall Chewable Vitamin C Tablets (Lot No. K12003) Squibb Ascorbic Acid Tablets (U.S.P.) (Control No. 037223 Purepac Vitamin C Tablets (Lot No. 0170K3) Squibb Ascorbic Acid Tablets (U.S.P.) (Control No. 0L561) Purepae Vitamin C Tablets (Lot No. 0167K3) C. P. Reagent Grade Ascorbic Acid (Eastman)
100
101f5
5
100
108 f 2
5
250
255 + 6
7
250
259 f 6
9
500
499 f 2
I
562
563 f 1
3
504 469
504 + 1 468 1
3 3
*
Volume 50, Number 9. September 1973 / 627
Table 2. Coulometric Analysis of Vitamin C Tablets
Sample Squibb Ascorbic Acid Tablets (U.S.P.) (Control No. 0B695) Squibb Ascorbic Acid Tablets (U.S.P.) (Control No. 0E722) Squibb Ascorbic Acid Tablets (U.S.P.) (Control No. 0L567) Purepae Vitamin C Tablets (Lot No. 0167K3)
mg Ascorbic Acid in samole (Manu- ma Ascorbic Number 'faiturer's - Acid of Label) Found Trials
50
50.4 + 1
3
100
101 + 1
7
250
255 -L. 4
6
500
498 + 2
2
of constant current electrolysis, and particularly amperometric end point detection. There was some concern that the accuracy of the coulometric method might suffer by omitting the separate glass frit cathode compartment often employed in this technique. The linearity of the calibration plot and the analytical results, however, indicated that this added mea-
628 1 Journal of Chemical Education
sure was unnecessary. We constructed and utilized two complete coulometric titration units and assigned a pair of students to each instrument. By staggering the laboratory schedule over several weeks, it was possible for 35 students to perform this part of the experiment. Acknowledgment Partial support for this work by a departmental grant from E. I. duPont de Nemours and Company, Inc. is gratefully acknowledged. The authors also wish to thank Mr. Hamy D. Wilson and Mr. Peter J. Bergonia, Jr. for their assistance in the construction of the coulometric equipment. Literature Cited (1) Psuling, I.C.. -vitamin C and The common Cold," W. H. Freeman and co., s a n Francisco. 1970. (2) The P h m a e o p e i a of The United Stat-, (18th Ed.), Beth-da, Md., 1970, pp. 5153. (3) H~nuit.. W- iEdit0,i. "Otfieisl Methods of Ans?vsi. of tho Amociatio" of Offici.4 Weshindrm.D.C.. Analvtieal Chemiaul."lllthEd.l.A.O.A.C.. . . . . . 1970.o. 777. (41 Whifa.V.R..andFitzgeraid. J.M.,Anai. Chsm., 44.1267(19721. 151 Sennella.. LT.. .and4moudeIis. C. J..Anvl. Chem.. 41. 171 119691 Krize, I.. Formotaiyo, i8.59 (1959). Schulek,E.,Kov&cs. J., andRasa,P.,Z.Anal. Chem.. 121.17 119411. Skwg. D. A,, and West. D. M., "Principle of Instrumental Analysis: Holt, Wnehart, and Winston. Ino.. NeuYork, 1971 p. 552. Kolthoff. I. M.. Sendell, E. B., Meehan. E. J., and Bruckenstoin. 8.. "Qusntitative Chemical Anslysia: Macrnillan Co.. London. 1969, pp. 820snd862. Skwg, D. A,. and West, D. M., "Fundamentala of Analytical Chemistry," Holt, Wnehert, and Winston, Inc., NeuYork, 1969, pp. 449-58. Evans, 0. H.. J. CHEM.EDUC., 45.88 (19681. Vineenf, C.A., and Ward,J. G..J. CHEM.EDUC., 46,613(19691.
