170 JOURNAL OF THE AMERICAN PHARMACEUTICAL ASSOCIATION

Iodimetric Titrimetric Determination of Ascorbic Acid*

By JANE ADAMS, MARGARET ACKER, and H. A. FREDIANIt

The iodimetric method for ascorbic acid determination is rec0mmende.d for routine testing. Excellent reproducible equivalent points, in colored as well as colorless solutions, may be obtained by the use of a vacuum tube potentiometer (titrimeter). The polarized platinum-platinum electrode system serves best in this titration. With a polarizing current of 1.5 microamperes (at 1 volt) the equivalent point is suffici- ently sensitive at a meter setting of -0.25 volts t o enable the use of 0.01 N iodine

solutions.

HE EXTENSIVE current literature (2) on the determination of ascorbic acid indi-

cates the widespread interest in methods of

EXPERIMENTAL

Various electrode pairs were investigated to de- termine whether the apparent lack of sensitivity in-

analysis for this vitamin. The apparent diE- culties encountered in applying polarographic ‘(3) or potentiometric (4, 8) methods to this determination have left modifications of the colorimetric method suggested by Mindlin and Butler (5 ) based on the 2,6 dichloro- phenol indophenol procedure of Tillmans and co-workers (10) as the preferred meth- ods (7).

It recently having become necessary in this laboratory to analyze large numbers of routine samples for added ascorbic acid con- tent on a colored pharmaceutical product it was decided to objectively review the cur- rently known methods before choosing one for routine work. Since the ascorbic acid to be used was to be initially assayed by the official U. S. P. (1 1) method utilizing a visual iodimetric titration, this method was inves- tigated potentiometrically and found to be admirably adaptable to the problem at hand. With the high sensitivity vacuum tube potentiometers currently available, a direct titration was found feasible obviating the back-titration recommended-bv Stevens

dicated by Kirk and Tressler (4) could be overcome. Figure 1 clearly indicates that for the calomel- tungsten, platinum-tungsten, and calomel-platinum electrode systems an e. m. f. change of but 0.05 to 0.15 volt could be expected a t the equivalent point. Use of a platinum-platinum system wherein a polar- izing current of 1.5 microamperes (at approximately 1 volt) was utilized showed a clear-out e. m. f. change of 0.68 volt indicating that the desired sen- sitivity could be expected under these conditions. The curves in Fig. 1 were drawn from data obtained by titrating known amounts of ascorbic acid in solu- tion with 0.1 N iodine. Starch solution indicator was initially added so as to compare the visual with the potentiometric end points. It was interesting to note that with a polarizing current of 1.5 micro- amperes the two end points coincided exactly. A single drop of iodine caused the “magic eye” to open completely and the blue coloration to appear. This was reversible since an added drop of equivalent strength ascorbic acid solution caused the “eye” to close immediately and the blue coloration to dis- appear. The potentiometric sensitivity was ‘re- tained when 0.01 N iodine was used as titrant al- though the visual sensitivity declined to the point where 4 drops of added reagent became essential. Initially a rather high polarizing current of 15 micro- amps (approx. 1 volt) was employed. Although the e. m. f. change at the equivalent point was of the same order of magnitude with the high and low cur-

(9). unquestionable one droi.equiva- lent point with iodine solutions as dilute as O.O1 indicates the method to be applicable over a wide range of ascorbic acid concentra- tions. The use of unstable dve solutions is

rents, it was found t h h the potentiometrk end point was more sluggish at the higher currents and did not appear until an of from o,2 to 0,3 ml, of iodine (visual observation) was present. Further investigation is being carried on along these lines.

Recommended Method.-Based on the informa-

rendered unnecessary and the to routine determinations enhanced.

tion gained from the preuminary electrode pair studies the following procedure was then used. Sufficient metaDhosphoric acid (2%) was added to - - the sample to be titrated to reduce effects of air oxi- dation as’recommended by Musulin and King (6) and Fujita and Iwotake (1). A 250-ml. Pyrex beaker

* Received Dec. 10, 1946, from Eimer and Amend, New

t Present address- Merck and Co., Rahway, N. J. York City.

SCIENTIFIC EDITION 171

dial was then set a t a reading of 75 (-0.25 volt). At this point the “eye” remains completely closed. The standard iodine solution was then added until a single drop caused the “eye” to open permanently. Standard iodine solutions of 0.1,0.05, or 0.01 N were used, depending upon the ascorbic acid content of

was used for titration vessel. Sufficient distilled water was added to have a h a 1 volume.of 100-200 ml, so that the stirring electrodes remained covered with solution while rotating. The Fisher titrimeter (Fig. 2) was then calibrated so that the potentiom- meter dial read from 0 to -1 volt. The instrument

35 40 45

m1.yfJ 1, Figure 1

TABLE I.-ASCOFUHC ACID DETERMINATION IN GRAVIMETRICALLY PREPARED STANDARDS

Original Sample G~./L.’ 7.3 7.3 7.3

Taken, Mg./MI. 7.32 7.32 7.32

Found, Mg./Ml. 7.32 7.30 7.32

7.3 7.32 7 . in 7.3 7.32 7.30 9.9 4.95 4.95 9.9 4.95 4.95 9.9 4.95 4.93 4.95 2.47 2.46 4.95 2.47 .2.45 4.95 2.47 2.46 ~. _ _ 2.47 2.47 2.38 2.47 2.47 2.38 2.47 2.47 2.41 0.99 0.49 0.42 0.99 0.49 0.47 0.99 0.49 0.42

0.01 0.002 0.002 0.01 0.002 0.002

0.01 0,002 0.002

the unknown. so that from 20 to 30 ml. of titrant were employed.

TABLE II.-ASCORBIC ACID CONTENT OF DARK COLORED PHARMACEUTICAL PRODUCT

Ascorbic Acid (Total)

Added, Mg. 0.00 0.00 7.30 7.30 7.30 7.30

10.90 10.90 10.90 10.90 18.20 18.20 18.20 18.20

Found, Mg. 0.00 0.02 7.28 7.28 7.26 7.28

10.82 10.87 10.82 10.86 18.13 18.15 18.15 18.12

172 T,IURM..~\L or: 7 I I V AMFRICAN PIIARMACRUTICAL ASSOCIATION

Figure 2

DISCUSSION

Tablc I indicates the reproducibility attainable in the potentiometric titration of buffered ascorbic acid solutions under the above. conditions. The iodine solutions used were initially standardized against Bureau of Standards arsenious acid.

The commercial pharmaceutical which was to be controlled contained, in addition to the added as- corbic acid, caramel (sugar coloring), potassium io- dide, and two fluid extracts. To ascertain whether

any iodine oxidizable material was present in atldi- tion to the ascorbic acid, blanks were initially dz- termined on experimental samples, from which the ascorbic acid was omitted. A zero blank was ob- tained.

These experimental samples were enriched by the addition of known quantities of ascorbic acid and were then analyzed. The results are summar- ized in Table 11.

CONCLUSION

Potentiometric means for equivalent point detection permits the use of the iodimetric mqthod for ascorbic acid determination in colored samples. The advantages of the

iodine titration method, aptly discussed by Stevens (9), are retained while the double back-titration found necessary to facilitate equivalent point detection is eliminated.

REFERENCES

(1) Fujita, A, . and Iwotake. D., Biochcm. Z . , 277. 293. (7) Nelson, W. L., and Somers, G. F., Jnd. Eng. Chem.. 295( 1935).

12) King. C. G . . I n d . E I I ~ . Chem., Anal. Ed. . 13, 225 (8) Ramsey, J . B . . and Cnlcichman, R . L , ibid. . 14, 319

(3) Kirk, M. M. . i b i d . . 13, 625(1941). (9) Stevens, J. W.. ibid.. 10, 269(1938). (4) Kirk, M. M.. and Tressler, D. K.. ibid., 11,322(1939). (10) Tillmans. J.. Hirsch, P., and Hirsch. W., Z. U~rier- (5) Mindlin, R. L., and Butler, A. M., J . B i d . Chem.,

122,673( 1937-1938). (!!) ‘Pharmacopceia of the United States,” Twelfth (6) Musulin, R. R., and King, C. G., ibid. , 116, 410 revision, Mack Printing Company, Easton, Pa., 1942,

(1938). p . 16.

A n d . Ed . , 17, 7.54(1945).

(1941). (1942).

such. Lcbcnsm., 6 3 , l(1932). -