Determination of Ascorbic Acid in Citrus Fruit Juices ROBERT BALLENTINE Physiology Laboratory, Princeton University, Princeton, N. J.
Mack and Tressler (1) modification of the Tillmans method (5). The indophenol dye, recrystallized from the Eastman product, was standardized by the method of Menaker and Guerrant (2). All the reagents used in the three methods were referred to a single primary standard solution, 0.1 A potas-
chemical methods for the determination of ascorbic the method of Tillmans, Hirsch, and Hirsch (5) employing 2,6-dichlorophenolindophenol or on some modification of the iodometric titration. In general the iodometric methods have been subject to criticism since they lack specificity and a sharp end point, although Stevens (3) has shown that by employing a double back-titration in the presence of a high concentration of hydrogen ions the iodine end point may be made very sharp. Further, Tauber and Kleiner (4) have demonstrated that the iodometric method is adequate for the determination of ascorbic acid in citrus fruit juices, since interfering substances are absent. However, in common with all iodometric methods, the reagents employed in Stevens’ method change their titer with time, requiring frequent standardization, and the double back-titration increases the volumetric error. In the method presented here, a direct titration with iodate in acid-iodide gives an end point comparable in sharpness with that of the above double back-titration method, and in addition avoids the necessity for frequent standardization. Only one accurately prepared solution is required, and this is stable over long periods of time. In addition to the iodate titration described below, the ascorbic acid content of the citrus juices was determined by the method of Stevens (3). The juices were also titrated by the acid (vitamin C) have been mainly' based on THE
Table I.
Determination
of
2.00
3.00
2.09
3.10
% 103.4 104.5 104.5 103.3
Mg. 1.88 1.90 2.96
Table II.
Mg.
% 102.8 94.0 95.0
1.80
1.77
2.08 2.07 2.97
98.6
Titration G./100 ml. 0.0460
103.9
103.6 99.0
of
Citrus Juices
G./100 ml. 0.0470
%
Difference
indophenol titration G./100 ml. 0.0460 0.0461 0.0434 0.0438
%
0.0462
0.0
Lemon juice,
1
Lemon juice,
2
0.0427
0.0465
Lemon juice, Lemon juice, Lemon juice,
3
0.0462
0.0460
4
0.0390
0.0397
0.2
0.0392
5
0.0416
0.0458 0.0448
8.8
Orange juice,
1
0.0416
0.0420
0.9
2.1
0.0471 0.0461
Orange juice,
2
Orange juice,
3
Grapefruit juice,
1
Grapefruit juice,
2
0.0388
0.0390 0.0388
0.0399
0.0346
0.0424
0.0461
0.0430
0.0470
0.0370 0.0370
0.0412 0.0416
0.0344
—
8.4 0.5
Difference Stevens’ iodine
titration
0.0
-2.1
2.
-6.2
1
0.5
1.0
-1.2
0.0424
1.9
-6.4
0.0419 0.0417
Cl. 5
-0.4
-
1.7
0.0387
-0.3
-
0.
Ó
0.0342
0.0341
0.0
-1.1
9.8
0.044G 0.0442
4.0
—
11.8
0.0372 0.0378
1.4
-9.4
89
as
determined by the iodate method
with that found by the indophenol titration. On the basis of these results the iodate method can be recommended for routine determinations of ascorbic acid in agrees
citrus juices. The iodate titration has the following advantages over the double back-titration method of Stevens: increased accuracy, stability of reagents, greater simplicity and rapidity, and more reproducible results.
Literature
0.0387
0.2
being up to 12 per cent too high. On the other hand, the ascorbic acid con-
tent
-Iodate Titration
indophenol titration
0.0420
A preliminary comparison of the three methods employed for the determination of ascorbic acid was carried out on solutions of synthetic, crystalline ascorbic acid (Merck). The results, presented in Table I, show that the indophenol and the iodate titration are in agreement, although the absolute amounts recovered were slightly high. On the other hand, the double back-titration of Stevens gave slightly low values for the ascorbic acid content. Analyses on citrus juices are recorded in Table II. The results obtained by the Stevens and iodate methods are compared with the ascorbic acid content as determined by the Mack and Tressler indophenol titration as a standard. Stevens’ method gives variable results that differ rather widely from those determined by the indophenol
___method,
Analyses
Indophenol
Results and Discussion
% 101.7
Stevens’ Iodine Titration Difference
Material
=
Recovery
Recovery
Recovery
Mg. 1.81 2.09
Potassium iodide, 10 per cent solution fresh daily. Sulfuric acid, 2 N. Starch indicator is made fresh daily from improved Lintner’s soluble starch, using 1 gram per 100 ml. with 2 grams of potassium iodide added. Procedure. Add to 5 ml. of citrus fruit juice 1 ml. of 10 per cent potassium iodide and 2 ml. of 2 N sulfuric acid. Titrate the resulting solution with 0.01 N iodate, adding the reagent dropwise near the end point. It is best not to add the starch until very near the end point. 0.88 mg. of ascorbic acid. 1 ml. of 0.01 N iodate
Iodate Titration Ascorbic
Iodine Titration Indophenol Titration Ascorbic Ascorbic
Taken Mg. 1.75 2.00
Reagents. Standard iodate, 0.1 N (0.0167 M), contains 3.567 grams of potassium iodate per liter. For use this stock solution is diluted to 0.01 N.
Ascorbic Acid
Stevens’
Ascorbic
sium iodate.
4. ó
Cited
(1) Mack, G. L., and Tressler, D. K., J. Biol. Chem., 118, 735 (1937). (2) Menaker, . H., and Guerrant, N. B., Ind. Eng. Chem., Anal. Ed., 10, 25 (1938). (3) Stevens, J. W., Ibid., 10, 269 (1938). (4) Tauber, H., and Kleiner, I. S., J. Biol. Chem., 108, 563 (1935). (5) Tillmans, J., Hirsch, P., and Hirsch, W. Z., Z. Untersvch. Lebensm., 63, 1 (1932).
