Determination of Ascorbic Acid in Citrus Fruit Juices ROBERT BALLENTINE Physiology Laboratory, Princeton University, Princeton, N. J.
T
Mack and Tressler ( 1 ) modification of the Tillmans method (6). 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 N potassium iodate.
H E chemical methods for the determination of ascorbic acid (vitamin C) have been mainly based on the method of Tillmans, Hirsch, and Hirsch (6) employing 2,6-dichlorophenolindophenol or on some modification of the iodometric titration. I n 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 ib 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
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 . 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 t o 5 ml. of citrus fruit juice 1ml. 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. I t is best not t o add the starch until very near the end point. 1 ml. of 0.01 N iodate = 0.88 mg. of ascorbic acid.
Results and Discussion
h 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 TABLEI. DETERMIXATION OF ASCORBIC ACID for the ascorbic acid content. Stevens’ Indophenol Titration Iodine Titration Iodate Titration Analyses on citrus juices are recorded in Table 11. The reAscorbic Ascorbic Ascorbic Ascorbic Acid acid acid acid sults obtained by the Stevens and iodate methods are comTaken found Recovery found Recovery found Recovery pared with the ascorbic acid content as determined by the MQ. Mg. % Mg. % M Q. 70 Mack and Tressler indophenol titration as a standard. Ste101.7 102.8 1.77 103.4 1.80 1.75 1.81 2.08 103.9 1.88 94.0 2.09 104.5 2.00 vens’ method gives variable results that differ rather widelv 95.0 2.07 103.6 104.5 1.90 2.00 2.09 from those determined by the indophenol 98.6 2.97 99.0 103.3 2.96 3.00 3.10 method, being up to 12 per cent too high. On the other hand, the ascorbic acid conTABLE 11. ANALYSESOF CITRUSJUICES tent as determined by the iodate method Iodate Titration--agrees with that found by the indophenol Stevens’ Iodine Titration Difference titration. On the basis of these results the Difference Differenpe from from from Stevens’ iodate method can be recommended for Indophenol indophenol indophenol iodine Material Titration titration titration titration routine determinations of ascorbic acid in G./100 nil. G./lOO ml. G./100 rnl. c7r ‘b citrus juices. Lemon juice, 1 0.0460 0.0470 0.0460 The iodate titration has the following 0.0471 2.1 0.0461 0.0 -2.1 advantages over the double back-titration Lemon juice, 2 0.0427 0.0465 0.0434 0.0461 8.4 0.043s 2.1 -6.2 method of Stevens: increased accuracy, Lemon juice, 3 0.046% 0.0460 - 0.5 0.0462 0 . (I 0 . .i stability of reagents, greater simplicity and Lemon juice, 4 0.0396 0.0397 0.2 0.0392 - 1. 0 -1 2 rapidity, and more reproducible results. 7 -
Lemon juice, 5
U.0416
0.0456 0,0446
Orange juice, 1
0.0416
0.0420 0.0420
Orange juice, 2
0,0368
8.8
0.0424
1.9
-6.4
0.9
0.0419 0.0417
(1
.5
-0.4
0.2
0.038i
0.0390 0.0368
0.03ST
Orange juice, 8
0 . 11899
0.0346 0,0344
1.7
Grapefruit juice, 1
0.0424
u ,0430
0.0461 0.0470
9,s
0,0370 0.0870
0.0412 0.0416
Grapefruit juice, 2
-0.5
0.0341 0.0446
0.6
-1.1
0.0442
1 0
-4.5
1.4
-9.4
0 . 037%
11.S
-0..i
0.084’
0.0378
89
Literature Cited (1) M a c k , G. L., a n d Tressler, D. K., J. Bid. Chem., 118, 735 (1937). ( 2 ) Menaker, M. H . , a n d G u e r r a n t , N. B.. IND. ENG.CHEM.,Anal. E d . , 10, 25 (1938). (3) S t e v e n s , J. W., I b i d . , 10, 269 (1938). (4) T a u b e r , H., a n d Kleiner, I. S.,J . B i d . Chem., 108, 563 (1935). f5) T i l l m a n s , J., H i r s c h , P., a n d H i r s c h , W. Z., 2 . Cntervrch. Lebensm., 63, 1 (1932).
