V O L U M E 2 5 , NO. 11, N O V E M B E R 1 9 5 3 Determination of Fruit Acids by Oxidation. Under the iecommended conditions of oxidation, the fruit acids react according to the following half-equations.
+ 3H20 + 12H+ + 12e + 2H2O +.4COs + 10Hf + 10e + 5H20 6C02 + 18H+ + 18e
HzC4H405 H2C4HlOs H3C~Hb07
+4 2 0 2
1639 error in determination of comparable quantities by the proposed method is about 0.1 mg. Furthermore, the proposed method is much more rapid. The classical procedure for the determination of any one of the major fruit acids requires about 2 days; but all three acids can be determined by the proposed method in 7 . 5 hours, during only 3 of which the operator's attention ip required.
-+
Thus the equivalent weights of malic, tartaric, and citric acids of the molecular weights, respectively. are 1/12, l/10, and A large excess of permanganate is required to oxidize the fruit acids quantitatively under the recommended conditions. Therefore, it was necessary to take a one-tenth aliquot of each fraction. Thus the minimum ratio of permanganate to fruit acid in terms of equivalents was 11, and quantitative oxidation occurred in the specified times in the 30' bath. Longer periods are not desirable because of possible decomposition of the permanganate. The periods required for quantitative oxidation in a 25" bath are 120 minutes for malic acid, 165 for tartaric, and 60 for citric. The temperature of the eolution rises sonieivhat upon the addition of 9 JI sulfuric acid, but no steps are required to cool the mixture except to keep it in the bath.
3.0- ELUTION OF MALIC, TARTARIC,
Table V. Analysis of AIixture of Grape and Peach Juices Malic Acid, Mg. 6 4 6.8
I n 3.00 nil. of grape juice In 3.00 ml. of peach juice Total expected Found Difference
Table VI.
CITRIC ACID
13.2 13.3
Tartaric dcid. Jlg. 11.7 0.6 13.3 12.4
+o. 1
TO.1
Citric .icid, JIg. 0 3 9 0 9.5 9.7 t o .2
Elution of Other Fruit Constitueiits Constituent
111. of Eluate Containing Constituent
Common s u ar, Succinic aci% .' Lactic acid Malic acid Tartaric acid Oxalic acid Citric acid
23- 91 91- 148 68- 240 383- 610 645- 793 690- 8 9 0 907-1100
F I G U R E I.
A minor disadvantage stems from the nonuniformity of commercial ion exchange resins. A different batch of Dowex-1 (100to 200-mesh) might have slightly different properties and hence require slight changes in the elution conditions. It is advisable, therefore, to test each column of Dowex-1 before using it for this determination by running separate elutions of known quantities of malic, tartaric, and citric acid. In each rase, small fractions of the eluate should be analyzed in order to determine the limits of eluate within which each acid is contained. ACKNOWLEDGMENT
VOLUME OF ELUATE, MI-. The oxidimetric method of determining the isolated fluit u i d s a a s tested by dissolving weighed quantities of a fruit acid in 250 ml. of the appropriate eluant and then applying the recommended procedure. The average error for each acid was thus found to be f 0 . 1 mg. Other Constituents of Fruit Juice. An error would be incurred in the analysis, of course, if any other reducing agent in the juice appeared in the second, third, or fourth fraction of the eluate along with the malic, taitaric, or citric acid. To investigate this point, small fractions of eluate were examined for the presence of several other possible fruit constituents, in order to find the total fraction of eluate within which the constituent was eluted under the recommended conditions. The results, summarized in Table VI, indicate that oxalic acid is the only constituent of fruit juice that is likely to interfere and that it would introduce an error in the determination of tartaric acid only. This is not serious, because oxalic acid is seldom present in fruit (6) and never in excess of 0.03% (equivalent to 0.1 mg. of tartaric acid per ml. of juice). Furthermore, oxalic acid can be distinguished from tartaric acid by the difference in the velocity of its reaction with permanganate. Advantages. The proposed method is much more accurate than the classical procedure-for example, a collaborative study ( 2 ) indicated an average error of 3 mg. in the determination of tartaric acid in apple jelly by the traditional method, whereas the
The authors gratefully express their indebtedness to the Rutgers Research Council for financial support during this investigntion and to R. C. Dell for assistance in the study of the effect of pH. LITERATURE CITED
(1) Assoc. Official dgr. Chemists, "Offic. Methods of Analysis,"
7th ed., Washington, D. C., 1950. (2) Hai.tman, B. G., J . Assoc. Ofic. Agr. Chemists, 14, 459 (1931). (3) Isherwood, F. A , , Biochem. J . , 40, 688 (1946). (4) Schenker, H. H., doctor's thesis, Rutgers University, 1952. (5) W e l a n d , T., and Feld, U., 2. angew. Chem., 63, 258 (1951). (6) Winton, A. L., and Winton, K. B., "Structure and Composition of Foods," T'ol. 11, Chap. 11, New York, John W'iley R- Sons,
1935. RECEIVED for reviem January
21, 1953
A c r e y t e d August 18, 1933
Colorimetric Microdetermination of Boron by the Curcumin-Acetone Solution Method-Correction I n the article on "Oolorimetric Microdetermination of Boron by the Curcumin-Acetone Solution Method" [Silverman. Louis, and Trego, Katherine, ANAL.CHEX, 25, 1264 (1953)l a t the bottom of the f i s t column on page 1265 the last sentence under "Stock Boron Solution" should read: One milliliter equals 0.1 mg. of boron.
LOWS SILVERMAN
