Standardization of 2,6-Dichlorophenolindophenol with Ferrous Compounds A. J. LORENZ AND L. J. ARXOLD California Fruit Growers Exchange, Los Angeles, Calif.
IN
Results
THE course of investigations i t was observed that the presence of ferrous ions in canned fruits and fruit juices interfered with the 2,6-dichlorophenolindophenoltiter for ascorbic acid. Tillmans, Hirsch, and Reinshagen (4)used ferrous sulfate in the presence of sodium-oxalate to reduce the indophenol. Although this interference of ferrous ions is known, the recent use of ferrous compounds as basic standards for the indophenol does not appear to be recorded. Tillmans, Hirsch, and Hirsch (3) mention the use of ferrous salts as standards, cautioning that the experiment should be performed in subdued light.
One mole of the dye (290 grams) is equivalent to two atoms of iron, or the iron equivalent of one mole of the indophenol is 111.68 grams. One mole of the dye oxidizes one mole or 176.064 grams of ascorbic acid. This yields the following equivalents: 1 mg. of ascorbic acid * 0.000634 gram of F e + f 1 mg. of ascorbic acid 0 003157 gram of FeSOa.7H:O 1 mg. of ascorbic acid == 0.002257 gram of FeC12.4H20 1 mg. of aworbic acid 0.00445 gram of FeS04(NH&304.-
6H~0 1 mg. of Nohr’s salt == 0.2247 gram of ascorbic acid
TABLE I. INTERFEREXCE OF FERROUS 103s Acid
Dye Vsed Ml,
Blank Citric hletaphosphoric Acetic Oxalic Phosphoric Sulfuric Hydfochlorio Sitric Trichloroacetic
0.05 0 05 2 95 0 05 2.95 0.05 0.05 0.05 0.05 0.05
Believing that the titration of ascorbic acid by the indicator and then of ascorbic acid by iodine is the most accurate method of standardization, the authors hal-e used it as giving the correct value of the dye. Table I1 reports a n experiment in n hich the various ferrous compounds in concentrations of 1 gram per liter have been compared as standards.
Speed of Reaction Very Very Very Verv
veri.
Slow very Very Very Very
slow slow rapid slow rapid slow slow slow slow
TABLE 111. COMPARISON OF METHODS (Iodine-ascorbic acid standard = 0 . 2 0 3 mg. per ml. of vitamin C equivalent of the 2,G dye) Sample Ferrous Method Error Sample Thio Method Error MQ./rnZ. .lfg./ml. % 10ml. 0.279 -4.4 2 ml. of 0.295 $0.68 hfohr’s 0,299 +2,00 0.279 -4.4 of dye $4.4 salt solu0.299 + n . 00 SKI 0.306 tion (1 0.295 +0.68 0.277 -5.1 gram per 0,290 -1.00 0.284 -3.0 liter)
The authors feel justified in presenting the present paper, because as late as 1936 (1) the belief n-as expressed that this interference occurs only in neutral solution. K o r k in this laboratory indicates t h a t this reaction also occurs in the presence of certain acids. Table I shows the results 77-hen 10 nil. of each acid (at concentrations giving a pH of 3.0) and 5 ml. of hlohr’s salt solution (1 gram per liter) were titrated with an unknown concentration of 2,6 dye. 811 solutions shon-a fading of the pink end point on standing. Only oxalic and metaphosphoric gave a quantitative reaction. The end point in oxalic acid fades rather rapidly. The most accurate end point is regarded as the first pink color which remains for 30 seconds.
Nenaker and Guerrant ( 2 ) , as n-ell as Buck and Ritchie, have recently presented a method for standardizing the indicator by oxidizing potassium iodide to iodine, which, in turn, is titrated with sodium thiosulfate. Their results show that the thiosulfate method gives values consistently 2 per cent lower than their iodine-ascorbic acid titration. A comparison of the two methods gave the results presented in Table 111.
TABLE11. STANDARDIZATION OF 2,6 DYE Standard Iodine FeClz FeSO4 Mohr’s
Concentration
Indophenol
N
M1.
0.01057 0.00503 0.00359 0.00255
5 Ob0 5.075 3.650
:
Summary
Error Mg./ml.
%
0.310 0.316 0.313 0.308
$1.90 +0.90
The use of ferrous compounds (particularly ferrous animonium sulfate) in the presence of metaphosphoric acid or oxalic acid is suggested as a basic standard for 2,6-dichlorophenolindophenol.
0.00 -0.64
Literature Cited
The use of ferrous ammonium sulfate (Mohr’s salt) as a possible convenient basic standard for 2,6-dichlorophenolindophenol has been investigated. The method used is as follows:
(1) Rekelen, bl. v a n , and E m m e r i e , A., Biochem. J . , 30, 25-7 (1936). (2) h l e n a k e r , M. H., and G u e r r a n t , N. B., IND.ESQ. CHEM.,A n a l . Ed., 10, 25 (1938). (3) T i l l m a n s , J., H i r s c h , P., and H i r s c h , F,, 2. Untersuch. Lebensm., 56, 272-92 (1925). (4) T i l l m a n s , J., H i r s c h , P., and R e i n s h a g e n , E., Ibid., 63, 1-21
Dissolve 1 gram of the ferrous salt in water, add 10 ml. of concentrated sulfuric acid, and make up to 1 liter. To 2 to 5 ml. add 5 to 10 ml. of oxalic acid (saturated, approximately 4 per cent) or metaphosphoric acid (3 per cent). Titrate with the 2,6-
(1932).
dichlorophenolindophenol.
RECEIVEDJuly 14, 1938.
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