232
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ANALYTICAL EDITION
Vd.
a, XO. 4
Determination of Phenol in Presence of Salicylates' E. H. Hamilton and C. M . Smith FOOD,DRUG,A N D INSECTICIDE ADMINISTRATION, U S. DEPARTMENT OF AGRICUI,TURE, WASHIKGTON, D . C.
IXCE the passage of the Federal Caustic Poison Act it has become necessary t o ascertain the percentage of phenol (carbolic acid) in many coal-tar insecticides and disinfectants. For this determination the Insecticide Control laboratory of the Food, Drug, and Insecticide Administration employs a method devised by Chapin (6) for use with saponified cresol solutions. Briefly this method is as follows: The product is dissolved in water and made to a known volume. Aliquots of the solution are placed in each of two test tubes and treated with Millon's reagent. A pink solution develops if phenol is present. Then formaldehyde is added to the solution in one of the test tubes and this changes the color to yellow. A pair of test tubes containing a standard aqueous solution of phenol are treated similarly. The amount of phenol in the unknown is determined by adding successive known quantities of the red standard solution to the yellow unknown solution and equal quantities of the yellow standard solution to the red unknown solution, comparing the colors after each addition, until the color of the two unknown solutions becomes the same. The proportion of phenol present in the unknown is then calculated from the number of cubic centimeters of the standard solution used. This method has given satisfactory results except with products containing salicylates, for, as Chapin ( I ) has pointed out, salicylates give the same color changes as phenol. Some insecticides, especially mineral oil solutions of phenols, often contain oil of wintergreen or oil of birch as a perfume, and both these essential oils consist largely of methyl salicylate. The first two products of this nature analyzed showed 12.5 and 11.3 per cent phenol by Chapin's original method using sodium hydroxide, 9.1 and 9.2 per cent when the sodium hydroxide was omitted, and 8.3 and 8.7 per cent by a procedure (suggested by E. L. Griffin of thi; laboratory) depending on saponification of the methyl salicylate, separation of the salicylic acid from the tar acids by means of a sodium bicarbonate solution, and estimation of the phenol by Chapin's method. The last procedure was presumed to give the correct results, but because it is rather long and tedious, and also because the omission of sodium hydroxide from Chapin's method resulted in fairly comparable values, it was decided to investigate the possibility that the kerosene originally in the product, or added to it, might be counted on to retain the methyl salicylate while the water used might extract the phenol. For this purpose it was desirable to know the partition coefficient of phenol between kerosene and water. As there appeared to be no information on this subject in the literature, the partition coefficient was determined as follows: A stock solution was prepared from c. P. phenol and water to coiitain approximately 4 grams of phenol per 100 cc., and from this solutions of one-half and one-fourth its strength were prepared. A 25-cc. portion of each solution was shaken with an equal quantity of kerosene and the mixture allowed to separate into two layers. The aqueous layer was drawn off, filtered through a wet filter to remove any residual kerosene, and the amount of phenol remaining determined by titration of an aliquot with a standard bromide-bromate solution. The results are given in the following table. No attempt was made to approach equilibrium from the other direction because of the low solubility of phenol in
S
* Received July 29, 1929.
kerosene. The results indicate that conditions are favorable for extraction of phenol from kerosene by means of water.
.
INITIAL SOLUTION Grams p e r
Grams p e r
100 cc. 4.05 2.02
1.01
KEROSENE PARTITION LAYER COSFFICIENT ( B Y DIFF.) Grams $er
100 cc.
100 cc.
3.20 1.62 0.81
0.85 0.40 0.20
3.8
4.0 4.0
Five kerosene solutions containing known proportions by weight of phenol, oil of birch, and para-cresol were then prepared. A commercial grade of birch oil and chemically pure phenol (congealing point 38.8" C.) were used. The para-cresol, of which a technical grade boiling at 202' C. and containing 0.2 per cent phenol was used, was added to increase the solubility of the phenol. The results given later prove that, even though this cresol aids in dissolving phenol in kerosene, it does not retard the extraction of the phenol by water. The phenol in these preparations was, for purposes of comparison, determined by four methods, as follows: A, directly by Chapin's method for unsaponified cresol solutions; B, directly by Chapin's method for saponified cresol solutions; C, by a method in which 50 cc. of kerosene were added to 10 cc. of each solution, after which the phenol was extracted by shaking three times with 100-cc. portions of water and the aqueous extracts were then shaken successively with 25- and 15-cc. portions of kerosene, filtered through a wet filter into a 500-cc. volumetric flask, and made to the mark with water, the phenol being then determined a6 in B; D, by a method exactly like C except that the washing of the aqueous extracts with kerosene was omitted. The composition of each solution and the percentage of phenol found by the various methods are given below.
1-
COMPOSITION SOLN. phenol
'2iZ
P-
KeroCresol sene
PHENOLFOUND Method Method Method Method A B C D
%
%
%
%
%
%
%
6
10
5
80
8.6
5.5 5.5 10.6 10.1 15.0 15.0 10.4 10.4 10.6 10.6
4.9
8.6 13.6 13.6
10
10
10
70
15
10
15
60
10
5.
10
75
10
20
10
60
18.1 18.1 12.1 12.1 16.6 16.4
10.1 10.1 14.8 15.0 9.9 9.8
10.1 10.2
% 5.2
5.1 10.0 10.1 14.8
14.8
10.2 10.2 10.2 10.2
It is evident that method A gives results which are very much too high, owing to the saponification of some of the methyl salicylate by the sodium hydroxide used. Method B gives results which, while quite close to the amount of phenol actually present in the solutions, tend to be slightly high. Methods C and D both give results agreeing with the theoretical within the limit of error of the methods on pure phenol solutions. As Method D is simpler than Method C in that the washing of the aqueous extract with kerosene is omitted and thereby the formation of troublesome emulsions avoided, it is preferable. It also offers a considerable saving of time and labor over the sodium bicarbonate extraction method. Literature Cited (1) Chapin, J. IND. ENG.CHEM.,12, 771 (1920). (2) Chapin, U. S. Dept. Agr., Bull. 1808 (1924).
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