T H E J O U R N A L OF I N D U S T R I A L A N D ENGINEERING CHEMISTRY
52
less soluble than the cresols and may for t h a t reason be considered as impurities. TOXICITY ASSAY
.. .. . . . . , . , , . . . . . . . . ,Purifiedpigs cresols . . .. .. ... .. .. .. .. .. .. .. .. .. ,. .. .. .. .. .. .Guinea .Subcutaneously
Sample., , , Animal.. , . . Method. , , WT OF ANIMAL 0.572 0.611 0.557 0.640 0.572
CRESOL DOSE PER KILO 0.6 0.6 0.7 0.7 0.8
RESULB Recovered Recovered Died Died Died
PHENOL
Recovered 0.5 0.437 Recovered 0.5 0.480 Died 0.6 0.446 Died 0.6 0.480 Died 0.6 0.570 0.340 Died 0.7 . .. Toxicity about 90 per cent of t h a t of phenol: Worth Hale, Hyg. Lab., Bulletin 8 8 ; James Leake and Hugh B. Corbin, Hygienic Laboratory,
Bwlletin 110.
GERMICIDAL ASSAY Sample, , , .Purified cresols Method.. , , . A . P. H. A. phenol coefficient method' Organism.. B. typhosus DILUTIONS ---TIME A N D RESULTS-SAMPLE 5 10 15 20 1-300 1-350 1-400 1-450 1-500 c PHENOL 1-100 f 1-1 10 1-120 1-130 1-140 Coefficient 3 6 1 Committee Report, Am. J . Pub. Health, 8 (1918), 506.
. .. ... .... .. .. . .. . .
~
-
-
-
-
-
-
+++
-+ +
-
++ +
-+
-
++ +
-+
-
+ ++
The cost of the process is inconsiderable, since no complicated chemical or mechanical steps are necessary. It is evident from observation of the steps in the process t h a t no unusual equipment is needed and only the commonest chemicals are employed. It is evident, therefore, t h a t here again the German chemists profited a t our expense for many years while the crude materials waited only for proper development. The logical place for the economical production of the refined cresols is where the crude material is first separated from the oils distilled from coal tar. These crude phenols, necessarily dissolved in alkali t o separate them from the neutral oils, can, a t t h a t point, by suitable means, be freed completely from their impurities, and after fractional removal of the phenol proper, the cresols could then be recovered in pure form with one operation. The production of purified cresols is, therefore, a logical opening for American enterprise, as well as American resources, for here, as in Europe, are immense supplies of coal t a r on which t o draw for crude materials. A NEW HEXABROMIDE METHOD FOR LINSEED O n 1 By Lawrence L. Steele and Frederick M. Washburn U.S. BUREAUOP STANDARDS, WASHINGTON, D. C. Received September 17, 1919 I-INTRODUCTION
I t has been recognized for many years t h a t the hexabromide, insoluble in ether, which is derived by the addition of bromine t o linseed oil or its fatty acid, is a characteristic compound, the quantitative determina1 Published
by permission of the Director of the Bureau of Standards.
Vol.
12,
No. I
tion of which should be of the greatest importance in the examination of this oil for adulteration. Several methods for the determination of the hexabromide yield of linseed oil and its f a t t y acid have been published. I n every case, the author obtained concordant results, but when the method was used by other analysts, the results reported were unsatisfactory. I n some cases different workers using the same linseed oil obtained hexabromide yields as widely divergent as 30 and 50 per cent, The authors of this paper first made a study of the published hexabromide methods and from the experience gained have developed a new hexabromide method by which i t is believed concordant results can be obtained. A preliminary study of the hexabromide compounds prepared from linseed oil as compared with the fatty acid hexabromide led t h e authors, for two main reasons, t o concentrate their attention on those methods which involve the preparation of the latter derivative. I n the first place, the hexabromide made from the fatty acids of linseed oil is more stable, has a sharper melting point and a better crystalline structure t h a n the corresponding hexabromide derived from the glyceride. I n the second place, our present knowledge of linseed oil does not tell us whether or not the linolenic acid is present quantitatively as a simple triglyceride. It is obvious t h a t if mixed glycerides are present in linseed oil the hexabromide derived from t h a t oil might be a mixture, instead of one definite compound. I n this article, only those hexabromide methods in which bromine is added to the fatty acids of linseed oil will be considered. 11-DISCUSSION
O F HEXABROMIDE M E T H O D S
I n 1909, a representative committee of linseed oil chemists1 in this country made a study of linseed oil in order to prepare specifications for its purity. One of the tests which was studied was the hexabromide yield of linseed oil glyceride b y Tolman's method.2 The results which were obtained were unsatisfactory. I n 1911 the committee tried for the first time a hexabromide test on the fatty acids of linseed oil, employing the method of Hehner and Mitchell as described by Lewkowitsch. I n brief, the method was as follows: The fatty acids of linseed oil were dissolved in glacial acetic acid and the mixture cooled to 5' C. and bromine to excess slowly added. The mixture was allowed to stand for 3 hrs., filtered on a Gooch and washed successively with 5 cc. each of chilled glacial acetic acid, alcohol, and ether. The precipitate was dried in a steam oven and weighed. The results obtained by the committee with this method were uniformly low, the hexabromide yield running from 28 t o 3 2 per cent. I n the light of the higher results obtained by the Eibner method, t o be described later, i t is evident t h a t this method was defective in the addition of bromine. I n other words, the linolenic acid was not quantitatively converted t o hexabromide under t h e conditions followed, or 1 1 8
A. S. T. M. Committee Reports, 1909. THISJOUXNAL, 1 (1909), 340. "Chemical Technology of Oils, Fats and Waxes," 1 (1904), 365.
Jan., 1920
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53
Lewkowitschl criticizes Eibner’s preparation of fatty acids in t h a t the method is unnecessarily elaborate in detail. It is believed t h a t if Eibner’s directions are carefully followed, the fatty acids thus prepared are pure, but a very much shorter method will give equally good results. Eibner dissolves the dried fatty acids in ether “dried over calcium chloride.” The alcohol and water content of ether so prepared will be variable, depending upon a number of factors: length of time The linseed oil was saponified with excess alkali, the fatty acids were liberated, washed with water until neutral and fil- of drying, amount of calcium chloride used, and size tered through dry paper. One gram of fatty acids was dissolved of particles of the drying agent. As will be shown later, in 50 cc. of absolute ether, the mixture cooled to 4-6’ C. and small amounts of alcohol in the solvent seem t o inbromine added slowly 10 excess. The mixture was allowed to fluence t o a marked degree the addition of bromine t o stand at 4-6’ C. for 30 min. and then filtered on a Gooch, wash- the fatty acids of linseed oil when they are dissolved ing with four 20-cc. portions of cold absolute ether. Washing in chloroform. It seems quite possible t h a t variable by decantation, using a centrifuge, was an optional method. results might be obtained in his method because of The precipitate was dried in a water bath and weighed. variable quantities of alcohol or water in different lots The results obtained by members of the committee of ether used as solvent for the fatty acids. The procedure of preparing a I O per cent solution of indicate t h a t in some cases a quantitative addition of bromine t o linolenic acid was attained, while it is the fatty acids in ether is open t o the objections t h a t believed t h a t the low results reported in other cases evaporation of ether will take place during the meascan be explained only by incomplete addition of bro- uring of the aliquot portion and the volume of ether mine or by incomplete crystallization of hexabromide varies quite appreciably with the temperature. Errors from the solvent used during t h e addition of bromine. in the amount of fatty acids taken for analysis would One of the members of the committee reported ex- be liable t o result from this procedure. cellent results by the use of the Eibner2 hexabromide The use of ether as a solvent for the fatty acids method, published t h e year before. This! new method during the addition of bromine is open t o the objection looked extremely promising and was given a thorough t h a t solid bromine derivatives separate from the solutrial by t h e committee in 1915 and again in 1916. tion before an excess of bromine has been added and One member reported consistently good results by it is conceivable t h a t partially brominized derivatives the new method, but other members working on the might crystallize and thus resist further addition of same linseed oil failed of agreement, and a t last the bromine and thereby lead t o low results. committee came t o the decision t h a t Eibner’s method After the addition of bromine t o the fatty acids, the possessed difficulties which a t t h a t time precluded its mixture is allowed t o stand with excess of bromine for use as a part of a linseed oil specification. z hrs. before filtration in order t o insure complete Since the Eibner method seemed t o be by far the crystallization of the hexabromide. It is possible best hexabromide method which has been published, t h a t under these conditions some substitution of hyt h e authors undertook t o study i t in detail. This drogen in the hexabromide by bromine might take method as published is exceedingly long and tedious, place. especially in t h e preparation of t h e fatty acids. For It is believed t h a t the most probable cause of variable the sake of brevity, this method will be only outlined, results is the method of filtration and washing of the b u t several points will be criticized and sources of hexabromide precipitate. A finely divided precipierror will be pointed out which i t is believed account t a t e would filter very slowly on the Gooch and confor the unsatisfactory results obtained. siderable amounts of ether would be lost by evaporation. I t would be hard t o wash such a precipitate The salient features of t h e Eibner method are: free from impurities, while a more crystalline preThe linseed oil is saponified with excess alcoholic potash, the fatty acids are liberated with mineral acid and extracted with cipitate in another determination would filter faster ether. The ether solution is dried over anhydrous sodium and could be washed more efficiently. Eibner does sulfate, the ether evaporated, and the fatty acids are dried in a not specify the quality of the wash ether. It is quite water bath in a stream of dry hydrogen. The accurately probable t h a t U. S. P. ether would give different washweighed fatty acids are dissolved in a definite amount of ether ing efficiency t h a n ether dried over sodium, for ex(dried over calcium chloride). An aliquot portion of this solu- ample. tion is cooled to -IO’ C., or below, and exactly I cc. of bromine Eibner considered the hexabromide sufficiently added very slowly, the low temperature being maintained. washed when it was white and free from yellow maThe mixture is then allowed to stand at a temperature between terial. It will be shown in the experimental part of -IO’ C . and -5’ C. for exactly 2 hrs., then filtered on a Gooch this article t h a t after a hexabromide precipitate has and washed with five 5-cc. portions of cold ether, suction being used after the last washing only. The precipitate is driedzat been washed free from yellow material, further washing with ether removes considerable amounts of white 80’ to 85’ C., cooled, and weighed. material which is not hexabromide. The fact t h a t he 1 Proc. A. S.T.M , 1913, p. 373. else the hexabromide which was formed did not quantitatively separate from the solvent in which addition of bromine was made. I n 1913 a new hexabromide method was formulated by the linseed oil committee1 but the results obtained were far from satisfactory, different analysts reporting as widely divergent hexabromide yields as 3 5 and 51 per cent on the same linseed oil. I n brief, this method was as follows:
* Farben-Ztg., November
23, 1912 (No. 8).
.
1
“Chemical Technology of Cils, Fats and Waxes,” 1, p. 573, 5th Ed.
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T H E J O U R N A L OF I N D U S T R I A L A N D ENGINEERING C H E M I S T R Y
dries the hexabromide a t 80' t o 85' C. and states t h a t drying a t 100' C. is apt t o cause sintering is very good proof t h a t the material is not pure hexabromide. As will be shown later, a properly washed hexabromide precipitate can be safely dried a t 110' C., with no sign of sintering or other deterioration. After the published hexabromide methods had been studied, an outline of a new method was developed which was designed t o eliminate the errors believed t o be inherent in the Eibner method and the other methods described. I t was proposed t o dissolve t h e fatty acids of linseed oil, prepared by a suitable method, in such a solvent as carbon tetrachloride or chloroform, in which the hexabromide was known t o be somewhat soluble, and t o add an excess of bromine t o t h e cooled mixture. The solvent was then t o be removed by evaporation and the resulting mixture of hexabromide contaminated witk other bromides washed until pure with a solvent like ether, in which the hexabromide was known to be only slightly soluble. I t was believed t h a t t h e procedure outlined above would eliminate errors due t o irregular addition of bromine. 111-EXPERIMENTAL
DEVELOPMENT
OF
NEW
HEXA-
BROMIDE METHOD
A. PREPARKTION O F F A T T Y ACIDS-The method for the preparation of t h e fatty acids of drying oils given by Lewkowitschl was tried and gave excellent results. This method is very simple and rapid, and the determination of the iodine number of the fatty acids indicated t h a t they were not appreciably oxidized. For example, a linseed oil having an iodine number of 181 (Hanus) yielded by this method fatty acids having a n iodine number of 185 (Hanus). This method was It therefore adopted without further investigation. was decided t h a t i t would be more accurate t o weigh out the sample of fatty acids for each hexabromide determination, rather t h a n t o make up a standard solution of the fatty acids in the solvent t o be used and then pipette aliquot portions. B. ADDITION OF BROMINE T O FATTY ACIDS-(I) Choice of Solvent. Chloroform was selected because i t was known t h a t the hexabromide was moderately soluble in this solvent and because it is commonly used as a solvent in various reactions involving bromine and the other- halogens. After some experimentation i t was found t h a t t h e hexabromide formed from one gram of fatty acids was sufficiently soluble in I O cc. of C. t o prevent its precipitation chloroform a t -5' during the addition of bromine. ( 2 ) Effect of Added Impurities i n Solvent. The first chloroform used was of U. S. P. quality. Later some chloroform was carefully washed with water t o remove alcohol and then dried with calcium chloride and redistilled. When this pure material was used as a solvent, the hexabromide yield suddenly dropped from an average value of 46 per cent t o about 28 per cent.2 1 "Chemical Technology of Oil4, F a t s and Waxes," 1, p 109, 5th E d . * All hexabromide yields given, unless otherwise stated, are on a single sample of raw linseed oil of known purity with a n iodine number of 181 (Hanus).
Vol.
12,
No.
I
After some time, this drop in hexabromide yield was traced t o the absence of alcohol in the chloroform. Different lots of chloroform were made up containing varying amounts of absolute ethyl alcohol. It was found t h a t the presence of 0.50 per cent of alcohol in the solvent increased the hexabromide yield from 28 per cent for pure chloroform t o 43 per cent. Mixtures with 3 and 5 per cent alcohol gave the maximum yield of 46 per cent. Amounts of alcohol as high as I O per cent lowered the hexabromide yield t o 4 3 per cent. I n order t o gain some insight into the action of alcohol in t h e solvent during addition of bromine t o t h e fatty acids, various other materials were added t o chloroform in small amounts and the mixtures were used as solvent in a hexabromide determination, I t was found t h a t 3 per cent of benzene gave a hexabromide yield of 29.6 per cent or practically the same as pure chloroform. Three per cent of methyl alcohol gave a 43 per cent yield, while 3 per cent of acetone gave 42.8 per cent. I n order t o see whether traces of hydrobromic acid catalyzed the quantitative addition of bromine t o the linseed fatty acids, some gaseous hydrobromic acid was passed into pure chloroform and the mixture used as solvent in a hexabromide determination. The yield obtained in this case was 36 per cent. The authors are a t the present time a t a loss t o explain why83 per cent of ethyl alcohol in chloroform gives t h e maximum and constant yield of hexabromide. ( 3 ) Conditions for Addition of Bromine. After a suitable solvent for the fatty acids during addition of bromine had been found, a study of other factors entering into the reaction was made. Determinations were made in direct sunlight, in ordinary diffused daylight, and in a dark room. No difference was noted between results obtained in t h e dark and in ordinary daylight, but results slightly variant were obtained when t h e addition of bromine was made in direct sunlight. I t is therefore specified t h a t direct sunlight should be avoided during addition of bromine. The length of time for the bromine-fatty acid mixture t o stand after an excess of bromine has been added was found t o be practically immaterial. The same results were obtained when t h e tube stood from 30 min. t o I hr. as when the time of standing was I O min. I n all known methods the standing of the brominized solution was important in order t o insure complete precipitation of hexabromide before filtration. Since in this new method the solvent is evaporated, there is no need of allowing the mixture t o stand t o induce crystallization of the hexabromide. It was decided t o allow the mixture, after addition of bromine, t o stand I O min. in order t h a t the reaction should be complete. I n all cases this length of time was found t o be sufficient. The optimum temperature for the addition of bromine was found t o be about -5" C. because it was low enough t o prevent local heating with subsequent substituting action by the bromine, and not so low as t o cause the hexabromide precipitate t o crystallize in a too finely divided condition, thereby increasing the difficulty of washing.
Jan., 1 9 2 0
T H E J O U R N A L OF I N D U S T R I A L A N D ENGINEERING C H E M I S T R Y
C. I S O L A T I O N OF THE HEXABROMIDE-(I) Evaporation of the Solvelzt. It was obvious t h a t if a solvent such as chloroform, in which t h e hexabromide was soluble during the addition of bromine, was used, the hexabromide could not be quantitatively removed b y filtration or decantation. I n the first determinations t h e chloroform and excess bromine were evaporated b y attaching t h e tube t o a vacuum pump. This proved t o be tedious because the evaporation cooled the solvent and greatly slowed up the process. Passing a stream of dry air through the t u b e aided t h e evaporation t o some extent, but the following better method was finally devised. Amylene was added t o absorb the excess of bromine. (This substance is an unsaturated hydrocarbon of low boiling point which readily reacts with bromine.) Heat could then be applied during the evaporation of t h e chloroform without the possibility of substituting action of bromine on t h e hexabromide a t elevated temperatures. This variation in the procedure gave very satisfactory results. Only a few drops of the amylene were necessary and any excess would be evaporated with the chloroform. The dibromamylene formed was found to be readily soluble in ether, so t h a t no difficulty resulted from contamination of the hexabromide by this halogen derivative. Experience showed t h a t it was necessary t o remove t h e chloroform quantitatively from the impure hexabromide before the washing with ether; otherwise low results were obtained through solution of the hexabromide in the ether-chloroform mixture. It was found t h a t all chloroform could be removed, if, after t h e chloroform solution of bromides was evaporated t o a thick paste, the tube was immersed for two-thirds of its length in water a t 60' C . for I S min., while the t u b e was connected t o a suction pump indicating a pressure not greater t h a n 30 t o 40 mm. of mercury. ( a ) Washing of the Hexahromide Pvecipitate. The first washing of the impure hexabromide was done with cold absolute ether. It was decided a t the start t h a t washing would be more efficient b y decantation, employing a centrifuge. I n t h e first determinations made, t h e washing was continued until the precipitate was white and t h e wash ether colorless, the criterion of purity of precipitate employed in the Eibner method. Three IO-cc. portions of ice-cold ether were found t o be sufficient t o give white hexabromide in all cases. Since it was recognized t h a t t h e hexabromide was somewhat soluble in ether, the washing was discontinued after the precipitate was white. A series of results was obtained by this method, the hexabromide yields varying from 48 t o j 1 per cent. It was believed a t t h a t time t h a t the varying results were due t o different conditions in the addition of bromine. A. series of determinations was made in which the conditions during t h e addition of bromine were varied in a large number of ways. An excess of bromine was added in direct sunlight, in diffused daylight, and in the dark, and the mixture in each case allowed t o stand for lengths of time varying from I t o 24 hrs. KO conditions could be found under which concordant results could be obtained.
55
Finally a study was made of the washing of t h e hexabromide, and here was found the explanation of t h e irregularities in the results, By evaporating t h e various portions of wash ether, and weighing t h e residue, i t was found t h a t even after three washings, when the hexabromide precipitate was snow-white, further washing would still remove white solid material t h a t was not hexabromide-enough other t h a n hexabromide t o lower t h e percentage yield from 50 or 5 1 per cent t o about 46 per cent. This fact, led t o the decision t h a t a more efficient washing of the hexabromide should be carried out. It was determined t h a t t h e hexabromide after washing with ether occupied a volume of about 2 . 5 cc. By experiment i t was found t h a t there were about 1.7j g. of oily bromides formed from I g. of linseelci f a t t y acids. Using 20-cc. portions of wash ether, it was calculated t h a t the first washing should remove approximately seven-eighths or I. 53 g. of the oily bromides. The second washing should remove seven-eighths of the amount remaining, or 0.19g., and the third. washing should remove seven-eighths of the amount remaining, or 0.028 g. The fourth washing should remove the small quantity of 0.0014 g., leaving the hexabromide practically pure. I n actual practice i t was found t h a t t h e theoretical efficiency of washing as given above was not reached. I n an experiment it was determined t h a t the following quantities of impurities were washed from t h e hexabromide formed from one gram of the^ f a t t y acids of linseed oil b y four washings of 2 0 cc. of ice-cold absolute ether. First washing. . . . . . . . . . . . . . . . . . . . 1.494g. Second washing.. . . . . . . . . . . . . . . . . 0.204 g. Third washing. . . . . . . . . . . . . . . . . . . 0 . 0 3 8 g. Fourth washing.. . . . . . . . . . . . . . . . . 0.012 g. The use of such comparatively large amounts of ether brought in the question of loss of hexabromide through solution in t h e ether. Lewkowitsch reports t h a t t h e hexabromide of linolenic acid is very slightly soluble in cold ether, but gives no figures. A determination (see experimental part) showed t h e solubility of pure, hexabromide in absolute ether t o be 0 . 0 0 0 2 6 g. per cc. a t o o C. and 0.000j6 g. per cc. a t 24.j oC. I n order t o obviate errors due t o solution of hexabromide in the wash ether, it was decided t o saturate the wash ether with hexabromide a t o o C. By this procedure loss of hexabromide through solution should be entirely eliminated. I n the preparation of the wash ether saturated with hexabromide, it was not found necessary t o use pure hexabromide recrystallized from xylene. If the hexabromide from previous determinations were used, it would contain very small amounts of other bromides which would dissolve in the ether during saturation but would do no harm because there would be no tendency of these compounds t o crystallize during washing of the hexabromide in the determination. The use of ether in washing the hexabromide precipitate was inconvenient in one respect: it was necessary t o keep the wash ether at some predetermined temperature during t h e whole process of washing.
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T H E J O U R N A L OF I N D U S T R I A L A N D ENGINEERING C H E M I S T R Y
owing t o the fact t h a t the solubility of hexabromide in ether varies considerably with the temperature. An attempt was made t o find some solvent in which the hexabromide was so slightly soluble t h a t washing could be carried on a t room temperature. Benzene (C. P., free from thiophene) was saturated with pure hexabromide a t 28' C. and the solubility determined. Results indicated t h a t benzene dissolved 0.000204 g. of hexabromide per cc. a t this temperature. A hexabromide determination was made with benzene saturated with hexabromide a t the room temperature as a washing medium. The results averaged 39.5 per cent, or about 6.5 per cent lower t h a n those obtained with the regular wash ether. It is believed t h a t this drop in hexabromide yield was due t o the appreciable solubility a t room temperature of hexabromide in a mixture of the oily bromides, contaminating the hexabromide, and the benzene. Washing experiments were made in which ordinary 9 5 per cent alcohol and absolute alcohol were substituted for ether, but pure white precipitates were not obtained and these materials for washing were abandoned. Washing experiments were made in which U. S. P. ether, instead of "ether over sodium," was saturated The results obtained were with hexabromide a t 0 ' C. variable and slightly higher t h a n those with the absolute ether and hence i t was conc1uded:that absolute ether should be specified in all cases as a washing medium. ( 3 ) Properties of Hexabromide. ( a ) Melting Points. Lewkowitsch gives values for the melting point of the hexabromide of the fatty acid of linseed oil as 180' t o 181' C. and 183 ' C. Various other values of this constant have been reported. The highest value is 183' C., while Eibner claims 177' C. t o be the correct value. The authors prepared some pure hexabromide by recrystallizing twice from hot xylene and washing with absolute ether until pure. When some of this material was heated slowly in a testLtube t o a temperature near its melting point evidence of decomposition was noticed and traces of hydrobromic acid were detected. On raising the temperature t o the melting point, the hexabromide gave off hydrobromic acid in considerable amounts and slowly turned brown. At a temperature slightly over the melting point, the material gave off clouds of hydrobromic acid and the liquid turned dark brown. This experiment shows t h a t the hexabromide is unstable a t temperatures near its melting point and slowly loses hydrobromic acid. Since a melting point determination cannot be accurately taken by the capillary tube method, unless the rate of heating is comparatively slow, the correct melting point of hexabromide cannot be obtained by the ordinary method. Using pure hexabromide, a double bath, and a calibrated total immersion thermometer, melting points varying from 180.3' t o 183' C. were obtained, depending upon the rate of heating; the lower value resulting when the rate of heating was slow, for example, a rise of one degree per minute.
Vol.
12, No, I
Some linolic tetrabromide was prepared from the f a t t y acids of cottonseed oil. The melting point of the product was 1 1 2 ' t o 113' C. A mixture of 95 per cent pure hexabromide and 5 per cent tetrabromide was dissolved in hot chloroform, the solvent evaporated, the solid material finely powdered and melting-point determinations made with a standardized total immersion thermometer, b y the usual capillary method. The mixture melted a t 176.5' to 177.2' C., while the pure hexabromide under the same conditions and same rate of heating melted a t 181.8' t o 182.3' C. It is believed t h a t this experiment explains the low melting point (177' C.) reported by Eibner for hexabromide, for the authors believe t h a t in the Eibner method approximately 4 t o 5 per cent of tetrabromide remains in the hexabromide precipitate. The criterion of purity of hexabromide in the Eibner method is freedom from yellow material. Since the tetrabromide is white and crystalline, i t is easily seen t h a t i t would be possible for i t t o be present in t h e Eibner hexabromide. The fact t h a t Eibner could not d r y his product a t 100' C. may also be explained by the presence of tetrabromide. ( b ) Bromine Content. Some pure hexabromide, recrystallized from xylene, was analyzed for halogen content by a method, believed t o be new, which depends upon the fact t h a t the hexabromide when heated with 95 per cent alcohol and excess of zinc, quantitatively loses halogen, linolenic acid resulting, together with a small amount of the ethyl ester and the zinc salt of the linolenic acid. The removal of halogen by means of zinc t o recover linolenic acid from t h e hexabromide is described by Erdmann and Bedf0rd.l About half a gram of finely powdered pure hexabromide was weighed and heated in a n Erlenmeyer flask under a reflux condenser with 30 t o 40 cc. of 95 per cent alcohol and about I O g. of go-mesh powdered zinc. After about 30 min. the hexabromide entirely disappeared and the solution became clear. It was then boiled for about I hr. longer, filtered through a folded filter, and the zinc washed three times with hot alcohol and twice with hot distilled water. The filtrate was then titrated by the regular Volhard method, as described b y Scott.2 Results on pure hexabromide: 63.04 per cent bromine 63.14 per cent bromine Theoretical for hexabromide (ClsHaoOzBr6), 63.27 per cent bromine. Some hexabromide from regular hexabromide determinations was also analyzed for bromine content by the method given above. Results on hexabromide from determinations: 62.8 per cent bromine . 6 2 . 6 per cent bromine The determination of the bromine content is practically useless as a criterion of purity of the hexabromide, since the impurity which is the most difficult t o remove, the tetrabromide of linolic acid, has such 1
Ber., 42, 1324.
* "Standard Methods of Chemical Analysis," 2nd Ed., p. 81.
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T H E J O U R N A L OF 1 N D U S T R I A L A N D ENGINEERING CHEMISTRY
Jan., 1920
a high bromine content as t o make the lowering of the percentage of bromine, due t o the presence of small amounts of this impurity, so small t h a t it lies within the limit of error in the bromine determination. This may be shown b y the following calculations: Theoretical for hexabromide (C18H3,,02Br6),63.27 per cent bromine Theoretical for tetrabromide (C18H3202Br4), 53.28 per cent bromine Assuming I per cent of tetrabromide t o be present: 0.99 X 6 3 . 2 7 = 62.64 0.01
x
53.28 = 00.53 63.17per cent
The above shows t h a t I per cent of tetrabromide would lower the bromine content from 63.27 per cent t o 63.17per cent. This lowering of 0.1per cent is well within the limit of error in the bromine determination, so t h a t even 5 per cent of tetrabromide, causing a lowering of 0.5 per cent in the bromine content, could hardly be detected. ( c ) Solubility in Ether and Ether-Oily Bromide M xture. The following procedure was employed t o deter solubility of pure hexabromide in absolute ether: Absolute ether which had been distilled over bright sodium was shaken a t room temperature a t intervals with a n excess of pure hexabromide (recrystallized twice from hot xylene and thoroughly washed each time with absolute ether) for 24 hrs. in a tightly corked bottle. The next day the mixture was maintained a t o o C. in an ice bath for 4 hrs. About 7 5 cc. of the solution were then rapidly filtered into a tared glass-stoppered weighing bottle and weighed. T h e weight of the ether solution divided by the specific gravity of ether a t o o C. gave the volume of ether solution. The ether was evaporated carefully on a hot plate and the weighing bottle dried t o constant weight in a n oven a t 100' C., cooled, and weighed. The total weight of t h e hexabromide residue divided by the volume of the ether solution taken gave the solubility per cc.'as 0.000261 g. Absolute ether was also saturated with pure hexabromide a t ~ 4 . C. 5 ~and filtered a t this temperature; 76.6 cc. of the filtrate upon evaporation yielded 0.0434 g. ,residue, giving the solubility of hexabromide a t this temperature as 0.000566 g. per cc. I n order t o determine whether loss of hexabromide takes place in a determination through solution in a mixture of oily bromides and ether during the first treatment of the impure hexabromide with wash ether, t h e following experiment was carried out: The oily residues obtained by the evaporation of ether from the first washings of previous hexabromide determinations were dissolved in a small amount of absolute ether and cooled t o approximately - I O O C. t o cause separation of any hexabromide which might be present. The ether solution was filtered and evaporated, and the oily bromides were entirely freed from traces of solvent by heating a t 60' C. in a vacuum for several hours. A mixture of 0.4927 g. pure hexabromide and 1.763 g. of the above oily bromide (these are the approxi-
57
mate amounts of hexabromide and oily bromide found in a determination) was weighed into a centrifuge tube and washed in the usual way a t o o C. with four 20-cc. portions of ether saturated with hexabromide. The tube was then dried and weighed and 0.4978 g. of hexabromide was recovered. This experiment indicates t h a t no considerable amount of hexabromide is lost by solution in the mixture of ether and oily bromide during the washing of impure hexabromide. The slight increase in weight of the hexabromide in the experiment described above can be attributed t o the fact t h a t a small amount of oily bromide was not washed out. IV-M
ETH OD PR 0 P 0 SED
From the experimental work described, the following procedure for the determination of the hexabromide yield of the f a t t y acids of linseed oil was developed: A. P R E P A R A T I O N OF REAGENTS-The following reagents are necessary: ( I ) Chloroform. Shake ordinary U. S. P. chloroform with several portions of water t o wash out all the alcohol. Dry the product with granulated anhydrous calcium chloride over night in order t o remove all traces of water. Decant from the calcium chloride and distill. Add t o the distillate 3 cc. of absolute ethyl alcohol for every I O O cc. of chloroform. Keep in a stoppered brown bottle. ( 2 ) Bromine Solutioiz. Mix one part by volume of C. P. bromine' with two parts by volume of chloroform prepared as above. This solution must be made up fresh each day because i t deteriorates upon standing. ( 3 ) W a s h Ether. Shake ordinary ethyl ether with I O per cent of its volume of ice-cold distilled water. Separate and repeat the washing three times. Dry the washed ether with fused calcium chloride over night. Decant the ether through a folded filter into another flask and add thin slices of sodium. Warm gently on a steam bath under a reflux condenser until the evolution of gas by action of the sodium has practically ceased and bits of freshly cut sodium remain bright in the ether. Distill the ether into a dry bottle and add a n excess (at least 3 g. per liter) of finely powdered hexabromide2 of the fatty acids of linseed oil, previously prepared. Shake a t intervals for 2 or 3 hrs. or allow the mixture t o stand over night. Then place the bottle in ice water so t h a t the ether solution will be a t zero or not above z o C. for 3 hrs. Decant the ether solution rapidly through a folded filter into a dry bottle and keep tightly corked in order t o prevent loss of ether by evaporation. 1 The authors have observed that samples of bromine marked "C. P." often contain considerable amounts of nonvolatile material. All bromine which is used must be redistilled unless it is found that 5 g. leaves no weighable residue upon evaporation. 2 If no hexabromide is on hand from previous determinations, it may be easily prepared as follows: I n a centrifuge tube dissolve about 5 g. of the fatty acids of linseed oil in 15 to 20 cc. of chloroform. Place the tube in a freezing mixture and add slowly with shaking bromine solution until a slight red color is permanent. Add a few dmps of amylene tu take up excess of bromine. Whirl in a centrifuge until the precipitate has settled and then pour off the chloroform. Rub up the precipitate with 20 cc. of cold absolute ether, whirl in a centrifuge, and pour offthe wash ether. Repeat the washing with 3 more 20-cc. portions of ether. After drying, the hexabromide is pure enough for the preparation of wash ether
58
T H E J O U R N A L OF I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y
Vol.
12,
No. I
( 4 ) Amylene. This material may be purchased the tube in a freezing mixture, as near -5' C. as from the Eastman Kodak Company. I t is one of the possible, made by adding a little dilute hydrochloric organic chemicals prepared in the laboratory of the acid t o finely cracked ice. Add bromine solution from University of Illinois. I t may be prepared in small a burette a t the rate of one or two drops per second, quantities from amyl alcohol by the method of Adams.' shaking the tube well during the addition. At first B. PREPARATION OF T H E FATTY ACIDS-Weigh ap- the bromine color will be rapidly discharged, but proximately 5 0 g. of linseed oil, place in a I ' / ~ liter later the mixture will assume a permanent orange color Florence flask, and add 40 cc. sodium hydroxide which indicates a slight excess of bromine. For most solution (sp. gr. 1.4) and 40 cc. alcohol. Place the fatty acids of linseed oil, about I cc. of the bromine mixture on a steam bath and heat for about 1/2 hour. solution will be found necessary t o give the orange Add I liter of hot distilled water and insert into the color. At this point run in rapidly 0.5 cc. more of the neck of the flask a 2-hole rubber stopper carrying a bromine solution, shake well, and allow the tube t o tube which projects into the flask so t h a t its end is stand in the ice mixture for I O min. Remove the slightly above the liquid, and pass a stream of carbon tube from the freezing bath and add amylene drop dioxide through the tube into the flask. The soap by drop with shaking until the bromine color has mixture may then be heated t o remove the alcohol, entirely disappeared. Usually five t o six drops of either over a free flame or on the steam bath. If amylene are sufficient, but a slight excess does no the free flame is used, a capillary "boiler" must be harm. The addition of bromine solution must never placed in the liquid, otherwise the soap solution will be done in direct sunlight. bump badly. If excessive foaming takes place, the Attach the tube t o a good water vacuum pump (one current of carbon dioxide should be increased until i t which will indicate a pressure not greater t h a n 40 is strong enough t o break up the foam. If the solu- mm. of mercury) by means of a new one-hole rubber tion is heated on the steam bath usually about 2 t o stopper. Evaporate the chloroform in a vacuum, 3 hrs. are required t o remove the alcohol, while if i t warming the tube in water a t 50" t o 60" C. t o hasten is boiled over a free flame, '/z hr. is usually sufficient. evaporation. The tube must be constantly shaken t o After the alcohol has been removed, cool the soap prevent bumping of the chloroform. Towards the solution and acidify with dilute hydrochloric acid end of the evaporation, when the contents of the tube (I : I). Insert a 3-hole rubber stopper, carrying two become more viscous, rotate and tilt the tube so t h a t glass tubes arranged as for a wash bottle, leaving the the oil will flow about half-way up the sides and thus third hole in the stopper open for a n outlet for the present more surface for evaporation. When praccarbon dioxide. The inlet tube should extend t o just tically all the chloroform has been evaporated, place above the layer of fatty acids, and the outlet tube the tube in a bath a t 55' t o 60" C. for 1 5 min., keeping should extend t o t h e bottom of t h e flask. It is essen- the suction on. tial t h a t the outlet tube should not extend down more Detach from the pump and place the tube in a bath t h a n an inch or two outside of the flask, otherwise of finely cracked ice and water. When the tube is siphoning would take place, causing the liquid t o boil cold, pour down its sides 2 0 cc. of cold wash ether. inside the tube. The wash ether should have been previously placed Pass a stream of carbon dioxide through the system, in four corked test tubes graduated a t 2 0 cc. by a file and boil gently, using a capillary boiler t o prevent mark and kept a t 0 " C. in a n ice bath. Thoroughly bumping, until the layer of fatty acids is clear. Plug stir and rub up the bromide mixture with a rod, breakthe hole in the stopper which acts as an outlet for the ing up all the lumps. Return the tube t o the ice bath carbon dioxide. The lower layer will be forced out for 2 min. and then whirl in a centrifuge until the prethrough the outlet tube by the pressure of the COZ. cipitate has settled into a hard cake and the superI n this manner remove as much water as possible natant liquid is clear. Return t h e tube t o the ice bath without losing any of the fatty acids, then remove the for 2 min., and then pour off the wash ether, making stopper and add about 500 cc. of hot distilled water, sure t h a t no solid material is lost. Repeat the washing shake thoroughly so t h a t the fatty acids are well of the hexabromide precipitate three times in exactly washed, allow the f a t t y acids t o separate and siphon the same way, using three 20-cc. portions of ice-cold off the wash water as before. Repeat the washing wash ether and rubbing u p the precipitate thoroughly until the wash water does not give an acid reaction each time. Use a weighed stirring rod and wash the with methyl orange. Before removing the last wash- precipitate adhering t o the rod into the tube with the ing, boil gent$ until the fatty acid layer is clear. wash ether a t each washing of the hexabromide. After the last washing, remove the stopper and suck 'Afterwards dry and weigh the rod plus the slight up with a pipette the last few globules of water. Filter coating of precipitate and add the weight of material the hot, fatty acids through a folded filter under an on the rod t o the weight of the main portion of hexaevacuated bell jar and keep in a well-stoppered bottle. bromide. After the fourth ether washing has been C. P R E P A R A T I O N O F HEXABROMIDES-weigh accu- poured off, carefully incline and t a p the tube and rately in a weighed centrifuge tube (approximately spread the hexabromide precipitate part way up the 6l/2 in. long by I in. in diameter) 1.00g. (plus or minus sides. Warm the tube in water a t 50' t o 60" C. until 0.05 g.) of linseed fatty acids, prepared as given above. most of the ether has evaporated. Attach t o the sucDissolve in I O cc. of chloroform (see A, I , p. 5 7 ) and place tion pump and place the tube in a bath a t 60' t o 7 0 " C. for 1 5 min. Detach the tube from the pump, cool 1 J. A m Chem SOC.,40 (1918), 1950.
Jan., 1920
T H E J O U R N A L OF I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y
in cold water t o room temperature, wipe dry with a towel and weigh a t once. Dry the tube t o constant weight in a n oven a t 100' t o 110' C. The total weight of the precipitate times I O O divided by the weight of fatty acids taken, gives the hexabromide percentage. The hexabromide should be pure white when dry. Special precautions should be taken as follows: (I) Have t h e chloroform dry and adjust its alcohol content t o 3 per cent. ( 2 ) Make sure t h a t all the chloroform is evaporated from the impure hexabromide before adding wash ether. This will be accomplished if the water pump indicates a pressure not greater t h a n 3 0 t o 40 mm. and t h e tube is heated in the bath a t 60' C. for twothirds of its length. (3) Make sure t h a t the wash ether is anhydrous and free from alcohol and t h a t it is saturated with hexabromide a t o o C. Unless the wash ether is allowed t o stand a t o o C. for a sufficient length of time before filtering off excess hexabromide, i t will be supersaturated a t o o C. and will give high results. Care should be taken t o prevent appreciable loss of ether by evaporation; it is well t o cool the stock bottle of wash ether on hot days before uncorking. (4) Make sure t h a t the centrifuge tube containing hexabromide and the wash ether are kept as near o o C. as possible during the process of washing. The finely cracked ice should be replenished a t intervals. V-RESULTS A.
LINSEED
OBTAINED B Y N E W METHOD
OIL-See
Table I.
TABLE I-RESULTS OBTAINEDBY NEWHEXABROMIDE METHOD (Values are expressed in percentage yields of hexabromide, calculated on the weight of fatty acids taken) Iodine Number = Analyst A.
.....
B
C . ..... D ......
E . .....
181 45.9 46.3 45.9 46.3 46.1 46.1 46.6 46.1 46.4 45.6 45.8 45.6 46.3 45.7
F. . . . . . 4 5 . 6
182
183
186
191
184
182
185
.... .... ............ ............ ............................ ,.., .................... ........ .... 46.2 .... . . . . . . . . . . . . 46.1 . . . . . . . . . . . . .... .................... ........ ............................ ........ ........ 45.9 .... ........ ........ ............ ........ . . . . 45.7 45.9 46.1 45.8 46.9 .... ........ ............................
G.. . . . . 46.5 TJp t o the present time analytical results have been obtained on eight different linseed oils and are given in the table above. The sample of oil with iodine number of 181 was OF known purity and results of seven different analyses working independently on this oil are given,
B. OTHER oms-Samples of cottonseed oil and tung oil gave no visible amounts of hexabromide b y the new method. .A sample of soy bean oil yielded 2.2 per cent hexabromide by the new method. c. M I X T U R E S O F LINSEED A N D O T H E R OILS-A mixture of 80 per cent raw linseed oil (average hexabromide yield 46 per cent) and 20 per cent soy bean oil (average hexabromide yield 2 . 2 per cent) gave an average hexabromide yield by the new method of 36.6 per cent. The following simple calculation gives the theoretical value which would be expected:
59
46 X 0.80 = 36.80 x 0 . 2 0 = 0,44
2.2
37.24per cent (Theoretical)
A mixture of 95 per cent raw linseed oil (average hexabromide yield 46 per cent) and j per cent soy bean oil (average hexabromide yield 2.2 per cent) gave an avergge hexabromide yield b y t h e new method of 44.1 per cent. The following simple calculation gives the theoretical value t o be expected for the mixture: 46.0 X 0 . 9 5 2.2
x
= 0.05 =
43.7 0.1
43.8per cent (Theoretical)
A mixture of 7 5 . 6 per cent raw linseed oil (average hexabromide yield 46 per cent) and 24.4 per cent tung oil (hexabromide yield nil) gave a n average hexabromide yield by the new method of 32.1 per cent. 46 X 75.6 = 34.8 per cent (Theoretical)
The results of hexabromide tests by the new method on mixtures of linseed oil with other oils indicate, i t is believed, t h a t i t may be possible t o detect as little as 5 per cent adulteration of linseed oil with oils low in hexabromide yield. All determinations of hexabromide yield of linseed oils examined so far in this laboratory have fallen between the values 45.6 per cent and 46.9 per cent. If further investigations show t h a t the hexabromide yield of pure raw linseed oil is a more constant value than the iodine number, i t may be possible t o make it quantitative determination in adulterated samples of linseed oil of the content of oils like soy bean oil, t h a t give low hexabromide yields. VI-
S U MMA RY
Various published methods for the determination of the hexabromide yield of linseed oils have been investigated, especially the Eibner method. An explanation is given of the observed fact t h a t this method does not yield concordant results. Experimental work leading t o the development of a new hexabromide method is given, the main features of which are the addition of bromine t o the fatty acids of linseed oil in a solvent in which the resulting hexabromide is soluble, the addition of a reagent t o remove excess bromine, the removal of the solvent by evaporation, and the isolation of the hexabromide free from contaminating bromides by thorough washing with a b solute ether saturated with hexabromide. A table of results, obtained by the new method, is given, showing concordancy between several analysts working independently. The results obtained so far indicate t h a t the hexabromide yield of pure raw linseed oil is a more constant value t h a n the iodine number. Results are also given on mixtures of linseed and other oils, such as soy bean oil, which indicate t h a t it may be possible t o estimate quantitatively adulteration of linseed oil with other oils which give a low hexabromide yield.
