H O L L A N D ON P U R I F I C A T I O N OF INSOLUBLE F A T T Y ACIDS. should not exceed 0.10 cc. iodine solution for the amounts employed. Some lots of bicarbonate have been found unfit for such work. Due recognition should be given the blank in calculating results. Considerable trouble is often experienced in determining insoluble matter with hydrochloric acid due t o the splitting off of white arsenic, especially with Paris green. To offset the difficulty i t was found advisable t o combine the determination with t h a t of preparing the arsenic solution b y simply filtering off the residue. The points noted above may be briefly summarized: Transfer z grams of finely pulverized sample together with 50 cc. of sodium acetate (1-2) t o a 500 cc. graduated flask and boil 5 minutes. Cool under tap, add about 60 cc. of hydrochloric acid (1-3) and shake until solution is effected. Make t o volume and filter. Pipette 2 5 or 50 cc. into an Erlenmeyer flask, neutralize with dry sodium bicarbonate, add 2 5 cc. of sodium potassium tartratel (1-10) to redissolve precipitated copper, approximately 3 grams of sodium bicarbonate, water sufficient t o make a volume of I O O cc., z cc. starch paste (1-200) and titrate with N / z o iodine to a permanent blue color. Toward the end of the reaction cork the flask and shake vigorously t o insure proper end-point. Calculate results as arsenous oxide. The residue in the graduated flask is brought onto the filter, well washed, calcined in a porcelain crucible and weighed as insoluble matter. The above process has given‘ excellent results with copper aceto-arsenite, copper arsenite and calcium arsenite. Sodium acetate does not prevent hydrolysis of copper and calcium arsenites as in the case of Paris green, but serves t o take up free arsenic. The presence of such impurities as cuprous and ferrous compounds, sulfurous and nitrous acids or other oxidizable substances is a source of error b y the iodine titration method. I O D I N E METHOD F O R ARSENATES.
The increasing use of lead arsenate as a n insecticide resulted in a demand for a rapid volumetric method for the determination of the arsenic acid. The Gooch and Browning process,z as modified b y Haywood,3 serves t o readily reduce arsenic acid t o arsenous in which form the iodine titration method is applicable. The process, in our hands, did not at first prove satisfactory b u t eventually yielded concordant results after minor changes. As the differences are largely a matter of detail not involving principle, only the modified process will be given. Transfer z grams of finely ground sample together with 60 cc. of nitric acid (1-3) t o a 500 cc. graduated flask, bring t o boil, cool, make t o volume and filter. Pipette 50 or I O O cc. into a 150 cc. Jena Griffin beaker, add I O cc. of sulfuric acid (2-I), evaporate, heat in ~ t o expel last traces of a n air bath a t I ~ O - Z O OC. moisture, and then on asbestos board to the appearance of dense white fumes t o insure complete removal of nitric acid. Add a small quantity of Used only with the copper arsenites. A m . J. Sci., 40, 66 ( 1 8 9 0 ) . Proc. Assoc. 017.Agr. Cltem.. 23, 165 (1906). of the Association. 1
2
Provisional method
171
water and when cold, filter through a sugar tube under suction into a 300 cc. Erlenmeyer flask and wash to about I O O cc. Add I O cc. of potassium iodide (165-1000) and boil until free iodine is expelled-solution practically colorless-with the reduction of arsenic t o arsenous acid. As,O, 4HI = As,O, 41 zH,O. Dilute, cool immediately, neutralize with sodium hydroxid and render slightly acid with sulfuric acid. Add starch paste and if any free iodine remains, add dilute ( N / s o ) thiosulfate carefully with vigorous shaking t o the absence of blue color. 21 zNa,S,O, = Na,S,O, 2NaI. Add excess of sodium bicarbonate and titrate as usual with N / z o iodine, reporting as arsenic oxide. The residue in the graduated flask is brought onto the filter, washed, caleined and weighed as insoluble matter. Care should be taken t o add sufficient sulfuric acid t o cover the bottom of the beaker when heated on asbestos. A decided excess of acid is also necessary when boiling with potassium iodide t o insure vigorous action and rapid volatilization of iodine. Neutralization with caustic soda is permissible under the conditions described. The hydroxid is a much more convenient and rapid agent than the carbonate. Practically no difference was noted in the titration when the lead sulfate was allowed t o remain, but the data a t hand does not cover a sufficient number of samples t o warrant a statement t h a t this will always hold true. The iodine method, as modified for arsenites and arsenates, has been given a careful study and proved repeatedly in the work a t the Massachusetts station to yield excellent results in the analysis of the insecticides mentioned, if reasonable attention is paid in following the details. While no radical changes in the method have been recommended, this article is offered in hopes t h a t some of the points noted may prove of assistance t o other analysts working along similar lines.
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DEPARTMENT O F PLANT AND ANIMAL CHEMISTRY, MASSACHVSETTSAGRICULTURAL EXPERIMENT STATION.
PURIFICATION OF INSOLUBLE FATTY ACIDS. ,
By E. B . HOLLAND.
Workers in oils and fats experience the same difficulty in obtaining chemically pure products as investigators in other lines of organic chemistry. The best insoluble fatty acids on the market-judging from our experience-are unsatisfactory in both physical characteristics and neutralization number. I n general appearance the acids t h a t are offered resemble granulated curd, though varying in color from white t o yellow, and contain considerable dust and dirt. The molecular weight, a s measured b y titration, in a n alcoholic solution, may deviate from the theoretical by 10-15 points. These statements apply t o chemicals marked “C. P.” and bearing the name of a reputable manufacturer or dealer. The writer required stearic, palmitic, myristic,
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T H E J O U R N A L O F I N D U S T R I A L A N D ENGINEERkNG ClYE3MISTRfi.
lauric, and oleic acids for certain tests and finding it impossible to purchase them of the desired quality was forced t o undertake a study of various methods for their purification. As the character of the unsaturated acids is so unlike t h a t of the saturated, only treatment of the latter will be considered a t this time. The methods t h a t seemed the best adapted for the purpose were ( a ) distillation of the fatty acids in vacuo, ( b ) crystallization from alcohol and ( c ) distillation of the ethyl esters i n vacuo and all were given extended trial.
(a)
D I S T I L L A T I O N O F T H E F A T T Y ACIDS I N V A C U O .
Direct distillation under reduced pressure was successfully employed a few years ago b y Partheil and Ferie* starting with Kahlbaum’s best acids. Upon careful test the writer found that the method possessed certain objectionable features which render it rather impracticable for ordinary use. If it was merely a question of distillation of the acids, the process would be less difficult but for fractionation, using a Bruehl or similar type apparatus, it proved almost impossible, in case of the higher acids, t o prevent solidification in the side neck (outflow tube). The danger arising from a plugged apparatus a t the high temperature involved has also t o be taken into account. An attempt was made to heat the tube and keep the acids liquid b y means of a hot water jacket, also b y a n electrically heated asbestos covering but neither process fully met the requirements of the case. The slow distribution of heat in vacuo is, of course, one of the obstacles in the way. For the distillation of solids of high melting point Bredt and A. van der Maaren-JansenZ devised a n elaborate piece of apparatus having a flask and receiver of special construction and a n overflow tube heated b y electricity b u t it is hardly suited for a general laboratory or for handling any considerable quantity of material. There are two other conditions necessary for a successful distillation of fatty acids, namely, absence of moisture and a current of hydrogen or carbon dioxide t o prevent bumping and to lessen decomposition. Overlapping of the acids in different fractions can not be obviated entirely and if an unsaturated acid was present in the original, i t will probably appear in nearly every fraction. Students under the direction of Professor Burrows, of the University of Vermont, have applied this process for a partial separation of the insoluble acids of several oils with a fair measure of success. With all due allowance for the possibilities of the method in the production of pure saturated fatty acids, the inherent difficulties render i t inadvisable in most instances. ( b ) CRYSTALLIZATION F R O M ALCOHOL. Crystallization in this connection is practically limited in its application t o the removal of a small amount of impurities, especially unsaturated acids. It can hardly be considered other than a supplementary treatment, though excellent for that purpose, t o follow either of the distillation methods. Dry neutral alcohol suitable for such work can be 1
Arch. Pharm., 241,545 (1903).
a Liebig’s A m . Chcm.. 567,354 (1909).
Mar., ~ g r r
prepared b y distillation after treatment with caustic lime. I n dissolving the acids care should be taken t o avoid heating to a higher temperature than is required for solution or to prolong the heating unduly as it will cause the formation of esters. Several minutes’ boiling of the different fatty acids in alcohol caused the following loss in neutralization number. Stearic a c i d . . . . . . . . . . . . . . . . . . . 1 . 7 0 Palmitic acid.. ................ 0.56 Myristic acid. . . . . . . . . . . . . . . . . . 2 . 2 4 Lauric acid.. .................. 0.89 Oleic acid.. . . . . . . . . . . . . . . . . . . . 0 . 2 8
Esterification undoubtedly causes a serious error by this process of purification. Under more careful treatment the change is not as rapid as shown above but is evidently cumulative and may even exceed the figures given. Further study may warrant the substitution of a more stable solvent such as acetone. For the filtration a water- or ice-jacketed funnel is almost necessary particularly for the acids of low melting point and suction is a time-saver. Repeated crystallization is needed to bring out the true crystalline structure and silvery luster of the leaflet. Vacuum drying a t a low temperature is one of the most efficient means for removing adhering alcohol and traces of moisture without injuring the structure. Crystallization as a whole is wasteful of acids and solvent unless both are recovered, but is essential for the production of a superior product. ( 6 ) DISTILLATION O F T H E ETHYL ESTERS IK VACUO.
As ethyl esters distil freely in vacuo, the process admits of a more ready application and to products of a greater range of purity than does a distillation of the acids. After considerable experimenting i t was found t h a t the esters are easily prepared by heating in an open flask equal parts (100 grams) of fatty acids and alcohol together with a small quantity (IO cc.) of concentrated hydrochloric acid, using capillary tubes to prevent bumping. The reaction requires about 30 minutes, after which the excess of hydrochloric acid can be removed with a separatory funnel. The distillation is conducted in a 500 cc. “low” side neck flask with a small (8 inch) Liebig condenser and a large size Bruehl fractionation apparatus. Heat is furnished by means of a linseed oil bath and suction by a pump of any type using a mercury manometer t o prove constancy of vacuum. The neck of the flask from the shoulder t o a n inch or more above the side tube should be wound with asbestos paper to prevent cracking, due t o sudden changes of temperature. The condenser should be kept full of water without circulation t o serve as a hot water jacket. The vacuum should be as high as the flask will safely withstand b u t above all uniform, otherwise the fractions are of questionable value. The temperature range of a n ester also varies with the distance between surface of liquid and side tube. At least one redistillation of like fractions is necessary. As the esters are very stable, more difficulty was experienced in finding some means for their quantitative decomposition than in any other portion of the work. Heating with mineral acids hydrolyzes
S I L V E R M A N 0-V FREEZIiYG POINT BY V1;C'EGARS. the fatty acids very slowly even under pressure. If, however, the esters are first saponified' b y heating over a naked flame with twice their volume of glycerol and an excess of caustic potash until all the alcohol is expelled, and then the resulting soap dissolved in water and heated on a water bath with a slight excess of sulfuric acid, the separation is readily accomplished. This plan was suggested b y the Leffmann-Beam saponification for volatile acids and after extended trial proved the most thorough and rapid means for decomposing the esters. The resulting acid should be washed in a separatory funnel with boiling water until clear and the cake allowed t o drain. As previously stated, several crystallizations are necessary if a crystalline product of satisfactory melting point and neutralization number is t o be secured. When crude acids are employed it is also advisable to crystallize a t the outset t o exclude a major part of the unsaturated acids which otherwise would prove troublesome. To summarize: saturated fatty acids may be purified by distillation of the acids or their ethyl esters. The latter method is less hazardous and easier t o manipulate although more steps are required. Crystallization is a finishing rather than a n initial process of purification. DEPARTMENT OF PLANTAND ANIMAL CHEMISTRY. MASSACHUSETTS AGRICULTURALEXPERIMENT STATION.
DEPRESSION OF THE FREEZING POINT BY VINEGARS AS A CHECK ON THEIR COMPOSITION. B y ALEXANDER SILVERMAN.
A number of years ago the writer, while visiting some agricultural laboratories in New York State, was informed t h a t '' Depression of Freezing Point '' determinations were made daily on samples of milk collected from various points in the district and that if the depression lay within certain limits (total variation about 0 . 0 8 ~C.) the analysis was dispensed with as i t had invariably been found t h a t a milk coming within this limit was of t h e composition required b y the state. This simple check on composition led to determinations of the depression of the freezing point b y four standard vinegars, samples of which were very kindly furnished b y the most reliable manufacturers of this country. I n each case a complete analysis was made as outlined in succeeding tables and by methods t h a t follow. Then a specific gravity determination and then the depression test. M E T Y O D S OF A N A L Y S I S .
Acidity.-s cc. of vinegar were diluted t o 5 0 cc. with distilled water in a 125 cc. Erlenmeyer flask, 3 drops of a I per cent. alcoholic solution of phenolphthalein added, and then N / r o NaOH solution from a burette until appearance of the pink color. If a n excess of NaOH had been added back-titration with N / I O HC1 was resorted to with subsequent addition of N / I O NaOH t o the pink end-point. The N / I O HC1 solution was prepared b y diluting Observing the usual precautions given for the determination of insoluble fatty acids. Mass. Apr. Expt. S f a .R p f . .11, 130 (1909).
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75 cc. of the concentrated chemically pure acid with an equal volume of distilled water, weighing 36 grams and diluting to one liter. The strength was determined b y precipitation of silver chloride, as directed in the "Official and Provisional Methods of Analysis of the Association of Official Agricultural Chemists " published b y the United States Department of Agriculture (Bulletin 107, Bureau of Chemistry). The acid was then diluted to exactly d\l/~o and again checked. The S/IO KaOH solution was prepared by dissolving 50 grams NaOH in water, diluting t o I liter and checking again the N / I O HC1, using 3 drops of I per cent. phenolphthalein as an indicator. After comparison the NaOH was diluted t o exactly N / I O and again checked b y the ~ Z ' / I OHC1. Solids.-Ten cc. of vinegar were placed in a small tared platinum dish, evaporated for two hours on a water bath and dried five hours in a steam oven. Ash.-Ten cc. of vinegar were placed in a platinum crucible, evaporated t o sirupy consistency and carefully incinerated below dull red heat. Moistiwe.-This was determined b y difference after deducting acidity and solids. Specific Gravity.-This was determined in a 2 5 cc. tared specific gravity bottle filled first with pure distilled water and weighed and then with the vinegar t o be tested. The weight of the latter divided by that of the water gave the specific gravity. Depression 0,'Freezing Point.-This was effected in the apparatus here illustrated when A is a Beckman thermometer usually covering a range of 5 or 6' C. and reading to I / I O O O C. The glass jar C was filled with a K mixture of I O parts of A cracked ice, I part of salt and I part of water which mixture gave a temperature of about -10' C. recorded on thermometer H . The mixture was stirred from time to time by stirrer D. The space between tubes E and F was filled with ethyl alcohol which gave better results than when tube E contained air. Twenty-five CC. of distilled water were placed in tube F and the mercury in bulb K adjusted, b y slightly jarring the thermometer, until the freezing point of water was recorded somewhere between j O and 6 O mark. The freezing point was recorded and the water was replaced by 2 5 cc. of vinegar which was next cooled to freezing and the temperature recorded. The differenceindicated the depression of the freezing point. While the water and vinegar were being frozen the
