J a n . , 1919
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
SOME NOTES ON ACETYLSALICYLIC ACID By HENRYL. DAHM Received May 27, 1918
Recent publications1 have brought t o light t h e difficulty of obtaining t r u e accurate melting points of bodies which decompose during t h e progress of t h e test, particularly in t h e case of acetylsalicylic acid. The following modification of t h e method described i n t h e U . S. P. has been used for t h e past year in this laboratory and gives consistent checking results b y different operators. A sketch illustrates t h e apparatus used. It consists of a 2 5 0 cc. C O z flask filled with paraffin oil, stirred by a small propeller, connected t o a Rabe water turbine or a small electric motor. A standardized thermometer provided with an air jacket for protection against air current, graduated in 0 . 2 ~C. a n d calibrated for a 3-in. immersion, is adjusted permanently in t h e oil bath with a 3-in. immersion when t h e temperature is 130' C. The melting-point t u b e is carried on a separate movable rod which easily slides into positic3n at 1;he proper time. The meltingpoint t u b e only is immersed in t h e oil, being drawn r a t h e r long for this purpose, t h e rod serving only as support, with z small r u b b e r band; t o hold t h e tube. The temperature is increased a t t h e uniform rate of L O per min. with constant stirring until the temperature has reached 130' C. when the melting - p o i n t t u b e is immersed i n the O J . With ~ this m e t h o d , samples of American- a n d foreign - m a n ufactured aspirin were found t o MELTING-POINT APPARATUS melt within the range of 133Oto 135' C. (corrected). Due t o t h e rapid decomposition of acetylsalicylic acid on heating and lack of consistent results when lower temperatures were used, 1 3 0 O C. was fixed as t h e proper point of immersion. Emery and Wright found a depression in t h e melting point of the pure substance of abouta I O for every 5 min. heating just
P
Proc. Assoc. of Agr. Chem., 1912. The initial temperature of the bath, before immersing the melting point Lube, should be 3' lower than the melting point of the sample. I n m y experience i t is very rare t h a t the initial temperature need be below 130' C. 8 1.3ureau of Chemistry, Dept. of Agr , Bulletin 162 1 2
29
below its melting temperature, so t h a t heating at the rate of I O per min. and starting, for instance, from 1 2 0 ' C. would mean a depression of about 2' in t h e true melting point. An increase in t h e rate of heating is also not advisable, due t o difficulty of control and possibility of overheating, especially with a thermometer having a considerable time lag. It is advisable t o use a thermometer fixed permanently in the bath, as t h e introduction of a cold thermometer carrying the sample would tend t o give low results, due t o cooling effects. The thermometer used for this work is graduated from 74' t o 150' C. in 0.2' intervals, is 1 5 in. long (Taylor Instrument Company, No. 1457 Precision Grade), and has a small expansion bulb which makes it impractical t o immerse i t with t h e sample and obtain rapid readings. But t h e y are more accurate and sensitive t h a n a longer thermometer with a wider range, large mercury bulb a n d no expansion bulb, as t h e stem correction will be less a n d barely noticeable. I t isimportant t h a t t h e propeller used have sufficient surface t o thoroughly agitate t h e oil. A set of color standards for determining t h e approximate amounts of free salicylic acid in aspirin were found quite useful in comparing t h e quality of various samples. The standards are very stable and when once adjusted can be relied upon for a long time. They were prepared as follows: A I O per cent solution of hydrated cobalt chloride was prepared by dissolving 10.000 g. CoClz zHz0 in 50 cc. water 5 cc. I O per cent HCl and diluting carefully to IOO cc. in a calibrated flask. By using various dilutions, as outlined below, standards were prepared. The standards are preserved in small, square or round, glass-stoppered bottles of uniform size and holding about 25 cc. of liquid.
+
+
'
STANDARD SOLUTIONWATER CoCL 2He0 COLOR cc. cc. Per cent No. 5.0 395 0.125 0 10.0 390 0.25 1 10.0 190 0.50 2 10.0 90 1 .oo 3 10.0 75 1.50 4 65 2.00 5 10.0 10.0 40 2.50 6
+
SALICYLIC IN
ACID EQUALVOLS. Gram 0.00005
0.0001 0.0002
0.00025 0.0003 0.00035 0.0004
They were standardized by using a 0.1per cent solution of sublimed salicylic acid, taking aliquots, adding 5 cc. of alcohol and diluting to a final volume of 2 5 cc. to which one drop of dilute ferric chloride solution ( I O per cent s o h , as per U . S. P.) was added. After standing for 5 min. in a flask similar to the standards, the color was matched to one of the standards and the per cent salicylic acid for that standard calculated. It is important to note that these results hold true only for similar quantities ( 2 5 to 3 0 cc.), after 5 min. time, and using the same amount of ferric chloride. If 5 cc. of a z per cent solution of acetylsalicylic acid in alcohol, diluted with 2 0 cc. H20 and placed in a similar bottle, are treated with I drop FeC13 solution and t h e color matched, t h e grams in t h e standard X I O *gives the per cent free salicylic acid in t h e acetylsalicylic acid.l If a colorimeter is available t h e same solution may be used, t h e volume ratios used giving t h e percentage ratio between sample and unknown. The following results were obtained on three wellknown American brands and one foreign brand of acetylsalicylic acid: 1 Color Standard No. 2 is the maximum limit allowable in a good sample of acetylsalicylic acid.
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
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1 133.5134.5” C. White crystals None Very faint trace Very faint trace None
Melting Point Color:
................. .............. Chlorides.. .............. Heavv metals ........... Reaction of aqueous s o h . Acid Free salicylic. .......... None Ash, per cent.. ......... 0 . 0 2 Odor.. Sulfates..
..................
Assay 99.95 ( a ) Foreign product.
2(a) 133.5134.5’ C: White powder None Very faint trace Very faint trace None Acid None 0.02 99.95
3 4 133.4133.8134.4’ C. 134.8’ C. White White crystals , powder None Acid Very faint Very faint trace trace Very faint Very faint trace trace Verv faint None tiace Acid Acid None None 0.015 0.03 98.86 99.59
I n carrying out these tests as well as others for the purity of various samples and brands of acetylsalicylic i t can be stated t h a t t h e same results and conclusions were obtained as those noted by Leech, with a n exception in higher melting points, though comparative results were the same. There is no difference in quality and purity between various samples of American products now on the market and the much vaunted patented foreign brand formerly on t h e American market.l SUMMARY
A modification of the usual U . S. P. method of taking melting points, applicable t o acetylsalicylic acid, is given. A set of permanent color standards for use in determining t h e approximate amounts of salicylic acid in acetylsalicylic acid, with directions for preparation, is described. Results of previous investigators as t o quality a n d purity of American and foreign aspirin were confirmed, by tests made over a period of one year. ANALYTICAL LABORATORIES MONSANTO CHEXICAL WORKS ST LOUIS,MISSOURI.
THE WLCANIZATION OF RUBBER AT CONSTANT TEMPERATURE AND BY A SERIES OF INCREASING TEMPERATURES2 B y G. D. KRATZAND ARTHURH.
FLOWER
It has long been known t h a t when the vulcanization of rubber is effected by heating for a period of time a t a definite and constant temperature, the rate of cgmbination of the sulfur with the rubber decreases with t h e time. I n this particular instance, however, as is recorded in the experimental part of this paper, we have endeavored t o maintain a constant rate of combination of the sulfur and rubber by a variation in t h e temperature. Our efforts have been confined primarily t o devising a method for calculating a series of temperatures by t h e use of which t h e rate of vulcanization might be accurately controlled. With this possible, we desired t o make a comparison of the physical characteristics of a rubber mixture vulcanized t o the same point by both methods. Although it is not within the scope of this article t o review in its entirety the literature upon the skbject or t o draw conclusions from the results previously obtained by others, certain of these should be briefly recalled. 1 J . phavm. chim., [ 7 ] 6 (1917), 213; P. N . Leech, THISJOURNAL, 10 (1918), 288. 2 Presented before the Rubber Section at the 56th Meeting of the American Chemical Society, Cleveland, September 10 to 13, 1918.
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The vulcanization of rubber a t constant temperature was regarded by Weber’ as consisting in a chemical reaction between t h e rubber a n d sulfur. Later, Skellon2 also recorded results which tend t o show t h a t t h e combination of sulfur with rubber is strictly a chemical reaction, which is first preceded by the melting of sulfur and its solution in the rubber. Likewise he maintains t h a t the rate of combination for unit t i m e and constant temperature decreases with the decrease in the active mass of t h e sulfur present. O ~ t w a l d ,on ~ the contrary, has regarded t h e vulcanization phenomenon as due t o a n adsorption of the sulfur by the rubber, t h e rate of which, when expressed graphically, follows the typical adsorption isotherm. Spence4 a n d his coworkers, however, have demonstrated t h a t Weber’s vulcanization curves, on which Ostwald based his calculations, are subject t o correction. They6 have also shown almost conclusively t h a t t h e vulcanization phenomenon is the resultant both of a n adsorption a n d a chemical interaction of the sulfur with the rubber, so t h a t the views of others are probably not entirely free from need of modification. Furthermore, from t h e results obtained by Spence it is quite obvious t h a t when vulcanization is effected a t constant temperature, the major portion of the sulfur combines with the rubber during the early stages of the reaction. And it is equally apparent t h a t a lowering of the initial temperature and subsequent increasing of it a t regular intervals would tend t o make t h e reaction proceed more uniformly. I n fact, for many years i t has been common technical practice t o employ this method, popularly known as a “rising cure,” but as King6 has recently pointed out, t h e use of the “rising cure” has been based mainly upon t h e fact t h a t it affords a means whereby the low heat conductivity of the rubber may be minimized rather t h a n for the above reason, for it is well established t h a t in the case of large bulky articles, unless the vulcanizing temperature is exceedingly low, or unless i t is initially low a n d gradually increased as the reaction proceeds, t h e outside surface may be overvulcanized before the heat has thoroughly penetrated t o the interior of the mass. With the former idea in mind it appeared t o us t h a t by employing slabs of a thickness such t h a t they would not be subject t o King’s contention, a series of increasing temperatures could be previously calculated which would effect the combination of a unit amount of sulfur in unit time throughout the period of vulcanization. The vulcanization-time curve thus would appear as a straight line. A mixture vulcanized in this manner might have widely different physical properties from those of the same mixture vulcanized a t a cons t a n t temperature. One of us7 has already shown t h a t there is a distinct and readily measurable relationship between. the time “Chemistry of India Rubber,” 1906 edition, pp. 85-88. India Rubber J . , 46 (1913), 723; Rubber I n d . , 1914. 8 Kolloid-Z., 6 (1910), 136. 4 I b i d . , 11 (1912). 2 8 ; Chem.-Ztg., 36 (1912). 1162; Kolloid-Z., 11 (1912), 274. 6 Ibid., 8 (1911), 304; 11 (1912), 28: 13 (1913). 265. 6 Met. and Chem. Eng., 18 (1918), 5. 7 G. D. Kratz, I n d i d Rubber Review, 16 (1916), 225. 1
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