Jan., 1922
THE JOURNAL OF I N D U S T R I A L A N D ENGINEERING CHEMISTRY
57
The Determination of Acid Number of Tung and Other Vegetable Oils’jz By L. L. Steele and G.G. Sward BUREAUO F
STANDARDS, DEPARTMENT OF COMMERCE, WASHINGTON,
Recently Jameson3 has shown that the heat test for the purity of tung oil (the length of time in minutes necessary t o cause polymerization or jellying of the oil a t an arbitrary temperature) is influenced greatly by the free fatty acid content of the oil. Gardner4has shown that the length of time necessary for the jellying of tung oil in the heat test is increased in direct proportion to the amount of free fatty acid of tung oil added. As Jameson points out, the free fatty acid content of tung oil is important and he recommends that the present maximum allowable percentage of free acid be reduced to 1 per cent. (Standard specifications of the American Society for Testing Materials allow not over 3 per cent.) I t was noticed a t the Bureau of Standards that different acid values, were obtained upon a given sample of tung oil when alcohol and alcohol-ben~ene,respectively, were used as solvents. Gardner and Coleman6 showed that the acid value of a varnish appears to be higher when alcohol-benzene mixture is used as a solvent than when alcohol alone is employed Jameson made use of a mixture of alcohol and benzene as a solvent in the titration of the free fatty acid of tung-oil samples. It seemed desirable to find out whether the higher acid values of tung oil in alcohol-benzene mixture were abnormal or whether the present standard use of alcohol6 as a solvent leads to low values. I n this connection it seemed desirable also to investigate the use of alcohol-benzene mixture as the solvent for other oils in the acid number determination. A third point included was the investigation of two other factors affecting the method of determining acid numbers of vegetable oils, namely, the possible influence of caustic soda, now commonly substituted for caustic potash, and the difference, if any, between the use of aqueous and alcoholic alkali in the titration of free fatty acid. METHODOF PROCEDURE The acid number of a sample of commercial tung oil was determined by the standard method as follows: Four portions of 5 to 10 g. each were weighed into flasks and heated on a hot plate under a reflux condenser for 30 min. with 50 cc. of ethyl alcohol, previously made neutral to phenolphthalein. After cooling, the four samples were titrated with 0 1 N aqueous sodium hydroxide, 0.1 N alcoholic sodium hydroxide, 0.1 .V aqueous potassium hydroxide, and 0.1 N alcoholic potassium hydroxide, respectively. Commercial samples of cottonseed and linseed oils were titrated in the same manner with the four standard alkali solutions. The titrations on the oils were repeated, substituting for the alcohol 50 cc. of a neutral mixture of equal parts by volume of ethyl alcohol and C. P. benzene. I n this case the oils dissolved in the alcohol-benzene mixture without heating, and titrations were made a t once (omitting the half-hour heating period). The fatty acids of each of the three oils were prepared by the usual method and each sample of fatty acid was titrated in both alcohol and alcohol-benzene as solvent, using the four standard alkali solutions. Weighed amounts of the fatty acids were added to weighed amounts of the corresponding oil, so as to form a series of oils of ascending acid 1 2
Received August 9, 1921. Published by permission of the Director, Bureau of Standards.
Analyst, 46 (1920), 328. Paint Mfrs.’ Assoc. U.S., Circ. 119. 6 Paint Mfrs.’ Assoc. U.S., Circ. 87. 6 A m . Soc. Testing Materials Standards, 1918, 577. 8 4
D.
c.
numbers. These samples of oil of known acid content were titrated with both alcohol and alcohol-benzene mixture for solvent and with the four different standard alkali solutions already described. By a comparison of the actual acid value obtained in each case with the known acid content of the oil, conclusions could be drawn as to the three points in question. EXPERIMENTAL OILS EXAMINED-Data were obtained upon the acid number of tung, linseed, and cottonseed oils, and of several mixtures of tung oil and rosin. REAGENTS: Sodium and potassium hydroxide--C. P. sodium and potassium hydroxide sticks were used. Alcohol and benzene-Ninety-five per cent by volume ethyl alcohol which had been recently redistilled was used. The benzene was C. P., free from thiophene. Both materials were neutralized immediately before use by titrating with alkali to a very faint pink color, using phenolphthalein as an indicator. Standard solutionsThe standard alkali solutions were prepared as for testing linseed oil,’ except that 0.1 N solutions were prepared instead of 0.25 N solutions. The weight of sample taken was so regulated in each case that approximately 15 cc. of the standard alkali solution were necessary for neutralization. RESULTS OBTAINED Table I includes values obtained upon the various oils and fatty acids. TABLE I-ACID VALUESOF OILS AND FATTY ACIDS -Value Found in Alcohol----Value Found in Alcohol-Benzene with Aqueous with Alcoholic with Aqueous with Alcoholic MATERIAL KOH NaOH KOH NaOH KOH NaOH KOH NaOH Tungoil 6.60 6.76 6.52 6.32 7.05 7.15 7.22 7.15 Fatty acid oftungoil 192.7 192.4 192.6 192.4 190.3 190.1 192.9 192.1 Linseed oil 4.46 4.39 4.52 4.52 4.66 4.62 4.64 4.69 Fatty acid of linseed oil 196.9’ 196.6 196.0 196.8 195.9 194.9 197.1 197.2 Cottonseed oil 0.68 0.70 0.72 0.72 0.75 0.74 0.71 0.76 Fatty acid of cottonseedoil 184.1 184 7 183.1 184.2 182.1 182.7 183.6 184.4 W. W.rosin 161.8 162.1 160.7 160.1 161.2 161.4 161.2 160.6
The determinations of the acid numbers of the fatty acids of tung, linseed, and cottonseed oils were in good agreement, except in the case where aqueous alkali was used for titration with alcohol-benzene as solvent. It is probable that here there was enough dilution by the water of the alkali solution to cause some hydrolysis, giving slightly low values for the acid numbers. For this reason the two low values in the case of each sample of fatty acid were omitted and the average of the six other values was taken as the true acid number of the sample. The acid numbers of a series of mixtures of the different oils with their fatty acids were calculated from the weights of oil and fatty acid mixed and the average acid value of the oil and its fatty acid, respectively. I n the case of tung oil the average acid value found in alcohol-benzene was assumed to be the true acid number. SUMMARY 1-The use of alcohol alone as a solvent in the determination of the acid number of tung oil leads to low values, “Recommended Specifications for Linseed Oil,” Bureau of Standards,
Circ. 8%
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THE JOURNAL OF INDUSTRIAL A N D ENGINEERING CHEMISTRY
especially in the case of oils with a low acid content. Alcoholbenzene mixture gives values which are correct within the limits of experimental accuracy of the determination. With rosin-tung oil mixtures, alcohol-benzene gives results nearer the theoretical than does alcohol alone. 2-Alcohol as a solvent in the determination of the acid number of linseed and cottonseed oils yields values which average slightly lower than the theoretical acid content. I n most of the determinations the average of the values obtained with alcohol-benzene mixture were closer to the theoretical acid content than when alcohol was used as solvent. 3-Alcohol-benzene mixture is preferable to alcohol as a solvent because the end-point in the titration is much sharper. 4-As was to be expected, no difference was noted in any case between the use of sodium and potassium hydroxide, within the limits of experimental accuracy of the titrations. 5-If the weight of sample is so regulated that approximately 15 cc. of alkali solution are required for neutralization, aqueous alkali can be used interchangeably with alcoholic alkali, with no appreciable difference in results within the limits of experimental accuracy.
Vol. 14, No. 1
&In the case of a material with a very high acid number (above loo), such as a fatty acid, there is evidence of some hydrolysis when aqueous alkali is used with alcohol-benzene as a solvent, and alcoholic alkali should be used for titration. TABLE11-ACID VALUESOF MIXTURESOR OIL AND FATTYACID 7 Value-----Value Found in Alcohol-Found in Alcohol Benzene 2' with Aqueous with Alcoholic with Aqueous with Alcoholic KOH NaOH KOH NaOH KOH NaOH KOH NaOH TUNG-OIL MIXTURES .1 15.33 14.40 14.37 14.72 14.20 15.22 15.33 15.38 15.41
g -zg
i2
2 3 4
29.25 28.3 59.1 58.8 99.3 99.2
28.3 58.5 98.9
28.7 55.9 99.0
28.6 58.3 98.7
29.3 59.2 99.3
29.1 59.2 98.7
29.4 59.3 99.4
29.3 59.2 99.2
LINSEED-OIL MIXTURES
1 2 3 4
10.61 24.22 53.2 100.4
10.45 23.76 52.86 100.2
10.41 24.01 52.88 99.6
10.55 10.42 10.54 10.52 10.51 10.60 24.10 23.85 24.13 24.25 24.32 24.20 52.97 52.85 53.15 53.13 53.05 53.18 99.9 100.0 100.6 100.2 100.6 100.5
1 2 3 4
11.29 11.07 25.4 25.07 50.7 50.4 101.5 101.7
11.05 24.79 50.5 101.4
1 2 3 4
15.93 29.7 60.7 102.8
14.81 15.33 15.53 15.75 15.87 15.75 15.98 28.33 28.25 28.75 29.32 29.13. 29.25 29.50 59.8 59.7 60.3 60.4 60.3 60.4 60.6 102.2 102.0 103.0 101.8 102.2 102.8 103.2
COTTONSEED-OIL MIXTURES
15.36 28.42 60.2 102.0
11.25 11.22 11.39 11.33 11.65 35.00 25.03 25.40 25.32 25.21 50.4 50.6 50.6 50.6 50.8 100.9 101.6 100.7 101.3 .100.6
11.34 25.26 50.8 101.4
TUNGOIL-ROSIN MIXTURES
Vapor Pressure Determinations on Naphthalene, Anthracene, Phenanthrene, and Anthraquinone between Their Melting and Boiling Points'#z By 0. A. Nelson and C. E. Senseman COLOR INVESTIGATION LABORATORY, BUREAUOF CHEMISTRY, WASHINGTON, D. C.
A search of the literature on vapor pressures reveals the fact that very few determinations have been made on most of the solid hydrocarbons between the temperatures of their melting and boiling points, or above. In view of this fact and also in response to the increased demand for more reliable physical constants, arising from recent researches into different types of reactions both in the liquid and vapor phase, the problem of determining the vapor pressures of some of the more common compounds was undertaken. I n this paper the method employed will be discussed, and some of the results obtained in naphthalene, anthracene, phenanthrene, and anthraquinone will be. tabulated. I n subsequent publications the results on other compounds, or mixtures of compounds, which are now under investigation will be given. Up to the present time vapor pressure determinations of compounds of the type in which we are interested seem to have been limited to comparatively low temperatures. Ramsey and Young3 determined the vapor pressures of ~ of a camphor between 0" and 180" C.; V a n ~ t o n e ,that mixture of camphor and borneol between 78" and 185" C., while Allen6 worked with naphthalene between 0" and 130" C. Perman and Davis6 determined the vapor pressure of naphthalene, and of mixtures of naphthalene and p-naphthol at 70" C., and Barker' determined the pressures of naphthalene at 20", 30", and 40' C. From the vapor pressure curves obtained by Stelzner,8 it appears that this investigator determined the vapor pressure of naphthalene between 75" and 170"and extrapolated the curve to 218" (b. p.) while in 1 Presented before the Division of Dye Chemistry at the 6lst Meeting of the American Chemical Society, Rochester, N. Y.,April 26 to 29, 1921. 2 Published as Contribution No. 53 from the Color Investigation Laboratory, Bureau of Chemistry, Washington, D. C . 8 Phil. Trans., 1884, I, 34. 4 J . Chem. Soc., 97 (1910),429. 6 Ibid., 77 (1900),412. a Ibid., 91 (1907), 1114. 7 Z . physik. Chen., 71 (1910),235.
* Dissertation, Erlanger, 1901, "Uber den Dampfdruck fester K6rper" (original article not available).
the case of anthracene observations were made for every 10" between 160' and 260" C., and on anthraquinone between 224" and 320". It appears, therefore, that in no case did the temperatures of the experiments even approximate the boiling point of the compounds or mixtures under investigation.
METHODSOF MAKINGVAPORPRESSURE DETERMINATIONS The static method of vapor pressure measurement and the dynamic, or air current method, have been used with different modifications so long and with so much success as t o become almost standard. Other methods have been proposed, such as the optical method devised by C. and M. Cuthbert~on,~ based on the assumption that the refractivity of a vapor was proportional to the vapor pressure; or the hygrometric method devised by Forbes.lo The dynamic method is most suitable for low pressures and low temperature work, and has been used with varying degrees of success by Regnault,l' Idnbarger,12 Perrnan,ls Derby, Daniels and Gutsche,l* and others. The method consists essentially in passing a known volume of air o r indifferent gas over the substance whose vapor pressure is to be determined, and by determining the amount of substance carried over, the vapor pressure is readily calculated from Dalton's law of partial pressures. The air drawn through the apparatus must necessarily be saturated with the vapors, and herein lies one of the chief difficulties. The temperature of the saturator must be kept very constant for considerable periods in cases where the substance under investigation has a low vapor pressure, and this alone is not easy to accomplish especially if the required temperature is comparatively high. Even if constant vapor baths are used, a change of 5 or 10 mm. in the atmospheric pressure Proc. Roy. SOC.London, A , 85 (1911),305. Chem. News, 106 (1912),88. 11 Ann. chim. fihys., [3]15 (1845),129. 12 J . A m . Chem. SOC.,17 (1895),615. 18 Pmc. Roy. Sac. London, 78 (1903),72. 14 J . A m . Chem. Soc., 86 (1914), 793. 9
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