THE HEAT OF COMBUSTION OF SALICYLIC ACID. 2747 CCCLXXV1.-The Heat of Combustion of Salicylic Acid. By ENDRE BERNER. BENZOIC acid naphthalene and sucrose are the substances generally used for the determination of the heat capacity of a bomb calori-meter. A number of recent excellent investigations have shown, however that only benzoic acid fulfils the requirements of a calori-metric standard. This acid therefore has been accepted as the sole standard substance for calorimetric work; its heat of combustion has been fixed at 6324 calories (15") per gram weighed in air or 6319 calories (15") per gram (vac.) (" Comptes Rendus de la Troisidme ConfGrence Internationale de la Chimie," Lyon 1922 p. 54). International acceptance of one or more other substances as secondary standards is highly desirable because the use of a single substance involves a certain risk of introducing an error in the value of the heat capacity of a calorimeter.Verkade and Coops (Rec. trav. chim. 1924 43 561) recently proposed salicylic acid as a secondary standard substance. They determined its heat of combustion and tested its applicability as a thermochemical standard but unfortunately they failed to take the precautions necessary to secure the utmost purity of their samples. The author has now determined the heat of combustion of the pure acid. Three different preparations of salicylic acid (Merck, Kahlbaum and von Heyden) were separately purified partly by crystallisa;tion from different solvents and partly by sublimation in a v&cuum until the heats of combustion of the products were constant; the values obtained 5233.8 5233.8 and 5233.9 cal.(15") per gram (vac.) were identical within the limits of experimental error. From the results of twenty experiments made with these samples, the mean value 5233.8 cal. (15)" per gram (vac.) was found for the heat of combustion of salicylic acid at constant volume. To this corresponds the value 5237.4 cal. (15") per gram weighed in air against brass weights. The ratio of the heats of combustion of benzoic acid and salicylic acid is 6319/5233.8 = 1.2073 (vac.). Verkade and Coops (Zoc. cit. p. 571) found a slightly higher value for the heat of combustion of salicylic acid vk. 5238 cal. (15") per gram (vac.) and consequently the ratio 1-2066. The discrepancy may be due to an error in the heat capacities of the calorimetera or to a difference in purity of the acids.The first explanation is highly improbable because both Verkade and Coops and the author used VOL. CXXVII. 6 2748 BERNER : benzoic acid from the Bureau of Standards Washington for the calibration of their calorimeters and obtained almost the same value [V. and C. 5238.1 ; B. 5237.1 cal. (15") per gram (vac.)] for the heat of combustion of Merck's salicylic acid (" Praparate von beatimmter Verbrennungswarme "). Verkade and Coops used salicylic acid from four different sources. Because the heats of combustion of those preparations all agreed and one of them-Mercks-gave the same value after a single recrystallisation from ether the authors contend that their preparations were '' absolutely pure." These facts however are of no practical value as proofs of the purity of their preparations.The combustion of salicylic acid (about 0-SO g.) in oxygen at a pressure of 35 30 or 25 atmospheres is complete; neither carbon monoxide nor the slightest trace of soot nor any smell due to products of incomplete combustion can be detected. Moreover the acid is neither hygroscopic nor volatile and is easily pressed into pellets which ignite readily. It should therefore be particularly suitable as a st'andard substance for calorimetric work. E X P E R I M E N T A L . Detailed descriptions of the calorimeter the method and the experimental results will be published elsewhere. The calorimeter was of the form recommended by the Bureau of Standards Washington (Bull.1915 11 189). The calorimetric bomb had a thick h e d gold lining and a capacity of about 300 C.C. Temperature measurements were made with a platinum resistance thermometer. The resistance was measured by Kohlrausch's method (" uebergreifende Nebenschlusse ") as used by Jaeger and von Steinwehr (Ann. Physik 1906,21,23) and by Fischer and Wrede (Sitzungsber. Prtuss. A M . Wiss. Berlin 1908 5 129). The cooling correction ur was calculated from the formula ur =a& (u - u,)&, in which a is the constant of the calorimeter u its temperature, u, the convergence temperature and t and t, respectively the time at the beginning and the end of the main period. The numerical value of the integral was in every case determined graphically.In some cases the cooling correction was also computed by the practical method developed by Dickinson of the Bureau of Standards (Zoc. cit. p. 229). This method gave practically the same values as those obtained by the complete computation and therefore may be used even in work of high precision. The total rise of temperature during a combustion was abou THE HEAT OF COMBUSTION OF SALICYLIC ACID. 2749 1". The temperature of the jacket was always regulated so that the cooling correction amounted to 2-3 parts in 10o0. The oxygen w&s prepared from liquid air ; it contained very little nitrogen-the correction for nitric acid was 06--0-7 ca1.-and no combustible impurities. Oxygen from the same source was used for the calibration of the calorimeter and for the combustion of the salicylic acid.Ignition of the substance was effected electrically by means of a very thin platinum wire; the heat developed by the current was about 0.8 cal. If the wire had been merely pressed into the pellet, ignition sometimes failed to occur. A part of the wire was there-fore covered with 06-1.5 mg. of benzoic acid by dipping it in the molten acid and placed close against the top of the pellet; under these conditions ignition never failed to take place. Heat Capacity of the Calorirneter.-This was determined by the combustion of two samples of benzoic acid (a) a standard sample (39b) obtained from the Bureau of Standards Washington and ( b ) a sample (517) purified as follows Benzoic acid (Kahlbaum's '' Praparate fur kalorimetrische Bestimmungen " ) was distilled twice at a pressure of 10 mm.of Hg crystallised twice from benzene and once from carbon disulphide finely powdered and dried for 3 days in a vacuum over phosphorus pentoxide. With 3700 g. of water in the calorimeter vessel 1 g. of water in the bomb and an oxygen pressure of 30 atmospheres the heat capacity was found to be 4066-3 calories (15") at a mean temperature of 18". To this value must be added a small correction for the differ-ence in heat capacity of the products of combustion and the oxygen necessary for the combust'ion. In some recent experiments the variation of the heat capacity with temperature was determined directly ; the heat capacity decreased by about 1 cal. for an increase of 1" a t 18".If the mean temperature during an experiment differed from 18" a correction for the divergence wits applied. Purificatioib of the Salicylic Acid.-1 . Merck's salicylic acid (" Priiparate von bestimmter Verbrennungswarme ") wit8 finely powdered and dried for 3 days in the way described above (sample 57 1). The same preparation was crystallised twice from water and once from chloroform powdered and dried for 3 days (sample 571 IIa). It was then dried for 6 days (sample 571 IIb) and sublimed in a vacuum (sample 571 III). 2. Salicylic acid (Kahlbaum D.A.B.5) was crystallised once from water sublimed in a vacuum crystallised once from chloroform, powdered and dried for 3 days (sample 572 In). This sample was then crystallised once from water once from benzene and 6 A 2750 THE HEAT OF COMBUSTION OF SALWYIXC ACID.once from chloroform powdered and dried for 3 days (sample 572 IV). 3. Salicylic acid (von Heyden's "Acidum Salicylicum volum. puriss.") was crystallised twice from water and once from chloro-form and dried for 3 days (sample 573 11). This sample was crystallised once from water once from chloroform and dried for 3 days (sample 573 111). TABLE I. Benxoic Acid. p = pressure of oxygon in atms.; h = heat capacity in cal. (15') of the calorimeter at 18O and 30 atms. of oxygen; d = deviation from the mean heat capacity in parts per 10,000. Sample. p. h. d. Sample. p. h. d. 39b 30 4066.7 +1*0 517 40 4066.6 +Om7 9 9 , 4065.6 -1.7 Y 30 4065.8 -1.2 Y , 4066-7 +1-0 ? , 4066.6 +O-7 35 4065-2 -2.7 Y 9 , 4066.2 -0.2 5 i j 30 4066.9 + l a 5 9 ) 4066.9 + l a 5 Mean 4066.3 TABLE 11.Salicylic Acid. p = Pressure of oxygen = 35 atms.; t = mean temperature; Q = heat of combustion per gram (vac.) in c a l (15"); D = deviation from mean heat of combustion in parts per 10,OOO. Salicylic acid (Merck) . Sample. t . Q. D. 571 18.0' 5237.3 +0.4 9 18.0 5236.4 -1.3 ,* 18.3 5237.3 40.4 Y 18.1 5237.0 -0.2 ?* 18.1 5237.3 $0.4 Mean 5237-1 Salicylic mirl (Merck) purified. 571 IIa 18.1" 6234-6 +1.5 57i'IIb 18-2 5233.3 -1.0 , * 18.1 5233-9 +Om2 , t 18.0 5232.9 -1.7 671 III 18-1 5234.5 +1-3 , 18.0 6233.1 -1.3 , 18-0 5234.4 +1*1 Mean 5233-8 18.1 5233.3 -1.0 * p = 30 atms. NORGES TEKNXSKE HOISKOLE, TBONDHJEM NORWAY. Salicylic acid (von Heyden) purified. 573 11 18-8' 5234.3 +0.8 17.9 6233.1 -1.5 57,; I11 18.0 62344 +I-0 9 17-9 5234-7 +1-5 Y Y 18.0 5233.2 -1.3 Me- 5233.9 Sample. t. Q. D. Salicylic acid (Kahlbaum), 572 III 18.2" 5234.1 ¶ 18-2 5233-3 18.1 6234.1 57;IV 18.5 6233.7 , 18-5 5234.6 ) 17.9 6233.0 , 18.0 5233-7 Mean 6233.8 purified. + 0.6 - 1.0 + 0.6 - 0.2 + 1-5 - 1.5 -0.2 7 p = 26 at-. [Received July 9th 1925.