April, 1922
THE JOURXAL OF INDUSTRIAL A N D ENGINEERING CHEMISTRY
hrs. before putting them in the 55" bath. The sample from Lot A gained a large excess of sulfur which could not be completely removed even by subjecting the sample to alternate heatings a t 120' and 55" C. The last point obtained was'apparently in error, owing to the same difficulties in sulfur migration as were noted a t the higher temperatures. There was little reason to doubt the accuracy of the three other points, and the line was drawn as indicated. HEAT OF soLuTIoN-solubility values for the rubber stocks having a vulcanization coefficient up to 7 per cent have been plotted in Fig. 4 for the three temperatures. That the curves are reasonably consistent is indicated by the fact that if, for any vulcanization coefficient, the log of the solubility is plotted against the reciprocal of the absolute temperature, the result is a straight line. From such a diagram drawn for a combined sulfur content of 3 per cent (Fig. 5 ) , neglecting heat of dilution (which is probably small), it can be calculated that the molal heat of solution of sulfur in rubber between 55" and 95" C. is 6200 cal. Assuming the molecular condition of sulfur in rubber to be SS,as it is in most organic solvents, the heat of solution of 1g. of sulfur in 100 g. of rubber becomes 24.2 cal.
321
It should be noted that at room temperature there is apparently not a very great difference in the solubility of sulfur in soft and hard rubbers. The indications are, therefore, that the non-blooming of hard rubber stocks at room temperature should be attributed more to the retarded migration of the sulfur than to any increased solubility for the sulfur. SOLUBILITY AT 140" C.-The solubility of sulfur in rubber a t vulcanization temperature, or 140' C., is of considerable practical interest. This value cannot be obtained directly by extrapolation of the line in Fig. 4 to 140" C., on account of changes in slope at 96.5" C., where rhombic sulfur changes to monoclinic, and a t 119" C., where monoclinic sulfur melt? to liquid sulfur. Theoretically, these changes in slope can be calculated from the heat quantities involved, but it is necessary to make several assumptions (for example, that the molecular condition for dissolved sulfur over this range is unchanged), which make a solubility figure thus obtained of doubtful value. From an inspection of such a diagram and of the vapor pressure curves of the various forms of sulfur over the same range, it would seem that the solubility of sulfur in rubber a t 140' C. is about 20 g. of sulfur per 100 g. rubber. This figure should be checked experimentally.
The Hygroscopicity of Picric Acid'" By L. Grier Marsha EXPLOSIVES CHEMICAL LABORATORY. BUREAUOB MINESEXPERIMENT STATION, PITTSBURGH, PA. Pure picric acid is only slightly hygroscopic. The purified product, crystallizedfrom water, absorbs from 0.25 to 0.37 per cent moisture in a safurafed atmosphere at 32" C. during 48 hrs. The hygroscopicify of the commercial maferial is dependent upon its purify, or the presence of water-soluble materials. Picric acid containing as much as 0.05 per cent SO8 (present as sulfate or free sulfuric acid) exposed in a saturated atmosphere at 32 C. for 48 hrs., increased 1.9 per cent in weight; under the same conditions, containing 0.16 per cenf of SOS, the increase was about 5 per cent.
T the close of the war the United States Government had on hand, and still has, large quantities of picric acid, of which it is endeavoring to dispose for industrial purposes. The value of picric acid as an explosive has been discussed by lCIunroe and Howell,* and as its industrial utility might be affected by its tendency to absorb moist'ure, the former suggested that the hygroscopic properties of the explosive be determined. The hygroscopicity of T N T had already been investigated by H u P in the explosives chemical laboratory of the Bureau of Mines, and the writer followed his method for picric acid.
A
ture will be absorbed by the salt; on the contrary, it will lose water to the air. By knowing the solubility of such a salt in water, the vapor pressure of its saturated solution at this particular temperature can be calculated by Raoult's Law, and from this the hygroscopicity of the salt, may be determined. From the nature of organic crystalline compounds, it is evident that their hygroscopicity is generally comparatively low. The rate of moisture absorption by T N T was found by Huff to be practically nil (less than 0.01 per cent). While no data on the hygroscopic properties or vapor pressures of saturated solutions of picric acid are available, the solubility of the compound in water at different temperatures has been fairly extensively investigated, and from this the measurement of its vapor pressure and hygroscopicity can be calculated. CALCULATED HYGROSCOPICITY According to Findlaye the solubility of picric acid in water is in part as follows:
The behavior of any salt, not forming crystalline hydrates, toward atmospheric moisture is dependent upon the solubility of the salt in water, and follows simple physicochemical laws. If the partial pressure of the water vapor in the air exceeds the vapor pressure of the saturated solution of the salt, the salt will absorb moisture from the air; but if the partial pressure of the water vapor in the air is less than the vapor pressure of the saturated solution of the salt, no mois1
Received January 16, 1922.
Temperature
Solubility Per cent
0 5 10 15 20 25
1.05
c.
THEORETICAL
26.5 30 40
1.07 1.10 1.16
1.22
1.37 1.42 1 55 1 98
At 32' C. the solubility of picric acid in water is assumed to be 1.636 per cent, and at the same temperature the vapor tension of water is given by Broch and Weibe7 as 35.372 mm. of mercury. From Raoult's Law, embodied in the formula
* Published by permission of the Director, U. S. Bureau of Mines.
* Assistant Explosives Chemist, Pittsburgh Experiment Station, Bureau of Mines.
"Picric Acid,as an Explosive," Bur. Mines, Serial 2243, April 1921. "The Hygroscopicity of TXT," Chem. Mel. Eng., 21 (1919), 570.
6
7
J . Chem. SOC.,81 (1902), 1219. Olsen, Van Nostrand's Chemical Annual, 1918, 532.
THE JOURATAL OF INDUSTRIAL AND ENGINEERING CHEMISTRY
322
where PI and p~ denote the vapor pressures of solvent and solution and n and N the number of moles of solute and solvent, respectively, the vapor pressure of a saturated solution of picric acid at 32” is found to be 35.3265 mm. of mercury, or 99.872 per cent of that of pure water. Since these are very nearly equal, it follows that picric acid is only slightly hygroscopic. (Incidentally, this hygroscopicity must be multiplied 62.5 times to become equal to that of pot,assium nitrate and 187.5 times before it would be necessary to enclose it in waterproofed inner packages.)* According t o Huff,s 1
a t 25” T X T is 2 as hygroscopic as potassium nitrate. 2300
EXPERIMENTAL Commercial picric acid nearly always contains impurities and occasional adulterations, but in practically negligible quantities. Sodium sulfate is sometimes added before crystallization, while occasionally9 oxalic acid and sugar have been reported. Free sulfuric acid is also a common impurity, and it is possible that a nitrophenolsulfonic acid exists in the lower grades of the explosive. Since small quantities of water-soluble impurities will necessarily affect the hygroscopic properties of any crystalline compound which is only slightly soluble in water, laboratory experiments were made on two different grades of commercial picric acid, as well as on the pure material. The latter was prepared by crystallizing from solution in hot water three times, washing the crystallized product thoroughly with cold water each time. For comparative purposes, the rate of moisture absorption of potassium perchlorate was also determined at the same temperature. The method used by Taylor and Copelo was followed. The picric acid was exposed in open porcelain crucibles (2 em. deep with flat bottoms 3.7 em. in diameter, so that the 2-g. sample taken covered about 11 sq. em. of surface) over water under carefully regulated conditions, and the increase in weight noted after various intervals. Ordinary quart jars, hermetically sealed with glass tops and spring clamps, were filled 2 em. deep Fvith distilled water, and in each jar was placed a small copper tripod to hold one crucible. The jars were then submerged under water in a large thermostat, and allowed to remain until the contents came up to the bath tempcrature. When sufficient time had elapsed for this, the jars were slightly elevated until the necks emerged, the glass tops removed, and the crucibles with contents placed on the small tripods within. The jars were then tightly closed as before, submerged, and allowed to remain for the periods desired. I n order to prevent possible condensation of moisture, the crucibles were warmed slightly above the bath temperature before being placed in the jar. T.4BL€
1
Commercial Commercial No. M-2308 NO. M-2330 119.5-121 .o 12o.c-121 .o Melting point, C.. . . . . . . . . . 120.0 120.7 Solidification point, C.. ..... 0.00 0.00 Moisture, per cent.. 0.002 0,002 Insoluble matter, per cent. . . . 0.049 0.16 Sulfur trioxide, per cent*. .... 0.00 0.00 0.00 Nitrates, per cent.. . . . . . . . . . 0.002 0.001 0.00 Ash, per cent.. The sulfur trioxide was present as a sulfate or free sulfuric acid.
.........
*
Pure 121.5-122.5 121.5 0.00 0.00 0.00
.............
The thermostat was a large tank of water, 30 X 26 X 20 in., heated by electric coils, electrically stirred, and controlled by a toluene thermoregulator within a range of 0.1’. The 8 Arthur Marshall, “Explosives, Their Manufacture, Properties, Tests, and History,” 1916, 340. 8 Marshall, LOC. c i t . , 576. 1Q “Hygroscopic Properties of Sodium, Potassium, Ammonium Nitrate, Potassium Chlorate and Mercury Fulminate,” Chem. Met. Eng., 16 (19161, 140.
Vol. 14, No. 4
jars were kept practically at constant temperature (32’ C.) throughout, and constant conditions of humidity, with the prevention of moisture condensation on the walls, were assured. Blank tests on empty crucibles were run at the same t’ime. When a crucible was taken from a jar it was covered immediately with a porcelain lid, whose weight was known, and weighed as rapidly as possible. Analyses of the different samples of picric acid are given in Table I. These samples were carefully dried in a desiccator over sulfuric acid for 3 days before the analyses and hygroscopicity tests were made. Results on the rate of moisture absorption are given in Table 11. TABI,E 11-APPARENT RATE OR MOISTURE ABSORPTIONI N SATURATED ATMOSPHERE AT 32’ c. Increase after Deducting for MATERIAL Pure picric acid..
HOCRS 5 17 24 41 48 5
.................
Commercial picric acid, M-2308.
....
Commercial picric acid,IM-2330.,
24 17 41 48
... 11;24 41 48
KC104
...........................
Empty crucible..
.................
i,B 24 41 48 5 17 24 41 48
.
Increase Grams 0.0051 0.0068 0.0076 0.0084 0.0099 0.0291 0.0651 0.0759 0.0954 0.1029 0.0113 0.0209 0.0256 0.0354 0.0407 0.0067 0.0151 0.0209 0.0339 0.0411 0.0021 0.0026 0.0020 0.0022 0.0025
Crucible Per cent 0.15 0.20 0.25 0.31 0.37 1.35 3.13 3.69 4.66 5.02 0.46 0.96 1.18 1.61 1.91 0.23 0.62
0.94 1.68 1.93 *.
.. *. .. ..
The above results are the average of five determinations.
DISCUSSION OF RESULTS I n comparing the solubility of potassium perchlorate and picric acid in water it is evident that the hygroscopicity of the former is greater, and increases more readily with rise in temperature than does that of picric acid. From Marshall’s” table of “relative humidity” and “relative deliquescence” of different compounds the comparative hygroscopicity of different substances can be calculated. Potassium nitrate is taken as a standard, and the relative deliquescence of this substance is taken as 1. At a temperature of 32’ C., the rela,tive humidity of picric acid was previously found to be 99.872 per cent. This gives a relative deliquescence of 0.016. From a calculated solubility of 3.60 per centI2 at 32”, of potasbiiim perchorate in water, the relative humidity and relative deliquescence can be similarly determined as 99.,532 and 0.0585. Theoretically then, potassium perchlorate should deliquesce about 3.7 times more rapidly than does picric acid at a temperature of 32” C. On examination of the results obtained, the deductions seem to be well verified, showing that the method gives results coming well within the range of experimental accuracy. ACKNOWLEDGMENT The writer wishes to acknowledge the constructive suggestions and helpful criticisms given by Dr. C. E. Munroe of the Kational Research Council, A. C. Fieldner, Supervising Chemist, and C. A, Taylor, Explosives Chemist of the Bureau of Mines, in the preparation of this article. 11 Lot.
c k , 340.
1% Comey-Hahn,
“Dictionary of Chemical Solubilities-Inorganic.”
