ASALYTICAL EDITION
58
in Tacuuni, saturated with dry oxygen at room temperature, and introduced into the oil flask without contact with air. Periodic weighings showed an initial rate of H,O evolution of about 0.200 gram per hour, falling regularly over a 6hour period to a constant value of 0.004 gram per hour, which corresponds with the blank obtained in the second run. I n all of these tests the oil was first dried by heating t o 250” C. while passing dry nitrogen through the flask during the heating and cooling period. Table I11 CaCh SATURATED RUN USED BLACK GEN WITH CO, TIME Grams Hours 1 None .. 0 . 3 7 0 0% N o 2.5 2 None Purified No 2.5 3 Oven-dried 10 Purified So 2.5 4 h’one , 0.37, 02 Yes 2.5 5 Degassed and saturated with 0 2 34 Purified Yes 6.5 a After deduction of corresponding blank. WEIGHT
BLACK
OF
h-ITRO-
..
.
APPARENT H20 Gram 7p . - of. blacka 0,1245 . 0.0115 0,0784 0:67 0.1122 .
.
.
0,5164
1.45
Discussion
The foregoing data show conclusively that oxygen in the nitrogen or adsorbed on the black may react with mineral oil under the test conditions. I n the description of the xylene-oil method the duration of the test and the purity and volume of the nitrogen used are not stated. It is understood, however,2 that the nitrogen was of high purity, that it passed over but not into the oil, and that all H?O mas evolved from the oil-black-xylene mixture in 10-30 minutes. This practically eliminates the nitrogen as a factor in the results, but the present data show the need for precautions on this point. It therefore seems probable that oxygen adsorbed on the black is the main factor. The rate of reaction observed in run 5, using 34 grams of black, might appear to be too low to account for the H20 evolved in 1030 minutes in the xylene-oil method using only a 5-gram sample. However, the kinetics of reactions in a liquidsolid-adsorbed gas system are so complex that it is difficult 2
Private communication from Carson.
Val. 2, h-0. 1
to predict the effect of the amount of solid present on the observed reaction velocity, while also this must depend largely on the particular character of the oil used. In the present case the oil was a refined paraffin wash oil (“straw” oil) and presumably more resistant to oxidation than the average petroleum fraction of like boiling range. Based on the apparent conversion of 0 2 to H20 observed in run 1, the 0.67 per cent H 2 0 evolved in run 3 would correspond t o 1.0 per cent oxygen adsorbed on the black, and the 1.45 per cent H 2 0 found in run 5 would correspond to 1.9 per cent oxygen. The work of Hulett and Cude (a) shows only about 0.14 per cent free O2 removable as such from channel blacks a t room temperature and 0.6-1.0 per cent removable as O?! C02, and CO a t 445” C., while their original analysis shows about 2.5 per cent total oxygen present. Johnson’s results (3) show about 3.0 per cent oxygen removable in all forms (except H20) a t 950” C. S o final conclusions can be drawn from these figures as to the original amount of adsorbed 0 2 , but from what is known of the niechanism of adsorption of 0 2 on carbon and other oxidizable surfaces, and of the mechanism of its subsequent removal (Langmuir’s general studies on oxygen films), it is probable that a large proportion of the total oxygen content is originally present as adsorbed 0 2 , regardless of the form in which it ultimately appears on heating and evacuating. All of the foregoing experimental observations and general facts are in agreement with the belief that the “additional moisture” shown by the xylene-oil method is mainly the product of the reaction of the mineral oil with the oxygen adsorbed on the black. Acknowledgment
These results are published with the permission of the Combustion Utilities Corporation, a t whose laboratories, at Linden, K. J., the work was carried out. Literature Cited (1) Carson, IKD.EKGCHEX.,Anal. Ed , 1, 225 (1929) (2) Hulett and Cude, Bur. Mines, Bull. 192, 76 (1922). (3) Johnson, IND.E N G CHEM, 20,904 (1928).
Anhydrous Magnesium Perchlorate as a Drying Agent’ S a m Lenher and Guy B. Taylor EXPERIMENTAL STATIOK. E . I.
DU
PONT
DE
T H E preparation of magnesium perchlorate and its use as a desiccating agent in the trihydrated and anhydrous forms have been described by Willard and Smith ( 2 ) . In a subsequent paper Smith, Brown, and Ross (I) advise the use of the trihydrate as a collector of water vapor in steel and organic combustion analysis, mainly because it can be more easily prepared in suitable physical condition. I n preparing the anhydrous salt, the trihydrate melts in its water of crystallization a t about 145” C. and passes slowly into the anhydrous form as the temperature is raised to 250” C. The result is a pasty mass, which is difficult to handle while being dehydrated and is not readily reduced to granules. We have found that if the trihydrate, “dehydrite,” is enclosed in a tube or bulb and a vacuum less than 0.1 mm. of mercury maintained while heating slowly to 250” C., the water of crystallization is removed so rapidly that the 1
Received November 5, 1929.
XEMOURS & COMPANY, W I L M I N G T O N , DEL.
salt does not fuse a t any stage of the operation. The reason for this is that the salt is partially dehydrated below its melting point, so that fusion cannot occur. The resulting material is in a better physical state for drying gases than the original “dehydrite.” Its capacity for water absorption is doubled, and its efficiency is greater where extremely low partial pressures of water vapor are significant. For evacuation during the dehydration a good oil-sealed pump of large capacity must be used to maintain the specified vacuum. A t a pressure of 10 mm. of mercury the salt melts on heating, just as it does in the open air. The dehydration of the trihydrate a t pressures below 0.1 mm. mercury a t temperatures of 140-250” C. is rapid. Our results indicate that under these conditions dehydration of a 10-gram sample is accomplished in 1 hour. Literature Cited (1) Smith, Brown, and Ross, IND. ENG.CHEM.,16, 20 (1924). (2) Willard and Smith, J . Am. Chem. SOC.,44, 2255 (1922).
