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iodine, but not to both of them). These results are compared to earlier published material, and a tentative reaction mechanism is suggested. REFERENCES CARTER, A. H., A N D WEISS,J.: Proc. Roy. SOC.A174,351 (1940). F. H.: J. Am. Cheni. SOC.60,2886 (1928). GETDIAN, I.,AND HARRIS; W.: J. Am. Chem. SOC.67,1484 (1945). KOLTHOFF, LEIGHTON, P.: Chem. Rev. 17, 431 (1935). J., AND LIVINGSTOX, R.: J. Phys. Chem. 60,176 (1946). MCBRADY, MONTIGNE, E.: Bull. SOC. chim. 6, 564 (1938). SCHNEIDER, E.: Z. physik. Chem. B28, 311 (1935). THUNBERC, T.: Handbuch der biologischen Arbeitsmethoden, Teil I, Heft 7. Urban una Schwarzenberg, Berlin (1920). WEISR,J.: Trans. Faraday SOC.34, 455 (1938).
DETERMINATION OF THE DENSITY OF A FINE POWDER’ W . R. RUBY
AXD
R. P. LOVELAND
Kodak Research Laboralories, Rochester, New E’orh:
Received February 16, 1946
I. INTRODUCTION In some work with fine powders, it became necessary to determine their true density with considerable precision. We were interested, moreover, in the relation between the density of a bulk solid and that of the same material when finely powdered and wished t o determine the two densities in the same way. For this purpose the liquid displacement method, with its direct weighings of a pycnometer, seemed to be fundamentally the simplest and most accurate procedure. Suitable modifications from the usual procedure were necessary because of the enormous surface per unit weight of the sample. We undertook an investigation of the method for such a determination, using selected quartz crystals as the test material, some of which were ground t o a fine powder as discussed later. Quartz is a non-porous solid whose bulk density has been very carefully determined, notably at the International Bureau of Weights and Measures in Paris (ll),and Sosman (10) has emphasized the constancy of the properties of clear transparent specimens. Some investigators have reported (1, 10) that extreme subdivision of even such a material as quartz alters its density appreciably, although Sosman declares that this must consist of a lowering by not more than f0.03 per cent. With other materials the change may be much greater, particularly in the case of glass, where there may be leaching of some components. This paper is a report of the method and the apparatus that mere developed, or modified, from that of previous workers. The work was interrupted by the 1 Contribution
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war before the last and more promising modification of the pycnometer was much used; therefore, most of the results discussed have been made with the older type. The method and the apparatus have been adequately tested, however. The use of the simple procedure, which is adequate for bulk material in relatively large pieces, gives increasingly erratic and erroneous results as the particle size is decreased, as is well recognized. The enormous surface of the material being assumed to be clean except for adsorbed air, which in itself may be a problem, the presence of the latter plus mechanically occluded air is the largest and most obvious difficulty. This has been eliminated by various workers principally in three ways: (I) boiling the immersion liquid in thE: presence of the sample, (2) preliminary use of an atmosphere, such as ammonia gas, which is freely soluble in the immersion liquid, and (3) use of a high vacuum. I n this case the third method was used. A second difficulty, one that was not recognized as soon as that remedied by the use of vacuum, was due to the compression of liquid because of capillary and adsorption effects. The use of a non-polar liquid will reduce this error, and previous workers have found that the apparent density was dependent upon the particular immersion liquid used and that the density values obtained with polar liquids (such as water) and powders of non-porous solids were appreciably larger than those for the bulk solid or Kith non-polar liquids. We therefore also chose a liquid of negligible polarity. Culbertson and coworkers (3, 4) have determined the densities of some powders, including quartz, utilizing a vacuum technique with various liquids of varying polarity. Since the liquids they used were relatively volatile, the vacuum was applied in a preliminary way to the sample, with the tip of the pycnometer being broken under the liquid surface, or the vacuum was used merely to obtain boiling of the liquid over the sample a t a lower temperature.
11. DETERMINATION OF POWDERDENSITY A. SUMMARY OF METHOD
The determination of the density of the purified and degassed immersion liquid a t the temperature specified is an independent operation of equal importance and required accuracy, but was carried out by one of two standard procedures discussed later (page 354 et sep.). To obtain the density of the solid, three values must be determined: A , the volume of the pycnometer; B , the weight of the solid sample; and C, the volume of the immersion liquid displaced by the sample, which, within the limitations just discussed, is the volume of the solid sample. For this purpose five independent weighings, each partially balanced by the same tare, had to be made for each density determination. The volume A was determined by subtracting the weight, l a , of the empty pycnometer from the weight, 2, of the latter filled with immersion fluid. The empty pycnonieter was again weighed, ib, and then another weighing, 3 , was made after addition of the solid sample. The final
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weighing, 4, was made after the immersion liquid had been added to the sample in the pycnometer. Values B and C were then obtained by successive subtraction of the weights and use of the density of the liquid. All weighings were reduced to vacuum by calculation (9). The general procedure is as follows (elements of technique will be discussed in later sections) : After the first tared weighing of a carefully cleaned pycnometer, it was attached to the evacuation apparatus and evacuated to at least the vapor pressure of the immersion liquid (less than 0.3 mm. of mercury). While still on the vacuum line the immersion liquid that had been thoroughly degassed previously was made to flow into the pycnometer until the latter was nearly filled. It was then disconnected and transferred to an accurate thermostat where, after 40 min., the slight amount of liquid required to more than fill the vessel was added. After another 30 min. the closure to the reproducible volume was made by the method described in the next section. The pycnometer "as carefully dried on the outside, allowed to sit in the balance case, and then weighed after suitable precautions. For the second step, the cleaned and reweighed pycnometer, l b , was partially filled with the bulk or powdered solid sample. I n the case of the fine powder, the stock had previously been baked at 750°C.for 6 hr. in a quartz flask, which was part of the vacuum system, a t a final pressure of less than a micron. When the transfer to the pycnometer was quickly made, at low or moderate humidity, a very short time was required to achieve a high vacuum again in either the sample in the pycnometer or the stock flask. However, after weighing the sample plus the pycnometer, weight 3, it was again baked for an hour a t high vacuum (
