302
INDUSTRIAL AND ENGINEERING CHEMISTRY
are, of course, different from those of Equations 1 to 3, inclusive. EFFECTOF THERMAL EXPANSION ON CALIBRATION EQUATION. It can be shown by dimensional analysis that in the case of a Pyrex glass capillary viscometer calibrated at 37.78" C. (100" F.) on known oils, and then used for routine measurements at 98.89" C. (210" F.), the calibration equation should theoretically be modified to
VOL. 8 , NO. 4
However, within proper limits of design and operation, net error by using a Pyrex viscometer interchangeably at these two temperatures (by using different heating media) is less than one part in 5000, and hence negligible in practical work, as compared to other possible sources of error. In applying a calibration equation over considerably wider temperature ranges, thermal expansion deserves study, as it may become important.
the hazards of making hose connections in order to carry out each viscosity determination. Once the manifold is connected it remains so throughout the life of the rubber hose. The bottom connection, in addition to affording a convenient means of filling and cleaning in conjunction with the manifold, serves as a means of easy and quick adjustment of the oil in the U-tube oil reservoir in case too much oil has been added. THE&MOMETER IN OIL RESERVOIR.The use of a second thermometer in the oil reservoir to indicate when oil is at the desired temperature for test has been found advantageous as to both speed and accuracy in a viscosity determination. Further, the use of a second thermometer serves to guard against a source of error not generally recognized-an increase in oil temperature above that of the vapor bath caused by the absorption of strong rays of light from solar or electric illumination. A rise of from 0.1" to 0.3" F. has been observed, depending upon the intensity of the light and the color of the oil.
Advantages
Disadvantages
kinematic viscosity = 1.0002 A100 t - 1.0004 Bioo t
The advantages in the use of the modified suspended-level viscometer are: RIGID METALMANIFOLD AKD THE BOTTOM CONNECTION. From the standpoint of routine viscosity work by nontechnical operators, the importance of permanently connecting all the tubes at the top of the viscometer with rubber hose to a rigid metal manifold, such as that previously described, cannot be overemphasized. It is by means of the rigid metal manifold and the bottom connection that all filling, operating, and cleaning operations are carried out in a viscosity determination. In other words, it amounts to transferring the operations from glass largely to metal, thereby eliminating
Unfortunately the suspended-level viscometer is not particularly suited for black oils, because of difficulty in observing the meniscus.
Literature Cited (1) FitzSimons, O.,IND.ENQ.CHEM., Anal. Ed., 7,345 (1936). (2) Herschel, W.H., Bur. Standards Technol. Paper 112 (1919). (3) Higgins, W. F., Nut. Phys. Lab. Collected Researches Repts., 11 (1914). (4) Ubbelohde, L., J.Inst. Petroleum Tech., 19,376(1933). RECEIVED December 19, 1935. Presented before the Division of Petroleum Chemistry a t the Mid-West Regional Meeting of the American Chemical Society, Louisville, Ky., Ootober 30 and November 1 and 2, 1935.
A Centrifugal Filtration Tube DAVID F. HOUSTON AND CHARLES PROFFER SAYLOR National Bureau of Standards, Washington, D. C.
C
.
ENTRIFUGAL filtration is recognized as a rapid and efficient method of freeing crystals from adhering liquid, but the use of a basket centrifuge for this purpose is often impracticable with small amounts of material. Centrifugal filtration tubes for h a n d l i n g s u c h s m a l l amounts have been described by Skau ( I , $ ) . I n these, the filtering disk rests on a circular constriction in the centrifuge tube; two of the three modifications discussed also have ground-glass j oilits, In the a c c o m p a n y i n g illustration is shown a simple centrifugal filtration tube designed by the writers. Although it can be used for fewer purposes than those described by Skau, it is sturdy, free from groundglass joints and constrictions, and is readily adaptable to any tube centrifuge. The ordinary tube of the centrifuge is fitted with a filtering crucible, a perforated support, and a container for receiving the filtrate. The tube is closed by a rubber stopper, squareshouldered to prevent wedging. The heavy glass receiving container fits loosely into the
tube, and is cushioned on a rubber pad. The top of this container is ground to support evenly the perforated disk, which also is ground to remove irregularities. A small wire loop is attached to the disk near the edge, so it can be readily removed. The filtering crucible may be a porousbottomed crucible, a Gooch crucible with or without a filter paper, or a tube with a fritted-glass bottom. Filtration is made a t any desired temperature by bringing the apparatus to that temperature, quickly transferring the wet crystals to the crucible, and centrifuging for about 5 minutes. An estimation of the amount of crystals obtained is readily had by transferring the filtering crucible and its contents to a weighing bottle and weighing. The crystals are then removed and a second weighing is made. This gives the weight of the material by difference, and no error is introduced by any liquid which may be retained by the porous bottom of the crucible.
Literature Cited (1) Skau, E. L., J. Phys. Chem., 33, 951-4 (1929). (2)Skau, E. L., and Rowe, L. F., IND.EXG.CHEM.,Anal. Ed., 3, 147-8 (1931). RECEIVED M a y 1 , 1936. Publication approved by the Director of the NEtional Bureau of Standards of the U . S. Department of Commerce.
