ANALYTICAL EDITION
INDUSTRIAL AND ENGINEERING
CHEMISTRY HARRISON E. HOWE,EDITOR
Surface-Tension Measurements of Viscous Liquids A. H. PFUND
AND
E. W. GREENFIELD, Johns Hopkins University, Baltimore, Md.
By allowing a small air jet to impinge on the surface of a liquid, there is produced a small depression whose depth is an inverse function of the surface tension of the liquid. This procedure is particularly suited to viscous liquids.
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Here the cell, B , which fits neatly between the blocks, DD, is mounted on a plate, E , to which a hinge, H, is attached a t one end and a screw, F , at the other. By means of this screw it is possible to raise or lower the cell bodily and thus to bring the image of the undisturbed surface into coincidence with the telescope cross hair, set to the zero reading of the micrometer. To facilitate cleaning, the cell is made out of a brass block, 10 mm. thick, from which a U-shaped portion has been removed. The glass plates forming the sides of the cell are simply held against the carefully machined sides of the brass block with small clamps (not shown).
HE usual methods for surface-tension measurements fail
when applied to highly viscous fluids. The following method, developed by one of the authors (A. H. P.) in 1932 and tested by the other (E. W. G.) in 1934 is based upon the observation that if a small air jet be allowed to impinge on the surface of a liquid, a n indentation or depression of the type shown in Figure 1 is produced. The depth of this depression has been found to be an inverse function of the surface tension. The method is at its best when applied to liquids which are nonvolatile, viscous, and transparent.
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The general arrangement of the apparatus is shown in Figure 2, where A is a glass tube drawn down to a bore of about 0.8 mm. at its lower end which is separated by about 3 mm. from the liquid surface, C. While there is nothing critical about the dimensions of the cell, B, the thickness of the fluid layer in the direction LM was chosen as 10 mm. The depth of the depression is 1 to 2 mm. since instability sets in when the depth is made considerably greater. Measurements of the depth of the depression are carried out with a short-focus Gaertner telescope, M , supplied with a micrometer eyepiece reading to 0.01 mm. 11lumination is effected by means of the small lamp, L, which is replaced by an arc lamp when the liquids are nearly black. Light coming over the top of the liquid is cut out by the opaque shield, S. Since it is necessary that the distance between the end of the glass tube, A , and the liquid surface be kept constanta condition difficult to realize by attempting to fill the cell always to the same level-an arrangement of the type shown in Figure 3 was adopted.
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FIGURE2. ARRANGEWENT OF APPARATUS
FIGURE3. ARRANGEMENT FOR MAINTAINING CONSTANT SEPARATION BETWEEN JET ORIFICE AND LIQUIDSURFACE
FIGURE 1. FORMOF DEPRESSIONWITH SucCESSIVE INCREMENTS OF AIR PRESSURE 81
INDUSTRIAL AND EKGINEERING CHEMISTRY
82
For the sake of brevity, no description of the apparatus used in the production and maintenance of constant air pressure will be given. A pressure of about 2.4 cm. of water was found best. Calib r a t i o n w a s e f f e c t e d by means of the known surface t e n s i o n s of pure liquidswater, glycerol, glycol, dibutyl phthalate, and several mineral oils-no corrections being a p p l i e d f o r density differences. The calibration curve obtained is shown in Figure 4, where the data are plotted on double logarithmic paper. The equation of this Surface & m a n (2ynm/cnj curve is FIGURE4. CALIBRATION y = Ka-lh CURVE,LIQUIDS OF KNOWN SURFACE TENSION 1, Water. 2, Glycerol. 3, Glycol. 4, Dibutyl phthalate. 5, 011 where y is the surface tension, 104. 6, Oil 108. K a constant, and a the depth of the dewession. An advantage of the air-jet method o;er other methods is brought out in a study of temperaturesurface tension relationships in oils used for the impregnation of cables. The specific case chosen for the sake of illustration relates to a mineral oil designated as No. 100/100. At 100" C. this oil had a viscosity of 0.5 poise and was therefore fluid: at 60" C., however, the viscosity has risen to 4 poises and the oil was thick. Surface-tension measurements made by the air-jet method and by the "Jolly balance platinum ring" method are in excellent agreement at temperatures above 60' C. As shown in Figure 5, measurements by the platinum ring method become very erratic a t 60", only to become meaningless a t lower temperatures. The jet method, however, is not subject to such limitations. Even a t 25" C. where the oil would not pour-the viscosity having risen to 85 poisesthe depression had reached a constant depth in 12 seconds and measurements could be carried out with the same ease. This method is obviously applicable also to molten metals and to opaque materials in general. By directing a jet of pure nitrogen (in place of air) against the surfaces of molten bismuth and tin, it became evident that the surface tension of these materials is much greater than that of water. Up to the present time no entirely satisfactory method for measuring the depth of the depression has been found. Another possible application of a modified jet method lies in the measurement of interfacial tension of water in contact with some nonmiscible, viscous liquid such as a heavy oil. In this case, a small, vertical jet of water, directed upward within the water and striking the oil-water interface perpendicularly, distorts the interface into a small hump or elevation whose height may be measured as before. Calibration
Tempcro fu,c'C.
FIGURE 5. xx 00
SURFACETENSION vs. TEMPERATURE (MINERAL OIL 106) Platjnum ring-Jolly balance method Air-jet method
VOL. 8 , NO. 2
may be effected through the use of liquids having higher fluidity and a known interfacial tension with water. In the absence of such information it is always possible to make measurements by creating electrically excited ripples (1) a t the interface between water and nonmiscible liquids of high fluidity. In connection with the preceding work it was noticed that air jets are extremely sensitive to short sound waves. If an air jet of the previously described type be directed vertically downward on a water surface-the separation between orifice and surface being 7.5 cm. and the pressure equal to 7.5 cm. of water-the depression immediately disappears as soon as a hissing sound is produced or if keys be rattled. There is a critical jet velocity and separation at which the sensitiveness is greatest. The behavior of the jet is therefore similar to that of the well-known "sensitive flame." The sensitive jet is well adapted for lecture-room demonstration : It is merely necessary to use a vertical projection lantern and to send the light through a flat-bottomed glass dish containing water. By focusing the water surface on the screen, it is found that a black spot appears where the water surface is deformed. This spot disappears immediately upon the production of short sound waves.
Literature Cited (1) Pfund, A.
H.,Phys. Rev., 32,326 (1911).
RECEIVED November 1, 1935. Presented a t the meeting of the Committee on Insulation (National Research Council) at Urbana, Ill., 1934.
A New Buret for Potentiometric Titrations L. S, KEYSER, University of Illinois, Urbana, 111.
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H E burets which are commonly used for potentiometric titrations are usually the o r d i n a r y s t r a i g h t t y p e or those which have the faucet type of stopcock. I n many titrations where a stirrer, electrodes, and other apparatus are necessary in or above the titrating r e c e p t a c l e , i t h a s b e e n found convenient to use the buret herewith illustrated. It has the advantage of allowing control of t h e s t o p c o c k a t a comfortable distance from the encumbering mechanism over the reaction vessel. Although the buret is not available from laboratory supply houses, it can be easily constructed from the ordinary straight buret by anyone possessing a minimum of glass-blowing ability. The tapered end of the stopcock tip is cut off a t its base as far from the stopcock as possible. The stopcock is then wrapped with asbestos cord or other suitable protection afforded and a length of glass tubing having the same approximate dimensions is sealed on. The tube is then bent into the shape shown and the end drawn off to give a tapered point. R B C ~ I VDecember ~D 31, 1935.
