Downloaded by UNIV OF NEBRASKA-LINCOLN on September 11, 2015 | http://pubs.acs.org Publication Date: January 1, 1929 | doi: 10.1021/j150314a017
THE DOUBLE-CAPILLARY NETHOII OF SURFACE TESSIOX MEASUREMIEIiT BY S . T. BOTVDEN
The determination of surface tension by the method of capillary rise involves, in general, the measurements of the rise of liquid in a capillary tube in communication with the liquid in a wider tube. I n this connexion, it has been shown by Richards and Coombs' that the diameter of the reference tube must be a t least 3.8 cm. in order to ensure a plane reference surface, Under these conditions, therefore, a somewhat large bulk of liquid is necessary for a determination. Sugden? developed a method involving the measurement of the difference in level to which liquid rises in two capillary tubes of different radii. More recently, Xills and R o b i n ~ o n adopting ,~ this principle have obtained accurate data on the surface tension of liquids by the use of three capillary tubes of different radii. If two narrow capillary tubes of radii rl and r? respectively are immersed in a liquid of density D and whose vapour has density d , we have by the application of the well-known formula for the rise of liquid in capillary tubes,
where hl and h ? are respectively the heights in the two tubeb. For the dlffercnce in height, h, we have
The fornis of apparatus which have hitherto been described for thc mensurprnent of surface tension by thi3 method necessitate a special setting of the capillnrics with reference to the containing-vessel for each determination. lIoreover, the arrangements are not easily cleaned, and no means is afforded for minimising the effects of surface contamination at the meniscus'? Thcse tlruxbacks have been obvi:ited to :I large extent in the appnratri.5 described below. A p , ~ r ~ t r j.0). t a -Yorc-r.cncii'w I.igliiii.,.--l'he apparatus shown in Fig, I has been designed for the mcaaurcnicnt of the density and surface tension on the s:inic s:i~iipIcof liquid. It consists essentially of a pyknometer, one limh of 1
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J . Am. Chrm Soc.. 37, 1656 , 1 9 1j: .J. Chem. Poc., 119, 1483 '1921 , J. C1ic.m Soc., 1927, 1 8 2 3 .
Downloaded by UNIV OF NEBRASKA-LINCOLN on September 11, 2015 | http://pubs.acs.org Publication Date: January 1, 1929 | doi: 10.1021/j150314a017
DOGBLE-CAPILLARY MEASUREMEST OF SURFACE TENSION
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which constitutes the capillary system. The wide portion, A , 5 cm. long and 1.2 cm. in diameter, is joined by means of 4 mm. quill tubing to the two capillary tubes, B and C. The capillary tubes, which are chosen with due regard to uniformity of bore, are 6 cm. long and of radii I mm. and 0.5 mm. respectively. They are set parallel to each other and to the tube, A . The upper portions of the tube, A , and the capillary system are joined to pieces of ordinary tubing, D and E bent as shown. The tube, D, is provided with a constriction upon which a reference mark is etched, whilst the tube, E, terminates in a short length of capillary tubing. Both limbs are provided with ground-on caps, and the over-all length of the apparatus is 15 cm.
n
FIG.I
FIG.2
For thr drterniination of the density, the apparatus is filled with liquid, anti the subsequent procedure is essentially that involved in ordinary pyknometric measurements. .I portion of the liquid is then forced out or rrmoved by suction from the apparatus, so that the lcvcl of the liquid falls midway in the limb, -4. The apparrttw is then.placed vertically in a thermostat and thc difference in lc,vel of the liquid in the two capillaries nieasured in accordance viith the usual capillarirrietric methods. By tilting the appar:itus, liquid may be transferred from l j to C , or altcrnntively, by :ippl’-inp slight pressure to the end of l;j the capillary liquid may be forced down to inis ivitlr that in A , and fresh liquid surfaceR produced in the capillary t u b e on releasing the pressure. Aippccrc2tiisf o r lr’cactiw Liprcids.--The method of capillary riw is particularly well-adapted for the measurement of the surface tension of reartivc or hygroscopic liquids, which need to be preserved in sealed vessels. 7 h e arrangement shown in Fig. 2 is very suitable for this purpose. The Tvide tube, A , has internal diameter of 7-10 mni. and the capillary tubes, B and C, arc zelected and fixed as previously indicated. The apparatus may be charged
Downloaded by UNIV OF NEBRASKA-LINCOLN on September 11, 2015 | http://pubs.acs.org Publication Date: January 1, 1929 | doi: 10.1021/j150314a017
1868
S. T. R O W D E S
with liquid in the usual manner or by direct distillation under ordinary or reduced pressure, and sealing of the tube effected a t a point, S,away from the capillary system. In this manner it is possible t o measure the surface tension of a liquid under its own vapour in the entire absence of air. Readings of the difference in level may be taken in the position shown and also in the inverted position. This apparatus permits the measurements to be carried out between moderate ranges of temperature on the same sample of liquid. Both types of apparatus are rigid, easily cleaned and require comparatively small amounts of liquid for the determination. Cnzi e r a z l ~Collegc. Cartlifl'. A p r z l 9 , 1930.
