edited by SHELDONH. COHEN
inflation fighter/
Washburn University Tooeka.Kansas66621
An Inexpensive Du Nuoy Tensiometer Manuel Sanchez-Rubio, Barbara Gordillo, and Dennis S. Rushforth' Centro de lnyestigaci6n y de Estudios Avanzados del I.P.N., Dept. de Quimica. Apdo. Postal 14-740, 07000 MCxico. D.F. Mexico
The Du Niioy type tensiometer which is described herein is a delightfully simple piece of equipment which yields results of excellent agreement with published values. Teachers may wish to use this tensiometer to challenge their students, either as a group or on a one-on-one basis to delve more into the area of surface tension. Apparatus Expensive tensiometers use a modified analytical balance to measure the maximum force applied before breaking the ring free from the surface. We use a much less sophisticated balance consisting of a plastic drinking straw as the crossarm, a needle as a pivot, and a wire loop counter balance to assure proper balance (see Fig. 1). Construction of the tensiometer is ouite simnle. . . and most students have no problems building their own. Formation of the ring is the most complicated part. (See Fig. 2). Our ring is made of 26 gauge nichrome resistance wire (80/20 alloy). The rinr is most easilv formed hv wraooina .. the wire around a %in. external diameter test tube. Jeweler's pliers are best used for making the necessary bends. Molding of the ring may require a little practice because you will want to avoid crimps in the wire; however, we have found that the second attempt is usually acceptable. (It is more important that the ring be planar than round). The less perfect first ring can he covered with aluminum foil and used as the balance pan (J in Fig.
Figure 1. Sketch of complete tensiometer and sample. A, clamp, 6. needle, C. Cork. D. ring stand, E. plastic drinking straw, F. wire loop counter balance, G. nylon thread, H. wire hook, I. ring, J. balance pan, K. lab jack, L, plastic wrap with hole, M. sample, N. constant temperature bath. 0. thermal insulation.
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Figure 1illustrates the tensiometer as we constructed it. We used the ring stand, clamp, and cork because we already had them. Cheaper and more convenient methods of mounting the tensiometer possibly could be found. Although a less expensive lab jack could be used, jacks are vital to the success of the design. Inexpensive weights for use with this balance can he bought from most scientific supply houses. Theory Du Niioy-type tensiometers are designed to measure the maximum force or weight that can he supported by a ring placed at the interface of a two phase system. The interfacial tension2 can then be calculated from the equation: Mgf 4aR
Y=-
Where M is the mass of the weight on the pan, g is the gravitational constant. and R is the radius of the rinr from the
Figure 2. Formation of the ring. The hooks should be clamped in place after the ring is completely formed.
on the ratio MlAp where Ap is the difference in density of the two ohases. This correction factor also deoends on the ratio of t i e radius of the ring, R , to the radius of the wire, r. In the case of liquid-liouid interfaces an exoerimentallv determined equationfor caiculating the correction factor i s used:
with c = 0.04534 - 1.679 rlR and a = 0.7250 and b = 0.0009075 as universal constants for all rings.3 Procedure Place the ring just under the interface being measured. Adjust the
(which is provided in tahuiar form in the literature2) depends
Methodsand details01 how you can save money by building your own equipment are outlined in this feature. If you have made something which is particularly useful or money saving, please share it with our readers by sending it to Ms. Arlyne Sarquis, Secondary School Section Editor. Miami University-Middletown. 4200 E. University Blvd., Middletown. OH 45042.
70
Journal of Chemical Education
Presented at 182nd Meeting of the American Chemical Society. New York, August 1981.
' To whom correspondence should be addressed.
Harkins, W. D. and Jordan, H. F., J. Amer. Chem. Sac., 52, 1751 (1930). Freud, B. 6. and Freud, H. Z., J. Amer. Chem. Soc., 52, 1772, (1930).
Comparison of Interfacial Tenslon Values at 20°C Determined by the Tensiometer with Those Reported in the Literature System H201air Ethanoiiair Benzenelair shexanelair odecanelair ~undecanelair n-dodecanelair Rtridecanelair ~tetradecaneiair shexadecanelair HzO/n-octane HnOibenzene HnOlethyiether H,O/sPentanoi Weasf.
y(exp)imNm-'
y(iit)lmNm-'
72.8 22.6 28.3 18.4 23.8 24.7 25.3 26.0 26.7 27.4 50.8 35.1 10.6 4.8
72.75* 22.75' 28.85# 18.45' 23.83" 24.66" 25.35' 25.9Sb 26.56b 27.47b 50.8 ' 35.0 10.7 4.8
R. C., Editor. "Handbook of Chemistwand Physics." sOthEd., CRC Press. Boca
Ration. FL. 1979. "meiian Petroleum lnstitvfe Research Project 44. "Selected Properties of Hydrocarboon and Related Compounds." Voi. 11. 1988. " Paliinglon.J. R.. "Advanced Treatise on Physical Chemistry." Voi. 11, Longmans. London. 1968. 0 . 170.
..
is added to the Dan.this second iack should eraduallv be lowered until
repeat the cycle until the maximum weight is determined Results
We found the reproductivity of our maximum weight determinations to be within f1 mg (which was the smallest weight we had available). The table shows a comparison of
interfacial tensions calculated from the data obtained with our tensiometer and those reported in the literature. The major sources of error in this interfacial tension determination were the cleanliness of the ring and the measurement of the diameter of the ring. Cleaning the ring with boiling detergent, rinsing well, and flaming the ring in a Bunsen burner immediately before using produced satisfactory results. The value used for the ring diameter was an average of four to eight values. In taking these diameter readings, one must guard against distorting the shape of the delicate ring which is easily bent. Conclusions
We have used this tensiometer in several teaching experiments and find that, in spite of the seemingly simple (i.e., it is not digital electronic) nature of the instrument, students have no nroblem in using it. As a honus. we have found that the exceilent precision g&es us a good opportunity to discuss experimental uncertainty and to review the differences between accuracy and precision. We consider this tensiometer to he a welcome addition to our tools. The satisfaction of building an apparatus and making precise measurements on it rewards the careful student and is highly motivating. Most important, this tensiometer permits the possibility of quantitatively demonstrating interfacial tension and the effects of the various agents that modify it. In fact, we often find that our discussion includes the commonly intriguing ability of water bugs to walk on water. Acknowledgment
One of us (BG) was supported in doing this work by an Avon undergraduate thesis scholarship.
Volume GO
Number 1
January 1983
71
