I X D U S T R I A L A N D ENGINEERING CHEMISTRY
1024
Vol. 15, No. 10
T h e Surface Tension Balance' Apparatus for the Rapid Measurement of Surface Tension By Robert Gladding Green UNIVERSITY OR MINNESOTA, MINNEAPOLIS, MI".
W
ITH the increas-
Type A balance is similar A new type of surface tension balance is described, an apparatus ing application of i n design to the apparatus utilizing the drop weight method of measuring surface tension of Physical and COldescribed in the PreliminaY liquids. Less than jive drops of liquid are necessary for measureloidal chemistry to biologiHere the balance ment. No calculations are used and the drop weights are not actually science and the indusbeam Swings to an allgular determined. The surface tension is indicated upon the scale ditries, surface phenomena rectly in dynes per centimeter. position when a drop of the are being recognized as of liquid to be measured is great importance and the dropped into the cup. The measurement of surface tension is accordingly becoming a adjustable scale is so placed that the end of the beam oppovery common procedure. Experimental work involving site the cup acts as a pointer and swings over the scale face large numbers of surface tension determinations has been indicating the surface tension in dynes per centimeter. At carried on in these laboratories for a number of years, the angular position of equilibrium the force acting upon and a special study has been made to develop apparatus and the balance beam may be expressed by the equation 8 c = wy methods suitable for biochemical investigation. While ra- cos0, where 8 = angle of torsion of the wire, c = torsion conpidity of measurement has been considered of prime im- stant of the wire, w = weight of liquid drop used, and r = portance, it has been required that the procedures give length of balance beam from torsion wire to cup. This may standard values of reasonable accuracy. For results of dif- be written = case, e as and are We also ferent investigations and of various investigators to be comparative, it is essential that data be reported in true standard have Tate's law, 7 = kw, where Y = surface tension and = constant. Combining the two equations, we have units. All methods of surface tension measurement require correction to give standard values, but as the cor= COS^' e rection in case of the drop weight method can be made so This relation indicates that a scale reading surface tension small as to be negligible, this method has been adopted for general use. For use with the drop weight method two types directb' in dynes Per centimeter when used on this type of of torsion wire balances have been developed. Such an balance must be so constructed that the size of the units will decrease as the angle of torsion increases. the relaapparatus has been termed a surface tension balance. A torsion wire balance was used by Searle2 for the measure- tion between e and C O S 0 is Purely geometrical, such a scale ment of surface tension, the degree of torsion being deter- can be accurately ConstPucted, and is Pictured in Fig- 1. Type A surface tension balance is the simplegt and most mined by the use of weights. DuNuoya introduced the production of torsion in one end of the torsion wire, and also rapid to operate. It has a limitation of accuracy as the its indication by means of a scale divided into degrees. I n function 0 / C O S e decreases rapidly with large values of 8. a preliminary description of this apparatus4 an adjustable scale has been introduced, which has made it possible to express the torsion of the wire in standard units of surface tension so that readings are made directly in dynes per centimeter. DESCRIPTION The two types of our surface tension balance are designated as Type A and Type B. The apparatus built as used in these laboratories is easily convertible from one type to the other. I n general, the apparatus consists of three mechanical parts, a dropping pipet, a balance beam mounted upon a torsion wire, and an adjustable scale. The dropping pipet is similar to those used for actual drop weights. The balance beam (CB, Fig. 2) supported by the torsion wire D carries a t one end a silver cup, C, pivoted upon watch jeuTels. The adjustable scale consists of a series of arcs of the same radius, but the segments are of different lengths and each segment is divided into the same number of units by lines normal to the arcs. This division into units is different in Type L4and Type B. The scale face is movable horizontally so that any one of the series of arcs can be moved under the end of the pointer. The scale is so constructed in relation to the size of the torsion wire that the units represent surface tension in dynes per centimeter. 1 Received 2
3
4
April 28, 1923
Proc. Cambridge Phd Soc , 17 285 (1914). J . Gee. P h y s i o l , 1, 521 (1919). J . Bact , 7, 367 (1922)
Copyvight. 1923, Robert G. Green FIG.I-SCALE O N TYPEA B&LANCE
October, 1923
I N D U S T R I A L A N D ENGIXEERING CHEMISTRY
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peated calibrations of the instrument will check within one or two tenths of a dyne. The number of drops used depends upon the size of the torsion wire; two or three drops have been found very convenient. The adjustability of the scale allows for a variation of 30-degree angular displacement of the pointer, and this coupled with the choice of size of torsion wire used and the number of drops employed gives the apparatus great flexibility. Evaporation and temperature are the gross sources of error. With water and watery solutions the evaporation error is minimized in that calibration and measurements are made under similar conditions of evaporation. The temperature error may be made small by control of room temperature or by frequent calibration. It is essential that the water used in calibration of the apparatus be very pure, as minute traces of some substance markedly reduce surface tension. It has been practice to use freshly redistilled water for calibration of this instrument, accepting the surface tension of this liquid as 72.8 dynes a t 20" C.
FIG.Z-TYPE
CobyYight, 1 9 2 3 , Robert G. Gvee?i B SURFACE TENSION BLOWER
This type of apparatus is calibrated by the use of a standard solution, preferably with a low surface tension, as benzene, and finds its application where rapidity and ease of manipulation is desired. Type B apparatus is pictured in Fig. 2. In this type the baIance beam a t equilibrium is in the horizontal position, the zero point being determined by the indicator K . Angular displacement of the balance beam due to the weight of the liquid dropped into the cup is counterbalanced with torsion produced by rotation of one end of the wire. As torsion is produced in the wire by means of the wheel W , the pointer P moves over the scale face and indicates the surface tension of the liquid in dynes per centimeter when the balance beam is again in the horizontal position. At equilibrium the balanced forces are expressible in the notation used above as 9 c = wr, and combining this with the law of Tate, y = k 0 , indicating that in this type of apparatus the units representing surface tension are proportional to the angles of torsion. Such a scale is shown constructed in Fig. 3.
CALIBRATION The pointer is set a t zero dynes and the balance beam brought opposite the horizontal indicator. Two or three drops of pure water (or other standard liquid of known surface tension) are dropped from the dropping pipet into the cup. The beam is balanced in the horizontal position by torsion in the end of the wire, the pointer moving to some angular position upon the scale face. I n the case of water a t 25' C., this angular displacement represents 72.8 dynes. On the scale face among the series of arcs there will be found one which will correspond to this angular displacement and will be divided into 72.8 units. The scale face is moved laterally until the reading off the end of the pointer is 72.8 and the apparatus is calibrated. This process is carried out quickly and conveniently each time the balance is used or the pipet cleaned, and it is also desirable to check the calibration after running a series of measurements. The pipet is the only part of the apparatus to be cleaned, as the cup need only be dried and the balance beam checked for zero reading. With pure water and careful manipulation re-
FIG. 3-SCALE
ON
Copyright, 1923, Robert G. Green TYPEB BALANCE
ACKNOWLEDGMENT The writer is indebted to Professor MacDougall, of the Department of Chemistry of the University of Minnesota, for many helpful and valuable suggestions.
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