5.1.1. Introduction. Because of t h e ease a n d rapidity with which t h e force necessary t o pull a ring out of t h e surface of a liquid can be measured, the ring method of surface tension measurement has h a d great popularity.
Like t h e drop weight a n d t h e m a x i m u m bubble pressure m e t h o d s this method is a dynamic one a n d requires a knowledge of t h e force necessary to r u p t u r e the liquid-air interface. T h e d a t a obtained b y t h e ring method are quite sensitive t o t h e size of wire a n d dimension of ring employed, so t h a t absolute values of surface tension cannot be obtained without applying corrections. I n fact it is rather surprising t o consider t h e large n u m b e r of ring surface tension determinations in which t h e shape of t h e surface was not taken into account. I t was not until 1 9 2 4 - 2 6 when Lenard ( 3 7 ) who used a straight wire instead of a ring and Harkins et al. ( 2 8 ) first correctly calculated surface tension values from t h e surface r u p t u r e forces t h a t the theory of the ring m e t h o d was p u t on a sound basis. W e are including t h e work of Lenard in this discussion as t h e problems involved in pulling a straight wire out of t h e surface are similar in m a n y ways t o those applicable t o work with circular rings.
5.1.2. Theory of the Ring Method. If a ring pulled a true hollow cylinder of liquid with a vertical plane of contact with the ring from the surface, t h e n at t h e m o m e n t t h e liquid surface ruptured, t h e weight of liquid supported t o t h e point of r u p t u r e above a flat level surface would be given b y t h e equation
W · g = 4T T# T (33)
where R is t h e radius of t h e ring measured to the center of the wire of which t h e ring is composed. However, the liquid surface will t a k e on t h e shape illustrated in Fig. 9 as t h e ring is raised (or t h e liquid lowered).
FIG. 10. Cenco Du Noliy tensiometer. (Courtesy of the Central Scientific Co.) Harkins and J o r d a n (27) m a d e an extensive s t u d y of t h e weight of liquid supported b y rings of various r a n d R values using liquids of known surface tension a n d found t h a t in t h e majority of cases t h e weight of liquid supported is greater t h a n t h e simple equation (33) would predict.
T h e y have published tables of F-factor corrections for t h e ring method which m a y a m o u n t to as much as 2 5 % under certain conditions. Thus, the correction factors m a y be extremely large. Knowing F, t h e surface
FIG. 9. Schematic diagram of the ring method.
tension is t o be calculated from t h e equation W · g
4Τ Γ# • F (34)
To determine F from the tables both R a n d r m u s t be known as well as V, t h e volume of liquid supported at the m o m e n t of break (V = m/p).
Freud a n d Freud (20) have carried out numerical integrations of Laplace's equation relating surface curvature to surface tension a n d t h u s succeeded in calculating t h e F-factors theoretically.
For this reason we can now regard t h e ring m e t h o d as being an absolute method.
5.1.3. Methods. For t h e most precise determi
nations using rings t h e technique of Harkins a n d J o r d a n (27) should be followed although Schwen-ker's straight wire m e t h o d (55) seems t o be some
w h a t more sensitive. T h e relative t w i n - r i n g m e t h o d of Dole a n d Swart out (12) described below has t h e greatest precision of all methods involving a r u p t u r e of t h e surface. Various commercial torsion force is calibrated b y known weights in advance of t h e surface tension measurement.
T h e Harkins /^-factor corrections should be applied, of course.
Returning t o the techniques of Harkins and Jordan, Fig. 11 illustrates their surface tension flask in which t h e p a n holding t h e liquid is sealed into another flask in order t o allow t h e whole t o be immersed in a constant t e m p e r a t u r e bath, and which has t h e side tubes A and Β so t h a t t h e surface can be renewed b y flushing through A a n d b y sucking t h e over
flow liquid out through B . During measurement t h e liquid is held stationary a n d t h e ring slowly raised b y using a mechanical gear arrange
m e n t t o raise t h e chainomatic balance on whose left beam t h e ring is supported. T h e chief difficulty comes from slight impurities, particu
larly in t h e case of aqueous solutions, which are picked u p b y t h e ring, thereby changing t h e contact angle. T h e pull on t h e ring at r u p t u r e is a rather sensitive function of t h e contact angle (44) as it also is of t h e angle of inclination of t h e ring (27). Precautions and experimental recom
mendations can be listed as follows.
FIG. 11. Surface tension flask of Harkins and Jordan (1930).
(a) Harkins (25) recommends raising t h e ring rather t h a n lowering the solution as t h e latter might produce disturbing ripples in t h e surface. Dole and Swartout (12), however, show t h a t a precision of 0.002% is attainable on lowering t h e liquid (the precision of Harkins a n d J o r d a n was a b o u t 0.2%).
(b) T h e diameter of the pan should be such t h a t it is 4 - 5 cm. greater t h a n the diameter of the ring.
(c) T h e angle of inclination of t h e plane of the ring with the free surface should be less t h a n 0.47° for the error of measurement to be less t h a n 0.1 %. A positive angle can be detected by carefully observing the ring as it approaches t h e surface of t h e liquid. If it makes contact with the liquid simultaneously all about its circumference, then no angle of inclination exists (assuming, of course, t h a t t h e ring is plane).
(d) Harkins and J o r d a n adjusted t h e volume of liquid in the p a n so t h a t the surface would be plane at t h e m o m e n t of rupture.
(e) T h e ring should be circular and all in a plane.
(f) T h e a p p a r a t u s should be cleaned with a hot mixture of nitric and sulfuric acids, rinsed with redistilled water, steamed (the steam generated from water t h a t had previously been refluxed with K M n 04) , and rinsed again. T h e p H of t h e rinse w a t e r should be checked to insure t h e complete removal of t h e acid.
These rigorous cleaning directions are of significance chiefly in t h e measurement of t h e surface tension of aqueous solutions.
(g) T h e ring is preferably m a d e of p l a t i n u m - 1 0 % iridium a n d should be ignited in a flame shortly before use.
(h) T h e dry weight of t h e ring must be known. T h e W of equation (34) is the weight at the moment of r u p t u r e less t h e dry weight.
T h e fact t h a t drops of liquid adhere t o t h e ring after t h e r u p t u r e is of no significance except t h a t it indicates a desirable wetting of the ring b y t h e liquid.