T H E DROP-WEIGHT METHOD FOR T H E DETERMINATION OF ELECTROCAPILLARY CURVES 0
S. R. CRAXFORD AND H. A. C. McKAY The Physical Chemistrg Laboratory, Balliol College and Trinity College, Oxford, England Received August 8, i934
Electrocapillary curves for non-aqueous solutions are of interest in assessing the r81e of the solvent in the double layer at a metal-solution interphase, but in an attempt to extend previous electrocapillary work to such solutions, it was found that the accuracy of a capillary electrometer was very much diminished, since the liquid no longer wets the glass perfectly and the angle of contact between the mercury and the glass is less than 180" by an uncertain and variable amount. In bad cases it is inipossible to determine an electrocapillary curve at all in this way. It is therefore a matter of importance to examine other methods for the determination of electrocapillary curves, methods that may not suffer from this defect, and can be used to obtain electrocapillary data for non-aqueous solutions. The drop-weight method was first used by KuEera (4)and has since been employed in electrocapillary work for special purposes in a limited number of cases, but only the following facts regarding its applicability and accuracy have emerged. (a) KuEera found that, if throughout the course of an electrocapillary curve, the dropping rate is kept constant and slow, the drop-weight-potential curve is roughly parallel to the corresponding electrocapillary curve. The only data for electrocapillary curves available at that time were those of Paschen ( 5 ) ) so that, by modern standards of accuracy, this rough agreement is not very significant. (b) In dilute aerated solutions a spurious maximum occurs on the positive branch of the drop-weight-potential curve, owing to the reduction of the oxygen present in the solution (Heyrowskjr (3)), but this can be avoided by using air-free solutions. In any case the effect only becomes large if the solution is very dilute. (c) When the interphase between the mercury and the solution is non-polarizable, the drop-weight method is particularly liable to lead to false results, but this is now well understood (see Craxford (2)) and need not be discussed here, since this paper is concerned only with the polarized interphase. Thus before confidence can be placed in electrocapillary curves obtained by the drop-weight method, it is necessary to develop a' reliable drop'
545
546
S. R. CRAXFORD AND H. A. C. MCKAY
weight technique for electrocapillary problems, and to compare curves obtained by it with the corresponding curves obtained by the use of an accurate capillary electrometer. Such a comparison is necessary, since a detailed application of the mathematical theory of the drop-weight method is impracticable, and it is therefore unjustifiable to assume, without experimental proof, that the surface tension is linearlyeproportional to the drop-weight over the whole of the electrocapillary curve. The requisite experiment,alproof of this assumption is given in this paper. EXPERIMENTAL DETAILS
Two forms of apparatus were used for the determination of drop-weight electrocapillary curves under air-free conditions, and they are sufficiently described by the diagrams (figures 1 and 2). The construction of the capillary, at the tip of which the mercury drops are formed, is however a rather more subtle matter. KuEera drew out a piece of ordinary thermometer tubing, and broke it off cleanly under water. The radius of the tip, which was the narrowest part of the capillary, was of the order of 0.002 cm., but with this arrangement KuEera was unable to extend his electrocapillary curves to potentials appreciably more negative than the electrocapillary maximum, on account of active electrolysis. Preliminary trials with such capillaries gave rather uneven curves, probably due to small changes in the effective radius of the capillary tip, arising from the very slow but unavoidable electrolysis that always takes place when a mercury surface is polarized. Such changes would become unimportant if the capillary tip were larger, and this would also effect a considerable saving of time, because for each measurement, a large number, fifty to one hundred, of the very small drops have to be counted out in order that their total weight may be large enough to minimize errors in weighing. Using wider capillary tips, only relatively few, ten to twenty, drops need be collected and weighed together each time. Hence the method used by Bennewitz and Delijannis (1) was tested, because this involves a capillary with a radius of about 0.1 cm., which is constricted above the tip so as to reduce the rate of flow by the desired amount. This gives smooth and reproducible curves only at the more positive potentials, in fact, at those potentials used by Bennewitz and Delijannis. As the mercury becomes more negative, very small amounts of gas form high up in the capillary and collect in the constriction, so breaking the thread of mercury. This insulates the drop of mercury that happens to be forming at that time, and thereby alters its potential, and hence its surface tension. The gas bubble is then swept down by the flowing mercury into the wider parts of the capillary, where it disperses to the walls and rises up into the constriction once more. This cycle is repeated many times before the bubble becomes large enough to be finally swept out of the capillary. While these proc-
DETERMINATION OF ELECTROCAPILLARY CURVES
547
esses are occurring the drop-weight is clearly unreproducible and useless. This difficulty has however been eliminated by the type of capillary shown in the figures, where a large bulb, A, containing about 10 cc. of mercury, is interposed between the capillary tip and the constriction B. The top of t8hetube C is connected to a mercury reservoir by means of a rubber tube. This arrangement effectively prevents the collection of gas in t h e constriction, and its use enables electrocapillary curves to be prolonged almost
FIG.1. APPARATUSUSED FOR
THE
DETERMINATION OF DROP-WEIQHT ELECTROCURVES
CAPILLARY
FIG. 2. APPARATUSUSEDFOR
THE
DETERMINATION OF DROP-WEIQHT ELECTROCURVES
CAPILLARY
as far into the negative region of potential as with an ordinary capillary electrometer. The interposed bulb also prevents foreign bodies or other impurities derived from the rubber tube from reaching the capillary tip, and so vitiating the surface tension measurements, because in the course of each series of experiments the volume of mercury that drops through the capillary is considerably less than the volume contained in the bulb A. For each series of .measurements, the apparatus is thoroughly cleaned,
548
S. R. CRAXFORD AND H. A. C. McKAY
filled with the necessary solution, a cork inserted at D instead of the capillary, and the air removed by bubbling oxygen-free hydrogen for several hours. Meanwhile the bulb A of the capillary is filled with pure mercury, and connected by means of a rubber tube to the mercury reservoir, taking care to enclose no air bubbles in the connection. The mercury used was and the rubber tube was freed from sulfur purified and redistilled in ZJCZCUO, compounds by boiling first in caustic soda solution, and then several times in distilled water. After being dried, it was shaken out several times with pure mercury before use. When mercury has begun to drop from the capillary, the latter is inserted into I?in place of the cork, and after standing for half an hour for everything t o come to a steady state, measurements may be commenced. The whole apparatus is fixed to a very heavy stand on a brick pillar, built in a basement laboratory, to avoid vibration, which KuEera showed to have a very large effect on the drop-weight. For each measurement the dropping mercury is polarized to the required amount by applying a suitable potential difference between the contact E and the polarizing electrode Kl, and hydrogen is bubbled through the solution for about ten minutes, and is then turned off. The potential of the dropping mercury is then measured against the calomel electrode Kt. The potential usually fluctuates slightly according to the state of the drop, but clearly, it is the value just before the drop breaks away and falls, that is connected with its surface tension. There is nodifficulty in measuring this potential, but in any case the fluctuation is small. The height of the mercury reservoir is then adjusted until the dropping rate becomes equal to any desired value, which must be kept fixed for each series of measurements. This rate must be a slow one, of the order of one drop every six or more seconds. The collecting spoon is then moved under the capillary, and twenty drops counted out and removed. The potential is again measured. The drops are poured into a 5-cc. crucible, washed twice with distilled water, and dried with strips of filter paper. The crucible is then placed in a vacuum desiccator, and its contents weighed next day. If, after each series of measurements, the capillary is washed thoroughly, it will continue to give satisfactory results for a long period of time. RESULTS
Table 1 contains the results of such measurements for N potassium nitrate solution. Column 2 contains the weights of twenty drops, and column 3 the surface tension for the,same potential, measured by means of an accurate capillary electrometer. The ratio of the surface tension at the maximum to the maximum drop-weight is 305.1, and this factor is then used to convert each of the drop-weight values to surface tensions, by multiplication, and the result is given in column 4. The agreement between columns 3 and 4 throughout the whole of the range of potential
549
DETERMINATION O F ELECTROCAPILLARY CURVES
employed shows,that the drop-weight method is accurately applicable to the determination of electrocapillary curves. A t any potential the surface tension, as given by this method, is always well within 1 dyne per centimeter of the value given by the capillary electrometer, and hence the surface tension is correct to within 0.2 per cent. ' TABLE 1 The electrocapillary curve for N potassium nitrate at 16°C.
POTENTIAL (N.CAL.)
DROP-WEIGHT (20 DROPS)
BURPACE TENSION MEASURED BY CAPILLARY ELECTROMETER
SURFACE TENSION CALCULATED F R O M DROP-WEIQHT
grams
dynes per cm.
dynes per cm.
-0.011 -0.046 -0.073 -0.145 -0.178 -0.211 -0.247 -0.286 -0.326 -0.361 -0.392 -0.433 -0.470 -0.516 -0.545 -0.578 -0.613 -0.649 -0.679 -0.714 -0.760 -0.816 -0.863 -0.907 -0.959 -1.008 -1.048 -1.107 -1.167
1.2390 1.2548 1.2673 1.2956 1.3077 1.3185 1.3306 1.3430 1.3530 1.3611 1.3667 1.3732 1,3764 1.3791 1,3807 1.3810 1.3803 1.3789 1.3752 1.3710 1.3625 1.3504 1.3388 1.3257 1.3093 1.2928 1.2736 1,2529 1.2288
378.3 382.2 386.8 395.7 399.2 402.6 406.0 409.3 412.3 414.8 416.4 418.5 419.8 420.8 421.2 421.3 420.9 420.2 419.2 417.6 415.2 411.7 407.9 404.0 399.2 394.0 389.4 382.3 374.4
377.9 382.7 386.7 395.3 398.8 402.3 406.0 409.8 412.8 415.2 417.0 419.0 420.0 420,7 421.2 421.3 421.1 420.7 419.6 418.2 415.7 411.9 408.4 404.3 399.3 394.4 388.6 382.2 374.8
SUMMARY
A technique for the determination of electrocapillary curves by the dropweight method has been developed, and the resulting curves are shown to coincide to within 0.2 per cent with the corresponding curves obtained in the usual way with a capillary electrometer.
550
S. R. CRAXFORD A N D H. A. C. M C K A Y
REFERENCES BENNEWITZ, K., AND DBLIJANNIS, A.: Z. physik. Chem. 126, 144 (1927). CRAXFORD, S. R.: Phil. Mag. 17, 54-64 (1934). HEYROWSK?, J., AND SIMBNEK,R.: Phil. Mag. [7] 7, 951 (1929). KUEERA,G.: Ann. Physik 11, 529, 698 (1903); Bull. intern. acad. sci. Bohbme, 1903. (5) PASCHEN, F.: Ann. Physik 39,43,51 (1890). (1) (2) (3) (4)
