846
G. L. MACK, J. K. DAVIS AND F. E. BARTELL
THE BOUNDARY T E S S I O S O F GALLIUW? G. L. MACK K e w Y o r k State dgrzcultural Ezperzment Statzon, Genetla, X e w Y o r k h\D
JAMES K. DAVIS
AND
F E BiRTCLL
Department of Chemzstry, Cnzuersity of Mllzchzgan, Ann A Ibor, Jfzchzgan Recezved December 16, 1,9.$0
Gallium is one of the few metallic elements which exist in the liquid state at room temperatures. From its other physical properties, gallium might be expected to have one of the highest surface tensions of any liquid at ordinary temperatures; hence accurately known values would be extremely useful in establishing surface and interfacial tension relationships. On account of the large free surface energy and chemical reactivity of gallium, the method of measurement must be chosen with care. It is essential that the surface be protected from contamination in an inert atmosphere and that a fresh surface be easily and quickly reformed. A method that appeared to be particularly adapted to these requirements has been developed recently by Hauqer and his associates (1). The image of the pendant drop or bubble is projected upon a photographic plate so its dimensions may be conveniently measured. Thc actual measurements consist of a determination of the maximum diameter of the pendant drop and of the diameter a t a selected plane, such a plane being located a t a vertical distance from the apex of the drop equal to the maximum diameter. Since the tension a t the interface is a function of the size and shape of the drop, its value in dynes per centimeter may be calculated from the equation:
where u is the difference in density between the two phases, g is the gravitational constant, de is the masimum diameter of the drop, and H is a n empirically dGermined function of S. S is the shape factor and is defined as the ratio of the diameter a t the selected plane, d,, to the maximum diameter, or S = d,/&. Other outstanding advantages of this method are that it is equxlly well suited to measurements of liquid-gas and liquidliquid interfaces, ihnt it is capable of a considerable degree of precision, and that it comprises a truly static measurement of the boundary tension. The apparatus differed in some details from that described by Hauser. 1 Prescntxl a t the Seventeenth Colloid Symposium, held a t Ann .4rbor, Michigan, J u n e G 8, 1040. 2 .ipprovc'd by the Director of the Xew York State Agricultural Experiment Station for publication as Journal Paper S o . 384, June 3: 1940.
BOUNDARY TEXSION OF G.kLLIUA4
847
The source of illumination was a 60-volt D.C. carbon arc lamp equipped with condensing lenses and a series of four glass heat filters. A shutter was also provided, so that the small amount of heat remaining in the illuminating rays could be completely shut out when necessary. The camera was a Bausch and Lomb Type H camera, fitted with vertical and horizontal focusing mechanisms and a specially designed lens and telecentric stop. The shapes of the pendant drops were recorded on 3$ x 4 t in. Wratten “121” plates, and their dimensions were measured with a coordinate comparator having an accuracy of h0.03 mm. The correction of the lens was tested by photographing an ocular glass disc with cross-hatched lines ruled upon it. Within a circle 3 in. in diameter the deviation from orthogonal projection was well within the accuracy of the coordinate comparator. A cylindrical Pyrex glass cell 10 cm. long with two parallel and optically plane windows 5 cm. in diameter was supported rigidly in an adjustable brass holder which fitted between cleats on a solid brass plate. The position of the cell in the holder was adjusted until the plane of the windows was just parallel to that of the photographic plate, using the method of superimposition of reflected images. The whole system was mounted on an optical bench so that it could be kept in permanent alignment. The cell was closed a t the top with a large ground-glass joint through which a smaller tube was insealed. The lower end of this inside tube was cut off and fitted with a ground joint into which a drop-forming tip could be inserted so that its lower edge would be located near the horizontal line passing through the center of the cell windows. The glass tips were turned down in a lathe until they were very nearly circular, and the end was ground down by hand until the inside edge appeared sharp under a magnification of twenty times. Two tips were used interchangeably, both having an internal diameter of about 2 mm. The drop-forming tip was connected through a stopcock to a 30-ml. gallium reservoir provided with a separate inlet and outlet for circulating an inert gas. A by-pass tube connecting the cell to the reservoir served to equalize the gas pressure. All ground-glass joints and stopcocks were fitted with mercury seals to exclude oxygen and water vapor. The cell and reservoir were inclosed in a constant-temperature air bath. The circulation of air was particularly efficient, and the heating and cooling elements were especially designed to maintain an even temperature. The temperatures never varied more than &0.05°C. during a determination. PURIFICATION O F .MATERIALS
The gallium as received contained considerable amounts of zinc and indium. It was dissolved in concentrated hydrochloric acid, the solution extracted with ethyl ether, and the gallium in the extract freed from insoluble sulfides according to the procedure of Hoffman (7). To insure
848
G. L. MACK, J. K. DAVIS AND F. E . BARTELL
complete precipitation of iron and other alkali-insoluble hydroxides, the modification of Stokes and Cain (10) was adopted. Gallium was deposited electrolytically from the alkaline filtrate, using platinum electrodes. Fractional recrystallization was not attempted, because it is not the most efficient means of purification (8), especially when only small quantities of material are available. The final purification, which removes indium quantitatively, was adapted from the procedure of Craig and Drake (2). Gallium is converted to the chloride and then fractionally distilled in a current of dry chlorine. The gallium chloride is dissolved in water, and the solution is made 10 per cent with respect to sodium hydroxide, and then electrolyzed between carbon electrodes (4) a t a potential of 5 to 7 volts and a current of 4 to 5 amperes. The carbon electrodes were purified by heating with concentrated hydrochloric acid, followed by prolonged rinsing in redistilled water. The hydrochloric acid coming in contact with the purified metal was redistilled, and the acetone was distilled from phosphorus pentoxide immediately before use. The water was triply distilled, being redistilled from alkaline permanganate, and finally from quartz vessels. EXPERIMENTAL PROCEDURE
About 5 ml. of gallium was quickly poured into the glass reservoir which had been filled with enough dry acetone to cover the metal. A drop of dilute hydrochloric acid was added to dissolve the oxide film. The gallium was then washed with fifteen or more portions of pure dry acetone until every trace of chloride was removed, taking care never to expose the gallium surface to the air. The dropping tip was inserted, the reservoir connected with the optical cell, and mercury seals a t all joints completed. The air in the entire system was then replaced with dry hydrogen and the acetone distilled off under reduced pressure. The inlet valve was closed and the entire system evacuated for an hour a t a pressure of 0.05 mm. of mercury or less. The outlet valve as closed, and dry hydrogen was admitted until the pressure was approximately equal to that of the outside atmosphere. Then the stopcock leading to the drop-forming tip was partially opened so that the gallium ran down and formed pendant drops. These were photographed a t an exposure of 0.01 sec. The first four or five drops were allowed to fall off, so that any slight traces of surface-active impurities would be removed. The drops were usually observed over a period of time varying from a few seconds up to 45 min., and those which exhibited any noticeable change in shape were discarded. Carbon dioxide was used instead of hydrogen in some of the experiments and appeared to be equally effective in preventing oxidation of the surface. After the plates had been developed and dried, the dimensions of the drop image were measured with the coordinate comparator. The magnification factor was determined separately on each plate, no attempt being made
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BOUNDARY TENSION O F GALLIUM
to maintain it constant. The external diameter of the dropping tip was one of the largest and most easily measured dimensions on the plate, and since its actual diameter was already known, the magnification factor could be calculated. RESULTS
The density of the gallium was determined in a 25-ml. pyconometer under benzene and also under water. The densities compared to water at 4OC. were as follows: a t 3OoC., 6.097; a t 35OC., 6.094; a t 4OoC., 6.090. Another sample which was used for the interfacial tension measurements had a density of 6.090 a t 35°C. These values are in good agreement with those recorded in the International Critical Tables and also with the recent and precise results of W. H. Hoather (6). TABLE 1 Surface tension of. gallium in a n inert atmosphere at different temperatures . ATMOSPRERE
SURFACE TEXSION I N D Y N E 0 PER CENTIMETER A T TEMPERATURE INDICATED
30.50'C.
35.00"C.
40.00'C.
Hydrogen
712 732
762 703 733
733 722 743
Carbon dioxide
807 696 728
755
Average
735129
,
738&20
733*7
The surface tension of gallium against hydrogen and carbon dioxide is shown in table 1. The large experimental error is caused principally by contamination of the surface. The tendency for impurities to be adsorbed a t an interface increases with the free surface energy, so that in this case considerable experimental difficulty is to be expected. If air is admitted into the apparatus, the observed surface tension falls rapidly to an approximately constant value of about 300 dynes per centimeter. In 1921 Richards and Boyer (9) reported a value of 358.2 dynes per centimeter for the surface tension of gallium against carbon dioxide. This figure has been widely accepted, but Einecke (3) has already pointed out that it must be much too low. Considering that Richards and Boyer used the sessiledrop method with no provisions for cleaning or renewing the gallium surface, their value is about what mould be predicted. The ionic parachor of gallium is therefore equal to about 60 instead of 50, as previously reported (11).
850
G. L. MACK, J. K . DAVIS AND F. E . BARTELL
Although the temperature coefficient of surface tension cannot be calculated from these results, its value would appear to be quite low in this temperature range. The interfacial tension of gallium against 0.1 N and 0.2 N hydrochloric acid is recorded in table 2. The average deviation from the value obtained for the interfacial tension of gallium against 0.1 N hydrochloric acid is 0.4 per cent. There is no significant difference between the results a t different concentrations of hydrochloric acid. Frumkin and Gorodetskaya (5), using a capillary electrometer, obtained a maximum value of 592 dynes per centimeter a t a potential of 0.9 volt. This value is in fair agreement with the interfacial tension found in this investigation.
CONCENTRATION OF
HCl
INTEBFACIAL TENBION
N
dunea per cedimeter
0.1
630 638 638 638 637 639
0.2
630 635 636 636&2.6
Average. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . , SUMMARY
1. The surface tension of pure gallium in an atmosphere of hydrogen or of carbon dioxide was found to be 735 =!= 20 dynes per centimeter between 30°C. and 40°C. Within that range the change in surface tension with temperature was less than the experimental error. 2. The interfacial tension of gallium against 0.1 N and 0.2 N hydrochloric acid was 636 f 2.6 dynes per centimeter a t 35.00"C.
The authors gratefully acknowledge financial assistance, in the form of a special grant, from the Rackham Foundation. REFERENCES (1) ANDREAS, J. M., HAWSER, E . A,, AND TUCKER, W. B.: J. Phys. Chem. 42, 1001 (1938). (2) CRAIG,W. M., AND DRAKE,G. W.: J. Am. Chem. SOC.56,584 (1934). (3) EINECKE, E . : Das Gallium, p. 38. Leopold Voss, Leipzig (1937).
EVALUATION O F SURFACE AREAS OF CATALYSTS
851
(4) Reference 3, p. 52. (5) FRUMKIN, k ~ .AND , GORODETSK*YA, -4.:z. physik. Chem. 136,215 (1928). W. H.: Proc. Phys. SOC. (London) 48, 699 (1936). (6) HOATHER, (7) H O F F M A X , J. I.: J. Research S a t l . Bur. Standards 13, 665 (1934). (8) HOFFMAN, J. I., ASD SCRIBNER, B. F.: J. Research Natl. Bur. Standards 16, 205 (1935). (9) RICHARDS, T. W.,A N D BOYER, S.: J. .Im. Chem. Soc. 43, 283 (1921). (10) STOKES, H. S . ,A N D CAIK,J. R . : J. d m . Chem. SOC.29, 409 (1907). (11) SUGDES,S.: T h e Parachor and Valency. G . Routledge and Sons, London (1930).
EVALUATION O F THE SURFACE AREA OF CATALYSTS O F CUBIC FORM BY THE ERfdKATIOh’ METHOD J. D. KURBATOV Department of Physics, T h e Ohzo State Cnzverszty, Columbus, Ohio Recezued December 6 , 1940 I. IKTRODUCTION
The determination of surface areas is obviously of great importance in catalysis. However, the problem of selecting a method with consideration for exactness and conditions of applicability has not yet been solved satisfactorily (1). Among the various methods proposed and described in the literature for the determination of the area of solid surfaces, the emanation method has recently been receiving increasing attention. It was suggested originally by 0. Hahn (6, 10, 11) for the investigation of various problems, and since then a new field of applied radiochemistry has been opened, Several series of papers dealing with different phenomena in solids, which have been studied by emanations, have appeared, so that now it i,c possible to discuss the method critically from the theoretical t o the practical point of view. This paper is such an examination of the emanation method, of its use in studying solids, and of its accuracy in evaluating surface areas in some cases. The cmanation method is applied extensively a t present to the study of oxides and hydrates (4, 5 , 8, 9), but this paper presents work which developed from the use of the method in the study of industrial catalysts, for which it was found to be a very sensitive tool. The advantage of the method lies in the rapidity of measurement and the breadth of the conditions of experiment, such as the temperature and
