EFFECTOF GASES ON THE SURFACE TENSION OF MERCURY
August, 1961
Chemical 99.9:;) were measured and added to the cryoscopic cell from the standard chemical vacuum apparatus. The experimental phase diagrams were made while attached to the chernical vacuum system. Thus any volatile compound formed by decomposition of the Me20-A1X3 phase during the experimental work could be detected and measured accurattaly. 90 Hz, CHI or H X ever were detected during the experimental work. When the solutions were heated above 100" a decomposition reaction yielding methyl halide as the only volatile product was noted. This is undoubtedly the reaction MezO:AlX, +2MeX AlOX A
+
!&0-NXS phases in the absence of air or moisture are quite stable a t room temperature and can be kept without detectible decomposition for months.
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Solubility Data.-The solubility of MzO: ,41X3 in paraffius was measured by stirring the two components vigorously for 1-2 hours together in a controlled temperature bath. The concentration of MezO:AlX3 in the paraffin phase waa determined by chemical analysis for A1 and halogen. The solubility of paraffins in MezO:AIXs was measured by adding the paraffin in measured increments to MezO: AlXs from the chemical vacuum system and observing the vapor pressure. Since MezO:A1X3 has a negligible vapor pressure, the composition at which the vapor pressure first became independent of liquid composition was taken to be the saturation point of the paraffin in Me2O:AlXJ.
Acknowledgment.-The author wishes to express his appreciation to Mr. E. L. Clark for his assistance in a number of the experimental measurements.
THE EFFECT OF VARIOUS GASES AND VAPORS ON THE SURFACE TENSION OF MERCURY' BY M. E. NICHOLAS, P. A. JOYNER, E. M. TESSEMAND M. I>. OLSON Honeywell Research Center, Hopkins, Minnesota R e d v e d Fcbruarv 86, 1961
The effects of He, Hz, Nz,0 2 , COZ,HzO, CH4, GH,, and pump oil on the surface tension of highly purified Hg have been determined, utilizing a large sessile drop and Worthington's equation. Kernball's value of 484 dynes/cm. for the surface tension of Hg in vacuo at 25' was confirmed prior to the study of gases and vapors. Contrary t o previously published results, it is found that He, Hz, Nz, 0 2 and COZdo not adsorb on highly purified mercury a t 25". Previous results are due to inadequate purification of mercury and bases, or to presence of stopcock grease, pump oil or other contaminants. Details of the experimental method and purscation techniques are given. The effects of H20. C3Hs and pump oil on the surface tension of mercury are discussed.
Introduction
A literature review of the adsorption of gases and vapors upon mercury surfaces, as measured by changes in the surface tension, revealed a wide range of reported effects. Indeed, the reported values for the surface tension of mercury in vacuo (in the presence of it's own vapor only) range from 400 to 516 dynes/cm. at 25". Table I contains some reported values for the surface tension of mercury in UCLCUO.
Our objective in this study was to establish whether or not certain gases and vapors adsorb on the surface of highly purified mercury and thc consequent effects on the surface tension. Experimental Procedure
The apparatus utilized by this Laboratory was patterned after the studies of Bradley9 and Kernball." The surface tension values were obtained by measuring the dimensions of a large drop of mercury resting on a cup or flat plate. The corrected Worthington equation'z "as used to calculate the surface tension. It is p g h 2 (1.641B) TABLE I = --Z- (1.641R 4- h) I~IWORTED VALIJESFOR THE SURFACE TENSION OF MERCURY Dynes/ where Tnvewigatnr Date cm.a Method y = surface tension Harkins and Ewing' 1920 476 Drop weight p = density of mercury Hogness?, 1921 4'76 Drop pressure g = gravitational acceleration h = max. height of the drop above the 1922 472 Drop weight Iredale' max. diameter Iredales 1924 464 Sessile drop R = radius of the drop a t max. horizontal CookB 1929 516 Sessile drop Burdons reported that this equation is consistent and ac1931 435 Sessile drop Kernaghan' curate only when a drop with a radius greater than 2 cm. ip Burdona 1932 488 Sessile drop utilized. This was confirmed experimentally by this LaboBradleyg 1933 498 Sessile drop ratory. The dimensions of the mercury drop were initially meas1036 476 Sessile drop Nernagh,mIo mired while it rested on a Pyrex cup within a Pyrex tpe speciKernball;,' 1946 484 Sessile drop ally fabricat,ed from 60 mm. Pyrex tubing. The two upper a Corrpcted to 2.5". ends of the tee, for holding the optically polished Pyrex windows were optically ground perprndicular to the center (1) Presented in part under the auspices nf the Division of Colloid line of the tube. The lip of the cup which had been opChemistry, American Chemical Society, at the meeting in Cleveland, tically polished was 46 mm. in diameter; it was aligned Ohio, April 11 -14, 1980. parallel to the same center line. A Gaertner cathetometer, (2) W. D. Harkins and W. W. Ewing, J . Am. Chem. Soc., 42, 2539 specially fitted with an Abbe-Lamont eyepiece and fine (1920). crosshairs, was aligned with a damped plumb line, so as to (3) T. R. liogness, ibid., 43, 1621 (1921). (4) T. Iredale, Phil. Mag., 45, 1088 (1923). ( 5 ) (a) T. Iredale, ibid.. 48, 177 (1924); (b) 49, 603 (1925). (6) S. G . Cook, Phys. Rev., 34, 513 (1929). (7) M. Kemaghan. ibid.. 37,990 (1931). (8) R. S.Rurdon, !Trans. Faraday Soc., 38, 866 (1932).
(9) S.Bradley, J . Phus. Chem., 38, 231 (1934). (10) M. Xernaghan, Phys. Re%, 49, 414 (1936). (11) C. Kernball, Trans. Faradau Soc., 42, 526 (1946). (12) A. M. Worthington, Phil. Mag., 2 0 , 51 (1885).
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M. E. NICHOLAS, P. A. JOYNER,B. M. TESSEM AND M. D. OLSON
traverse in vertical and horizontal directions. The Pyrex window a a s aligned perpendicular to the cathetometer, and thus perpendicular to the plane of the lip of the cup. Next, the Pyrex cup was leveled by horizontal movement of the cathetometer, parallel to the window. This alignment procedure was necessary before each determination. The height of the mercury drop, and thus the size, was controlled by using a U-tube. Mercury was spilled gently into the U-tube from a reservoir by means of a Pyrex bulb containing an iron ring. Thus, mercury was displaced into the U-tube as the bulb was lowered into the mercury with an electromagnet. Mercury of the highest purit,y was utilized for this study. Information oertainine: to the actual Duritv of commercial grade mercuiy is quiTe limited. Oxidation and chemical treatment, followed by diptillation, seemed the best method of purification. Table I1 illustrates the relative efficiencies of some processes in the removal of metals.
The inverted U-tube still was fabricated from Pyrex glass. This system is greaseless since no valves are required. The still was protected from back diffusion of vacuum pump oil by a liquid nitrogen cold trap and carbon pellets. The distillate was gravitationally drained into a clean receiver. The flasks used for the chemical purification, being well rinsed with mercury, made good storage containers. Pure oxygen was bubbled gently through the hot mercury during distillation. The oxygen was purified by passing it through silica gel and carbon to remove water and organic vapors, respectively. There was no buildup of oxide films on the surface of the mercury in the pot, on the sides of the pot, or in the receiver during the long periods of usage, indicating that oxide forming metals were efficiently removed by the chemical treatment. Therefore, the use of oxVgen during distillation may not be necessary. All mercury was distilled a minimum of three times. The easiest method of keeping the still clean and the glassware rinsed was by continual usage. Therefore, the mercury was cycled from the TABLE I1 receiver back to the pot during those periods when mercury REXOVALOF METALS FROM MERCURYIN DESCENDINGwas not needed. There are unresolved questions a8 to the best container for ORDER O F EASEO F REMOVAL'3 storing high purity mercury. Soft glass and pure iron are Absorption of Metals Metals Metals commonly used. Plastic containers, polyethylene specifiremoved by removed h y oxygen by removed by cally, will transfer vapors to the vacuum svstem. These "Os vacuum distn. KOH amalgams vapors cannot be removed by pumping. Pyrex suction Na Sn Mg Au flasks were used exclusively for the storage of mercury in this Mg Zn A1 Pt research because it would contact Pyrex during experimentation. The containers were cleaned with hot concentrated Zn Pb Cr Ag nitric acid and rinsed with deionized water. However, all Cd Mn Cu Pyrex surfaces do not react the same with mercury and Pb Cd Sn only selected Pyrex containers were satisfactory; others Sn Ni Pb readily interact with mercury to form an amalgam film. The glass surface was completely wetted by mercury in some Zn T1 Sn cases. Cd Pb All gases except water vapor were meticulously purified cu by the purification train utilizing the stages shown in Table Oxidation will form insoluble oxides of the alkali and less 111. The gas was gently purged through the train t o the noble metals. Treatment with sodium hydroxide and nitric atmosphere for several hours before being admitted to the acid will remove a large group of base metals. Distillation measuring system. A further precaution was the use of a second U-tube cold is the most effective method for reducing the concentration of noble metals. Consequently, the following procedure was trap. This was next to the entrance valve but on the used to purify the mercury. A series of solutions were measuring system manifold. This trap was heated a t the placed over the mercury and pure air was bubbled through conclusion of several experiments, and in no case caused a the mercury and the solution. The solutions were: (a) change in the surface energy of the mercury, demonstrat3 M sodium hydroxide, (b) 3 M nitric, and ( q ) 0.001 M nitric ing the adequacy of the purification train. Water was purified by a different procedure. A liter acid. The air oxidizes some of the impurities and stirs the mercury. Thus, it continually renews the interface be- beaker was cleaned and thoroughly rinsed with distilled tween the mercury and the solution. The progress of purifi- water. A volume of approximately 500 ml. of distilled cation was followed by observing the clarity of the solution water was degassed by placing it in the beaker and boiling above the mercury. The solution picked up impurities it to 200 ml. The water container, to he attached to the valve of thc rapidly and had to be changed frequently during the first
TABLE 111 Impurities removed
Agent
JVater rust Carbon dioxide Kater Organic vapors Hydrogen or oxygen \\'ater Water ?Titrogen Hydrogen Any residual vapors
Silica gel Ascarite Silica gel Carbon granules Deoxo Unit 125" Silica gel Cas04 and Mg(Cl0a)z Mg 350' Ca 425" Cold trap 0"
Container material
Copper Pyrex Pyrex Pyrex Iron
I
H2
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x s
s s
x
Y.d c n d .
S c i . , 66, 370 (1957).
0 2
x X x s
X X X XI
X
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co2 X
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s x x
7
1%e
CJHS
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