NOTES
418
an equilibrium, possibly similar to that demonstrated kinetically for ethylene glycol by Duke and Bulgrim7 By analogy to their work, we suggest the following scheme which accounts for the observed data. AE
+ IO4-
C
C +products
Experimental
However, the method used for determining the presence of the activated complex C in the ethylene glycol work7 fails with 2-aminoethand due to the very low concentration of C which would be present a t any time. Table 11: Thermodynamic Functions of Activation a t 0" for the Periodate Oxidation of 2-Aminoethanol via the Reaction AE
+ 104-
kn
-+products -4.3 =t0 . 2 11.0 f 0 . 0 -56 f 1
AH*, kcal./mole AG *, kcal. /mole AS*, e.u.
Many mechanisms can be postulated which would account for the observed kinetics. Although no conclusive experimental evidence was found for it, one of these schemes is worthy of note a t this time, for it suggests a new reactive species, the protonated form of the aminoalcohol. The negative temperature effect and the first-order pH dependence can be accounted for by a pre-equilibrium between the protonated form of the aminoalcohol and the periodate species, followed by the disproportionation of the complex in a ratedetermining step, uix. AEH'
+ 104-
electrolyte solutions, an interesting effect was observed to occur a t the mercury-glass interface. This was the transport of the aqueous electrolyte solution, between mercury and the wall of the containing tube, when the mercury was polarized negatively with respect to the aqueous solution.
-C+H+
The apparatus used is shown in Fig. 1. The mercury contained in the U-tube was connected to the negative pole of a d.c. source and the mercury pool (C) was connected to the positive pole. Transport of the aqueous solution was observed, with the solution moving between the mercury and the walls of the U-tube, from the interface at h to the opposite side of the U-tube. The effect has been observed with U-tubes of glass, Tygon, and Teflon, and with solutions of tetramethylammonium chloride, tetramethylammonium iodide, hexadecyltrimethylammonium bromide, sodium chloride, lithium chloride, hydrochloric acid, and mixtures of these substances, a t concentrations of 0.001 to 0.1 M . It was observed with triple-distilled mercury and with 99.99999% pure mercury (purchased from United Minerals and Chemicals Co.). As voltage was increased from zero, the contact angle (measured through the aqueous phase) at the mercury solid-aqueous solution line decreased markedly. The voltage a t which the contact angle reached zero was apparently the threshold voltage for transport, which was approximately 1.5 v. in glass tubes for the systems tested. This threshold showed variations of only 0.1 or 0.2 v., typically, among the various solutions in glass; it was reproducible in a particular system to within about 0.1 v. (This degree of repro-
f-
C -+ products
Acknowledgment. The authors gratefully acknowledge the financial assistance of the Research Corporation in support of this work.
I --
Transport of Aqueous Solutions at a MercuryGlass Interface, Induced by Electric Polarization
SOLUTION
by Robert J. Good and William G. Givens GeneTal Dynamic /Astronautics, S a n Diego 18,California (Received August 7 , 1965)
I n the course of a study of the effect of electric polarization on the interfacial tension between mercury and The Journal of'Physical Chemistry
-----
MERCURY
NOTES
ducibility was due in part to hysteresis and in part to uncertainty of observation of the start of transport.) At voltages high enough to cause continuous pumping, e.g., about 2 v., a definite waist, shown exaggerated in Fig. 2, was visible below the top of the mercury column. Teflon showed a threshold of around 2 v., as did Tygon which had been washed with a hot detergent solution. Untreated Tygon in one case showed a threshold of 0.5 v. with 0.01 M tetra,methylammoniuni chloride; while we were not able to repeat this low value, addition, of organic oils such as dinonyl phthalate to the solution, or to the plastic tubing, decreased the threshold from near 2 v. to approximately 1 v. (It was difficult to obtain consistent values of this reduction in threshold voltage, however.) U-tubes having inside diameters from 0.28 to 7 mm. were used, with transport occurring in all. I n the smaller tubes, with i.d. about 1mm. or less, the mercury column tended to break up and pulsate in the tube, whereas in the larger tubes continuous stable flow was obtained. Removal of the dissolved oxygen from the solhion by bubbling nitrogen through it did not change the effect qualitatively. The current was reduced somewhat, and the depth to which the aqueous solution was carried below the mercury surface, at a given voltage, was increased. The transport of water could be made to occur against a very considerable pressure head. At 1.75 v. in a 4mm. glass 'U-.tube, deoxygenated 0.01 M tetramethylammonium chloride penetrated to a depth of 37 mm. in about 2 hr. (This appeared to be the limiting depth, under these conditions.) At 3.0 v., the solution penetrated past the 37-mm. depth in about 10 min. Both the rate of trmsport and the head against which the pumping could take place increased rapidly with increasing voltage. One measurement of the rate of
41!2
transport was made. With a U-tube 7 mm. in diameter and a mercury depth of 2 mm., 0.016 ml./min. of 0.01 M tetramethylammonium chloride was transferred to the other side of the U-tube, when 3 v. was applied. The current was 0.24 ma. Analysis showed no change in the chloride concentration (within 5%) and a pII change from 6.5 to 8 between the starting electrolyte and that transported under the mercury. The effect has also been observed with liquid galllium, in glass, though with considerably greater difficulty. The threshold voltage was about 0.5 v. higher than with mercury; and with several samples of gallium it was not, possible to get the pumping to start a t all. There usually remained regions of Ga-glass interface where the solution had not displaced the metal. Presumably this behavior of the gallium occurred because of the strong tendency of gallium to form oxide coatings which adhere to glass.
Discussion Changes in the mercury-aqueous solution interfacial tension with an applied potential are well known, as KS the circulation of a mercury surface under the influence of interfacial tension Hence, a mechanism for the transport may be proposed in terms of the interfacial tension gradients. If the contact angle in the aqueous phase is zero, and if the interfacial tension at the edge of the meniscus is greater than that nearer the center, a surface flow will occur toward the edge of the meniscus. The mercury and aqueous phase near the surface will be carried along and the aqueous phase can be dragged in between the mercury and the wall. The waist in the mercury surface shown in Fig. 2 provides visible evidence of circulation beneath the mercury surface, since such a profile could not be stable if the system were static. The contact angle in the aqueous phase is normally positive, as indicated by a sharp edge to the meniscus on observation with a microscope. With increasing voltage, the contact angle visibly decreases and the edge disappears into a continuous curve (indicating a zero contact angle) when the voltage is reached at which transport sets in. This decrease in contact angle is to be expected, since the interfacial tension between the mercury and the aqueous phase is reduced by the negative polarization of the mercury (past the electrocapillary maximum) and this causes the spreading co-
G. W. C. Milner,"The Principles and Applications of Polarography," Longmans, Green and Co., London, 1957, p. 70 ff. (2) I. M. Kolthoff and J. J. Lingane, "Polarography," Interscienae Publishers, Inc., New York, N. Y., 1952, p. 168 ff. (3) J. J. Bikerman, "Surface Chemistry," Academic Press, New York, S . Y., 1968, p. 441 6.
(1)
Figure 2. Configuration of mercury surface at voltagee above 3 v. (schematic).
Volume 68, Xumber d
February. 2964
420
NOTES
eEcient for the aqueous phase to increase. An analladium-alumina catalyst could not be brought back to ogous effect was reported4 for the mercury-wateroriginal activity by regeneration with air. In view of hydrogen gas contact angle. the loss of activity of platinum catalysts which have undergone growth of the platinum metal particles,l The proposed gradient in surface tension would and the somewhat lower Tamman temperature of be generated by a gradient in polarization and adsorppalladium (460’ us. 540°), crystal growth might be tion over the surface. The geometrical restriction of expected to be a major problem. Johnson and Keith2 the available path for electric current, near the edges have explained the various data obtained on platinum of the tube, will cause a lower current density near by showing that an increase in oxidation severity will the edge of the meniscus than in the center, with the result in more platinum oxide-alumina complex formalowest current density nearest the edge where the tion and, therefore, provide a more dispersed platinum mercury comes in contact with the tube walls. The upon reduction, if one does not exceed the temperature polarization of the interface decreases with decreasing a t which the oxygen pressure is less than the dissociacurrent density, and with it the adsorption of countertion pressure of platinum oxide. ions, so the interfacial tension will be higher at the edge On this basis, similar crystal growth would not be exof the meniscus than nearer the center. pected below 790°, since it has been demonstrated that The magnitude of the driving force leading to the obthe dissociation pressure of palladium oxide is 0.2 served pumping action could be estimated with the aid atm. at 789°.3 However, our results indicate crystal of published electrocapillary curve^^-^ if the exact growth of palladium a t temperatures below 400’ in potential distribution over the surface were known. the presence of air. Therefore, it must be concluded The increase of negative potential past the electrothat the explanation of crystal growth for platinum capillary maximum causes a very marked decrease in interfacial tension, e.g., as much as 100 dynes/cm. v . ~ put forth by Johnson and Keith2 is not applicable to palladium. If a difference in tension acted on a sufficiently thin layer of liquid, a very considerable pressure could be Experimental developed. For example, a 1 dyne/cm. (interfacial) A commercial palladium-alumina (Harshaw Pdtension acting along a film lo4 8. thick corresponds to 0501) catalyst containing 0.3% palladium in 3-mm. a pressure of about 8 em. A detailed hytablet form was used for all experiments. drodynamic analysis would be required to give a quanAdsorption experiments were run on approximately titative account of the rate of flow in relation to the 20 g. of catalyst. The catalysts mere heat treated in actual surface tension gradient ; but this calculation a muffle furnace (flow system for oxygen treatment) by shows that the magnitude of the force available is gradually raising the temperature to the desired level quite adequate to account for the effect observed. and holding for 48 hr. The catalysts were then reduced in a flow of hydrogen for 2 hr. and then evacuated Acknowledgment. This work was supported by the for 2 hr. Carbon monoxide was then introduced to U. S. Atomic Energy Commission under Contract the evacuated system in 1.426-cc. increments a t 5AT(04-3)-297. Thanks are expressed to Dr. Donald min. intervals and the adsorption calculated from the Lewis for stimulating discussions which led, serendipipressure increase. tously, to the work here reported. Analysis of fresh and regenerated catalysts showed no (4) H. G . RIoller, Ann. Physik (4)2 5 , 725 (1908). change in palladium content and no increase in foreign (5) J. T. Davies and E. K. Rideal, “Interfacial Phenomena,” metal content. Attempted determination of crystal ilcademic Press, New York, N. Y . , 1961, pp. 97, 361-362. size by both X-ray and electron microscopy failed to detect palladium. Pore volume and surface area determination on a catalyst which had been through ten production-regeneration cycles showed a definite Deactivation of Palladium-Alumina Catalysts but small collapse of the support. It is felt that such an effect on one treatment is insignificant. by Donald G. Manly and Fred J. Rice, Jr. The John Stuart Ressarch Laboratories of The Quaker Oats Company, Borrington, Illinois (Received September 12, 1g 6 3 )
In the course of a catalytic study carried out in these laboratories it was observed that a deactivated palThe Journal of Physical Chemistry
G. A. Mills, 6 . \Teller, and E. B. Cornelius, “Second International Congress on Catalysis,” Vol. 11, Paris, 1960, Paper 113. M. F. L. Johnson and C. D. Keith, J . Phys. Chem., 67, 200 (1963). L. Wohler, 2. Elektrochem., 11, 839 (1905); C. €3. Alcock and G. 1%’.Hooper, Proc. Roy. Soc. (London), A254, 551 (1960), concluded that the vapor pressure of PdO was probably less than that of the metal.
