A HYDROGEN ELECTRODE IN ICE
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A Hydrogen Electrode in Icela
by P. N. Krishnan,lb I. Young, and R. E. Salomon Department of Chemistry, Temple University, Philadelphia, Pennsylvania
(Received December 6 , 1966)
The preparation and behavior of a hydrogen electrode in ice is reported. I n order to compensate for the slow diffusion of hydrogen in ice, powdered platinum was placed on the ice-gaseous hydrogen interface. The cell potential followed the Nernst equation at the temperature of the measurements. The net reaction only involves the transport of hydrogen from the high- to the low-pressure side.
Introduction The hydrogen electrode is normally regarded as an oxidation-reduction electrode a t which equilibrium is established between the electrons in a noble metal, hydrogen ions in an aqueous solution, and dissolved molecular hydrogen.2 I n practice, the noble metal must serve the role of a catalyst and for this reason, as well as for reasons of its nobility, platinized platinum is used. The solution is saturated with hydrogen by continuous bubbling in the vicinity of the platinum solution interface. The operation of the electrode depends upon the existence of both dissolved hydrogen and hydrogen ions at the platinum solution interface. The extension of the temperature range of this electrode to below the freezing point of the solution would require that fairly rapid transport of hydrogen from every metal-ice interfacial region to the gaseous hydrogen phase could occur. Such a transport could in principle take place either through the ice or through the platinum; however, the rates of diffusion would be too low to be of US^..^,^ Platinum and palladium electrodes can be frozen into ice and in various forms are used as ohmic contacts for conductivity measurement^.^-* The potential difference between a pair of these electrodes in the same piece of ice is at the mercy of the accidental difference in hydrogen concentrations in them. A potential of the order of 10 mv is almost always found to exist between two apparently identical platinum wires when they are frozen into the same sample of pure or electrolyte-doped ice. Simple metallic electrodes cannot be used, therefore, in any potential measurements in ice involving a few millivolts. For instance, the results of measurements of thermogalvanic potentials in ice using brass electrodes9 now appear to be
in doubt.l0 It was therefore of interest to develop a thermodynamically reversible electrode for use in ice. In the absence of a known reversible electrode, which could be used as a reference, it was necessary to prepare two such electrodes and place them in the same sample and then to compare the experimental potential difference with that predicted by theory. Since protons are the only mobile species in pure and impure ice,11z12 the obvious electrode to use is a hydrogen electrode. The major experimental problem to be solved was the development of a method whereby gaseous hydrogen can enter and leave every region separating metal and ice. In order to achieve this end, it is necessary to keep the diffusion path at as small a value as possible. It was found that precooled fine platinum powder, (1) (a) This work was supported by the Office of Saline Water, U. S. Department of the Interior. (b) This research is part of a dissertation to be submitted to the Temple University Graduate Board in partial fulfillment of the requirements for the degree of Doctor of Philosophy. (2) J. G. David and G. J. Janz, “Reference Electrodes-Theory and Practice,” Academic Press, New York, N. Y . , and London, 1961, p 71. (3) W. Kuhn and M. Thurkau, H e h . Chim. Acta, 41, 938 (1958). (4) R. M.Barrer, “Diffusion In and Through Solids,” The Macmillan Co., New York, N. Y . , 1941, p 168. ( 5 ) R. S. Bradley, Trans. Faraday Soc., 53, 687 (1957). (6) G. W. Gross, private communication. (7) H. Engelhardt, Ph.D. Thesis, Physics Department, TechnischeHochschule, Munich, Germany, 1965. (8) J. H. L. Johnstone, Proc. Trans. Nova ScotianInst., 13,126 (1912). (9) J . Latham and B. J. Mason, PTOC. Roy. Soc. (London), A260, 523 (1963). (10) G. W. Gross, J . Geophys. Res., 70, 2291 (1965). (11) J. C . Decroloy, H. Granicher, and C. Jaccard, Helv. Phys. Acta, 30, 465 (1957). (12) E. J. Workman, F. H. Truby, and W. Drost-Hansen, Phys. Rev., 94, 1073 (1954).
Volume 70,Number 6 M a y 1966
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P. N. KRISHNAN, I. YOUNG,AND R. E. SALOMON
sprinkled on the surface, was suitable in this regard. The largest diffusion path required would have a length of the order of the grain circumference. The powder would be kept in equilibrium with a controlled pressure of hydrogen.
Experimental Section The 10-4 M NaCl solutions were placed in the U tube shown in Figure 1 and boiled under reduced pressure for several minutes to remove dissolved air. The tube was closed off and the bottom tip was placed in a Dry Ice-acetone bath for the purpose of seeding. The seeded tip was placed in a constant-temperature bath maintained a t -6". The bath level was slowly raised (4 mni/hr) by the dropwise addition of coolant from an external reservoir. In this manner a flat surface was maintained during the entire growth period and the resultant sample was free of gross flaws and was found, by observation through crossed polaroids, to be strain free. Powdered platinum black (99.98% pure, with an average particle size of 15 p, supplied by the J. Bishop Co.) which was precooled to -6" was sprinkled on the surface to a height of about 2.5 mm. A broom of fine platinum wires, spot-welded to a heavy platinum wire, made contact to the powder. The heavy wires were sealed into side arms and brought to the outside where they were connected to a Cary Model 31 vibrating reed electrometer. The U tube was surrounded by a grounded concentric copper tube for purpose? of shielding. All leads were shie'ded. Two plastic plugs with small openings which were kept above the platinum-ice interface helped to reduce convect:on. Both sides of the tube were evacuated and the stopcocks were closed. The tube was then slowly brought to - 10" and kept a t that temperature for several days. During this time the potential difference varied erratically. When hydrogen at 1 atm pressure was introduced into both sides of the tube the potential difference reached zero after a period of about 30 min. When one side arm was maintained a t a fixed pressure and the pressure of the other varied, the potential responded sluggishly and reached a steady value after a 30-min period. The results obtained in a typical run are given in Tahle I. Attempts to study the temperature dependence of the cell potential were limited by the contraction of the sample and the subsequent leakage of hydrogen dong the tube walls. Discussion The electrode system described differs from one in which two hydrogen electrodes are immersed in an aqueous liquid electrolyte in one important respect. The Journal of Physical Chemistry
ELECTROMETER
--
v Figure 1. Diagram of the cell.
Table I : Potential of the Cell at Various Hydrogen Pressures Pressure (Pl),
Pressure (Pa),
log
om
om
(Pi/Pa)
Potential, calcd
Potential, obsd
12.5 17.6 31.2 72.0 77.2 77.2 77.2 4.1 30.2
0.7896 0.6410 0.3924 0.0290 0.8964 0.5472 0.0938 1.2737 0.4065
20.64 16.6 9.2 0.78 -23.4 -14.3 -2.43 33.0 10.6
20 16 12 2 24 - 15 -6 30 10
77.0 77.0 77.0 77.0 9.8 21.9 62.2 77.0" 77.0"
-
" Repeated using the same system. I n both pure and doped ice the current is carried entirely by protons and the passage of 1 f of charge results in the transport of 1 equiv of hydrogen without any other changes in the ice. Hence, corrections for liquid junction potentials are not required even if significant concentration gradients exist across the ice sample. Unlike its relative in aqueous solution, whose potential can be varied by varying the hydrogen ion activity, the ice-hydrogen electrode potential depends only on the hydrogen pressure (and, of course, the temperature). In the fourth column of Table I, the potential of the cell calculated on the basis of the Nernst equation is given. I n spite of a precision which was limited to about 1 mv, the agreement between theoretical and experimental cell potentials is fairly good. The cell resistance was quite high and was found to be due to the intergranular resistance of the platinum powder. This high resistance (typically of the order of 108 ohms) limited the precision with which the measurements could be carried out.
DECARBOXYLATION OF OXALIC
ACIDIN POLAR SOLVENTS
A factor which may be of significance in the operation of this cell is the liquid-like layers purported to exist on an ice interface at temperatures below O 0 . I 3 These films of water on ice have often been invoked to explain the phenomena of regelation and would be expected to enhance the transport of hydrogen to every interfacial region. These same layers may be responsible in some
1597
measure for the spurious potentials observed when various electrodes are frozen into ice. The electrode described should be of use in the measurement of thermogalvanic potentials in ice. The results of this investigation are in accord with the position that protons are the only mobile carriers in ice. (13) W. A. Weyl, J. Colloid Sci., 6 , 389 (1951).
Further Studies on the Decarboxylation of Oxalic Acid in Polar Solvents
by Louis Watts Clark Department of Chemistry, Western Carolina College, Cullowhee, North Carolina
(Received December 6 , 1966)
Rate constants and activation parameters are reported for the decarboxylation of oxalic acid in propylene glycol, 1,4-butanediol, and 2,3-butanediol. The results of this investigation are compared with previous data on the reaction in seven additional solvents, as well as in the vapor phase. Inductive and steric effects of the various solvents are discussed.
I n the past, kinetic studies have been carried out on the decarboxylation of oxalic acid in at least 15 nonaqueous solvents. These include dioxane,' g l y ~ e r o l dimethyl ,~ sulfoxide, triethyl phosphate, aniline, N-methylaniline, N,N-dimethylaniline, q ~ i n o l i n e , ~ 6-methylquinoline, 8-methylq~inoline,~o-cresol, mcresol, p-cresol, ethylene glycol, and 1,3-butanedi0l.~ Recently, Haleem and Yankwich have repeated the experiments using the solvent glyceroL6 Lapidus, Barton, and Yankwich have also studied the decarboxylation of oxalic acid in the vapor phase and have proposed a unimolecular mechanism for the reaction.' In an effort to gain a better understanding of the mechanism and energetics of this reaction, further experimentation has been carried out in this laboratory on the decarboxylation of oxalic acid in three additional polar solvents, propylene glycol, 1,4-butanediol1 and 2,3-butanediol. A comparison of the results of this investigation (reported herein) with previous data sheds light on the mechanism of the reaction and leads to a better understanding of other heterolytic reactions.
ExperimentaI Section Reagents. (1) Anhydrous oxalic acid, reagent grade, 100.0% assay, was used in this research. To ensure perfect dryness it was stored in a desiccator containing sulfuric acid. (2) The solvents were highest purity quality and were redistilled at atmospheric pressure directly into the dried reaction flask immediately before the beginning of each decarboxylation experiment. Apparatus and Technique. The details of the apparatus and technique used in this research have been described previously.* In these experiments, a sample (1) A. Dinglinger and E. Schroer, 2. Physik. Chem., A179, 401 (1937). (2) L. W. Clark, J. Am. Chem. Sac., 77, 6191 (1955). (3) L. W . Clark, J. Phys. Chem., 61, 699 (1957). (4) L. W . Clark, ibid., 62, 633 (1958). (5) L. W. Clark, ibid., 67, 1355 (1963). (6) M. A. Haleem and P. E. Yankwich, ibid., 69, 1729 (1965). (7) G . Lapidus, D. Barton, and P. E. Yankwich, ibid., 68, 1863 (1964).
Volume 70,Number 5 M a y 1966
