NOTES
Feb., 1957
255
TABLE I
RESULTS OF Atom
N CI CI! c 3
Cc
X
THE
LEAST-EQUARES ANALYSIS OF T M P AND COMPARISON WITH PREVIOUS FOURIER ANALYSIS
Fourier analysis
Y
Least. squares analysis
Z
0.092 0.096 -0.069 ,010 .139 .032 - .090 .041 .lo4 .020 ,297 .067 - .181 .OS8 .220
Standard dev.
X Y Z of position, A. B X l O l e om. 0.0956 f 0.0004 0.0061 f 0.0004 -0.0693 f 0.0003 0.006 3.56 f 0 . 1 8 .032G f .0004 .007 3 . 1 1 f .20 .0080 f .0005 .I403 f .0004 - .OS69 f .0005 .0419 f .0004 .lo54 f ,0004 ,007 3.26 f .20 .0710 f .0005 .009 4.78 f .28 .0205 f ,0006 .2956 f .0005 ,1802 =k .0006 .OS83 f .0005 .2202 f .0005 .009 4.51 f .25
-
GALVANIC CELL MEASUREMENTS IN THE COPPER-SILVER SYSTEM’
electrode. Thus, one may calculate from the data listed by Brewer, et CZZ.,~ that the equilibrium Cu AgCl = Ag CuCl lies to the right as is required, BY RUSSELLIC. EDWARDS, JAMES H. DOWNING AND but that the equilibrium constant is only about 3 a t DANIEL CUBICCIOTTI 1428°K. The chloride electrolyte might become Contribution fvom the Department OJ Chemistrv, Illinois Inatitute of reasonably satisfactory for the cells involving CuTechnologg, Chicago, Illinois Ag electrodes more dilute in Ag, where the activity Received August S O , 1.966 of Ag would be reduced. I n this general regard, The therniodynaniics of the liquid Cu-Ag system the oxide electrolyte is much to be preferred for one have been reported by Edwards and Downing,2 can estimate from the oxide thermodynamic propbased on vapor pressure measurements by the ef- erties given by Brewer4 that the comparable equifusion method. Previously we had conducted an librium is overwhelmingly toward the Cu20. Howinvestigation by the galvanic cell method in view ever, CuzO a t low temperatures is known to be an of the potentially greater precision often realized excess oxygen semiconductor. 4 ~ 6 Presumably this by this method. After considerable exploration effect might well be minimized by working with a our coiifidence in the method was lessened with re- dilute solution of CuzO in a liquid oxide solvent a t gard to its applicability for the study of the Cu-Ag high temperatures and subjected to the reducing system ; however, the results were generally sup- action of the liquid Cu electrode. porting to those of Edwards and Downing2and are Experimental here reported in that connection. Tungsten wires were used to connect the electrodes to a The limiting factors involved in obtaining reliable Brown Electronik recording potentiometer or a Rubicon galvanic cell measurements are sometimes to a de- Portable Precision potentiometer. The tungsten lead wires were isolated from the electrolyte by porcelain progree inverted as one proceeds to the high tempera- tection tubes. The electrolytes were dried and fused under tures required for the present study. Firstly, the vacuum, but the runs were made in an argon atmosphere to attainment of true equilibrium between the elec- minimize electrolyte volatilization loss from the hot zone. trode interior and surface is certainly more favor- The several different t y es of cells are noted below for identilater with the &we presenting the results obtained. able for the case of liquid metal electrodes as com- fication Cell Type A.-The anode and cathode were simply conpared to solid metal electrodes. However, the tained in small porcelain crucibles separated from each other problem of selecting a suitable electrolyte and con- and within the main porcelain container. The crucibles ducting species becomes often formidable. The were spaced from each other by a graphite holder. Cell Type B.-The anode was concentric with the cathode electrolyte must (a) be sufficiently non-corrosive and only by the alumina or zirconia crucible toward the container and of sufficiently low vola- whichseparated contained the central anode. tility to permit observation of the cell behavior over Cell type C.-A Vycor cell of conventional “H” cell dea period of time, (b) provide a medium toward sign was used. Type D.-The cell consisted of a porcelain tube which which one of the electrode components is appreci- hadCell been fabricated to simulate a n “H”cell in that it had ably more noble in behavior than is the other com- legs to se arate the anode and cathode compartments. ponent, and (c) must not display electronic conThe &l electrolyte with about 5% CuCl was used with duction in addition to the ionic conduction required cells A, B, and C. Several different oxide mixtures were used for the electrolyte in cell D. Most of them were too in the cell reaction. corrosive toward the porcelain container so that the activThe two types of electrolytes chosen for use were ity results in these cases do not warrant reporting. One (I) CuCl dissolved in liquid KC1 and (2) CuzO dis- mixture with which activity results were obtained consisted solved in a liquid oxide solution. The desired elec- of 60 g. of borax with 55 g. of soft glass, and the liquid mixture was saturated with porcelain a t the int,ended operating trode reactions are temperature of the cell. Then 15 g. of CupOwas dissolved in Anode Cu(1iquid. puro) = C u t
+ e-
CU+
Cathode = C U ( C ~liquid) A~,
+ e-
The chloride electrolyte suffers from the probability of permitting a simultaneous opposing reaction involving the transfer of Ag from the Cu-Ag electrode to the electrolyte solution and to the Cu ( 1 ) (a) This work was supported by the U. S. Office of Naval Research, U. S. Navy, through Contract N7-onr-329, Task Order 11,and Contract NONR 1406, Task Order 11. (b) Based on part of a thesis by J. H. Downing, submitted to the Illinois Institute of Technology in partial fulfillment of the requirements for the Ph.D. degree, May, 1954. ( 2 ) R. K. Edwards and J. H. Downing, J . Phgs. Chem., 6 0 , 108
(1056).
+
+
the oxide mixture to constitute the electrolyte. This electrolyte, having been presaturated with porcelain, was much less corrosive to the porcelain container and permitted cell observations €or several hours. A Pt-Pt( 10% R h ) thermocouple was used to obtain the cell temperatures. The voltage of the galvanic cell was followed as a function of time until it arrived at a steady state value or failed due to excessive volatilization of the electrolyte or corrosion by the electrolyte. Runs of five to (3) L. Brewer, L. A. Bromley, P. W. Gilles and N. L. Lofgren, Paper 6, National Nuclear Energy Series, Vol. IQB, edited by L. L. Quill, McGraw-Hill Book Company, Inc., New York, N. Y . , 1950. (4) L. Brewer, Chem. Reus., 6 2 , 10, 30 (19.53). (5) N. F. Mott and R. W. Gurney, “Electronic Processes in Ionic Crystals,” 2nd Ed., Univmsity Pross, Oxford, 1018.
COMMUNICATIONS TO THE EDITOR Results and Discussion
256
1
Fig. 1.-Activity
\,
50 I( Atom %Ag. of Cu in liquid solutions at 7.0 X 10-4
l/’K.
(1428’K.).
eight hours were obtained in several cases. One of the disturbing features of the voltage behavior was that i t generally continuously oscillated about a mean value with a n amplitude of about one to two millivolts and a frequency of up t o 30 oscillations per minute, even when the so-called steady state was reached. It appeared that periodic polarization was taking place. Purposeful polarization away from the steady state values was generally followed by a rapid return of the cell to its steady state reading, although still accompanied by the usual oscillating behavior. Cells which had larger electrode-electrolyte interface areas usually showed decreased oscillation amplitudes and frequencies. Cells employing the oxide electrolyte were much less subject to this type of disturbance. Examination of the copper anodes after extended runs using the chloride electrolyte showed that some silver had been transferred from the alloy electrode in line with the factors discussed in the introductory paragraphs.
Vol. 61
The results are shown in Fig. 1 where observed activities are plotted for the temperature 1428°K. and are compared with the experimental curve of Edwards and Downing2 for the same temperature. The present results were in general obtained at slightly different temperatures but have been recalculated to the temperature 1428°K. by use of the temperature coefficient data of Edwards and Downing.2 In several cases the thermocouple protection tube, and consequently the thermocouple, failed during the course of the run and it was necessary to estimate the temperature thereafter. The uncertainties shown in Fig. 1 are ample to cover such temperature errors, and also the oscillation error for the particular cells involved. The dashed line in the figure is the “ideal solution” reference. For one rather well behaved cell (54 atom % ’ Ag alloy), it was possible to make observations a t several temperatures. From these data a relative partial molar enthalpy for Cu of 1,610 calories was obtained. The galvanic cell activity data are seen to lend general support to the vapor pressure activity data in that they, too, indicate that the liquid Cu-Ag solutions show positive deviation from ideal solution behavior. If a correction could be made for the contribution of the competitive silver reaction, the effect would be such as to bring these activities into closer agreement with the vapor pressure results, It is also to be noted that the effect of the competitive silver reaction should likely decrease at higher silver dilutions in the alloys, and such seems to be observable in Fig. 1. I n spite of the limitations as to the quality of the cell, the relative partial molar enthalpy for Cu obtained in the one case is in good agreement with the work of Edwards and Downing2
COMMUNICATIONS TO THE EDITOR CRYSTALLOGRAPHIC EVIDENCE FOR T H E TRIHYDRATE OF ALUMINUM FLUORIDE Sir:
Dr. Benjamin Post has pointed out that, in the absence of single crystal evidence to the contrary, the sin2 0 values which I recently reported‘ for MF3.3HzO may be equally well fitted to a tetragonal unit cell with c half the value which I assigned. ( 1 ) R. D. Freeman, THISJOURNAL,60, 1152 (1958).
His comments are quite valid. Therefore, until single crystal data for MF3.3H20 are presented, the u?it cell dimension! should be taken as a = 7.734 A. and c = 3.665 A., the 1 values of the planar indices2 should be halved, and the space group assignment is not yet definite. DEPARTMENT OF CHEMISTRY OKLAHOMA A. AND M. COLLEQE STILLWATER, OKLAHOMA
ROBERTD. FREEMAN
RECEIVED JANUARY 7 , 1957
