COMMUNICATIONS TO THE EDITOR Results and Discussion
256
1
50 Atom %Ag.
Fig. 1.-Activity
\,
I(
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 an amplitude of about one to two millivolts and a frequency of up to 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 Downing2for 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 THE 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 A. AND M. COLLEQE OKLAHOMA STILLWATER, OKLAHOMA
ROBERTD. FREEMAN
RECEIVED JANUARY 7 , 1957
