GOLD-CHLORINE AND GOLD-BROMINE EQUILIBRIA IN FUSED SALTS
be made until the supernatant liquid remains colored after shaking with the zinc sulfide, but the powder remains white. Decolorization of the dye added from acidic solvents to unfired, acid-precipitated zinc sulfide must therefore be attributed to protonation a t active acidic sites in the zinc sulfide surface in equilibrium with the solvent. The situation appears similar to that discussed by Walling,31 who utilized the ability of surfaces of powdered calcium fluoride, calcium sulfate, etc., to convert adsorbed indicating dye bases to their differently colored conjugated acids to measure the acid strengths of solid surfaces. In Walling's experimenta, as in those reported1 here on zinc sulfide, the surface acid strength dependied on the medium from which the indicator was adsorbed, weakly basic solvents such as
847
water and the other members of group 1 depressing the acid strength of the surface and promoting adsorption in the unprotonated form. These examples of proton transfer between surface sites and adsorbed molecules appear to be analogous to recently studied examples of electron transfer between surface sites and adsorbed molecules. 2 4 , 3 2
Acknowledgments. We wish to acknowledge the assistance of Mr. Frank Grum in securing the reflectance spectra of the adsorbed dye layers, of Mr. .D. Pearlman in providing samples of zinc sulfide, and of Dr. L. G. S. Brooker, for supplying the dyes. (31) C . Walling, J . Am. Chem. Soc., 72, 1164 (1950). (32) J. J. Roomy and R. C . Pink, Trans. Faraday Soc., 58, 1632 (1962).
Gold-Chlorine and Gold-Bromine Equilibria in Fused Salts
by Denis D. Durnan, Alan D. Mighell, Edward J. Zapolski, and Reuben E. Wood Department of Chemistry, The George Washington University, Washington, D. C. (Received October $6, 1963)
The reaction between gold and chlorine in fused zinc chloride and in fused XaCl-KC1 and the reaction between gold and bromine in fused KaBr-KBr have been studied. Equilibrium concentrations have been determined and, from these, thermodynamic data have been calculated.
Introduction Most of the existing thermodynamic data concerning fused salt solutions have been obtained by cell potential measurements. The relative scarcity of directly obtained data on chemical equilibria can be attributed to analytical difficulties inherent in the study of equilibria involving extremely small concentrations of some reactant and to the difficulty or practical impossibility of varying equilibrium concentrations sufficiently to permit reliable extrapolations to infinite dilution or to unit mole fraction, for example. The gold-halogen equilibrium studies reported here were
not hampered by the first difficulty. Because of the second difficulty, however, data for making the mentioned extrapolations were not obtained. This paper reports the results of studies on these three reactions 2Au
+ Clz +2AuCl
(1)
in fused zinc chloride a t 736OK. 2Au
+ Clz
2AuCl
(2) in fused equimolar NaCI-KC1 in the temperature range 1025 to llOO°K., and --f
Volume 68, Number 4 A p r i l , 1964
D. DURNAN, A. MIGHELL,E. ZAPOLSKI,AND R. WOOD
848
2Au
+ Br2
observed were less than * 5 O . Control and maintenance of the chlorine flow was not entirely satisfactory; in fused equimolar NaBr-KBr in the temperature the flow was interrupted several times. Samples range 1035 to 1200OK. taken during or shortly after the interruption of chlorine flow showed lessened gold content, a fact presumExperimental ably resulting from escape of chlorine to the air and There was much similarity in the procedures used consequent reduction of chlorine concentration in the in studying the three reactions. The reaction mixtures solution. This interpretation is supported by the were heated in an electric furnace supplied through facts that after chlorine flow interruption, very fine a constant-voltage transformer. The gold used was platelets of gold precipitated and this precipitate respecified to be of 99.95% purity. The chlorine was dissolved rapidly when chlorine flow was resumed. specified by the manufacturer (Natheson) to be of a t The data are shown in Fig. 1. The dotted portions least 99.3% purity. It was used without further of the curve indicate periods where the chlorine Row purification except for drying in the cases of the XaC1was, or recently had been, cut off, KC1 experiments. Other chemicals including the broGold-Chlorine Reaction in Fused NaC1-KC1. Equimine were all of reagent grade. The reaction vessel molal NaC1-KC1 was used as the solvent. The reIn the zinc chloride experiments was Pyrex glass. In action was found to be much faster than in zinc chlothe NaC1-KC1 and NaBr-KBr experiments, the vessel ride but the temperature was also much higher. The was an alundum tube. Temperatures were measured data from this experiment are shown in Table I. with a platinum-platinum rhodium thermocouple. These represent two different runs. In the table, the In each case the halogen was bubbled under prevailfirst column is the time from the beginning of the run. ing atmospheric pressure through the fused salt in The second is the observed temperature a t the time of which the gold was immersed. One atmosphere was sampling. One entry is missing from this column used in calculations as the halogen pressure. Actual because the thermocouple had been removed a t the departures from this value would lead to insignificant time of this sampling. The third and fourth columns changes in the results of such calculations. In the express the concentrations in milligrams of gold per case of the chlorine experiments, the exit gas was gram of sample and mole fraction of aurous chloride, trapped or exhausted through the fume hood system. respectively. In the case of the bromine experiment, the gas was obThe last column gives the ratio of gold precipitated tained by boiling liquid bromine in an apparatus in by treatment of the sample with water only to this which the exhaust bromine was condensed and regold plus the gold which was reduced by the sulfur cycled. This system contained anhydrous %lg(C104)2 dioxide. The significance of this column is that it conas a desiccant. In all cases the fused salt was dried firms what would be predicted by extrapolation of lower by the passage of the halogen through it. In each temperature data,l-3 namely, that most of the gold case, the reaction vessels were arranged to permit the insertion of a sampling pipet. The analytical procedure was gravimetric. The sample of fused salt, withdrawn into a pipet, was cooled and weighed. It then was mixed with water in which most of it dissolved, leaving a residue of precipitated gold. The dissolved gold was precipitated by sulfur dioxide. In all cases the total precipitated gold was weighed. In the NaC1-KC1 experiments and in some of the NaBr-KBr experiments the gold precipitated before and after treatment with sulfur dioxide was weighed separately. ---t
2AuBr
(3)
Results Gold-Chlorine Reaction in Fused ZnClz. This was the first of the three reactions studied. The reaction was found to be very slow. The run reported here covered a period of 53 days during which 27 samples were taken and analyzed. The temperature of this run was 736 f: 5'K. The temperature fluctuations The Journal of Physical Chemistry
0
I
10
I
20 30 40 Duration of experiment : Days
Figure 1. Approach to equilibrium in reaction between gold and chlorine in fused zinc chloride a t 736 f 5'K. Dotted lines indicate intervals during which chlorine flow was or recently had been int,errupted.
50
GOLD-CHLORINE A N D GOLD-BROMINE EQUILIBRIA IN FUSED SALTS
Table I : Data of the Gold-Chlorine Experiments in NaCl-.KCl
Run
I
l;:
Time, hr.
Temp., "X.
2.25
1025 1033 1020
I1
1 (
1.25 5.25 3.0 7.25 9.25
Conon. of AuCI,
mg./g.
mole fraction
Ratio
1019
296 529 528 535 542
0.133 0.322 0.322 0.329 0.337
0.645 0.600 0.619 0.665 0.638
1096 1089 1097 1094 1089
457 464 468 468 481
0,251 0.257 0.262 0,262 0.272
0,657 0.655 0.654 0,655 0.630
...
25.5
Conon. of Au,
849
third and fourth columns express the observed concentrations in milligrams of gold per gram of sample and mole fraction of aurous bromide, respectively. The last column again presents, for the cases in which the requisite data were obtained, the ratio described in connection with Table I.
Table I1 : Data of the Gold-Bromine Experiments in NaBr-KBr
I
is gold(1) at the temperatures of these experiments. The proportionation reaction
3AuCl+
2Au
1
19.5 21.5 22.5 24.0
1057 1057 1062 1058
173 168 173 174
0.114 0.110 0 114 0.115
3.0
1174 1175 1186
136 129 142
0.0865 0.0815 0.0909
1032 1031 1038 1047 1044
186 192 186 191 194
0.124 0.129 0.124 0.126 0.131
1201 1202 1206
138 138 143
0.0881 0.0881 0.0916
1032 1033 1034
192 191 178
0.129 0.126 0.118
+ AuC13
which presumably occurs when the fused salt sample cools indicates 0.667 as the number to be expected in the ratio column if in the melt all the gold were gold(I). The fact that the experimental ratios are somewhat less than this and that they fluctuate more than the values for total gold concentration could be abcounted for in part, a t least, by assuming that dissolved chlorine reacted with some of the AuCl during the cooling process. The fact that the lower temperature run gave a bit lower average ratio is of dubious significance although reliable data are not yet available to rule out the possibility that the observed difference could result in appreciable part from the greater concentration of gold(II1) at the lower temperature. Gold-Bromi?ae Reaction in Fused NaBr-KBr. Two experimental runs are reported. In the first experiment the temperaturla was first held at 1059 f 2.9"K. After eqdilibrium had been attained, as determined by sample analysis, the temperature was raised to 1178 f 2.4"K. In the second experiment, the tern-. perature was initially maintained at 1038 rt 4.3"K.; it was then raised to, and maintained at, 1202 f 3.1 "K. ; then it was again lowered and the final data were obtained a t 1037 f 5.9"K. These temperatures are the means observed at 0.5-hr. intervals during equilibrium periods. The data from these experiments are presented in Table 11. ~h~ firstcolumn is the time elapsed from the start Of the experimerltal run at the particular rheostat setting which controlled the temperature of the reaction cell, The second column is the observed temperature of the melt just before sampling. The
[
1
1
9.0 10.0 13.0 16.5 17.5
18.0 19.5 23.0
0 614
0.663 0.662
0.670
Treatment of Results Several standard thermodynamic properties may be estimated from the data presented in Fig. 1 and in Tables I and 11. The standard state to which these properties will be referred will be the hypothetical unit mole fraction solution having properties extrapolated from those of the solutions actually studied. The work with the ZriCle solvent, although an expenment of long duration (53 days), involved essentially a single temperature and therefore only a value for L. Brewer, L, A. Bromley, I,, w, Gilles, and N , L, LoEgren., "Chemistry and >fetallurgy of PIiscellaneous Materials: Thermodynamics," L. L. Q,uill, Ed., McGraw-Hill Book co.,Inc., New York, N. Y., 1850. ( 2 ) W. J. Hamer, 51. S. Malmberg, and B. Rubin, J . Electrochem. Soc., 103, 8 (1956).
(3) A. Glassner, "The Thermodynamic Properties of the Oxides, Fluorides and Chlorides to 250OCK.," U. S. Atomic Energy Commission Report ANL-5750 (1957).
Volume 68, Number
4
April, 1.964
850
D. DURNAN, A . MIGHELL,E. ZAPOLSKI,AND R. WOOD
AGO of formation of AuCl can be calculated. This value, like the values for AGO calculated hereafter from the data in Tables I and 11, is based on the familiar relation AGO = -RT In K , K in these cases being taken simply as the mole fraction of the aurous halide corresponding to the equilibrium solution. For AuCl in ZnClz at 73G"K., then, A G O = 4.6 kcal. mole-l. Since the reaction between Au and Clz and Br2 in the corresponding alkali halide eutectics involved a range of temperatures, more calculations could be made in these cases and certain extrapolations and interpolations were possible. First, by least-squares treatment of the primary data, empirical equations were developed for the equilibrium constants (equilibrium mole fraction of AuCl or AuBr) of the reactions
XaCl/KCl-YaCl PbClz/Pb. They reported a cell potential of 1.00 0.02 v. in the temperature range 700-738 ". Flengas and Ingraham5report values
+ 0.5C1, +AuCl (in fused XaC1-KC1) Au + 0.5Brz --+AuBr (in fused NaBr-KBr) Au
as a function of temperature. From these empirical equations and applicable thermodynamic relations, values at 1000°K. for the A G O , A H " , and A S " of formation of the gold halide in the specified solvent were calculated. For these calculations the approximation was made that AH" (and then likewise AS") was constant in the temperature range to which the calculations were applied. The results of these calculations are shown in Table 111. ~~
Table 111: Values Computed from Data in Tables I and I1 AuCl
log K log Kiooo
A
G
AH'iooo ASoiooo
(1.55 x 1 0 3 ) / ~ - 2.00 -0.45 ~ 2 . 1~ kcal.~ mole-' ~ ~ - 7 . 1 kcal. mole-' - 9 . 2 cal. mole-ldeg.-'
AuBr
(1.20 x 1 0 9 1 ~ 2.07 -0.87 4 . 0 kcal. mole-I - 5 , s kcal. mole-' - 9 . 5 cal. mole-1 deg.?
For oxidation-reduction reactions, much of the experimental work and the published data are in terms of cell potentials. We therefore ,give in Table IV some cell potential figures to which our experimental data correspond.
Discussion The only previously published experimental results we have found which can be compared directly with our data involve measurements by Gitman and Delimarskii4 of the potential of the cell: Au/AuCl KC1-
+
Tha Journal of Physical Chemistry
+
*
Table IV : Calculated Cell Potentials Temp.,
Cell
0
K.
Au-ClZ in ZnClz 736 Au-C12 in NaC1-KCl 1000 Au-Brz in NaBr-KBr 1000 Au-C12 in XaC1-KC1 736 (extrapolated to 736°K. for comparison)
EO, volt
-0,100 -0,042 -0,087 +0.02
for the Pb, P b f 2 and C1-, C1z electrodes in fused S a C1-KC1, the difference between which, interpolated to 727" (lOOO"K.), is 1.225 v. This, combined with our value of -Eo for the Au-Clz reaction in the same solvent (Table IV) would lead to a prediction of 1.26 v. for the Gitman-Delimarskii cell. If thermodynamic data for the pure gold halides were known with sufficient accuracy at the temperatures of our experiments, activity coefficients could be calculated. However, consideration of ref. 1 and 3 and of the dissociation pressures of the gold halides reported by Meyer6 lead us to conclude only that our values of AGO for the gold halides in solution are the same as those for the pure compounds within the range of uncertainty of the pure-compound A G O values and that because of this considerable uncertainty, activity coefficients could be unity or either significantly greater or less than unity with the pure compound as reference state. The results with zinc chloride compared to those with the alkali chloride melt are just the opposite of what one might predict on the basis of a simple common-ion effect and the assumption that zinc chloride contains relatively few chloride ions. The fact probably is that complex ion formation is importantly involved in these systems and that the zinc competes with gold more effectively than the alkali metals for the complexing chloride ions. (4) E. B. Gitman and Y. K. Delimarskii, U k r . K h i m . Zh., 22, 731 (1956). ( 5 ) S. N. Flengas and T. R. Ingraham, J . Electrochem. Soc., 106, 714 (1959). ( 6 ) Meyer, International Critical Tables, Vol. 7, 1933, p. 273.
