THE SOLUBILITY O F SILVER I N MERCURY. I1 ROBERT E. DERIGHT Chemical Laboratory, The University of Rochester, Rochester, New Yorb Received October 2.9, 193.9
Determinations of the solubility of silver in mercury at various isolated temperatures have been reported by several investigators (5, 8, 17, 14, 12, 15, 24), but Joyner (9) was the first to study the problem systematically. Several years ago Sunier and Hess (22) published some precise work on the solubility of silver in mercury over the range 80°C. to 200°C. Their results agreed remarkably well with those obtained by Joyner, who used a rather crude method. Since that time Murphy (13) has published a complete phase diagram for the system silver-mercury. He was interested, however, in the composition of the solid phase and cites the data of Sunier and Hess (22) for his liquidus curve at low temperatures. Since Joyner (9) is the only person offering systematic data below 8O”C., (four determinations) and his data in this range of temperature pointed to a marked flattening out of the solubility curve below 50”C., it was thought advisable to extend the precise work of Sunier and Hess (22) below 80°C. MATERIALS
The mercury used was first purified by dropping it through a five-foot Meyer column containing 6 N nitric acid and was then washed with distilled water. The dried mercury was distilled in an all glass apparatus according to the method of Hulett and Minchin (7). Samples of this mercury yielded no weighable residue when evaporated according to the method of analysis of the amalgams in a stream of hydrogen. The silver used was “1000 fine” foil, kindly supplied by the Philadelphia Mint. This foil was used as the solid phase either directly or after treatment by methods described later in this paper APPARATUS
The solubility tube used, a modification of the one used by Sunier and Hess (22) and similar to the tube used by Sunier and Gramkee (21), was made of Pyrex glass. It differed in size from that of the latter, as the smaller solubility necessitated the employment of larger volumes of mercury. The shaking mechanism differs little from that used by Sunier and White (23) with the exception that in one run a set of sixteen rather than eight 405 T E E JOURNAL OF PHYSICAL CHEMISTRY, VOL. XXXVII, NO. 4
406
ROBERT E. DERIGHT
tubes were run a t the same time. The apparatus brought about complete transfer of the contents of the tube from one end to the other by rotating it through an angle of sixty degrees. The tubes were immersed in a heavily lagged thermostat similar to the one described by Sunier and White (23). The bath was heated by electricity and steam, and was provided with a copper cooling coil. In the early runs, manual control of the temperature was resorted to, but in later runs a mercury thermoregulator of the type described by Clark (1) was used. When manual control was used the constancy approached =tO.O2"C.,while with the thermoregulator the temperature was held within the range of =kO.Ol"C. Temperatures were read from mercury thermometers of the double diamond label manufactured by Hiergesell Brothers, and graduated in tenths of a degree. These thermometers were compared with thermometers standardized by the Bureau of Standards. The ice points were taken frequently but showed no change. Temperatures could be read to 0.02"C. with ease. Changes of temperature during the run were noted from a Beckmann thermometer inserted in the bath. The weights used were calibrated according to the method of Richards as described by Fales (4)and compared with a weight checked by the Bureau of Standards. EXPERIMENTAL PROCEDURE
After the solubility tube had been washed with 6 N nitric acid and distilled water, it was dried in an oven a t 160°C. overnight or flamed with a Bunsen burner in a stream of air. The tube was then charged with 135 to 155 g. of mercury in the manner described by Sunier and Gramkee (2) and two to three hundred per cent excess of the silver required a t that temperature, using Joyner's (9) data as a guide. The tubes were flamed and evacuated with a Cenco Hyvac pump to a pressure of 0.01 t o 0.1 mm. of mercury read on a MacLeod gauge. To determine the solubility at a certain temperature a total of eight tubes was made up (16 tubes in Run N). The run was made in the manner described by Sunier and White (23), four tubes being rotated a t 5°C. above the solubility temperature preliminary to the insertion of the remaining tubes. Later in the research it became evident that equilibrium had not been attained in the time allowed, so that all of the tubes were inserted a t the same temperature and rotated for periods of from eight to two hundred and fifty-six hours. Sampling was carried out in the manner described by Sunier and White (23). The method of analysis was that used by Sunier and Gramkee (21) in the analysis of gold amalgams. A large Pyrex tube with a ground glass
SOLUBILITY O F SILVER IN MERCURY. I1
407
stopper, as shown in figure 1, was supported in an electric furnace. A mercury trap was provided to catch the condensed mercury swept through by the gas admitted at the opposite end. The capsules were supported on Sillimanite slabs, hence there was no possible contamination of evaporated mercury. I n order to test the applicability of this procedure to silver amalgams, various experiments were carried out. It was found that the mercury used left no weighable residue when evaporated by this procedure. Weighed pieces of silver foil lost no weight when heated for periods of twenty hours a t 550OC. in a stream of hydrogen. Mercury was placed on this foil and then driven off by heating at 270OC. to 300°C.;the residues were then heated to 550°C.for several hours. When air was used the residues were consistently several tenths of a milligram heavier than their original weight. This could not be due to silver oxide formed, for it would
FIQ. 1. EVAPORATION TUBE
be unstable a t the temperatures employed, according to the free energy relationships cited by Lewis and Randall (11). Owing to the finely divided condition of the crystallized silver, occlusion suggests itself as a possibility. Steachie (20) refers to the occlusion of gases on finely divided metallic surfaces. When hydrogen, purified by bubbling through saturated solutions of potassium permanganate, sodium hydroxide, and 95 per cent sulfuric acid was used the residues came down to their original weight. To test the entire experimental procedure, eight tubes containing weighed quantities of silver and mercury were made up and carried through the entire procedure. The results of this experiment are tabulated in table 1. It is to be noted that the solubility of silver is much less than that of gold, so that, of necessity, much smaller residues must be weighed, and the amounts of mercury used must be much larger. Despite these
408
ROBERT E. DERIGHT
disadvantages, from the composition found, the average deviation 'was less than one part per thousand. TABLE 1
I
TUBE
1 2 3 4 5 6 7 8
Average - =
SILVER TAKEN
MERCURY TAKEN
ATOMIC PER CENT TAKEN
grams
grams
0.0912 0.0986 0.0877 0.0859 0.0949 0.0899 0.0940 0.0930
143.928 147.909 145.417 143.349 144.816 145.311 137.OS8 145.897
ATOMIC PER CENT FOUND
DEVIATION
parts per thousand
0.1177 0.1238 0.1120 0.1113 0.1217 0.1148 0.1273 0.1184
0.1182 0.1228 0,1124 0.1122 0.1218 0.1142 0.1267 0.1179
4.2 8.1 3.5 8.1 0.8 5.3 3.9 4.2
0.1184 0.1183 Deviation of average composition = 0 5 per 1000
4.8
TABLE 2 Run A Time of run 6 hours (low side-4 hours); temperature 40.11"C. (high side-45.3"C.); silver-foil (mint) ; mercury-triple distilled. TUBE NUMBER
-
EILVER AT START ,
WEIQHT OF AMALGAM
0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5
142.991 144.533 143.692 142.416 146.070 142.993 143.285 142.772
grams
A-1 A-2 A-3 A-4 A-5* A-6* A-7* A-8'
Average =
WEIGHT OF
SILVER
ATOMIC PER PER CENT OF SILVER
grama
DEVIATION FROM MEAN
REMARKS
p.p.t.
0.0880 0,0886 0.0872 0.0932 0.0896 0.os80 0.0879 0.0867
0.1144 0.1139 0.1128 0.1216 0.1140 0.1150 0.1140 0.1129
Low side, 0.1137 High side, 0.1140
@lo,1139
5 0 11 X
Exclude
1 11 1
10 per
looo
Number of determinations, 7. * High side. EXPERIMENTAL RESULTS
In table 2 will be found a sample table of data for one run. In table 3 will be found the table of data for Run N in which sixteen tubes were used and the solid phase varied as described later in this article.
TABLE 3 Run N
Time of run 256 hours; temperature 25.28"C.; silver as noted; mercury-triple distilled. * TUBE NUMBER
WEIGHT OF SOLID PHABE
prams
WEIGHT OF AMALGAM
ATOMIC 'ER CENT OF SILVER
WEIGHT OF SILVER
DEVIATION FROM MEAN
REMARKS
p.p.t.
grams
grams
A-1 A-2 A-3 Ex. E-1 E-4
1.7 1.6 1.6 1.0 2.0 1.0
144.951 143.811 146.676 155.377 140.474 138.439
0.0606 0.0620 0.0685 0.4891 0.0631 0.3189
0.0777 0.0801 0.0868 0.5841 0.0835 0.4275
11 35 102
E-2 E-3
c-1
2.0 1.o 2.0
141.603 0.0570 0.0748 134.842 0.0673 0.0928 143.694 0,0695 0.0899
18 162 133
c-2
2.0
141.549 0.0570
0.0749
17
c-3 c-4 H-1 H-2 H-3 H-4
2.0 2.0 1.o 1.0 1.0 1.o
85.435 148.444 143.853 143.467 144.120 144.399
0.0748 0.0754 0.0908 0.0799 0.0765 0.0757
18 13 142 33 1 9
0.0766
21.1 per 1000
0.0346 0.0602 0.0703 0.0617 0.0593 0.0588
Average =
Exclude No glass wool Exc1u d e Filtered through chamois Exclude Exclude
X
69 X
No glass woo1 Poor vacuum Exclude
Number of determinations, 9. * With the exception of the tube marked Ex., column 1. TABLE 4 RWN
PEMPERATURn
SOLUBILITY
DEVIATION
NUMBER OF TUBES
TIME ~~
degrees C.
A B C D E F H I J K L N
40.11 50.02 60.26 70.54 30.15 80.94 19.01* 29.93* 8.92*
25.60* 18.17* 25.28*
h0ur8
parte per
thousand
0.1139 0.1450 0.1901 0.2404 0.0965 0.2892 0.0636 0.0881 0.0641 0.0792 0.0643 0.0766
5 4
27 32 91 4
6 13 64 14 10 21
7 6 8 8 8 6 8 5 6 6 5 9
* All tubes approached saturation from the same side. 409
6 7 7 7 7 Definite trend 7 8 9 9 78 75 256
410
ROBERe E. DERIGHT
In table 4 will be found the summary of all runs. Figure 2 is a plot of the log of the solubility versus the reciprocal of the absolute temperature. The slope of the line was determined from a large scale plot and the line corresponds to the equation
+
log N = ___ 1105'8 0.5894 T
Of a total of one hundred and twenty-eight determinations made, thirtyeight, have been rejected. Of this apparently large number of rejected
-26
-26
t" 8
U
-30
-3.2
34
32
30
/OUO/7-
Fro. 2. THE SOLUBILITY OF SILVER IN MERCURY 0 Joyner; 0 Sunier and Hess; 0 DeRight.
tubes, sixteen are accounted for by Runs G and M, where equilibrium was not attained. Of the remaining twenty-two excluded, four tubes were broken, three amalgams bumped during evaporation, and difficulty was experienced in the filtration of four tubes. Therefore only eleven tubes were excluded, merely because their deviation was greater than four times the average deviation.
SOLUBILITY O F SILVER IN MERCURY. I1
411
DlSCUSSION O F RESULTS
It was realized a t the inception of this investigation that, owing to the slight solubility of silver, it would be necessary to employ large tubes and to weigh the residues extremely accurately. After the completion of the blank run and the satisfactory results obtained, it was felt that little further difficulty would be experienced. The solubilities at temperatures above 40°C. were first determined. Although in some cases rather large average deviations were found, the determinations seemed to be in fair agreement, and when plotted fell on a straight line. The slope of this curve was somewhat less than that determined by Sunier and Hess (22). This change in slope is not unusual a t lower temperatures. The determination a t 80"C., which was the lowest temperature a t which Sunier and Hess (22) made measurements, was in good agreement with their result. Furthermore, in the range of temperature above 40"C., the data was in agreement with that of Joyner (9). A hint of the difficulty present in the behavior of amalgams below 40°C. was given by Joyner's data. There was a marked break in the slope of his curve a t 40°C. The slope of the curve at the lower temperatures approached zero. I n Run E at 30.12"C. there was a marked discrepancy between the four tubes of the high side and those of the low side. This seemed to be a criterion that equilibrium had not been attained. A very marked discrepancy existed between the two sides in Run G a t 1935°C. This disagreement was so marked that this run was omitted from table 4. It was decided to make a second run near the temperature of Run G, rotating all eight tubes, from the low side, for eight hours (Run H) a t a temperature of 19.01"C. This run yielded very satisfactory results, since all eight tubes gave an average deviation of only 6 p.p.t., a very low figure for such a small solubility. Furthermore the point, when plotted, showed no deviation from the straight line drawn in figure 2. Run E was repeated in the same manner in Run I. Unfortunately, two of the tubes were broken and a third excluded. The remaining five were in good agreement. Run J yielded a curious result. It was carried out in the same manner as Runs H and I. Although the six determinations retained were not in good agreement, their average deviation of 64 per thousand pointed to some regularity. The apparent solubility was greater than that obtained at 20°C. and showed some agreement with Joyner's results. The impression gained from a first reading of Joyner's paper was that he had rotated his tubes in a constant temperature bath for periods of a fortnight. Upon closer inspection of his paper it was found that he had rotated his amalgams containing both silver and tin for the time indicated, but he made no specific statement of the time of rotation in the silver-
412
ROBERT E. DERIGHT
mercury system. Furthermore, he claims the solubility he obtained was the equilibrium of AgsHgr in contact with mercury, rather than of silver in contact with mercury. Murphy (13) states that silver amalgams attain equilibrium several times faster at 100°C. than a t room temperature. Data is also available as to the rate of penetration of mercury into a silver surface at different temperatures (8). Therefore it seemed advisable to continue the runs at lower temperatures for longer periods of time. Run K was to continue for seventy-five hours t o note any difference in the solubility obtained at 25°C. Likewise new mercury was used in two tubes to see if any error could creep in, owing to the use of mercury evaporated from previous runs. The results of this run were in fair agreement among themselves and the mean result fell very close to the straight line obtained at higher temperatures. Run L was a determination of the same type, a t 18°C. Unfortunately, the contents of one capsule bumped, silver being transferred to two other capsules, so that it was necessary to exclude three values. The mean solubility obtained in this run was not in disagreement with previous runs, and fell on the straight line. This proved that the runs at 20°C. and 30°C. for a shorter period of time had practically reached equilibrium. Run M was to continue for one week in the hope that equilibrium at 10°C. might be attained. This temperature was made possible through the use of a thermoregulator, cooling being brought about by pumping water from an auxiliary thermostat containing ice, through the copper cooling coil. I n four of the tubes the solid phase consisted of silver which had been completely dissolved in mercury at a high temperature and then allowed to cool. The results of this run were very disappointing. There was too little agreement among the tubes to accord any credence to this determination. The four tubes, containing the solid phase mentioned above, filtered with great difficulty, the solid caking a t the mouth of the capillary. Furthermore, the results showed a higher solubility than the other four, supporting the conviction that solid passed through with the liquid phase. Owing t o the disagreement among the tubes this run was omitted from table 4. At this point in the research a critical survey was made of the fourteen previous runs, with a view toward detecting any errors in procedure and introducing any helpful innovation. The following changes in procedure suggested themselves: (1) variation of the form of the solid phase, (2) variation of the liquid phase, (3) design of the tubes, i.e., the method of filtering, (4) the time of shaking, ( 5 ) the number of tubes. Murphy (13), in his paper, states that equilibrium is much more readily reached when finely divided silver is used. Furthermore, it is to be noted that in the runs of Sunier and Hess (22) in which they obtained their smallest average deviation, silver filings rather than foil was used. Joyner claims
SOLUBILITY O F SILVER I N MERCURY. I1
413
his data to be that of Ag3Hg4 in equilibrium with mercury, and Murphy postulates the formation of this compound from the liquid phase a t a higher temperature. It was decided to make use of this solid phase to explain the behavior of the amalgams. The design of the tube would be rather dependent on the type of solid phase used. The use of glass wool above the capillary filter, or filtration through sintered glass, suggested themselves. A method of double filtration or decantation was considered, but did not appear to be applicable a s finely divided silver, when wet with mercury, seemed to be dispersed quite uniformly throughout the liquid phase. The use of glass wool above a very fine capillary was deemed the most logical procedure. An identical tube support was made so that sixteen tubes might be run a t the same time. The supplementary Run N was then made. A temperature of 25°C. was chosen because it could be easily attained and would establish the validity of the data to 20°C. The tubes used were identical with those previously used, with the addition of glass wool above the capillary. Although no objection to the use of the evaporated mercury could be found, it was decided to use new mercury. The time of shaking was increased to two hundred and fifty hours and sixteen tubes were used. Three distinct solid phases were used. (1) Mr. G. H. Reed,l working in this laboratory several years ago on the solubility of silver in mercury a t higher temperatures, made up several t8ubescontaining silver and mercury. One of these tubes had been carefully preserved, but for some reason had not been analyzed. The silver had been in contact with the mercury for several years at room temperature. When opened, the solid phase was granular and brittle and retained none of the characteristics of the original foil. When all of the excess mercury was squeezed from the solid through a chamois, and a portion analyzed, it was found to have a composition approximating Ag3Hg4. (2) Since Murphy cite's the formation of Ag3Hg4,it was decided to attempt to prepare this compound. Weighed amounts of silver and mercury in the proportion of Ag3Hg4were sealed in a tube and evacuated; the tube was so constructed that, by inversion, the contents could be transferred from one eng to the other through a capillary. This acted as a criterion when the contents were entirely liquid. This tube was heated a t 250°C. for forty hours, then raised to 500°C. and the contents filtered three times at intervals of one hour, thus insuring homogeneity of the contents. The tube was then cooled t o 300°C. and held for twenty-four hours, cooled to 250°C. and held for fifty hours, and kept in an oven at 100°C. until introduced into the tubes, The contents of the tube were entirely solid, and had a bright crystalline appearance. 1
See footnote ( a ) , Sunier and Hess (22).
414
ROBERT E. DERIGHT
(3) Finely divided silver was prepared in a manner similar to that described by Tartar and Turinsky (25), who refer to Lewis (10). Some modifications were made. The silver employed was the “1000 fine” foil, and c . P. chemicals were used. The silver carbonate was heated in a stream of hydrogen at 250°C. to 300°C. for six hours, and then raised to 500°C. until there was no further loss in weight. The temperature throughout the run was very constant, the thermoregulator being used. Two tubes were broken at the inception of the run, The contents of one was transferred to a tube with no glass wool, but yielded a very high result. The contents of one tube refused to filter, so they were removed and quickly squeezed through a chamois skin. The very high silver content obtained in this case is of significance. The average solubility obtained from this run agreed very well with the straight line plot, hence establishing the straight line function to 25°C. or lower. Furthermore, the different variables used seemed to clear up any question of variable phases employed. A consideration of available data leads one to some interesting speculations. First of all, the term “solubility” must be defined. Richardson (18) states, “a solution is a body of homogeneous character, the composition of which may be varied continuously within certain limits.” Lewis and Randall (lla) give a more fundamental definition. Definitions may vary but the idea of homogeneity is always retained. Next the criterion of homogeneity must be defined. In ordinary aqueous or organic systems, this idea is overlooked, for the appearance of the solution is an immediate criterion. A cloudy or translucent appearance postulates a colloid sol, while a clear appearance is evidence of a real solution. The effect of light, then, is the means of classification in these systems. In the mercury system, the opacity prevents determination of the clearness, so that the amount of material that passes through the filter in the liquid phase has been a criterion of solubility. Too little attention has been paid to the size of the filter or conditions of filtration, Le., pressure. It is this fact that may account for the many discrepancies in data reported. Russell (19), working on amalgams other than silver and gold, reports some interesting findings. His method of analysis relies on the preferential oxidation of metals less noble than mercvry, by shaking the amalgam with a solution of potassium permanganate. In one study he filtered identical amalgams through a Jena glass sintered filter and through chamois skin. He found the use of the chamois skin very unreliable, the apparent solubility inconstant and several times larger than when filtered through the sintered glass filter. This is in agreement with the result obtained when a chamois skin was used in Run N. A large majority of the excluded determinations in this paper have been “high.” Until there is some method of determining a homogeneous phase, other
SOLUBILITY O F SILVER IN MERCURY. I1
415
than filtration, the term “solubility” in a metallic system has a limited meaning, for particles may vary in size from atomic proportions to large aggregates. If this is true, and there is not a sharp gradation, solubility will be dependent on the conditions of filtration. Apparently, in the silvermercury system, there is a distinct difference in size of silver particles above 40”C., while below that temperature there seems to be a gradual gradation. It seems, therefore, that the greatest credence should be accorded to the lowest results, provided that enough time has elapsed for equilibrium t o be reached. There is a possibility that a measurement of the light reflected from an amalgam, or some other optical means may be used by which homogeneity may be determined. SUMMARY
1. Amalgams as dilute as 0.06 atomic per cent have been prepared, and analyzed with a precision approaching one part per thousand. 2. One hundred and twenty-eight determinations of the solubility of silver in mercury in the range 20°C. to 80°C. have been made. 3. Several forms of silver and intermetallic compounds have been used as the solid phase. 4. In the range of temperature from 20°C. to 80°C. the solubility changes according to the equation log N =
-1105.8 + 0.5894
5. The relation of the term “solubility” to particle size has been briefly discussed. In conclusion the writer wishes to express his sincere appreciation to Professor Arthur A. Sunier who has so inspiringly and unselfishly directed this research. REFERENCES (1) CLARK:Determination of Hydrogen Ions. The Williams & Wilkins Company, Baltimore (1917). (2) DE SOUZA:Ber. 8, 1616 (1875); 9, 1050 (1876). A N D HILDEBRAND: J. Am. Chem. SOC. 36, 2020 (1914). (3) EASTMAN (4) FALES: Inorganic Quantitative Analysis, paragraph 82, p. 103. The Century, Co., New York (1925). (5) GOUY:J. Physics 4, 320 (1895). (6) HARTECK: Z.physik. Chem. 134, 1 (1928). (7) HULETTAND MINCHIN:Phys. Rev. 21, 388 (1905). (8)HUMPHREYS: J. Chem. SOC.69, 243 (1896). (9) JOYNER:J. Chem. SOC.99, 195 (1911). (10)LEWIS:J. Am. Chem. SOC.28, 158 11906).
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ROBERT E. DERIGHT
(11) LEWISAND RANDALL: Thermodynamics, p. 480. McGraw-Hill Book Company, New York (1923). (lla) Ibid. p. 10. (12) MAYEY:Z. Physik 60, 209 (1905). (13) MURPHY:J. Inst. Metals, March, 1931. (14) OGG:Z.physik. Chem. 27, 285 (1898). AND JOVANOVICH: Gam. chim. ital. 49, 6 (1919). (15) PARRAVANO (16) PIERSAL:Phys. Rev. 23, 785 (1925). Z. physik. Chem. 64, 609 (1906). (17) REINDERS: (18) RICHARDBON: General Chemistry. Henry Holt and Company, New York (1928). (19) RUSSELL:J. Chem. SOC.32,835 (1932); 32, 891 (1932). (20) STEACHIE: J. Phys. Chem. 36, 2113 (1931). J. Am. Chem. SOC.61, 1703 (1929). (21) SUNIERAND GRAMKEE: (22) SUNIERAND HESS:J. Am. Chem. SOC.60, 662 (1928). AND WHITE:J. Am. Chem. SOC. 62, 1842 (1930). (23) SUNIER (24) TAMMANN AND STRASSFURTH: Z. anorg. Chem. 143, 357 (1925). AND TURINSKY: J. Am. Chem. SOC.64, 580 (1932). (25) TARTAR (26) VANHETTEREN: Z.anorg. Chem. 42, 128 (1904).
