THE TERNARY SYSTEM: SIJ,VEFC BROMIDE-POTASSIUM BROMID&-j~~AATER”
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BY RAYMC~TDH, ]LAMBERT
Some idea of the solubility rela - % i 7 . a ternary system can be obtained from a study of the three pairs of binary systems, provided the data for the binary systems are avai1able.l I n the system instanced in the title such is the case and since the binary systems are very simple, it is possible to develop the entire surface between solid and liquid phases with only a very little extra data. From the data in the literature*, the systems may be arranged as in Fig. I . The dotted portions of the diagram indicate imaginary boundaries not existing at atmospheric pressure but real and nearly identical a t sufficiently high pressures. An examination of the three pairs of binary mixtures leads to some interesting conclusions in regard to the ternary system. At least two of the three pairs of binary mixtures i.e., systems, KBr-H20, KBr-AgBr, show distinct eutectic points, the former at 6.6 molar percentages of potassium bromide and the latter a t 44 molar percentages potassium bromide. If in a ternary system each pair of binary systems contain only a single eutectic mixture, the phase diagram is very simple, For a non-variant system both temperature and concentration are fixed, this temperature being lower than for any other equilibrium mixture. ,4t this point the liquid phase will be in equilibrium with three solid phases of the pure components. Compound formation obviously makes the diagram more complex. Hellwig concluded that no compound or double salt between silver bromide and potassium bromide is found to exist in precipitates from water s ~ l u t i o n . ~ Reference will be made to this later in the paper. There is also no evidence of hydrates between either salt and water although many alkali halides do combine with water. In this ternary system, the phase diagram may be even further simplified since any eutectic in the binary system, AgBr-H20, lies so very near the axis of the pure water component that it has never been detected. I n any case it can be assumed to lie on the pure water axis. The lowest temperature a t which equilibrium will exist between solid and liquid will then be a t one of the other two binary eutectics, in this case, the KBr-H20 eutectic a t - 1 3 O C and 6.6 molar percentages of potassium bromide. From this assumption, one may proceed further and predict a eutectic valley leading from the AgBr-KBr side of the triangular diagram, figure I , t o the KBr-H20 side exists. This valley should be a smooth path, which on a four dimensional system, temperature being represented by height, falls in * Communication No. 273 from Research Laboratory, Eastman Kodak Company
’ Roozeboom:
(‘Die heterogene Gleichgemichte,” 3 I. C. Sandonnini: Atti Accad. Lincei, 2 1 11, 197 (1912). Systems KBr-HZO and ilgRr-H?O. Landolt-Bornstein Tabellen. 4Karl Hellwig: Z. anorg. Chem., 2 5 , 182 i1900).
* System KBr-AgBr.
974
RAYMOND H. LAMBERT
temperature and composition from z ~ o ° Cand 56 molar percentages of silver bromide and no water, the eutec:tic point of system, KBr-AgBr, at a continuous rate down to - 13OC and zerc, silver bromide, the eutectic point of system, KBr-H20. For two salts in eqbljlibriufi with water, if one is extremely insoluble compared with the other, as is tru: with silver bromide and potassium bromide, the valley will be conca. to tht axis (in the triangular diagram) of the comparatively insoluble comjIlA, d e . , AgBr-H20, and convex to the KBr-HSO axis. One would expect L~ I ' : degree of curvature in the eutectic whose difference in solubility, when valley would be greater for a pair of s. alone in water, is greater and that for o salts equally soluble in water, the eutectic valley would approximate to a straight line in the composition diagram.
FIG.I
Since interest lies mainly in the eutectic valley a t temperatures of low vapor pressure of water, a study was made only of the solubility of silver bromide and potassium bromide co-existent in water solution in the presence of both solid salts with change of temperature up to I O O O C . The method used was that suggested by Fritz Fejgll whereby the solution in equilibrium with solid silver bromide and potassium bromide was filtered into a porous cup which was also used as a stirrer in order to bring the system more quickly to equilibrium, Fig. z is a diagrammatic sketch of the apparatus. An ordinary liquid thermostat was used. A red dye soluble in oil was added to the bath to prevent light decomposition of the silver bromide. The layer of oil prevented excessive evaporation of water from the bath a t the higher temperatures. Equilibrium was obtained in about 2 4 hours, due to efficient stirring, after which portions of the solution were drawn off by means of a pipette which Fritz Feigl: Z. angew. Chem., 31, 168 (1918).
SILVER BROMIDE-POTASSIUM
975
BROMIDE-WATER
several hours before sampling had been placed in the hollow glass tube to which the porous cup was fastened. This brought the pipette to the same temperature as the bath. The special pipette made of thick walled glass tubing had a very short stem so that the liquid did not leave the porous cup until the pipette was ready to be drawn entirely from the bath. The liquid was quickly placf 'n tared weighing bottles and even 4 le: highest temperatures employed error was caused by sampling. The cup was alundum having the desired porosity for filtration. It was chosen from about a dozen cups.
!reus
Chemically pure potassium bromide and silver bromide were three times recrystallized together a t IOOOC from water solution. Samples were removed from the apparatus at ten degree ntervals for increasing temperatures and at twenty degree intervals on the decreasing scale. In the latter experiments, the porous cup was not inserted until the lon-er temperature had been established for some tinie.
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FIG.2
TABLE I Per cent by Mol.
Per cent by Weight Temperature AgBr
6.71
H20 93.29
Ratio KBr/AgBr oc
9.33 9.21
90'4 90.54
37.32 38.06
0.489
9.77 10.78 10.50
89.88 88.70 89.01
28.75 20.93 21.47
51.56
0.685
11.42
87.90
16.67
0
KBr -
-
1.67 1.636
39.87 39.54 40.88 43.09 42.43
56.88 53.66 54.45
0.515
50
2.24 3.25 3.12
60
4.20
44.24
5.80
- I3
30
40
H20
AgBr
KBr
0
58.50
0.250
58.82
0.242
0,339
70
5.60
45.48 45.53
48.72 48.87
0.990 0.954
12.26 12.24
86.75 86.80
12.38 12.83
80
6.72
46.91
46.38
1.192
13.12
8j.69
11.01
90
8.47 9.00
48.15 47.98
43.39 , 43.115
1,580 1.687
14.16 14.20
84.26 84.11
8.96 8.42
10.55
48.66
40.79
2.080
14.98
82.96
7.27
100.0
976
RAYMOND H. LAMBERT
Table I contains the da.ta both in weight and molar percentages. In Fig. 3 are plotted the weight percentages of potassium bromide and silver bromide with temperature as the abscissa. The upper curve is that for the solubility of potassium bromide alone in water, the data being taken from Landolt and Bornstein's Tabellen. I t is interesting to note that IO. j per cent by weight of silver bromide dissolves in a potassium bromide solution in equilibrium with solid potassium bromide at IOOOC.
The ratio of potassium bromide to silver bromide is of interest since at zgo°C, Sandonnini gives the eutectic of the system, KBr-AgBr as j 6 molar
percentages of silver bromide. Figure 4 shows the ratio varying with temperature. The upper end of the curve should be asymptotic to the Y-axis a t - 13OC If the eutectic valley be projected on a plane for the ternary system, the result is thae shown in figure 5 . Horizontal lines measure molar percentages of silver bromide with zero silver bromide on the base line. Molar percentages of potassium bromide are lines parallel to the line on which molar percentages of potassium bromide is written. The third series of lines obviously corresponds to molar percentages of water. Here check points lie on a smooth curve but
SILVER BROMIDE-POTASSIUM
BROMIDE-VATER
977
are not together. This would indicate an error in temperature reading, Extrapolation of the curve ends at the eutectic point of the binary mixture, KBr-HzO, as was to be expected. Figure 6 is an expression of the ternary system including temperature relations. The photograph is that of a plastic model and was taken expressly to shorn the eutectic valley. As previously stated, the system is only stable
for pressures greater than atmospheric since water should boil away at the high temperatures. Below I O O O C , hon-ever, the model expresses actual facts. The bend in the curve is convex to the KBr-H20 axis of the diagram as previously predicted would be the case. In order, however, to test the assertion that the curvature of the eutectic valley is greater for salts having a greater difference in solubility, an experiment was carried out with silver chloride and potassium chloride. Silver chloride is comparatively only slightly less soluble than silver bromide and therefore should not affect the
FIG.j
SILVER BROMIDE-POTASSIUM
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BROMIDE-WATER
979
in relative solubilities is not the only factor of degree of curvature. If it were the only cause, then silver chloride should be more soluble in saturated potassium chloride than silver bromide in saturated potassium bromide at any temperature. Some predictions can be made of solubility of silver bromide in saturated lithium bromide and sodium bromide. Lithium and sodium bromide1 form mixed crystals with silver bromide in all proportions. I n both cases, however, the alkali salts form water of crystallization, thus complicating the phase diagram a t low temperatures. KO very distinct eutectic can exist in the ternary system with both salts present in the solid phase and therefore no constant ratio of salts exist in solution a t a given temperature as shown in the system, KBr-AgBr-HzO in Fig. 4. I n order to ascertain more definitely that no compound existed between potassium and silver bromides from precipitates, powder x-ray diffraction photographs were made of the eutectic mixtures of fused silver and potassium bromides and of precipitates from saturated solutions of both salts from 100°C to 7 j"C, 7 j"C to 5o°C and from 50°C to z j"C. These photographs show only the diffraction pattern of pure components, thus verifying the statement of Hellwig's previously referred to. The author wishes to express his thanks to Mr. R. B. Wilsey for the x-ray examination of the various mixtures and to Dr. S. E. Sheppard for advice and criticism. Rochester, K. Y . April, 7 1926.
A. Sandonnini and G. Scarpa: Atti Accad. Ljncei, 22 11, 519-520(1913).
