EQUILIBRIUM IN SYSTEMS CONSISTING OF LEAD HALIDES AND PYRIDINE' BY GEORGE W. HEISE
The object of this work is to study the systems lead iodide-pyridine, lead chloride-pyridine and lead bromidepyridine in the light of the phase rule. A compound of lead iodide and pyridine has been described by Pincussohn, l while various compounds of pyridine with lead chloride and lead bromide have been prepared and analyzed by Pincussohn, Classen and Zahorski, Reitzenstein3 and go ebb el^.^ However, no systematic quantitative study of the solubilities of these compounds in pyridine has hitherto been attempted. The present article is really a part of a series of investigations on the equilibria in systems of various salts and pyridine, carried on in this laboratory under the direction of Professor Kahlenberg .
Equilibrium between Lead Iodide and Pyridine The pyridine used was a purified (Merck) preparation, dried one week over fused caustic soda and then distilled. Only the fraction passing over between 114' C and 115' C under a barometric pressure of 742 mm; was employed. Lead iodide was prepared from pure lead nitrate (Merck) by precipitation from aqueous solution with pure potassium iodide. The lead iodide thus obtained was carefully washed with cold water, dissolved in boiling water and filtered hot. Pure lead iodide crystallized out on cooling. The supernatant liquid was then drained off, and the yellow flakes of lead iodide were dried, first between sheets of filter paper and Zeit. anorg. Chem., 14, 379 (1897). Ibid., 4, I O I (1893). Ibid., 18, 289 (1898). Ber. chem. Ges. Berlin, 28, 794 (1895). Jour. Phys. Chem., 12, 283 (1908); 13, 4 2 1 (1909); 14, 189 (1910).
George W . Heise
374
finally for several weeks in a vacuum desiccator. All of the work was done by diffused daylight or weak artificial light.. The solubility determinations were performed in a glass tube fitted with a glass stirrer. For determinations at or near room temperature, a large water bath was used as a thermostat. For determinations between room temperature and zero, the reaction tube was tightly fitted into one neck . of a three-necked Wolff bottle. The temperature was kept constant by admitting a carefully regulated stream of ice water through a second neck, while an equal amount was drawn off through the third. Freezing mixtures of snow and salt were used from o o down to -zoo C. For lower temperatures, solid carbon dioxide was necessary. For determinations above 30°, the reaction tube was carefully fitted into a large side-neck test tube provided with a reflux condenser, Boiling liquids in the outer tube kept the temperature constant. The following series of liquids was employed : ________ -~ ~ - ~-_~~ ~ _ ____ _ _ ~ Liquid
Ether Carbon bisulphide Acetone Methyl alcohol Ethyl alcohol Water Isobutyl alcohol
1
I I
I
Boiling point in degrees C
35 46
57 66 78 IO0
~
I 06
An oil bath was used as a source of heat in all determinations above 100' and also in securing temperatures lying between the boiling points of the substances above mentioned. Temperatures below -20' were read on a toluene thermometer. An ordinary mercury thermometer served for higher temperatures. I n all cases the solvent was stirred with an excess of salt for 5 hours, to ensure saturation. After the stirring, the very finely divided solid was allowed to settle. This generally required a t least an hour. The supernatant liquid was then siphoned directly into a weighing bottle. To hold back any
Equilibrium of Lead Halides and Pyridine
375
solid particles, the upper end of the siphon was carefully capped with a little cotton and muslin. The solubility of lead iodide proved t o be very slight and consequently the following procedure for determining the strength of the solutions was adopted: After the solution was weighed, the pyridine was carefully evaporated off over an oil bath heated t o about 1 5 0 ~ . The flask was thoroughly wiped, cooled and weighed. A number of blank tests with samples of known composition proved that this method yields satisfactory results. All the results recorded in the following table are the average of a number of concordant readings and, judging from the corresponding curve (Fig. I ) , they must be near the true values. Equilibrium between saturated solution and solid PbI2.3C,H,N _ -_.-_- -_
- _ _ _ ~
Temperature in degrees C
~
i
- 43.5 Freezing point of saturated solution - 37 - 28 - 20
f
_ _ _ _ _.~ ~
____
Grams PbI, is IOO grams of pyridine
-
9
0.166 0.168 0 . I75 0.186
0
0.200
3 6 (transition point)
0.225
I5 35 57 77 92 98 I05 I08 II2
~
0.215
I
I I
0.208 0 . I88 0.190 0.228 0 .'290 0.340 0.370 0.410 0.445
At - 4 3 . 5 O , solid pyridine and salt separate out together; this is the freezing point of the saturated solution.
376
George PV. Heise
This temperature marks a quadruple point; the four phases of the non-variant system in equilibrium are: solid pyridine, solid salt, saturated solution and vapor. The solubility increases very gradually from -43.5' to + 6 O . Between these temperatures, the solid in equilibrium with the saturated solution is PbI,.3C,H5N. It appears as minute, white crystals which are fairly stable even a t room temperature.
Fig.
I
Analysis of the solid showed 33.95 percent of pyridine, while the calculated amount in PbI,.3C,H5N is 33.96 percent. Though the solid forms a thick paste in pyridine, it can readily be dried on a porous plate. Between 6 O and 6 5 O the solubility remains almost unchanged, but from- 65' t o the boiling point of pyridine it rises steadily. Between these temperatures, the solid in equilibrium with the saturated solution is PbI,.zC,H,N. It does not differ from the first-mentioned compound in appearance, having the same finely divided form and peculiar
Equilibiium of Lead Halides nnd Pyridine
377
chalk-white color. The particles were too small to enable any conclusions to be drawn concerning their crystalline structure. Upon analysis, the solid was found to contain 25.6 percent of pyridine, which coincides with the calculated value for Pb12.2C,H,N. At 6' there is then a quadruple point at which vapor, saturated solution, PbI,.3C,H5N and PbI,.2C5H,N are in equilibrium. In the work on lead chloride and lead bromide, the apparatus and method were the same as described for lead iodide. Both salts were made by precipitation from solutions of pure (Merck) lead nitrate by means of the corresponding potassium halide, and in each case the product was purified b y recrystallization from hot water. Equilibrium between Lead Chloride and Pyridine The experimental results obtained in the case of lead chloride are given below. In each case, the reaction mixture was stirred a t least 3 hours t o ensure equilibrium in the system. Equilibrium between saturated solution and solid PbC1,.2C5H,N
-~ ~ _ _________
_
_
_
~
____ -____
I
Temperature in degrees C
- 20 0
+ 22
44 65 76 90 94 I02
Grams PbCI, in IOO grams of pyridine
0.303 0.364 0.459 0.559 0.758 0.893 1.07 I . I2 I
.31
The solubility rises more or less regularly with the temperature. As the curve (Fig. 2 ) indicates, there is but one solid in equilibrium with the solution. This solid has the composition PbCl,.zC,H,N, it separates out on cooling a hot, saturated solution in well-defined, needle-like crystals.
378
George W . Heise
Analyses showed a pyridine content of from 33-35 percent, the calculated amount for PbC1,.2C6H,N being 36.2 percent. The compound loses pyridine very rapidly in air, a fact which makes it difficult to obtain concordant 'analyses and would account for the discrepancy between the calculated and the observed values.
Fig.
2
There are three compounds of pyridine and lead chloride 4PbC1,.3C,HaN1 mentioned in the literature, viz., PbClz.C5H5N3, and 3PbC1,.C5H5N.2 None of these has been found in the course of this work and their existence consequently seems very doubtful. It is possible that the fact that the compound, PbC1,.2C5H,N, decomposes very rapidly in air misled previous investigators, for none of them mentioned the temperature a t which t.heir work was conducted. Classen and Zahorski: LOC.cit. Reitzenstein: LOC.cit.
Equilibrium of Lend Halides and Pyridine
3 79
Equilibrium between Lead Bromide and Pyridine F o r lead bromide the following results were obtained: Equilibrium between saturated s o h tion and solid PbBr2.3C,H,N
__ -~
_
_
_
~
Temperature in degrees C
'
- 26
i
- 10 - 5
1
!
0
1
I3
19 (transition~point) -
1
_
_
_
~
Grams PhBr, in IOO grams of pyridine I .02
0.89 0.84 0.800
0.661
-
Equilibrium between saturated solution and solid PbBr2.2C,H,N 26 45
0.583 0.661
64
0.800
77 95
0,969 1.33 1.44 1.56
IO0 10.5
Fig. 3
George W . Heise
380
As will be seen from the corresponding curve (Fig. 3), the solubility of lead bromide undergoes some rather remarkable changes with changes in temperature. Below I+', the salt becomes less soluble with rise in temperature, the solid in equilibrium with the saturated solution being PbBr,. 3C,H,N. On waFming a saturated solution, the solid separates out in minute crystals, showing by analysis a pyridine content of 37 percent. The calculated amount of pyridine in PbBr,. 3C5H5Nis 39.2 percent. The discrepancy of 2.2 percent is probably due to the fact that the compound is exceedingly unstable, losing pyridine very rapidly in the air, even a t temperatures 30' or more below the transition point. Above 19', the solubility rises rapidly until the boiling point of the saturated solution is reached. The solid in equilibrium with the solution is PbBr,.2C,HjN, analysis showing a pyridine content of from 28-30 percent. The calculated amount is 30.1 percent, and a s this compound is also very unstable, the agreement is probably as good as could be expected. A t 20' there is a transition point a t which PbBrz.3C,H,N, PbBr2.2C,H,N, saturated solution and vapor are in equilibrium with each other. Three compounds of lead bromide and pyridine have been mentioned previously, namely, PbBr,.C,H,N,' PbBr,. 2C,H,N' and gPbBr,.7CsH,N.' But one of these compounds, PbBr,.2C,H,N, w7as actually found in this work. . The fact that the compounds previously recorded show a smaller percentage of pyridine than those described in the present series of determinations would seem to point t o the explanation that the investigators analyzed samples that had lost pyridine of crystallization. Summary In this work, the equilibrium between pyridine and halides of lead has been studied. Lead iodide forms two crystalline compounds with pyridine, PbI,.2C,HjN and PbI,. Goebbels: LOC.cit.
Equilibrium of Lead Halides and Pyridine
381
3C,H,N. The latter is stable below 6 ', the former is the stable salt above that temperature. The crystals are minute in both cases. They are fairly stable and may be dried and analyzed very readily. Lead chloride forms but one compound with pyridine, PbC1,.2C,H,N, within the range of temperature of the into +IIO'. On cooling the vestigation, i. e., from -20' saturated solution, the compound separates out in welldefined, needle-like crystals. In air, the crystals are exceedingly unstable, losing pyridine very rapidly. Lead bromide forms two compounds with pyridine bePbBrz.3C,H,N, stable below ~ g ' , and tween -26' and IIO', PbBr,.zC,N,N, stable above the latter temperature. In both cases, the crystals are small and extremely unstable. It is rather interesting to note that the solubility of lead bromide rises sharply as the temperature is lowered below the transition point. Of the compounds mentioned above, the analyses of all but one, PbBrz.2C,H,N, are here reported for the first time. My thanks are due to Professor Kahlenberg, at whose suggestion and undef whose direction this work was done. Labo~atoryof Physical Chemistry, University of Il'isconsin, Madison, March, 1912
