SOLID POLYIODIDES O F CESIUM H. W. FOOTE, W. M. BRADLEY,
AND
MICHAEL FLEISCHER
Department of Chemistry, Yale University, New Haven, Connecticut Received Novem.ber 2i, 1982
It has recently been shown conclusively by Briggs, Greenawald, and Leonard (1) and by Briggs (2) that cesium iodide and iodine form two binary addition products having the formulas CSISand CsI4 and that the compound Cs15 does not exist, a t least above 25°C. Water solutions were used by these investigators in preparing the compounds a t 25"C., but as the latter were anhydrous, they are undoubtedly the only stable binary compounds which can form a t 25"C., independent of what solvent is used, or indeed of whether any solvent is used. On the other hand, Abegg and Hamburger (3) working on the system cesium iodide-iodine-benzene showed from solubility results that there was evidence of a higher polyiodide which they believed to be CsIs. They made no attempt t o determine whether a solvated ternary compound existed. Since it has recently been shown (4) that potassium iodide forms such a ternary compound with benzene we have investigated the system cesium iodide-iodine-benzene at 6°C. and a t 25°C. and also the system containing toluene in place of benzene. The cesium iodide used was an exceedingly pure sample which had been prepared by heating the higher polyiodide. 'The latter had been prepared by crystallization from water containing an excess of iodine. Benzene, toluene, and iodine were prepared by the methods described previously by Foote and Bradley (4). The apparatus and methods are also fully described in the same paper. T H E SYSTEM TOLUENE-CESIUM
IODIDE-IODINE
I n this system, qualitative tests showed that no solvated compounds were formed and that cesium iodide, like potassium iodide, is insoluble in the solvent. The investigation was therefore relatively simple. The original components were accurately weighed and by determining iodine in the solution after equilibrium had been reached, the composition of the solid residues, whether mixtures or pure compounds, could be calculated with probably greater accuracy than could be obtained by direct analysis. The results obtained are given in table 1. The results show clearly a t each temperature that there are three univariant points. A t each, the solubility is constant and the residues consist 21
22
H. W. FOOTE, W. M. BRADLEY, AND MICHAEL FLEISCHER
of a variable mixture of two solids; the two pure compounds, which are in equilibrium with solutions of varying composition, exist between the univariant point's. The average of the six results obtained for free iodine in Cs14is 59.35; and of the four results for CsI3 is 49.73. The corresponding calculated percentages are 59.44 and 49.42. The results confirm those of Briggs and his coworkers (1) a t 25"C.,using water as the solvent, and show that the same compounds also exist a t 6°C. No solvated ternary compound with toluene exists. TABLE 1 T h e system toluene-cesium iodideiodine T
T
= 26°C.
Iodine in solution
Iodine in residue
weight per cent
weight per cent
0.060 0.047
16.66 48.18
0.77 1.72
Unsolvated solid residue contains
CsI and Cs13 CsI and CsI3
= 6°C.
Iodine in solution
Iodine in residue
weight per cent
weight per cent
0.024 0,024
11.18 46.00
50.13 49.57
0.54 1.08
49.70 49.50
2.53 2.54 2.54
51.42 53.80 57.21
1.53 1.53
52.30 56.25
3.09 9.90 13.45 14.66
59.25 58.53 59.17 59.58
4.35 6.74
59.76 59.79
10.39 10.43
64.06 89.77
15.46 15.37
62.24 94.32
THE SYSTEM BENZENE-CESIUM
-
Unsolvated solid residue contains
IODIDE-IODINE
Qualitative tests showed that a t the iodine end of the system the solids contained benzene, showing the existence of a ternary compound. The simple method of calculating the composition of the residue which was adopted with toluene therefore could not be used in this case and it was necessary to analyze the residues, a t least when the pure solvated compound was present. The Schreinemaker method of determining the composition of the solids by the analysis of the wet residues was entirely unsuitable in this case on account of the position and form of the solubility curve. We were therefore forced to determine the composition of the compound by
NO.
TABLE 2 T h e system benzene-cesium iodide-iodine
1 1 ,
's"o"Lb",",d~
1
SOLID RESIDUB
Iodine
:&;$
I
Benzene
RESIDUE CONTAINS
T = 25°C. per cent
per cent
per cent
1 2
0.049 0.045
10 IO0 42.14
90.00 57.86
none * none*
CsI and cs13 CsI and cs13
3 4
0.77 1.34
49.38 50.00
50,62 50.00
none* none*
CsIa
5 6
2.35 2.34
53.15 56.68
46.85 43.32
none* none*
cs13and cs14 CsIa and cs14
7 8
3.89 5.27
59.40 59.40
40.60 40.60
none* none*
9 10
7.76 7.74
58.6 66.2
33.3 14.7
8.lt 9.lt
CSII and T. C.$ CsI4 and T. C.$
11 12
8.88 12.19
75.54 73.23
15.96 16.22
8.50$ 10.55$
T. C.$
14.09 13.83
79.9 91.8
12.5 5.1
7.6t 3.lt
T. C.! and Iz T. C.$ and IZ
13 14
1
T
=
per cent
cSI3
T.C.$
6°C.
15 16
0.022 0.024
12.07 47.42
87.93 52.58
none* none*
CsI and csI3 CsI and cs13
17 18
0.55 1.20
49.25 49.41
50.75 50.59
none* none*
CsIa
19 20
1.31 1.30
53.98 56.85
46.02 43.15
none* none*
21 22
2.00 3.41
59.37 59.16
40.63 40.84
none* none*
CsIa CSIl
23 24 25
3.80 3.80 3.81
59.8 67.3 71.31
39.9 27.1 20.15
0.37 5.6t 8.54t
csI4 and T.C.$ csI4 and T. C.$ CsIa and T. C.5
26 27 28
4.55 6.25 6.60
74.22 75.23 76.33
16.74 15.23 15.64
9.043 9.54$ 8.03$
T.C.$ T.C,$
29 30
8.59 8.58
81.2 92.3
11.7 4.8
7.lt 2.9t
1
.
cSI3
T. C.$ T. C. and Iz T. C. and 1 2
* Composition of the solid calculated as in the toluene system. Residue unsolvated. t Composition of the solid obtained graphically. 1Composition of the solid determined by analysis (benzene by difference). $ Ternary compound. 23
1
24
H. W. FOOTE, W. M. BRADLEY, AND MICHAEL FLEISCHER
analysis, after removing it from the solution and freeing it from the mother liquor by pressing rapidly between filter papers. The compound loses its benzene rapidly on standing. On the other hand, the color changes appreciably as it decomposes, so that it was not difficult to determine rather closely from the appearance when the mother liquor was removed. In analyzing the compound, free iodine was determined by titration with thiosulfate and cesium iodide by heating to remove free iodine and benzene. The latter was determined by difference. The composition of the residues a t the univariant points could be determined graphically with sufficient accuracy when the composition of the solvated compound was known. The results given in table 2 were obtained. The method used in determining the composition of the residues is indicated in each case. The data in table 2 show in an entirely satisfactory manner the existence of the two compounds Cs13 and Cs14 a t both 6°C. and 25°C. Besides TABLE 3 Showing the ratio CsI:I:C6& for each of the five residues in which the pure ternary compound was present. T h e ratios are calculated f r o m the analytical data in table 1 NO.
11 12 26 27 28
I
RATIO
CSI
I
CsHa
1.0 1 .o 1.0 1 .o 1.0
9.64 9.24 9.07 10.10 9.99
1.77 2.16 1.79 2.08 1.71
Average = 1.0
9.52
1.90
-
these two compounds, however, there is a third solvated compound. It was present in pure condition, as shown by the solubility results, in Nos. 11 to 12 at 25°C. and in Nos. 26 to 28 at 6°C. The ratio CsI: I : C6H6 has been calculated for each of these residues, and the results are given in table 3. In considering these ratios, it must be borne in mind that the material uked in the analysis was very finely divided, and was of necessity contaminated with iodine from the solution, since the benzene in the latter was exceedingly volatile and deposited iodine as it evaporated. The ratio CsI :C6H6 appears to be definitely 1:2, and allowing for the fact that the analytical results should be high in iodine, the ratio Cs1:I is nearly 1:9, so that the most probable formula, based on the above analyses, is CsIlo. 2C6Hs,in which the ratio CsI :I is less certain than the ratio CsI: CeHe. It is evident, however, that the ratio Cs1:I can be calculated just as in the unsolvated residues, provided the ratio of CsI: CaHa in the residue is known. Judging from the results on the unsolvated residues, this method should give more reliable results than direct analyses. In table 4 we give
25
SOLID POLYIODIDES OF CESIUM
the charges used, the calculated composition of the residue assuming the ratio CsI :CaH6= 1 :2, and the ratio CsI :I calculated from the composition of the residues. The ratio I/CsI is nearly 9, and the formula is therefore CsIlo.2C6H6 or 2CsI.912-4C6Hs.The writers are not unaware that this formula is most unusual and a priori highly improbable. The composition as determined by analysis, and as calculated in table 4, however, seems t o exclude any simpler ratio. The corresponding ternary compound of potassium has the formula K19.3C6H6and is therefore of different type, the only evident relationship being that the cesium compound may be derived from the potassium by the substitution of one atom of iodine for one molecule of benzene. TABLE 4 Stiowing the weight of each component in the original mixtures; the calculated per cent of cesium iodide and of iodine in the residues, assuming the ratio CsI:CeHs = 1:2; and the molecular ratio 1:CsI in the residues NO.
11 12 26 27 28
CESIUM IODIDE
IODINE
BENZENE
CALCULATED CSI I N RESIDUE
CALCULATED 12 IN RESIDUE
gram8
gram8
grams
per cent
per cent
0.67 0.67 1.22 1.22 1.22
6.12 7.02 6.70 7.168 7.637
30.833 30.795 30.134 30.488 29,917
15.85 17.30 16.82 17.09 16.20
74.62 72.30 73.06 72.62 74.05
1
RATIO
CUI 1
9.63 8.55 8.89 8.70 9.35
Average = 9.02
At any univariant point in the systems investigated two solids are present, and their dissociation pressure of iodine is equal to the partial pressure of iodine in the solution. The latter is closely proportional to the molar fraction of iodine in solution, as all the solutions are dilute. The dissociation pressure of any polyiodide compared with pure iodine (or the
C in which C is activity) is therefore given very closely by the expression CLl the molar concentration of iodine in solution a t a univariant point, and COis the corresponding value for pure iodine. The latter is practically identical with that at the univariant point where iodine is one of the solid phases, as cesium iodide is insoluble. C are given at In table 5, the necessary data and the values of the ratio -
CO 25OC.and at 6°C.for both the benzene and the toluene solutions, so far as
the data are comparable. The data for the point in the benzene system in
26
H. W. FOOTE, W. M. BRADLEY, AND MICHAEL FLEISCHER
which the solids consist of CsIG and the ternary compound have been omitted, as there is no comparable point for toluene.
C
The differences between comparable values of - for the benzene and CO
toluene systems is of the same order as the error in determining the compoTABLE 5 Values of the ratio C/C, at 25°C. and at 6°C. WEIQET PER CENT OF IODINE I N SOLUTION
SOLIDS PRESENT
Benzene.
Iz and T. C.
I2 and &I4 Cs14 and CSIS CsIs and CsI
If and T. C. Cs14 and CsL CsIs and CsI
I
= 25°C.
4.75 (CO) 0.73 0.014
0.155
0.0030
Toluene. T = 25°C. 15.42 2.54 0.054
?::: 0.023
Toluene.
Iz andCsI4 cs14 and CSIB CsIa and CsI
T
13.96 2.35 0.047
Cs14 and CSIS CsIs and CsI
C
MOLE PER CENT OF IODINE I N SOLUTION
10.41 1.53 0.024
1
T
6.20 (Co) 0.94 0.020
0.151 0.0032
2.81 (Co) 0.41 0.0071
0.145 0.0025
= 6°C.
4.04 (Ca) 0.56 0.0087
0.139 0.0022
sition of the solutions. Assuming the vapor pressures of iodine a t 6°C. C and 25°C. are 0.0546 mm. and 0.313 mm.,l and the values of - in table 5,
co
the dissociation pressures of Cs14a t 6°C. and 25°C. are respectively 0.00775 mm. and 0.0479 mm. For Cs13, the values are 0.000131 and 0.000970 mm. From these values, the heats of dissociation of CsI4 and CsI3, calculated by van’t Hoff’s equation, are found to be respectively -15,800 and - 17,500 calories. These values, as well as those for the dissociation pressures, are based on the solubility data. Since some of the solubilities are 1
By interpolation from the data given in the International Critical Tables.
SOLID POLYIODIDES O F CESIUM
27
very low, the relative error in them causes a considerable error in these calculations. This error appears to be less than f 5 per cent. SUMMARY
The systems cesium iodide-iodine-benzene and cesium iodide-iodinetoluene have been investigated a t 6°C. and at 25°C. With toluene, no ternary compound exists, and the two binary compounds found were Cs13and CsI4, confirming the results of Briggs and his coworkers at 25°C. With benzene, in addition to the two binary compounds, there is also a ternary compound. The results on the dissociation pressures of the binary compounds show close agreement when calculated from the data on the benzene and on the toluene systems, REFERENCES (1) BRIGGS, GREENAWALD, AND LEONARD: J. Phys. Chem. 34,1951 (1930). (2) BRIGGS:J. Phys. Chem. 34,2260 (1930). (3) ABEGGAND HAMBURGER: Z. anorg. Chem. 60,403 (1906). (4) FOOTE AND BRADLEY: J. Phys. Chem. 36, 673 (1932).
