THE POLYIODIDES OF RUBIDIUM I. Iodine and Rubidium Iodide BY T. R. BRIGGS AND E. 8. PATTBRSON
Temperature-composition data for the system iodine and potassium iodide and for the system iodine and cesium iodide, a t a pressure of approximately one atmosphere, have been reported in earlier papers' in this series. In the first of these systems, no solid compound (such as KIg) was found in equilibrium with the melt, whilst in the second system the compounds (polyiodides) cs13and Cs14were definitely shown to be produced.2 The present communication presents the data for the system iodine and rubidium iodide, which system, as it will appear in the sequel>turns out to be intermediate as regards the other two. The experimental technique employed in this investigation was the same as that described in the earlier papers and needs no further comment. The data required for the construction of the phase diagram were obtained from temperature arrests in the cooling curva, from the boiling points of mixtures of known composition and from analyses of saturated liquid phases (melts). The method used in the analysis will be found in the earlier papers. The iodine was purified by being sublimed and it was dried carefully before it was used. The rubidium iodide (from Kahlbaum) was recrystallized once from water and then dried. The combined iodine in the product was then determined by analysis and it was found to be 97.8 per cent of that required by the formula RbI. The salt was thus considered to be sufficiently pure for the purpose of the investigation. The various types of data which were obtained are presented in Tables I, I1 and 111. The temperature-composition diagram is shown in Fig. I. The solid triiodide RbIs certainly exists below 1 8 8 O , but there is no indication of a higher polyiodide such as RbI4, analogous with CsIa. The triiodide melts incongruently at about I S S O , decomposing into R b I and iodine above this temperature. The solubility of R b I in iodine, like that of KI and of CsI, is almost independent of the temperature; and because of this, no temperature arrest due to the separation of solid RbI was discernible in the cooling curve for Mixture 1 2 in Table I. The (constant) boiling point of melts saturated with R b I lies a t about 238' and a special determination of the eutectic point made it possible to place the latter at 80.8' and 17.4mole per cent RbI. Briggs and Geigle: J. Phys. Chem., 34, 2250 (1930); Briggs: 34, 2260 (1930). * Cf. also the ternary system iodine, cesium iodide and water. Briggs, Greenawald and Leonard: J. Phys. Chem., 34, 1951 (1930).
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T. R. BRIGGS AND E. 8. PATTERSON
TABLE I Temperature Arrests ("C) Mixture Number
Mole Per Cent RhI
1st Arrest
I
0
2
5
114 108.8
3 4
IO
100.0
15 I7
87 . o 81.8 I03 134 749 167 178
5 6 7
20
8
25
9
30 35
IO
40
I1
42.5 45 47.5
I2
13 I4 15 16 '7 18 I9
50 52.5
55 60 65 70 85
20
2nd Arrest
none 80.6 80.8 80.8 80.8 80.7 81 . o
81 . o 80.0 81 . o 81.5 79.0 79 . o
185
187.2 187.8 I88 187 ' 5 187 ' 5 189
none none none none none none none
I88 I90 189
TABLE I1 Composition of the Saturated Liquid A. Original Mixture containing 42 Mole Per Cent RbI Temperature Mole Per Cent RbI
81 (eutectic) 142 160 I75
B. Original Mixture containing 55 Mole Per Cent RhI Temperature Mole Per Cent RhI
17.44 26.35 32.16 37.60
I95 206 220
238.5 (B. P.)
44.71 45.23 45.62 46.23
TABLE I11 Boiling Points ("C) Mixture Number
Mole Per Cent RbI
Boiling Pdint
Mixture Number
Mole Per Cent RhI
Ib zb
0
3b 4b 5b
184 187 I95
6b
IO
30 40
50 60 70 80 90
20
207 225
*;Constant, others rising with time.
7b 8b 9b rob
Boiling Point
*236 *238.5 *238. j *238 *239
THE POLYIODIDES OF RUBIDIUM
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Wells, Wheeler and Penfield,’ in their celebrated investigation of the polyiodides, reported the existence of Rb13 as solid phase in aqueous systems of iodine and rubidium iodide. They were unable to prepare a higher polyiodide of rubidium, such as RbI6, analogous with the higher polyiodide of cesium, to which they had assigned the formula CsIsa2 They stated that the triiodide
FIQ.I The System I t -RbI at agroximately
I
atm.
“melts” a t 194’ in an open tube and at 190’ in a closed tube, and that it “whitens” at 270’ because of the loss of the polyhalide iodine. The “melting” point of Wells and his coworkers is evidently the transition point (188’) for the system Rb13, RbI and melt. Their temperature of whitening, however, is unnecessarily high, for the melt saturated with RbI boils at 238’ and the vapor must consist virtually of pure iodine. The present investigation thus confirms in all essential respects, the findings of Wells, Wheeler and Penfield. It also confirms the work of Foote and Chalker? who showed that a single polyiodide, presumably Rb13, exists as solid phase in the ternary system iodine, rubidium iodide and water at 2 5 ’ . Abegg and H a m b ~ r g e r in , ~ their work on the ternary system containing benzene, reported the higher polyiodides Rb17 and RbI, in addition to the Am. J. Sei., (3), 43, 475; 44, 43 (1892). a Cf. Briggs, Greenawuld and Leonard: J. Phys. Chem., 34, 1951(193o);Briggs:34,2260 l
( 1930). 8
Am. Chem. J., 39, 561 (1908). Z. anorg. Chem., 50, 403 (1906).
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T. R. BRIGGS AND E. S. PATTERSON
triiodide RbI3. There is no indication of these higher polyiodides in the binary system, and if they do exist in the ternary system, it seems reasonable to suppose that they must be ternary compounds containing benzene.' In that part of their paper which deals with the binary system iodine and potassium iodide, Abegg and Hamburger stated that a mixture of iodine and rubidium iodide, corresponding in composition to the supposed compound RbI,, melted a t 81.3'. It is now apparent that this is the eutectic temperature and not the melting point of a compound; Abegg and Hamburger were not dealing with RbI,, as they supposed, but with a mixture of iodine and RbIa. They made the same mistake here thac they made in the system iodine and potassium iodide-that is to say, they took the eutectic temperature to be the melting point of a polyiodidc. The probable explanation of this curious error and the consequences which it produced have been discussed at length in the paper by Briggs and Geigle. In connection with the work reported in this paper, various other systems containing a halogen and an alkali-metal halide have been investigated. These systems were: iodine and sodium iodide, iodine and sodium chloride, iodine and sodium bromide, iodine and potassium chloride, iodine and potassium bromide, bromine and potassium bromide. The temperature arrests and the boiling points of mixtures of known composition showed that in every case the components are immiscible. The same is true of calcium iodide and iodine as a binary system, but it is interesting to find that investigation of the ternary system iodine, calcium iodide and water, now under way in this laboratory, shows that iodine is extremely soluble in mixtures of calcium iodide and water, and that at least one hydrated calcium polyiodide is present as solid phase in the ternary system a t 2 5 ' . We thus find no polyiodide of calcium in the binary system and not even any dissolving of calcium iodide by liquid iodine, yet the miscibility is extraordinarily great in the ternary system containing water and there is evidence of a polyiodide when water is the third component. This is strong confirpation of the position taken by Grace and accepted by Foote and Bradley.
Summary The results of this investigation may be summarized as follows: The temperature-composition diagram for the system iodine and I. rubidium iodide has been determined from 60' up to the boiling point of the saturated melt (238'). The solid phases in contact with melt are iodine, rubidium triiodide 2. (Rb13) and rubidium iodide. The triiodide melts incongruently at 188". No indication of a higher polyiodide, analogous for example with cesium tetraiodide, was obtained. 3. Iodine and rubidium iodide, as one would expect, is thus intermediate between iodine and potassium iodide (solid phases iodine and KI) and iodine and cesium iodide (solid phases iodine, CsL, CsI3 and CsI). Cornell University, June, 198.8. 1
Cf. Grace: J. Chem. Soo., 1931, 594; Foote and Bradley: J. Phys. Chem., 36, 683
(1932).
