640
H. W. FOOTE AND MICHAEL FLEISCHER
and Fraser (7). The older values were used so that the dissociation pressures calculated would be consistent with those given in our earlier papers. SUMMARY
The systems rubidium iodide-iodine-toluene and rubidium iodideiodine-benzene have been studied at 6' and 25°C. The binary compound rubidium triiodide is the only compound stable a t both temperatures in the toluene system. In the benzene system two ternary polyiodides, RbI. 61. 4CsH6and RbI .71.4C&, are stable phases a t both temperatures. The dissociation pressure of rubidium triiodide a t both temperatures has been calculated from the solubility results. REFERENCES (1) (2) (3) (4) (5) (6) (7) (8) (9)
ABEGGA N D HAMBURGER: Z. anorg. Chem. 60,403 (1906). BRIGGSA N D PATTERSON: J. Phys. Chem. 36, 2621 (1932). FOOTE A N D BRADLEY: J. Phys. Chem. 36, 673 (1932). FOOTE, BRADLEY, AND FLEISCHER: J. Phys. Chem. 37, 21 (1933). FOOTE AND CHALKER: Am. Chem. J. 30, 561 (1908). FOOTE AND FLEISCHER: Unpublished data. GILLESPIEAND FRASER: J. Am. Chem. SOC.68, 2260 (1936). GRACE:J. Phys. Chem. 37, 437 (1933). WELLSAND WHEELER:Am. J. Sci. 44, 43 (1892).
ADDITION COMPOUNDS OF IODINE WITH ALKALI BROMIDES AND THIOCYANATES H. W. FOOTE
AND
MICHAEL FLEISCHER
Department of Chemistry, Yale University, New Haven, Connecticut Received June $0, 1969
Addition compounds of iodine with the iodides of potassium (4), ammonium (5), rubidium (7), and cesium (6) have been studied in this laboratory by the solubility method. It seemed desirable to extend our work to study the addition of iodine to alkali bromides and thiocyanates in order to widen our'knowledge of the stability relations of this type of addition compound. We used benzene and toluene as solvents, since our previous work had shown that only binary polyiodides are formed with toluene as solvent, but that there are a number of solvated ternary compounds composed of an iodide, iodine, and benzene. 1 This article is taken in part from a dissertation submitted by Michael Fleischer to the Faculty of the Graduate School of Yale University in partial fulfillment of the requirements for the degree of Doctor of Philosophy, June, 1933.
ADDITION COMPOUNDB OF .IODINE:
641
The experimental procedure used and the method of purification of the iodine, benzene, and toluene have been described (4). The bromides were recrystallized from water solution and dried a t 105OC. Their purity was checked by determination of the bromide content by precipitation of silver bromide. The thiocyanates were recrystallized from water solution and dried over phosphorus pentoxide. Analyses by titration with standard silver nitrate solution gave results close to the theoretical values for the thiocyanate content of each salt. ADDITION OF IODINE TO ALKALI BROMIDES
The only addition compound of the alkali bromides and iodine reported in the literature is CsBrIz, prepared by Wells (9) by crystallization from water or alcohol solutions. Attempts to prepare analogous compounds with potassium bromide and rubidium bromide failed (10). The cesium compound was also obtained by Cremer and Duncan (3). The only equilibrium study on such systems was made by the latter workers, who studied the system potassium bromide-iodine-carbon tetrachloride a t 25OC. and found that no compounds were formed. TABLE 1 The system rubidium bromide-iodine-benzene at 6'C. IODINE IN BOLUTION
IODINE IN REBIDUE
BOLJDB PREBENT
weigh1 pcr c d
8.67 8.70 8.74
We have studied the systems sodium bromide-iodine-benzene, potassium bromide-iodine-benzene, rubidium bromide-iodine-benzene, thallous bromide-iodine-benzene, cesium bromide-iodine-benzene, and cesium bromide-iodine-toluene a t 6OC. The system with thallous bromide was included because thallous iodide adds iodine to form two higher iodides, Tl& and TlIs (1). No compounds were found in any of the systems except the two systems containing cesium bromide. The data for the four other systems are essentially identical and only that for the rubidium bromide system will be given (see table 1). These results show clearly that no compounds are formed. In these systems and in the two cesium bromide systems given in table 2, the composition of the solid residue was calculated in each case from the known original mixture and the analytically determined concentration of iodine in the solution after equilibrium had been reached, as qualitative tests showed that no solvated phases were present and that the bromides were insoluble in the solvents. The results show clearly that only one
642
H. W. FQOTE AND MICHAEL FLEIscHER
compound, CsBrIz, is stable in both systems. The average of the five results in which the pure compound was present is 54.48 per cent iodine; TABLE 2 The system cesium bromide-iodine-solvent
Toluene; t = 6°C. ~
weight per cent
weightpercent
0.99 0.97
24'20
47.48
/i 1
CsBr and CsBrIz
1.51 9.63 10.41 10.31 Benzene: t = 6°C.
9 10 11
1.43 5.82 7.74
TABLE 3 Showing the ratio C/CO i n toluene and i n benzene
1
SOLID0 PEPBIN&
1
IODWBWBOLUTION
I
C/M
Toluene; t = 6°C.
In and CsBrIz. . . . . . . . . . . . . . . . . . . . . . . .
.I
CsBrI, and CsBr.. . . . . . . . . . . . . . . . . . .
w&ht pa cent
mole pa cml
10.36
4.026 (CO) 0.358 (e)
0.98
~
Benzene; t = 6'C.
1 ::: I
and CsBrIz.. . . . . . . . . . . . . . . . . . . . . . . CsBrIz and CsBr.. . . . . . . . . . . . . . . . . . . . 12
2.845 ( C O ) 0.264 (c)
1
0.0892
the theoretical value is 54.41 per cent. No ternary compound containing benzene was formed, although such a compound was found in the system cesium iodide-iodine-benzene (6).
ADDITION COMPOUNDS OF IODINE
643
The dissociation pressure of CsBrI2 a t 6°C. has been calculated from these results by the method previously explained (6). The values of C/CO for both solvents are given in table 3. From the average of these values of C/CO, the dissociation pressure of CsBrIt a t 6°C. is calculated to be 0.00486 mm., taking the vapor pressure of iodine a t 6°C. to be 0.0546 mm.2
ADDITION OF IODINE TO ALKALI THIOCYANATES
The remarkable similarities in the behavior of thiocyanates to that of iodides, bromides, and chlorides have often been pointed out in the past. Iodides and thiocyanates, especially, have been found to behave in almost the same fashion towards many reagents, a striking example being the formation of addition compounds with sulfur dioxide by both iodides and thiocyanates (8). Some years ago, in this laboratory, Dr. Joseph Fleischer observed that aqueous solutions of thiocyanates were remarkably good solvents for iodine. The solutions were not stable, however, as a slow reaction gave free sulfur, some thiosulfate, and other products. This reaction is well known and has been studied (2) for dilute iodine solutions, but, as far as we know, there is no mention in the literature of the fact that the solubility of iodine in thiocyanate solutions is of the same order of magnitude as its solubility in iodide solutions. It seemed probable that the alkali thiocyanates would form addition compounds with iodine similar to those formed by the alkali iodides. The reaction mentioned above precludes the study of such compounds in aqueous solution, but it was thought possible that the reaction would be sufficiently slow in non-aqueous solvents to allow solubility equilibrium to be reached before the side reaction had used up much iodine and thiocyanate. Accordingly, we have studied the systems sodium thiocyanateiodine-toluene, sodium thiocyanate-iodine-benzene, potassium thiocyanate-iodine-toluene, potassium thiocyanate-iodine-benzene, ammonium thiocyanate-iodine-toluene, and ammonium thiocyanate-iodinebenzene at 6°C. The extent to which the side reaction was consuming iodine was determined for each system by making up mixtures such that the solution was nearly, but not quite, saturated with iodine, rotating for 4 days in the thermostat, and analyzing the solution for iodine. In every case, there was a deficiency of iodine. In the four systems with potassium thiocyanate and sodium thiocyanate, from 2 to 3 per cent of the total iodine had been used by the side reaction; in the two systems with ammonium thiocyanate about 8 per cent of the iodine had reacted with the thiocyanate. The results obtained are therefore subject to some uncertainty. However, the results showed clearly that no compounds were formed in any
* By interpolation of
the data in the International Critical Tables.
644
H. W. FOOTE AND MICHAEL FLEISCHER
of the six systems studied, excepting the system potassium thiocyanateiodine-benzene. The data obtained in the five other systems are essentially identical and need not be' given in detail, the data given below for the system po-
TABLE 4 The system potassium thiocyanate-iodine-toluene at 6°C. IODINE IN B O L U ~ O N
IODINE IN REBIDUE'
w&At par cent
w&At per cent
10.21 10.29 10.18
38'60 63.97 92.48
BOLID PEAAM PRMEHT
}
KSCN and 1,
* Calculated from the amount of iodine originally added and the analytically determined concentration of iodine in the solution. The results have not been corrected for the iodine used by the reaction with thiocyanate. TABLE 5 The system potassium thiocyanate-iodine-benzene at 6°C. Y-OD
OF
BOLID REBIDUE
BOLlD PEASE8 PREBENT
COYPOBI-
I
1
KSCN
I2
weight puunt
weight percent
par cmt
wdqht p a cent
1
7.34
1m.o
0.0
0.0
2 3 4 5
8.15 7.93 8.11 7.91
78.63 27.89 22.74 12.86
15.16 51.14 54.79 61.80
6.21 20.97 22.47 25.34
6 7 8
8.30 8.45 8.59
8.47 8.12 7.83
65.78 63.61 64.09
l::;;
28.08
c
9 10
8.70 8.69
7.67 6.60
67.67 72.24
24'66 21.16
1I E
w&At
___ -~ a* b b b b
KSCN
I} 1)
KSCN and pound
ternary com-
Ternary compound
Ternary compound and It
* a, Residue unsolvated, composition calculated as in toluene system; b, composition of residue obtained by algebraic extrapolation; c, composition of residue obtained by analysis. tassium thiocyanate-iodine-toluene being typical of all five. It is evident that there is no compound formation in this system. Qualitative tests showed that the solid residues at the iodine-rich end of the system potassium thiocyanate-iodine-benzene were solvated. As
645
ADDITION COMPOUNDB OF IODINE
shown by tmheresults in table 5, a ternary compound is formed in this system. It resembles those formed by the alkali iodides, but is much less stable. At a temperature below room temperature and estimated to be approximately 12°C. it melts incongruently and loses both iodine and benzene very rapidly. This increased the difficulty of analyzing the compound, as samples for analysis had to be dried and weighed extremely rapidly. The residues from Nos. 6, 7, and 8 were analyzed for available iodine by titration with thiosulfate solution, and for potassium thiocyanate by TABLE 6
Compound formation of R X compounds m'th iodine at 8°C. i n the presence of benzene
R
x-I
-
X
X-WN
Na . . . . None
None
None
K . . . . . No binary compound KI.81.3CdIa
No binary compound KSCN.61.4CsHs
None
None
Noqe
"411 N o ternary compound
"4..
-
Br
Rb .... RbIi S.bI.GI.4CsHe RbI. 71 * 4C&
None
cs.,. . car*;CSIC
CsBrIl No ternary compound
CSI *QI*2csHs T1 . . . . TlsI,; TI18 N o ternary compound
None
-
evaporating with sulfuric acid and igniting to potassium sulfate in a current of ammonia gas. The molecular composition of the' ternary compound from the analyses is as follows: nmsu. NO.
6 7 8
COMPOlITION OF TEBNABY COMPOUND
KSCN 5.951 :3.79CsHs KSCN :6.001:4.33CdIa KSCN:6.271:4.46CdIs
Assuming that the ratio KSCN: C a s in the residue is 1 :4, the composition of the residue in each of these three experiments has been calculated from
646
H. W. FOOTE AND MICHAEL FLEISCHER
the original charges and the analytically determined iodine concentration in the solution, with the following results: u r n " 1:KSCN
11
wsioW ga cent
weight w Cant
8.28 8.27 8.26
6 7 8
65.11 65.16 65.24
6.01 6.03 6.06
These results confirm the probable formula derived from the analyses. The compound KSCN .61.4CsHe is analogous to RbI .61.4CsHe, found by us (7) to be one of two ternary compounds of rubidium iodide, iodine, and benzene. It is improbable that the error due to the side reaction could be large enough to cause much error in the formula determined, although the experiments in which both potassium thiocyanate and the ternary compound were present (Nos. 2 to 5 ) show much more variation than usual in the determined iodine concentration in the solution. Table 6 gives a comparison of the results obtained with iodides, bromides, and thiocyanates at 6°C. It seems safe to predict that rubidium thiocyanate and cesium thiocyanate will form ternary compounds with iodine and benzene, and that cesium thiocyanate will probably form a binary compound with iodine. It is also possible that thallous thiocyanate will form addition compounds with iodine. SUMMARY
A study at 6°C. of systems composed of an alkali bromide (or thiocyanate), iodine, and benzene (or toluene) has shown that sodium bromide, potassium bromide, ammonium bromide, thallous bromide, sodium thiocyanate, and ammonium thiocyanate form no compounds with iodine. Cesium bromide adds iodine to form CsBrIz, and potassium thiocyanate The dissociation forms the solvated ternary compound KSCN .61 4C&. pressure of CsBrIe 6'"C. has been calculated from the solubility results.
tt
e
REFERENCES (1) ABEQQAND MAITLAND: 2. anorg. Chem. 60, 403 (1808). (2) ANQELESCU AND POPESCW: 2. physik. Chem. AlM, 258 (1931). (3) CREMER AND DUNCAN: J. Chem. SOC.1931, 1857, 2243. (4)FOOTEAND BRADLEY: J. Phys. Chem. 37, 673 (1933). (5) FOOTEAND BRADLEY: J. Phys. Chem. 37, 29 (1933). (6) FOOTE,BRADLEY, AND FLEISCRER: J. Phys. Chem. 37, 21 (1933). (7) FOOTEAND FLEISCHER: Unpublished data. (8) FOOTE AND FLEISCHER, J.: J. Am. Chem. SOC. 63, 1752 (1931);64, 3902 (1932). (9) WELLS:Am. J. Sci. 43, 17 (1892). (10) WELLS AND WHEELER: Am.J. Sci. 43,475 (1892).
