SOLID POLYIODIDES OF POTASSIUM* BY H. W . FOOTE AND WALTER 11. BRADLEY
The literature of this subject has been summarized quite recently in two papers' which appeared almost simultaneously while our own work was in progress. The conclusions in the two papers are somewhat at variance, particularly in regard to compound formation from water solution. It has been shown by Briggs and Geigle2that the two components, potassium iodide and iodine, exhibit a simple fusion diagram and that no polyiodide forms at temperatures where fusion occurs. At lower temperatures, probably the simplest way to determine whether a polyiodide forms or not is to introduce a liquid component and determine the solubility relations. If the liquid component does not enter into the composition of the solid phases, the solubility results are sufficient to show whether or not a two-component polyiodide can exist at the temperat'ure of experiment. Using the solvent tetrachlorethane, Grace has concluded that potassium iodide and iodine do not form a binary addition product at 2 5 ' . The liquid component may, however, unite with the others to form a solvated ternary addition product, when no binary compound exists. The results of Grace, at z s ' , indicate that water forms two such hydrated addition products, while Bancroft, Scherer and Gould, with less experimental evidence, claim that no addition products of any kind form in this system. Benzene also forms such a ternary compound, which we shall discuss in more detail below. We have determined two solubility isotherms in each of t,he three systems of potassium iodide and iodine with toluene, chloroform, and benzene. Our results with the first two solvents confirm the results of Grace that potassium iodide and iodine form no two-component addition product at z jo,and extend With benzene, we have found, as did Grace, that a this conclusion to 0.7'. ternary compound exists, but' our results do not confirm the composition of the compound found by him. Materials and Methods The three solvents, toluene, chloroform, and benzene were dried over phosphorus pentoxide and distilled before use. Potassium iodide and iodine were of suitable purity and were merely dried thoroughly. In each system, weighed amounts of the three components in varying proportions were introduced into bottles with specially ground stoppers. Before placing the bottles in the thermostat, the stoppers were coated with * Contribution from the Department of Chemistry, Yale University. Bancroft, Scherer and Gould: J. Phys. Chem., 35, 764 (1931); N. S. Grace: J. Chem.
1
SOC.,1931, 594. 2
Briggs and Geigle: J. Phys. Chem., 34,
2250
(1930).
H. W. FOOTE AND WALTER M. BRADLEY
674
a quick-drying enamel which was not attacked by the three solvents used, and then dipped in melted paraffin, to make certain that no water entered the bottles from the thermostat. As potassium iodide is extremely insoluble in all three solvents, it was anticipated that equilibrium might be reached slowly if addition products formed. The two solids were therefore finely pulverized and either glass pearls, glass rods, or pieces of broken porcelain introduced in the bottles to promote reaction between the solids and prevent lumps from forming. This precaution was probably only necessary with the benzene solutions. The broken porcelain was preferable to the glass pearls or rods. With porcelain we never found any difficulty in reaching equilibrium after three days of shaking, though most of the experiments ran considerably longer. Solutions for analysis were removed through a filter of glass wool and iodine determined in the weighed sample by titrating with sodium thiosulphate. Preliminary determinations showed that the amount of potassium iodide in all solutions was negligible. To determine the composition of the solid residues in equilibrium with the solutions, either of two methods was used, depending on conditions. I n the systems with toluene and chloroform, qualitative tests showed that the residues were not solvated and the composition of the residues could be obtained, either graphically or by calculation, when the composition of the solution and the gross composition of the original mixtures were known. In the system containing benzene, the composition of those residues which consisted of the pure compound alone was always obtained by analysis. The Schreinemakers method of analyzing wet residues was unsuitable in this case on account of the position and form of the solubility curve. The empirical composition of mixtures of two solids in this system could be determined graphically with sufficient accuracy as in the other systems, when the Composition of the addition product was known. The isotherms at 0.7' and a t 6' were carried out in a low-temperature thermostat which has recently been described.' Following are the results with toluene and chloroform. All data represent per cent by weight. System : CsH6CHy-KI-12 Temperature, z jo "/o I o In Residue
% I* in Solution
yo IZ in Residue
15.89
98.04
9.12
94 47
15.93 15.76 15.76
91.04 82.56 54.02
9.11 9 15 9 10
90 59 50 7 0 40 66
16.01
35.14 8.25
15.88 1
Temperature, 0.7'
yo l2 in Solution
Foote and Akerlof: Ind. Eng. Chem., Anal. Ed., 3, 389 (1931)
67.5
SOLID POLYIODIDES O F POTASSICX
System: CHCla-KI-12 Temperature, Zj" yo I ZIn Solution Iz in Residue
L~
Temperature, o 7' Is in Holutlon 5 I2 in Residue
2.95
95.54
1.20
2.99
9 0 . I3
1.21
95.47 90.95
2.98 2.97
56.08 21.18
1.28
jq.62
1.20
14.77
The constant solubility with widely varying residues s h o w that potassium iodide and iodine form no two-component addition product at either temperature investigated. The results at 2 5 ' confirm those of Grace obtained with tetrachlorethane. System: C6H6-KI-12.-This system was first investigated by Abegg and Hamburger' who showed by solubility determinations that one or more addition compounds formed. They had difficulty in reaching equilibrium, however, and did not suspect that a solvated compound formed. Grace has recently investigated the system again and found one ternary addition product. He analyzed only one sample of the compound, and derived the formula KI.312.zCaHe.Our own investigation was well advanced when the work of Grace appeared, and as our results did not entirely agree with his, we have taken some care to corroborate them. We found at 25' and 6' that only one addition product forms, but it appears to have the composition KI.412.3CoH6. A series of solubility determinations a t 6' and 25' on mixtures of the components in widely varying proportions showed that at bot'h temperatures there were two univariant systems. As the gross composition of each original mixture was known, the ratio of potassium iodide to free iodine in each residue could be determined graphically with sufficient accuracy after the composition of the addition product had been determined by analysis. We give below (Table I) the results obtained. The solubility shows the parts by weight of iodine in IOO parts of solution, and the molecular ratio gives the ratio of free iodine to potassium iodide in the residues. The residues also contained benzene of crystallization due to the presence of the addition product which was present in all residues. The data in Table I show, at each temperature, that a solid addition product exists, but appear to exclude the possibility of a solvated compound in which the ratio of KI:12 is I :3, which is the ratio found by Grace. To obtain the composition of the pure addition product, a series of mixtures was prepared in such proportions that the solubility fell between the two univariant points. After analyzing the solutions, the solids were rapidly freed from adhering solution by pressing between filter paper, and analyzed. Free iodine was determined by titration, and potassium iodide by heating t o remove free iodine and benzene. Benzene was determined by difference, Ahegg and Hamburger: Z. anorg. Chem., 50,403 (1905)
676
H. W. FOOTE AND WALTER M. BRADLEY
TABLEI System: CsHs-K1-12 Showing the solubility at the two univariant points at each temperature and the molecular ratio of free iodine to potassium iodide in the residues Temperature, 25' Molecular Ratio, Solution I z : KI, in Solid
5% 11 in 9.07 9.16 8.99 9.07 9.15 9.03 9.09 9.12 9.08 9.12 9.11 9.12 8.99
.03 .04 .34 .83 I .89 2,26 2.45 2.62 2.79 3.34 3.65 3.74
14.22 1 4 . I9 14.18
4.27 4.90 8.45
I.20
Temperature, 6' Molecular Ratio, Solution Iz : KI, in Solid
% 11 in
4.77 4.65 4.69 4.65 4.64
0.26 2.42
8.54 8.60
4.46 11.95
2
.go
3.31 3 ' 56
We give below, in Table 11, the results obtained on every residue analyzed, including one which was allowed t o stand too long in the air and had begun t'o lose benzene.
TABLEI1
Showing the composition of the solutions with which the ternary compound is in equilibrium, and the analyses of the compound Analyses of Compound % IZin Solution 7 0 KI 7c 1 2 70 C6Hs Temperature, 6" I 2
3
4* 5 6
5.26 5.80 6.13 6.30 6.76 7.53
11.65 11.81 11.56 12.46 12.13 11.38
71.82 70.98
16.53 17.21
71. I7
17.27
75'70 71.81 71.45
11.84 16.06 17.17
11.80 11.42 11.59 11.67
70.83 70.96
17.37 17.62
70.20
18.21
71.15
17.18
11.73
71.72
16.55
TernCerature, 25'
7 8 9 Average (excluding Calculated for K I . ~ I z . ~ C &= Calculated for KI.312.2CBHe =
10.69 12.14 13.31 No. 4)
15.32 70.27 14.41 'The solid had begun to lose benzene before analysis. Ratio of KI to I t is 1:3.97.
677
SOLID POLTIODIDES O F POTASSIUM
The results of the analyses lead to the formula K I 41z 3CeH6 rather than to the formula K I 3 1 z d A H 6proposed by Grace. The analytical results on the compound can be checked indirectly since the weights of the three components were accurately known in all experiments. By assuming the composition of the addition product, the composition of the solution can thus be readily calculated and compared with the composition found by analysis. I n Table I11 we give the weights of each component in the original mixtures, the calculated per cent of iodine in solution assuming (a) the formula proposed by Grace and (b) the formula which we have proposed. The per cent of iodine found by analysis is in the last column.
TABLE I11 Showing the weight of each component in the original mixtures, the calculated per cent ofiodineinsolutionassuming (a) thesolidhas theformulaKI,312,zCSHe proposed by Grace, (b) the solid has the formula KI~412~3C6H6. The per cent of iodine found by analysis is given in the last column. ICI gms.
0.429
0.704 0,429
0.82 0,704 0.82
Calculated 70 I l i n Solution (b)
R 11in Solution (Analysis)
4.73 5.62 5.69 6.05 6.61 7.32
5.26 5.80 6.13 6.30 6.76 7.53
13.62
10.43
10.69
15.02
11.90
12.14
16.07
13.07
13.31
Calculated % I p in Solution (a) Temperature, 6'
If
CsHs
gms.
gms.
4.069 6.043 4.396 6.87 6.36 7.29
29.756 30. I93 29.984 29.937 30.014 29.980
8.39 8.86 9.33
30. I45 29.625 29.861
6.68 8.70 7.59
9.63 9.64 10.78
Temperature,
0.82 0.82 0.82
25'
I t will be noticed that the solubilities calculated by assuming that the residue is K I , ~ I z , ~ agree C ~ Hrather ~ closely with the solubilities determined, while if the residue is assumed to be KI~312~2C6H6 (Grace formula) the calculated solubility is very different from the results of analysis. It would appear, therefore, from the data on the univariant systems (Table I),from the analyses of the pure compound (Table 11),and from the calculated solubilities (Table 111) that the ratio of K1:Iz is 1:4. The only reliable data on the molecular ratio of benzene is contained in Table 11, but the results of eight of the nine analyses show three molecules of benzene to be present and in the one exceptional case (No. 4) the salt clearly showed by its appearance that it had begun to decompose.
IT. W. FOOTE AND WALTER M. BRADLEY
678
In concluding, it seems well to point out that these results are not in disagreement with the results found by Grace using water as a solvent. He found, as did Foote and Chalker' much earlier, that there were three univariant points in the water system, with two corresponding addition products in which the ratio of KI to I p is I : I and I :3, respectively. It is evident, however, that these compounds must be hydrated, since no binary addition product exists, and Grace assigned the formulas K18.H20and K17,H20. The small percentage of water in these compounds escaped detection in the earlier work. Bancroft, Scherer and Gould appear not to have considered the possibility of hydrate formation in their discussion.
summary The solubility results on mixtures of potassium iodide and iodine with chloroform and with toluene show that between 25' and 0.7', no solid binary addition product of potassium iodide and iodine exists. 2. With benzene, a ternary addition product exists, having the formula KI.412.3C8Hs. 3. These results are not in disagreement with the results of Grace on the ternary system with water, who showed, as have others, that there are two addition products in which the ratio of K I to 1 2 is, respectively, I : I and I : 3. These, however, must be ternary hydrated compounds, as Grace pointed out. I.
New Haven, Connecticut.
Foote and Chalker: Am. Chem. J., 39, 561 (1908).
