POLYIODIDES O F AMMONIUM. I1
THETERNARY SYSTEM AMMONIUM IODIDE, IODINE, AND WATER T. R. BRIGGS, K. H. BALLARD, MISS F. R. ALRICH, A N D J. P. WIIG3WO Department of Chemistry, Cornell University, Ithaca, New York Received June 18, 1089
While there is now no valid reason for questioning the reality of solid ammonium triiodide (cf. 7, 3) or for doubting that it can crystallize from aqueous mixtures containing ammonium iodide and iodine (cf. 12), no quantitative phase rule studies of the system ammonium iodide-iodinewater have yet been made, though such studies (hitherto always isothermal) have been published for the systems containing the iodides of potassium (15, 9), cesium (5), and barium (16). Accordingly, the investigation to be described in the present paper was undertaken; it consists of two parts,-one a preliminary isothermal study made by Miss Alrich and Mr. Wikswo, and the other an elaborate polythermal study carried out subsequently by Mr. Ballard. The latter's polythermal phase diagram is the first of its type for any polyhalide system.
EXPERIMENTAL PART I.
ISOTHERMAL STUDIES
The isothermal studies were somewhat exploratory in nature and were completed before the polythermal work was begun. Their primary purpose was to find out what solid polyiodides of ammonium are formed in the aqueous system, and to try to identify these solid phases by the method of indirect analysis. The low temperatures (0' and -15OC.) were chosen for the studies because it was thought that they might be favorable for the discovery of new solvated forms. This expectation was borne out by the facts, as will be seen. The procedure used was essentially the same as that already described in connection with the cesium iodide system ( 5 ) . The constant-temperature bath was similar to the one developed by Foote and Akerloff (6). Because of the low temperatures which were employed, special precautions had to be taken regarding the sampling of the saturated liquid and the wet solid residues, as well as to prevent condensation of moisture from the air during these operations. 325
326
BRIGGS, BALLARD, ALRICH AND WIKSWO
The work a t 0°C. was carried out by Miss Alrich. Her data appear in table 1, and her isothermal diagram, showing the details of the extrapolaTABLE 1 Ammonium iodide-iodine-water at OOC. * BOLUTION
WETBOLID
1
SERIAL NO.
EXTRAPOIATID SOLID
NHJ
I
NHJ
1 2 3 4 5 6
60.8 57.4 54.5 51 .O 49.2 46.8
0 8.4 14.4 22.4 26.1 31.3
78.1 72.5 69.6 67.0 66.5
13.7 16.9 19.8
7 8 9 (Mean.. . . . .
46.6 46.5 46.1 46.4
31.0 31.3 32.0 31.5)
60.6 52.2 50.0
27.8 38.3 42.1
10 11 12 13 14 15
41.9 39.1 34.0 33.3 28.8 25.9
37.1 40.9 49.1 50.3 58.0 63.8
38.5 37.4 35.9 35.6 34.5 32.7
55.4 56.4 59'0
68.0
"411
and "41s
"416
(1) and iodine
1 1 4.3
61.5 63.6
25.8
64.0
27.1
17 18
25.8 25.0
63.9 64.2
24.2 23.8
19
24.1
65.4
22.8
70.2
20 21 22 23 24 25 26 27
24.0 23.8 23.5 23.3 22.2 17.4 12.2 3.5
64.9 61.4 56.4 52.7
11.7 9.6 9.5 11.1 8.2 4.6 3.3 0.9
83.2 85.2 84.0
* Data by
27.6 15.0 2.9
,
"41
i::: 81.5 82.0 80.9
and
"41s
' "418
58.2
16
45.6
NHJ
, (?)
'
)
Iodine
,
Miss Alrich.
tions through the wet solid residues, appears in figure 1. Compositions are in percentages by weight. (theory 36.3 per cent "41) The diagram shows definitely that "413 is one of the solid phases and that it is congruently soluble. But a more
POLYIODIDES OF AMMONIUM.
327
I1
interesting matter is the clear indication of a second polyiodide, the solubility curve of which is very short. The data are not numerous enough to permit one to be sure about thc formula of this new polyiodide, but the evidence (based upon two tie lines) points to a pentaiodide. If so, it is not congruently soluble. It will be noticed that Miss Alrich’s diagram has been marked “partly metastable.” This is because it was later found in the polythermal study that her isothermal curves in the region of the polyiodides, while correct
0
FIG.1. Isothermal diagram a t 0°C. insofar as they represent solid-liquid systems in equilibrium, do not correspond to the stable conditions. This matter will be considered later in more detail. The work a t -15OC. was done by Mr. Wikswo. His data appear in table 2 and his diagram in figure 2. The latter indicates that the solid phases are ice, ammonium iodide, and a polyiodide. The isothermal is apparently stable throughout. The test mixtures used in this work were kept in the thermostat for a t least one month before the first analysis was made, and during that time they were shaken each day and the solid residue ground fine with a glass rod.
328
BRIGCIS, BALLARD, ALRICH AND WIKSWO
Fortunately, in the work at -15°C. the trouble due to metastability was detected in time so that the stable diagram could be obtained. The liquidus curves for the ice and for the ammonium iodide were determined first, and what later proved to be metastable systems were found in the TABLE 2 Ammonium iodide-iodine-water at -15°C.' SOLUTION
WET SOLID
BERIAL NO.
EXTRAPOLATED BOLlD
NHJ
I
NHd
I
53.38 56.91 55.14 53.96m 49.76m
0 4.25 8.85 11.23m 20.95m
80.78 78.38 73.90 69.63
1.94 4.43 6.39 12.81
6 7 8 (Mean.. . . .
54.67 54.61 54.58 54.61
10.08 9.93 9.94 10.00)
41.74 67.05 49.05
38.40 14.96 29.87
9 10 11 12 13 14 15
50.55 47.83 45.81 42.03 40.47 39.12 37.08
9.91 9.68 9.78 9.93 10.30 10.19 10.52
41.56 39.69 37.02 37.25 34.47 35.62 33.86
32.97 34.03 40.53 32.36 41.80 32.17 37.13
"413
16 17 18 (Mean, , . . .
37.12 37.02 36.72 36.9s
10.67 10.52 10.74 10.64)
30.51 28.03 22.23
32.69 22.20 8.53
NH113.3H20 and ice
31.74m 34.54m 36.30m 37.03 m 38.61 39.54
34.38m 21.10m 13.63m 11.06m 4.40 0
23.30 25.85 23.62 24.62 23.36
24.91 15.70 8.92 7.22 2.70
-
1 2 3 4 5
19 20 21 22 23 24
* Data by
3H20
Ice
J. P. Wikswo; m, metastable.
course of this work. The metastable systems are serial Nos. 4 and 5 (saturated with ammonium iodide) and Nos. 19, 20, 21, and 22 (in equilibrium with ice) in table 2. They give points on the metastable extensions, respectively, of the liquidus curves of ammonium iodide and ice, as shown in figure 2 by means of the broken lines. Finally-and purely by chance-one of the mixtures (serial No. 5 in
POLYIODIDES OF AMMONIUM.
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329
table 2) gave the invariant solution (serial No. 7) later found to be in equilibrium with ammonium iodide and the polyiodide. Solid taken from this mixture was thereupon used to inoculate other mixtures with the polyiodide (obviously the solid which was failing to appear); whereupon No. 4 changed to S o . 6 (ammonium iodide-polyiodide invariant), and 370s. 19, 20, and 21 changed to Nos. 16, 17, and 18, respectively (ice-polyiodide invariant). After this, it was a simple matter to locate the solubility curve of the polyiodide itself (Nos. 9 to 15, inclusive).
FIG.2. Isothermal diagram a t -15°C.
The tie lines used in ascertaining the composition of the polyiodide a t The points numbered 2 , 3, and 4 in the diagram correspond respectively to the dihydrate, trihydrate, and tetrahydrate of NH413. I t is evident from the diagram that the tie lines converge in close proximity to the point (32.0 per cent ammonium iodide, 56.1 per cent iodine) representing the trihydrate; hence the poiyiodide has the formula KH413.3H20. I t is not congruently soluble a t - 15°C. It is quite evident, from what has gone before, that the isothermal studies were productive of several important results. They showed that - 15°C. have been drawn with great care in figure 2 .
330
BRIGGS, BALLARD, ALRICH AND WIKSWO
a t least three polyiodide phases were to be expected in any polythermal study of the ternary system,-Le., the anhydrous triiodide "118, the 3Hz0, and a third polyiodide which is possibly hydrated triiodide "413. a pentaiodide. They also gave warning of the difficulties arising from suspended transformation, especially as regards the hydrated triiodide, and indicated that systematic inoculation would have to be used in all future work. PART 11.
T H E POLYTHERMAL SYSTEM
A . The binary systems The solubility of iodine in water has been determined at a number of temperatures by Hartley and Campbell,' and is so small that it may be neglected in the present work. The ice-iodine eutectic temperature is therefore virtually the same as the melting point of ice, and the eutectic solution contains almost no iodine. TABLE 3 Solubility of ammonium iodide -
NBI
TEMPERATURE
'C.
p a cmt
00.
pa cent
-20 - 10 0 10 15 20 25 30
57.6 29.1 60.5 62.0 62.7 63.1 63.9
40
65.6 66.6 67.6 68.6 69.6 71.4 73.1 75.0
64.4
50
60 70
80
100 120 140
When the present investigation was begun, temperature-composition data for the system ammonium iodide-iodine were non-existent and had to be found by means of a special investigation. The results have been presented in a separate communication (3). As expected, the compound NH41a was found; it decomposes a t 175OC. (incongruent melting point) into solid ammonium iodide and melt. The temperature of the eutectic between iodine and NH413is 88.g°C., and the eutectic melt contains 10.5 per cent ammonium iodide by actual analysis. As regards the system ammonium iodide-water, Smith and Eastlack (17) have determined the solubility of the salt, which forms no hydrates, between - 19' and 136°C. Their data, interpolated for rounded temper-
* See reference 11; the solubility is 0.028 per cent at 18°C. and 0.092 per cent at 55°C. Cf. also the more complete study of the system iodine-water by Kracek (J. Phys. Chem. 36, 417 (1931)).
POLYIODIDES OF AMMONIUM.
33 1
I1
atures and converted into per cent ammonium iodide, are given in table 3. According to Guthrie (10) the ice-ammonium iodide eutectic is a t -27.6"C. and 55.6 per cent ammonium iodide. Data enabling one to locate the fusion curve of ice, however, were lacking, and had to be determined separately. The data, for rounded temperatures down to the eutectic point, are given in table 4. At this point mention should be made of the fact that in no case, either in the binary systems containing ammonium iodide or in the ternary system, was there found any indication of an amqonium iodide transition point. While Bridgman (2; cj. 17) has shown that there is a second polymorphic modification which is stable below an estimated temperature of -17.6"C. at normal pressure, it is formed too slowly to be observed under the conditions prevailing in the present work. TABLE 4 Freezing points of ammonium iodide solutions TEYPEEATUEO
'C.
0
-3 -6 -9
-13
1
"J pa edn(
0 11.8 20.7 28.0 35.8
I/
TEMPmATUEI
'C.
-16 -21
-24 -27.4 E
1
NHJ p a cant
40.7 47.7 51.5 55.5 E
B. Procedure The procedure employed in the polythermal study was basically the same as that described by Briggs and Geigle (4)in their work on the system potassium iodide-iodine. Cooling curves of various mixtures of the three components contained in a specially constructed jacketed tube were obtained with extreme care. The tube, which was partly immersed in a suitable cooling mixture contained in a large Dewar flask, was fitted with a cork stopper (painted with collodion) carrying the thermometer, a highspeed stirrer, and an opening for sampling and inoculation. All thermometers (marked for stem immersion) were calibrated by intercomparison and by means of a number of standard fixed temperatures. The first attempts to obtain reproducible cooling curves resulted in complete failure a t the low temperatures. Undercooling was extreme,so much so that a firat arrest w~lsoften followed by a second which was as much as 10°C. higher. The trouble, however, disappeared immediately after a suitable system of inoculation was employed. The inoculating solid was obtained by dipping a number of fine glass rods into the fused mixture being cooled, and keeping these rods in a stoppered tube held well below the temperature of the arrest being determined.
332
BRIGGS, BALLARD, ALRICH AND WIKSWO
The procedure finally adoptcd was to makc a preliminary dctcrmiriatioii of the cooling CUI'VO bo as to rstahlish thc approximatc position of each arrest, and to follow this with a sccond dctrrmination accompanicd by repeated inoculations begun well above the temperature of the expected arrest. Perfectly reproducible cooling curves were thus obtained, in which consecutive arrests differing from each other by as little as 0.5"C. could be found with certainty. After the cooling curve had been determined to the satisfaction of the operator, the mixture was remelted, cooled slowly, and subjected to analysis of the liquid phase a t appropriate temperatures as determined by a consideration of the cooling curve. Since the operation is really a determination of solubility, care was taken to hold the temperature of the mixture constant a t the point decided upon for a sufficient length of time. A period of 15 min. (with rapid stirrings) was found by test to be sufficient for the accuracy necessary in work of this kind. Inoculation, of course, was again essential.
C . Crystallization jrom ternary melts Before proceeding to the data, the general considerations governing crystallization (i.e., separation of solid phases) in ternary systems will be reviewed. Although the matter was considered in detail by Geer (8) many years ago, it will be presented here again, but in a somewhat different way. Figure 3 shows a typical triangular projection diagram of a system giving a ternary compound, ABC, melting incongruently. The direction of falling temperature along each boundary line between adjacent fields is indicated on the diagram and is in agreement with the theorem stated by Bancroft (1) and attributed by him to van Rijn van Alkemade (18). Points E and F are therefore ternary eutectics and T is a transition point, while P and Q are positions of maximum temperature (Le., dystectics) on their respective boundary lines. As temperature falls along the boundary DE, solid A and solid B separate simultaneously from the melt and the composition of the latter follows the boundary to E, where a third solid phase (compound ABC) appears and solidification proceeds to completion a t constant temperature and constant composition of the melt (Le., E is an end point of crystallization). If one takes any point on this boundary (such as the one indicated on the diagram by the pair of diverging arrows), and draws through this point the tie lines to A and B, respectively, it is easy to see that separation of A from the melt tends to shift the composition of the melt along the corresponding tie line away from A, while separation of B tends to shift the composition along the other corresponding tie line away from B. The direction of Lhese individual composition shifts may be represented by
POLYIODIDES OF AMMONIUM.
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333
means of arrows (Le., composition vectors) as shown on the diagram; whence it becomes obvious a t once that the net effect or resultant of the two simultaneous separations of solid is a shift in the composition of the melt (necessarily along the boundary line) in the direction of the eutectic a t E,-that is to say, in the direction of falling temperature. Along the boundary DE, of course, the two solid phases separate simultaneously as the system is cooled. Such is not the case, however, along the incongruently saturated boundary EQT, for, as one of the two solids R
FIG.3. Typical triangular projection diagram of a system giving a ternary compound, ABC, melting incongruently
separates (ABC in this case), the other (B) actually dissolves. The latter phenomenon will be called negative separation (or negative crystallization) in order to distinguish it conveniently from the opposite case where the solid actually separates, which case will be called positive. In figure 3, as a matter of fact, we have positive separation of A and B along D E during cooling, positive separation of A and C along H F , of B and C along TG, of ABC and C along FT, and of A and ABC along E F ; but along ET we have positive separation of ABC and negative separation of B. The composition of the melt will remain on the boundary ET during cooling, only
334
IIRIGGS, IIALLARD, ALRICH AND WIKSWO
so long as a supply of solid B remains to be dissolved. When the solid B is used up, the melt departs from the boundary. Several instances of such a departure were encountered during the work on the ternary system, and until it was clearly understood, the cooling curves were difficult to interpret. Departure from a boundary is possible whenever one of the two coexisting solid phases exhibits negative separation along the boundary. It is therefore necessary to be able to predict the type of separation which prevails along any part of any given boundary, and in doing this one will find it extremely helpful to make use of the semi-quantitative vectorial method which has been indicated above. The general rule gov‘erning the method may be stated as follows: “The vectors (arrows) drawn a t any point on a boundary curve in order to represent the individual composition shifts which result from the separation, either positive or negative, of the respective solid phases during cooling, must conform to the facts that their resultant is tangential to the boundary a t the place un.der consideration, and is pointed in the direction of falling temperature along the boundary.”
The direction of falling temperature, of course, is taken from the point under consideration on the boundary; and the direction must be known from the experimental facts or from the application of van Alkemade’s theorem. It is then a simple matter to apply the rule, give the vectors (arrows) their proper direction a t any point on a boundary, and find whether the separation required of each solid is positive or negative. Several examples of the application of the rule are shown in figure 3 and are discussed in the next two paragraphs. If we take any point on the congruent boundary FT in figure 3, or any point other than the dystectic P on the congruent boundary FPE, it is evident that the rule requires that the vectors (arrows) be drawn and directed as shown on the diagram. Positive separation of both solid phases (ABC and C, ABC and A for the two boundaries, respectively) is thus indicated during cooling. It should be observed that at the dystectic P the tangential resultant has become equal to zero and there is neither a temperature fall nor a change in the composition of the melt while the two solids are separating. As regards the boundary EQT, the only possible arrangement of the arrows which conforms to the rule is also as shown on the diagram; hence ABC separates positively and B negatively. At the dystectic Q the same kind of separation takes place, but the resultant composition shift is again zero and the melt remains a t Q (temperature constant) as long 9s any solid B remains undissolved. I n figure 3A part of the same triangular diagram is reproduced on a larger scale, so that some of the crystallization phenomena may be more easily followed. Lines have been drawn (also in figure 3) from M (ABC)
POLYIODIDES OF AMMONIUM.
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335
to the three apexes A , B , and C, respectively, dividing the triangular diagram into three delimiting areas which enable one to tell whet final solid phases will result when any given melt is cooled to the point of complete solidification. Thus, if the composition of the melt lies in the area bounded by AMCA, the final solid phases must be A, ABC, and C, and the end point of solidification (crystallization) must be the eutectic F. Similarly, A, ABC, and B must be the final solids in the area A B M A and the eutectic E the end point, while in the area BMCB, C, ABC, and B must be the final solids, and the end point must be the transition point T,even though B separates negatively a t that point.2 All of this, of course,
FIG.3A. Crystallization paths
is predicated on the assumption that equilibrium between the various phases is constantly maintained. We are now ready to deduce a few theoretical crystallization paths, beginning, for example, with melt of the composition x in figure 3A. Since x lies in the delimiting area giving A, ABC, and B as the final solids, the eutectic E must be the end point of crystallization. Since x also lies in the field of B as solid phase, the latter will be the first to appear on cooling. The temperature will fall continuously as B separates, and the composition of the melt will move along the tie line 2 A t a eutectic, of course, all three solids separate positively, but at other threesolid invariants depending on the type, we have negative separation of one solid or negative separation of two solids.
336
BRJGGS, BALLARD, ALRICH AND WIKSWO
Bzz toward z. When z (on the boundary EQT) is reached, solid ABC will begin to separate, and B will begin to redissolve (Le., separate negatively). The temperature will continue to fall until the eutectic a t E is reached, where solidification to a mixture of A, ABC, and B will go on to completion a t constant temperature. Between z and E, however, some but not all of the solid B separating between 2 and z will have redissolved. The cooling curve will show a partial arrest a t (separation of B), a second partial arrest a t z (separation of ABC), and a complete and final arrest a t the eutectic E. Melt of the composition y (below the critical line E M ) follows a more complicated crystallization path in which departure from a boundary line occurs. Since y happens to lie in the delimiting area AMCA, the final solids must be A, ABC, and C, and the second eutectic a t F is the crystallization end point. As before, solid B appears first on cooling (note however that B cannot be one of the final solid phases), the composition of the melt shifts to z and then along the boundary EQT in the direction of E . It never reaches E , however, but instead a t the point r (on the extension of the tie line joining y with M) , the melt leaves the boundary line and crosses over the ABC field to the boundary FPE a t s. At the “critical” point r the system consists of melt, solid ABC, and the last trace of solid B, for the solid B which separated between y and z is about to disappear on account of negative separation between z and r. It is not difficult to understand why this must be so, if one keeps in mind the fact that the composition shift from y to the critical point r could have been made stoichiometrically by the removal of ABC alone from the original melt. At the point r, of course, the system gains a degree of freedom. The rest is obvious. When the critical point is reached, the melt moves across the ABC field to s, the temperature falling continuously and ABC separating positively. At s solid A also begins to separate positively, and finally the eutectic is reached a t F . The cooling curve is also worth considering. A partial arrest will be observed a t y when B begins to separate, and another at z when ABC starts to form. Some change in the slope of the cooling curve should be noticed a t the critical point r, followed by a third partial arrest a t s. There will be, of course, a complete and final arrest a t the eutectic F . Melt of the composition z‘ in figure 3A lies in the delimiting area giving A, ABC, and C as the final solid phases and the transition point T as the end point of crystallization. By following the same line of reasoning as before, it is not difficult to see that the crystallization path follows the tie line Bz’z’ to z’, and then the boundary EQT to T . Similarly, melt of the composition y’ (which lies in the delimiting area giving A, ABC, and C as the final solids, and F as the end point of crystallization) follows the path y’ to z’, the path z’ to r’ (critical point where solid B disappears),
POLYIODIDES OF AMMONIUM.
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337
then departs from the boundary EQT across to s’, and finally follows the boundary FT to F. If one has followed this discussion carefully, it is now apparent that the region (hereinafter called a “critical” area) which is bounded by EQT and the straight lines EM and M T , includes all possible melts which show departure from the boundary EQT on cooling. The recognition of these critical areas is important for the purposes of the present investigation, since several areas of the type appear in the ternary diagram, ELS will be seen later. For further details, the paper by Geer should be consulted.
D. The ternary diagram Table 5 contains the temperature-composition data, the compositions being expressed in percentages by weight, as before. The data marked “W” are taken from table 2. The ordinary type of polythermal projection diagram is shown in figure 4. The value given for each invariant is the mean of several independent determinations. Thus, for example, the invariant K was found during four different surveys, with the following results:
.I 1
Temperature, “C .... , 0.1 Ammonium iodide. , . . . 23.84 Iodine.. . . . . . . . . . . . . . . . 65.38
0.2 23.90 65.40
~
0.2 23.98 65.51
~
0.2 23.91 65.40
1
23.91 +Z 0.04 (mean) 65.42 0.04 (mean)
Table 6 contains the various binary and ternary invariants, collected together for the sake of convenience. Some of these, of course, are taken from the paper by Briggs and Ballard on the system ammonium iodideiodine (3). Most of the actual composition data are shown on the triangular diagram (figure 4), except in the pentaiodide region, where they have been omitted for the sake of clearness. Figure 5 is the corresponding Janecke projection on the ammonium iodide-iodine plane. The invariants B and L are eutectics; P and Q are dystectics. The peculiar appearance of the ice-iodine boundary SL on the Janecke projection should be noted. Some of the boundary lines merit further comment. A discussion of these boundaries follows. The boundary S L (ice-iodine). Mention already has been made of the fact that the position of this boundary is most unusual. If one applies the vectorial method described in the preceding section of this paper, one fmds that iodine separates negatively along all but a negligible part of the boundary, the critical region being the area inclosed by the boundary and a straight line drawn from the invariant L to a point which is virtually the same as S. Any mixture having a composition within this area will ultimately depart from the boundary during cooling, the crystallization path
338
BRIGGS, BALLARD, ALRICH AND WIKSWO
TABLE 5 Boundary lines POWI’ION ON DIAQRMI.
LIQUID
PEyPEBAlWRI
NHJ
I
‘C.
S SL SL SL SL SL L
0 -1.9 -3.2 -4.5 -5.9 -6.9 -8.3
0 7.81 13.48 18.53 21.51 22.80 23.20
(0.016) 8.30 18.36 31.22 43.50 48.63 52.48
LB LB LB LB LB LB LB LB LB LB LB LB LB B
-8.1 -7.6 -7.6 -7.7 -8.1 -9.0 -10.5 -12.8 -15.0 -16.9 -19.1 -23.4 -27.9
23.21 23.61 23.82 23.51 23.82 24.20 25.90 28.91 33.12 36.95 W 39.52 43.22 48.70 53.50
51.50 44.35 43.40 43.00 36.81 32.60 23.84 17.10 12.32 10.64 w 8.97 7.09 5.76 4.60
BC BC BC BC BC BC BC BC BC BC C
-23.4 -19.1 -16.9 -15.0 -14.9 -12.8 -10.5 -7.2 -4.2 -2.2 -0.8
54.41 54.62 54.61 54.62 W 54.50 54.13 53.48 52.22 50.49 48.50 46.72
5.92 7.49 8.61 10.00 9.75 11.20 13.33 17.08 21.91
46.12 44.79 43.22 42.19 37.92 36.41 29.92
31.42 35.11 39.02 41.89 53.02 55.72 70.13
CD CD CD CD CD CD D
2.0 9.7 18.0 25.0 61.5 78.7 175
* The data marked “W” are taken
Ice and iodine
Ice and NH&.3Aq
w NHJ and NHJa.3Aq
28.35 30.02
from table 2.
NHJ and N H ~ I
POLYIODIDES OF AMMONIUM.
339
I1
TABLE &Continued '
1
LIQUID
POBlTION ON DUORAM
NHcI
I
B O U D PBABIB
t
'0.
CG CG CG CG CG CG CG CG CG CG CG CG CG G
0.9 2.9 4.4 5.9 6.7 7.0 6.6 6.0 5.6 5.1 4.1 3.4 2.9 2.4
44.02 40.31 37.38 34.61 32.61 30.78 29.31 28.31 27.91 27.30 26.58 25.92 25.51 25.08
33 72 39.21 43.82 48.28 51.52 54.13 56.51 58.93 59.90 60.41 62.12 63.52 64.23 64.64
GF GF GF GF GF
F
3.0 5.0 5.3 7.0 7.6 9.8
24.91 24.41 24.45 24.02 23.90 23.22
65.10 66.13 66.05 67.00 67.10 68.75
GK GK GK K
1.6 1.2 0.6 0.2
24.78 24.61 24.07 23.91
65.10
I
-0.3 -0.8 -1.8 -2.7 -3.8 -5.0 -6.9 -8.3
24.00 23.90 23.81 23.82 23.81 23.80 23.63 23.20
64.71 64.21 63.00 61.75 60.50 58.90 55.40 52.48
KF KF KF KF KF KF KF
2.2 3.2 4.2 4.4 5.7 6.0 6.8 8.7 8.9 9.8
23.71 23.82 23.71 23.58 23.50 23.62 23.40 23.30 23.16 23.22
66.12 66.50 66.96 67.00 67.00 67.21 67.90 68.13 68.45 68.75
KF F
Pentaiodide and
)
NH113.3Aq and iodine
"413
65.23 65.42
KL KL KL KL KL KL KL L
KF
)
and iodine
340
BRIGGS, BALLARD, ALRICH AND WIKSWO
TABLE %Concluded POBITION ON DIAQBAY
LIOUID
TBYPBRATUR10
BOLID PHMBEIJ
NHJ
I
22.93 22.80 22.41 22.02 21.52 20.03 17.88 15.13 10.52
69.10 69.41 70.17 71.56 72.62 75.03 78.22 82.46 89.48
“C.
FE FE FE FE FE FE FE FE E
11.3 12.0 15.1 20.1 25.0 33.4 45.6 59.2 88.9
’ ”118
and iodine
,
FIG.4. Polythermal projection diagram then cutting across the ice field to the boundary BL. An interesting study of the theoretical crystallization paths may be made in this region. The boundary LB (ice-NHJs.3Hn0). The vector test shows that both solid phases separate positively during cooling. The experimental data show a definite though slight temperature maximum (Le,, the dystectic
POLYIODIDES OF AMMONIUM.
I1
341
TABLE 6 Invariant points POSITlON ON DId9EAY
UOIJID
IZYPIBA-
mxm
NEJ
I --
-27.4 0 175 88.9 114
55.5 0 29.9 10.5 0
0 (0.016) 70.1 89.5
-27.9 -8.3 -0.8 2.4 0.2 9.8
53.50 23.20 46.72 25. Os 23.91 23.29.
I
EOLID PHABBEB
'C.
A
9 D E I €3
L C
G K F
100
4.60 52.48 30.01 64.64 65.41 68.7s
Ice and "41 Ice and iodine "41 and "41s "411 and iodine Iodine
Ice, NH&.3Aq, and N H J Ice, XH411.3Aq, and iodine "11, NHJa*3Aq, and "41s NH41~.3Aq,"418, and pentaiodide NH&.3Aq, pentaiodide, and iodine Pentaiodide, "418, and iodine
FIG.5. Janecke projection on the ammonium iodide-iodine plane
P ) , not far from the invariant L. When the data used in constructing the Janecke projection (temperature versus per cent iodine in the total of ammonium iodide and iodine) were plotted separately on a suitable scale,
342
BRIGGS, BALLARD, ALRICH AND WIKSWO
the position of the maximum temperature (- 7.6"C.)was estimated to be a t about 64 per cent iodine. Since the theoretical iodine content of ammonium triiodide is 63.7 per cent, all this is evidence (based upon the van Alkemade principle) that the polyiodide present a t this boundary is a triiodide. Since it cannot be the anhydrous triiodide, it must be a hydrated form-evidently the trihydrate discovered by Mr. Wikswo in his isothermal study. Further evidence supporting the trihydrate formula will be presented later on. The boundary CD (NHJ-NHaI3). This boundary is of interest because the vector test shows that solid ammonium iodide separates negatively throughout the whole length from D to C. The critical region is the area between the boundary and lines drawn from R to C and from R to D, respectively. The boundary CG (NH&-3H&NHI13). The vector test shows that the anhydrous triiodide separates negatively along the whole boundary. There is a pronounced temperature maximum a t Q. The critical region is the area enclosed by the boundary and straight lines drawn from H to C and from H to G , the whole giving a system like that shown in figure 3A. Test mixtures were found to depart from the boundary a t the points required by the crystallization theory, and then to cut across the field of the hydrated triiodide,-thus failing to show the invariant arrests a t C, G , or K , according to circumstances. For instance, in one experiment a mixture lying in the area QHG failed to give the arrest a t G but did give the arrest a t K , while in another experiment the mixture failed to give both these arrests, and crossed instead directly from the boundary QG to the boundary K L . Occurrences of this kind were not easy to interpret during the early stages of the investigation. The Janecke projection data make it possible to locate with considerable precision the position of the temperature maximum on this boundary, the estimate based upon the actual data being 7.0"C. and 64 per cent iodine in the total of ammonium iodide and iodine. The latter number again corresponds very closely to the iodine content of ammonium triiodide (63.7 per cent) and shows that the two coexisting solids are triiodides. T h e "melting points" of the pure polyiodides. Each of the three polyiodides found in the present system melts incongruently when heated. or incongruent-of any solid The (stable) melting point-congruent compound appearing in a ternary diagram is always the point of highest temperature on the (stable) liquidus surface for that compound in the space model; this follows easily from the theory of crystallization in ternary systems if one considers the fact that the melting point of the pure compound (whether congruent or incongruent) must be the same as the end point of crystallization of a mixture the total composition of which is stoichiometrically identical with that of the compound. By pure compound is meant the solid free from any adhering liquid phase.
POLYIODIDEB O F AMMONIUM.
I1
343
Accordingly, the compound NHIIl. 3H20 melts incongruently at the temperature (7.OOC.) corresponding to the dystectic &,--the highest temperature anywhere on the liquidus surface of the trihydrate. As i t melts, it gives liquid of the composition Q and solid of the composition R (figure 4),that is, solid "*I3. The latter, in turn, melts a t D (175"C.), giving liquid of composition D and solid "41. However, the invariant point F (9.8"C.) is the highest temperature in the field of the pentaiodide; hence this compound melts incongruently a t 9.8"C., giving liquid of the and iodine. composition F and two solids, Le., "413
E. T h e indirect analysis of the solid polyiodides Further work on the chemical identification of the polyiodides was done during the polythermal study in order to check the results obtained in the isothermal investigations. However, the usual method of analyzing liquid and wet solid residues was largely replaced by the alternate method involving the determination of crystallization paths. The latter method is simple experimentally, since it requires the sampling and analysis of only the liquid phase; but it suffers from the fact that in fields of limited area the crystallization paths are necessarily short and extrapolation is therefore uncertain. As was to be expected, therefore, the best results were obtained in the field of the hydrated triiodide, and the poorest in the field of the pentaiodide. The three polyiodide fields will be taken in order. T h e NH& field. Only crystallization paths were determined in this field; the data are collected in table 7. The paths (excepting Nos. 8 and 9) converge over a wide angle to a very satisfactory focus which is close enough to point R (figure 4) to show that the solid is NH413,thus confirming the isothermal extrapolations a t 0°C. It is not deemed necessary to present a graph showing the crystallization paths in this case. T h e "413'3HzO field. The crystallization paths in this field were in some instances supplemented with analyses of the wet solid residue. The data appear in table 8 and the extrapolations, numbered to accord with the table, in figure 6. With one exception, the extrapolated tie lines pass through the point corresponding to the formula NH413.3H20, thus confirming the isothermal results a t - 15OC. T h e pentaiodide field. The data are given in table 9 and the graph in figure 7. The crystallization paths (surveys 1 and 3) are necessarily very short, and little is to be expected from the method unless the analytical technique can be greatly improved. The greatest weight should therefore be given to the extrapolations through the wet solid residues, and these, though only two in number, are taken to indicate that the compound is probably a pentaiodide containing one molecule of water, i.e., hTH416.HzO. If this conclusion is correct, the compound is closely analogous to the hydrated higher polyiodide of potassium, KIT.HzO, reported by Grace (9).
344
BRIGGS, BALLARD, ALRICH AND WIKSWO
F . Interpolated isothernals Attention should be called to the fact (see the Janecke projection) that the liquidus surfaces in the ice and NHJ3.3H20 fields are comparatively TABLE 7 Crystallization paths an the "11,
&ld
LIQUID
PA=
10.
TDYPE3RAlWRE3
NHJ
I "C.
1
2
3
4
5
6
7
8
9
10 11
36.5 36.5 36.8
52.4 48.8 44.6
25.0 4.6
34.8 34.5
51.0 48.3
6.0
33.3 32.4
55.3 51.6
6.8
32.0 30.8
66.2 64.1
7.0
30.0 29.2
57.6 56.6
6.6
28.6 26.9
61.4 61.0
5.0
27.0 25.9
63.0 63.0
3.6
27.2 25.9 25.0
64.8 64.7 64.5
20.4 2.4
26.0 24.0
67.0 67.3
7.4
24.0 22.8
69.0 69.5
12.3
24.2 22.0
70.4 71.6
flat in respect to the temperature axis (the other fields are steep). Accordingly, it was thought worthwhile to locate, a t least with a moderate degree of accuracy, a number of contour isothermals in these two fields. This was done by carrying out several special thermal surveys-the details of
1
I
I
PO
30
I 40
I
50
I GO
70
P E P C€NT IODIN€
FIG.6. Extrapolations for the NH4111.3H~0 field
TABLE 8 Crystallization paths i n the NH41,.3H10 jield TEYPEBATUBE
PATE NO.
I
"11
'C.
5.9 4.4 2.0
1
34.60 36.02 38.40
48.35 44.40 37.50
2
42.13 44.41 45.57
25.32 19.11 15.18
3
34 45 35.40 36.62 33.60
44.26 40.82 34.83 48.50)
3.9 2.4 0.2
33.03 33.40
34.20 34.83 35.43
43.02 38.47 29.46 19.38 13.83
2.4 0.2 -3.1 -8.4 -12.1
5
31.30 30.58
50.45 47.50
4.8 2.4
6
27.87 26.80
57.44 58.00
25.76
58.50
4.9 2.7 0.4
58.35 58.75
2.3 0.2
(Wet solid., . . . . . . . . . . .
26.40 25.43 30.00
8 (Wet solid . . . . . . . . . . . . .
29.24
(Wet solid... . . . . . . . . . . 4
7
9
I
-2.6 -5.6 -8.4
56.60)
25.26 26.00
24.70 345
58.90 56.80)
0.1
61.70 63.40
3.3 1.3
346
BRIGGS, BALLARD, ALRICH AND WIKSWO
which need not be given-and combining the results with the data already available from tables 5 and 8. The final data for the various isothermals are collected in table 10; figure 8 shows the isothermals themselves. The isothermals in the ice field are especially interesting, for if one takes a point on the ice-ammonium iodide side of the field and from i t moves along the tie line leading to the iodine apex of the triangular diagram, one finds that the temperature rises. This means, of course, that iodine raises TABLE 9 Indirect analysis i n the pentaiodide $eld WET SOLID
LIQUID
SURVEY NO.
NHJ
I
NHiI
I
24.45 24.73
66.30 65.02
23.46
69.35
2
24.18
65.01
23.39
68.80
3
23.58 23,92
66.93 65.40
1
* Temperatures
l G5
l
not recorded in this field.
l
l
I
/
I
l
I
I
70
l 75
/
/
I
I L
P€R C € N T / O O I N €
FIG.7. Extrapolations for the pentaiodide field
the freezing point of a solution of ammonium iodide in water. This elevation of the freezing point was observed almost fifty years ago by LeBlanc and Noyes (13; cf. also 14) (not with ammonium iodide solutions but with potassium iodide solutions, which behave similarly); and the fact that the freezing point was not lowered as expected was explained, it will be remembered, by postulating the formation of Is- ions in the solution. Figure 8 also contains the isothermals in the field of the trihydrated triiodide, except that a short section of the -8°C. isothermal near the
POLYIODIDES O F AMMONIUM.
347
I1
TABLE 10 Interpolated isothernlals ICE FIELD TEMPERATURE
NHiI
-
NHd
1
"C.
-24
51.5 48.8
0 5.6
54.4 48.8
5.6 5.6
-20
46.4 43.6
0 7.1
54.6 43.6
7.1 7.1
- 16
40.7 38.0
0 9.8
54.6 38.0
9.5 9.8
- 12
34.0 32.8 31.6
0 7.2 13.8
54.0
46.8 35.4 31.8
12.0 12.0 13.1 13.8
-8
25.8 24.5 23.8 24.0
0 9.2 22.3 33.5
52.8 45.5 34.9 24.0
16.1 15.6 20.0 33.5
-4
15.0 14.8 15.0
0 7.0 15.0
50.2 43.3 38.9 34.2 25.6 23.8
22.3 22.0 24.5 28.3 44.7 60.2
0
45.3 40.4 33.5 29.0 27.6 25.6 24.0
31.8 32.0 37.7 44.7 48.1 58.7 65.2
38.1 36.6 32.8 31.1 29.6 27.6 26.3
42.5 43.0 46.5 49.5 51.5 57.7 62.6
0
4
0
eutectic L (-8.3"C.) has been omitted because i t cannot be shown satisfactorily in the drawing. The approximate position of the 25OC. isothermal has also been included; likewise the isothermal a t 0°C.
348
BRIGGB, BALLARD, ALRICH AND WIKSWO
It is apparent from the diagram (figure 8) that Miss Alrich's 0°C. isothermal is quite different from the one obtained in the polythermal study. It 1s now evident that she should have obtained the anhydrous triiodide along only a very short portion of the isothermal near the invariant C, that she should have found the hydrated triiodide along an extensive and strongly curving intermediate portion, and that she should have just missed the pentaiodide, in all probability. Actually, she missed the hydrated triiodide entirely.
FIG.8. Isothermals However, if one recalls the trouble experienced in getting the hydrated triiodide to form even at -15"C., and the similar difficulty which confronted the polythermal work until inoculation was used, it is easy to see that Miss Alrich was dealing with metastable systems in the polyiodide region. The diagram shows clearly (cf. figure 8) that, allowing for small experimental discrepancies, the position of her isothermal is what it should be on the assumption of an extreme lag in the formation of the hydrated triiodide and little or no lag in the formation of the anhydrous triiodide or of the pentahydrate, an assumption which has plenty of experimental facts to support it. The OOC. isothermal is not incorrect, but it does not represent the stable system. Before this paper is concluded, the various polyiodides of ammonium
POLYIODIDES OF AMMONIUM.
I1
349
which are formed in the presence of water will be compared briefly with those formed by the alkali metals. The work already accomplished in this laboratory and elsewhere, while in no single instance so complete as is the present study of the ammonium system, nevertheless indicates fairly conclusively that cesium and rubidium3 form only unhydrated polyiodides. These are CsI,, cs14, and Rb13, respectively. On the other hand, potassium and sodium-and undoubtedly also lithium, which however has not been studied systematically-seem to form only hydrated polyiodides. In the case of potassium, for example, Grace has reported the compounds K13.Hz0 and K17.Hz0, and in a polythermal study now approaching completion in this laboratory, two additional solid forms have been discovered. Both of these new polyiodides are hydrated solids, and one is definitely known to be KI, -2H20. The system containing sodium iodide is also being investigated; and in this system three curiously complex and strongly hydrated polyiodides have been found. The ammonium system is the only one giving both hydrated and unhydrated polyiodides; i t is therefore the intermediate case in the series cesium, rubidium, ammonium, potassium, sodium, and lithium. SUMMARY
The more important results of this paper may be summarized as follows: 1. The temperature-composition diagram of the ternary system ammonium iodide-iodine-water has been completed. The diagram is restricted to solid-liquid systems in air at approximately normal pressure. 2. The solid phases are ice, NH41, iodine, "413, NR4Is.3Hz0, and probably a pentaiodide having the formula NH4Ia.HzO. Each of the polyiodide fields is incongruent. 3. Pure solid NH413melts incongruently a t 175"C., decomposing into binary liquid and solid NH41. Pure solid NH413.3H20 melts incongruently a t 7.0°C., decomposing into ternary liquid and solid ?rTH&. The pure solid pentaiodide melts incongruently a t 9.8"C., giving ternary liquid, and solid iodine. solid ",I, 4. A convenient graphical method of predicting the separation of solid phases along field boundaries has been described and applied to the general problem of crystallization from ternary melts. 5. The problem of suspended transformation has been considered. Of the three polyiodides, the trihydrated triiodide is by far the slowest to form. 6. As regards its tendency to form hydrated polyiodides, ammonium is a Unpublished isothermal studies carried out in this laboratory show that RbIs (anhydrous) is the only solid polyiodide in the system rubidium iodide-iodine-water a t 0" and 25°C.
350
BRIGGS, CLACK, BALLARD, AND SASSAMAN
the intermediate case in the series cesium, rubidium, ammonium, potassium, sodium, and lithium. 7. The polythermal diagram shown in this paper is the first for any ternary polyhalide system. REFERENCES
(1) BANCROFT: The Phase Rule, p. 149. Journal of Physical Chemistry, Ithaca, New York (1897). (2)BRIDGMAN: Proc. Am. Acad. Arts Sci. 62, 136 (1916-17). (3)BRIGGSAND BALLARD: J. Phys. Chem. 44, 322 (1940). (4) BRIGGSAND GEIGLE:J. Phys. Chem. 34, 2250 (1930). (5) BRIGGS,GREENAWALD, AND LEONARD: J. Phys. Chem. 34, 1951 (1930). (6) FOOTE AND AKERLOFF: Ind. Eng. Chem., Anal. Ed. 3, 389 (1931). (7) FOOTE AND BRADLEY: J. Phys. Chem. 37,29 (1933). (8) GEER:J. Phys. Chem. 8, 357 (1904). (9) GRACE:J. Chem. SOC.1931,594. (10) GUTERIE:Phil. Mag. [4] 49,213 (1875). (11)HARTLEY AND CAMPBELL: J. Chem. SOC.93, 741 (1908). (12) JOENBON: J. Chem. SOC.33, 397 (1878). (13) LEBLANC AND NOYES:Z. physik. Chem. 6 , 385 (1890). (14) OSAKA:Z.physik. Chem. 38, 743 (1901). (15)PARSONS AND WEITTEMORE: J. Am. Chem. SOC.33, 1933 (1911). (16)RIVETTA N D PACKER:J. Chem. SOC.1937, 1342. (17) SMITEAND EASTLACK: J. Am. Chem. SOC.38, 1500 (1916). (18) VAN ALKEMADE, VAN RIJN: Z. physik. Chem. 11, 289 (1893).
POLYIODIDES OF POTASSIUM.
I1
THE TERNARY SYSTEM POTASSIUM IODIDE-IODINE-WATER T. R. BRIGGS, K. D. G. CLACK, K. H. BALLARD, AND W. A. SASSAMAN Department of Chemistry, Cornell University, Ithaca, New York Received July 84, 1959
The present paper-a companion to the one published several years ago on the binary system potassium iodide and iodine (3)Aescribes an isothermal and polythermal phase rule study of the system potassium iodideiodine-water. This study, which has been in progress for several years, consists of two preliminary isothermal surveys-one a t 25OC. by Mr. Sassaman and another at 0°C. by Mr. Ballard-followed by a much more detailed polythermal survey carried out for the most part by Mr. Clack, who employed the procedure already described in the paper on the system ammonium iodide-iodine-water (2). The investigation in all instances
