POLYIODIDES OF SODIUM.
I
595
POLYIODIDES OF SODIUM. I
THE SYSTEM SODIUM IODIDE-IODINE-WATER T. R. BRIGGS, W. F. GEIGLE,
AND
J. L. EATON
Department of Chemistry, Cornell University, Ithaca, New York Received December 16, 1940
The present investigation (completed in 1939) is the third in the current series of phase rule studies on the aqueous polyiodides being carried out in the senior author's laboratory (1, 2). It consists of the usual exploratory isothermal surveys, followed by a determination of the major part of the ternary space model. The work as a whole has been restricted to solidliquid systems in air under approximately standard pressure, and no important changes have been made in experimental methods.' Up until a few months ago (Le., July. 1940), when a paper by Cheesman, Duncan, and Harris ( 7 ) appeared on the system sodium iodide-iodinewater a t O"C., very little on the formation of polyiodides of arliem in the presence of water was to be found in the published -il About the only significant reference to the subject was contained in s paper by Grace (8), who reported the results of a brief and unfinished investigation of the system sodium iodide-iodine-water a t below room temperatures. He obtained a hydrated polyiodide in the form of acicular crystals and, on what was extremely doubtful evidence, suggested the formula NaI, .4H20. The work of Cheesman, Duncan, and Harris is much more complete. They give the diagram of the system a t 0°C.and show conclusively that two hydrated polyiodides are formed a t that temperature. In view of the fact that their work was not published until after our own work had been completed and presented for publication,Z we shall defer the discussion of their polyiodide formulas and return to the subject later. Cheesman, Duncan, and Harris, however, find no evidence in support of Grace's hydrated triiodide. THE EXPLORATORY ISOTHERMAL SURVEYS
The diagram of the system sodium iodide-iodine-water a t 25°C. has been determined by Strelnikov (12). The only solid phases are NaI.2H20 and iodine, and the composition of the single ternary invariant solution (determined for us by E. J. Blau and G. W. Waring) is 63.8 per cent iodine, 1 The isothermal surveys (by W. F. Geigle) preceded the polythermal work on the space model (by J. L. Eaton) by about three years. Some of the experimental determinations also were made by Messrs. E. J. Blau, G. W. Waring, and S. S. Hubard, whose help is gratefully acknowledged. * T h e original paper has been withdrawn and replaced with the present communication.
596
T. R. BRIGGS, W. F. GEIGLE, AND J. L. EATON
25.3 per cent sodium iodide, and (by difference) 10.9 per cent water. compositions in this paper are percentages by weight.
All
The surveys at 0" and -15°C. The experimental procedure has been described before (4) and requires little further comment here. However, in dealing with mixtures containing the polyiodides which were found to be present a t 0" and -15"C., it proved to be extremely difficult to obtain a suitable sample of the wet solid residue for the convergence analysis, owing to the fact that there was so little difference in composition between the saturated liquid and the pure crystals. Various procedures for removing the mother liquor were tried ; in many cases the crystals were dried on filter paper before analysis, the TABLE 1 The invariant solutions at 0" and -16°C. P O I m IN AND 4
?raunma 1.2,
LIQUID BOLID PEUBICB
I
NaI
NaI.2Ho0, X X, iodine
43.9 56.0
35.1 26.8
D
IiaI.2H90, Y
E
y, z Z, iodine Ice, iodine y, x X, iodine
8.5 39.3 48.6 35.8 47.8 51.8
54.9 34.8 26.0 23.7 30.3 27.8
At 0°C.:
B C
At -i6"C.:
F G B' (m)' C' (m)
*m = invariants known t o be metastable in the isothermal surveys.
paper resting on the bottom of a stoppered glass container which was held suspended in the constant-temperature bath. All analyses were made by the distillation method (determination of free and combined iodine), which, though less satisfactory in principle than the "eompleteJ' method for sodium iodide, iodine, and water used by Grace and by Cheesman, Duncan, and Harris, had nevertheless proved accurate in earlier work done in this laboratory (cf. 4;also 1, 2). Aside from the ternary invariants which are shown in table 1 (each invariant the mean of several determinations), we shall not present the mass of analytical data that was accumulated during these isothermal surveys. Instead, we shall show the general results in graphical form (figures 1, 2, 3, and 4). A single polyiodide-to be designated hereinafter as "polyiodide X"-was found a t 0°C. (cf. figure l), and two additional
POLYIODIDES O F SODIUM.
I
597
polyiodides--"polyiodide I"' and "polyiodide Z"-were found at - 15°C. (cf. figure 2). Polyiodide X separates in the form of blue-black needles and is believed to be identical with Grace's polyiodide; the crystals of polyiodide Y are blue-black plates, while those of polyiodide Z are characteristically granular and gritty. Both isot,hermal diagrams (cf. figures 1 and 2) are partly metastable. This fact was not definitely known to us when the diagrams were first determined, but became evident later after the binary system sodium iodidr-water (cf. 3) had been more thoroughly studied, and the polythermal ternary diagram (figure 5 below) had been obtained. We shall speak more explicitly about this mat,ter later on. Definitely metastable conditions, however, were encountered and duly recognized during the work on the - 15°C. isothermal; these may be seen with the aid of figure 4 (Le., lines EB', B'C', and C'F). B'C' is the solubility curve of a metastable polyiodide, the crystals of which were the fine needles that are so characteristic of polyiodide X. The metastable invariants a t B' and C' are shown in table 1. The convergence diagrams obtained from the data a t 0" and -15°C. are given in figure 3 (polyiodides X and Z) and figure 4 (polyiodide Y), the solid points shown on the various KaI:Iz ratio lines indicating hydrated forms which differ by a single molecule of water. The indicated formulas are: 4KaI. 51z.13--15Hz0for polyiodide X, 5 S a I . 31z.17--19H20 for polyiodide Y, and either 5 S a I . TI2.26-28€IzCi or 2XaI. 312.IO-12H20 (t,etraiodide formula) for polyiodide Z. Such complex formulas as those just given were obviously disconcerting, but though every effort was made to find out, if some kind of systematic error was present, none such could be discovered. Polyiodide X (apparently a hydrated form of the complex Sa13.b)did not conform to Grace's triiodide formula, while polyiodide Y (appareiitly a hydrated form of the complex Na12.p)indicated a new type of polyiodide for univalent metals,-i.e., one containing less than three atoms of iodine per atom of metal. A special experiment was carried out on a sample of polyiodide X, as follo~w:A qiantity of the needle-like crystals was taken from one of the test rnixhres a t 0°C. and given the filter paper treat,nient a t that, tcmperature to remove as much of the mot,her liquor as possible. The product was then analyzed. The temperature was next raised a few degrees and, after further treatment with filter paper, the analysis was repeat,ed. As this process was continued, little change occurred in the analyses until the temperature reached 14°C.. above which point there was a sharp increase in sodium iodide and a corresponding decrease in iodine,-all of which indicates that the particular saniple of polyiodide X contaminatd with n s n i d amount of mother liquor began t o decompose into solid (XaI.2Hz0) and liquid a t about 14°C. Since Grace (8) reports that his polyiodide
FIG. 1. The experimental isothermal for sodium iodide-iodine-water at 0°C.
.---
io
PO
30
40
50
60
\
70
80
90
IO0
FIG.2. The experimental isothermal for sodium iodide-iodine-water at -15°C. 598
POLYIODIDES OF SODIUM.
599
I
(which undoubtedly contained rather more of the mother liquor as well as mother liquor of another composition) decomposed a t 11.8OC.' a not
40
45
50
55
GO
65
70
75
" E 6 C € A T /OO/N€
FIG.3. The isothermal indirect analyses for polyiodides X and 2
FIG.4. The isothermal indirect analysis for polyiodide Y
very different temperature, the experiment is fairly good proof that Grace's polyiodide and polyiodide X are identical compounds. A similar experiment carried out with polyiodide Y and begun at - 15°C. showed a similar decomposition into solid (NaI. 2Hz0) and liquid some-
600
T. R. BRIGGS, W. F. GEIGLE, AND J. L. EATON
where between -2" and O'C., and a third experiment with polyiodide Z (also begun a t - 15°C.) indicated decomposition into solid (iodine) and liquid between -8" and -6.2"C. We shall return to these experiments later when we. come to the polythermal ternary diagram (figure 5 below). THE POLYTHERMAL SURVEY
A . T h e binary systems The iodine-water system (cf. 9) requires no discussion here. The diagram of the system sodium iodide-water is given in a recent paper from this laboratory (3). The solid phases are ice, NaI.5H20, NaI.2Hz0, and NaI. The binary invariants are given later in table 3. In an earlier paper (5) from this laboratory it was stated that anhydrous sodium iodide is almost insoluble in liquid iodine. Recently, however, Plotnikov, Fialkov, and Chalii (11) have published some results on the conductivity of sodium iodide in liquid iodine which would seem to indicate that the salt dissolves freely (up to 25 per cent) in liquid iodine a t 130140°C. We have therefore reinvestigated the solubility, with the following results: Five test mixtures, each containing about 10 g. of dry iodine and 2 g. of finely pulverized anhydrous sodium iodide, were heated a t 120-140°C. for various lengths of time (10 min. to 48 hr.). The liquid phase was then filtered through a plug of glass wool and analyzed for combined iodine by the distillation method. The average sodium iodide content waa found to be 0.29 f 0.14 per cent; hence the two components appear to be almost immiscible. In view of the foregoing results, it seems unlikely that Plotnikov and his associates could have been dealing with the concentrations of sodium iodide claimed in their paper. An examination of this paper fails to show that any of the supposed solutions of sodium iodide in iodine were ever directly analyzed in order to make sure that the salt, the crystals of which are not easily visible in the opaque liquid, had really dissolved; indeed, we are led to infer from the paper that the "concentrations" were merely the total compositions of various sodium iodide-iodine complexes. Furthermore, Plot,nikov and his associates found that the conductivity of these Complexes, besides being more or less independent of the sodium iodide content, was also not very different from that of liquid iodine itself-a further indication, of course, that the solubility is small.
B. T h e ternary system Since the general procedure employed in investigating the ternary system has been adequately described in two preceding papers (1, 2), we shall pass directly to the experimental data. The latter have been assembled in tables 2 and 3 (the data marked "BG" in table 2 are from the paper by
POLYIODIDES O F SODIUM.
601
I
TABLE 2 The temperature-composition data for the ternary boundaries SERIAL NO.
?OSITION ON DIAQRAY
LIQUID
E YPE R AT llR
I
~
N a I I
S O U D PRASES
‘C.
S 1
2 3 4 5 6 7 8 9 10
SL SL SL SL SL SL
SL
SL SL SL
0 -1.6 -3.8 -5.7 -7.3 -8.5 -9.7 -13.4 -16.2 -20.3 -22.9
trace 6.0 13.2 19.0 23.1 25.7 28.2 32.9 36.4 39.9 40.8
O 7.0 13.1 16.5 18.8 20.1 21.0 22.4 23.9 24.9 25.2
11 12
L L
-24.7 -24.7
42.3 42.1
i:::
13 14 15 16 17
LB LB LB LB LB
-26.9 -28.1 -29.2 -30.1 -30.8
36.9 34.0 31.2 29.3 27.6
32.9 33.8
18 19
B B
-33.3 -33.3
22.4 22.2
37’3 37.3
20 21 BG
BA BA A
-32.4 -31.7 -31.5
15.6 9.0 0
47.1
BG 22
I
-12.3 -13.2
0 4.6
60.2 57.3
23 24
H
-13.9 -13.9
9.8 9.6
54.2 54.4
25 26 27 28 29 30 31
HJ HJ HJ HJ HJ HJ
-14.0 -14.5 -15.2 -16.3 -17.0 -18.7 -20.4
9.9 11.1 13.3 15.2 18.0 22.4 25.2
53.8 52.6 50.5 49.0 46.9 43.6 41.5
J J
-21.8 -21.8
27.6 27.7
39.7 39.8
32 33
IH H
HJ
I1 Ice, iodine
,
1)
1)
i:::
Ice, iodine, and 2
Ice, Z, and NaI.5H10
Ice and N a I . 5 H 2 0 and ) NaI.2H10 NaI.5H20 Y, and ) NaI.2H20, NaI.5H20
Y, NaI.5H20
1)
Y, Z, and NaI.5Hz0
602
T. R. BRIGGS, W. F. GEIGLE, AND J. L. EATON
TABLE 2-Continued SERIAL NO.
'08IZION ON DIAQRAM
LIQUID lMPERATUI
I
NaI
11
SOLIDPHABES
'C.
34 35 36 37 38 39 40 41
JK JK JK JK JK JK JK JK
-19.4 -17.8 -15.6 -15.0 -14.0 -13.2 -11.7 -11.0
31.7 34.1 37.9 38.6 39.7 41.7 44.2 45.4
36.7 37.8 35.1 31.9 34.2 33.4 32.3 31.6
42 43
K K
-10.1 -10.0
47.0 47.0
31.1 31.0
44 45 46
KC KC
-9.0 -8.5 -7.8
49.1 50.0 51.6
29.9 29.2 28.3
47 48
C C
-7.0 -7.0
53.7 53.7
i;:!
49 50 51 52 53 54 55 56 57
CL CL CL CL CL CL CL CL CL
-7.5 -7.6 -10.2 -13.2 -15.4 -17.4 -19.5 -20.9 -23.4
53.4 53.3 51.3 49.1 47.8 46.5 45.5 44.6 42.8
26.5 26.2 26.1 26.1 26.0 25.9 25.7
58 59 60
BJ BJ BJ
-30.8 -28.7 -24.5
22.8 23.9 26.0
61 62 63 64 65 66 67 68 69 70
HM HM HIM HM HM HM HM HM HM HM
-12.1 -7.8 -5.9 -4.4 -3.2 -2.4 -1.4 0.0 0.8 1.3
11.o 16.4 20.1 23.7 27.2 29.9 32.8 37.9 40.5 42.4
71 72
M
1.9 2.0
45.2 45.0
KC
hl
46.3 44'4
42.8 41.0 38.2 36.8 35.8 34.5 34.4
! )
Y'
J
1) j
1)
x1" X,
z
X, Z, and iodine
~
1
Z, iodine
,
/I
WaI.2H20, Y
i
J
0
X, Y, and XaI.2H20
POLYIODIDES OF SODIUM.
603
I
TABLE 2-Continued BBRIAL NO.
POSITION ON DIAQBAM
I
LIQUID EMPERATUR
I
T
I
“C.
73 74
MK blK
0.0 -4.1
45.8 46.4
33.7 32.4
75 76 77 78 79 80 81
MD AID MD MD MD MD MD
3.5 5.5 7.7 11.3 14.4 14.8 15.9
46.5 47.7 49.6 52.9 56.4 57.1 59.3
28.9 28.4 27.2
82 83
D D
16.4 16.3
60.6 60.4
26.3 26‘3
84 85 86 87 88 89
DC DC DC DC DC DC
15.0 12.2 10.2 5.1 0.0 -3.4
60.3 59.7 59.2 57.3 55.8 54.7
26.1 26.3 26.7 26.8
90 91 92 93 94 95
DF DF DF DF DF DF
18.3 19.4 21.8 25.6 30.9 35.0
61.3 61.6 62.4 63.2 64.7 65.7
26.0 25.9 25.7 25.3 24.6 24.3
96 97
F F
37.6 37.6
66.1 66.1
24.4 24.2
FE
42.2 44.6 46.2 49.5 53.0 55.7 58.2 60.3 62.3 63.1 63.8 64.4 65.0 65.5 66.1
63.7 62.1 61.0 58.0 54.6 51.3 47.4 43.1 36.6 32.8 28.5 23.6 18.2 14.8 9.3
26.1 27.3 28.2 30.2 32.5 34.8
98 99 100 101 102 103 104 105 106 107 108 109 110 111
112
FE FE FE FE FE FE FE FE FE FE FE FE FE FE
67.3
SOLID PEASES
1)
x, Y
1
1 1
X, NaI.2H20, and iodine
X, iodine
NaI.2H20snd iodine
1) 1 1
1
J
S a I , SaI.2H20, and iodine
604
T. R. BRIGGS, W. F. GEIGLE, AND J. L. EATON
TABLE 2-Concluded LIQUID
I
1
NaI
'C.
113 BG
FE
114 115 116 117 118 119 120
FG FG FG FG FG FG
E
G
1
BOLID PEABbE
-
66.8 68.1 43.7 48.3 54.2 64.8 70.0 75.2 113.5
68.4 70.0 72.4 75.8 78.2 80.7 :a. 99.8
22.7 21.4 19.9 17.6 15.6 13.7 cq. 0 . 2
NaI, iodine
Briggs and Geigle on sodium iodide-water) and the resulting diagrams appear in figures 5 and 6. The latter is the Janecke projection on the sodium iodide-iodine plane. The eight fields appearing in the phase diagram belong respectively to ice, iodine, NaI, KaI.2Hz0, N a 1 . 5 H ~ 0 and , the three polyiodides XI Y, and Z,-no additional polyiodides having been discovered in the polythermal survey. The field for X is M D C K M , the field for Y is H M K J H , and the field for Z is J K C L B J . Each polyiodide melts incongruently. Of the nine three-solid invariants (table 3) only one (i.e., B) is a ternary eutectic; there are also no dystectics. In figure 5 the isothermal contour lines-except for the contour a t 25°C.- have been drawn only in the fields for ice, NaI. 5H20, and the three polyiodides, where they are spaced a t 5-degree intervals between -30" and 15°C. The other fields are generally much steeper in relation to the temperature axis (cf. figure 6). As solutions are cooled along the several ternary boundaries, there appears to be positive separation of both saturating solid phases along all of A B , B L , B J , J K , M K , CD, and DF. Iodine separates negatively along most of S L and all of L C ; NaI .2Hz0 separates negatively along I H , H M , and M D ; sodium iodide separates negatively along EF (and possibly along part of FG) ; and polyiodide X separates negatively to a slight extent along all of KC. Polyiodide Y appears to separate negatively along part of H J near J , and positively (with positive separation of NaI.5H20) along the remaining portion. As already stated, each polyiodide melts incongruently when heated. According to figure 5, polyiodide X gives invariant liquid, NaI .2H20, and iodine at 16.4"C. (point D ) ;polyiodide Y gives invariant liquid, NaI 2H20, and polyiodide X a t 2.0"C. (point M ) ; polyiodide Z gives invariant liquid, polyiodide XI and iodine a t -7.O"C. (point C). None of these decomposition points, however, has been directly tested by heating the polyiodides, e
POLYIODIDES OF SODIUM.
605
I
because of the difficulty of preparing these compounds free from mother liquor. It will be recalled that in the isothermal studies a sample of polyiodide X, contaminated with a small amount of mother liquor, began to decompose into N a I . 2 H z 0 and liquid a t about 14OC. The preparation was made a t OOC.; hence the composition of the mother liquor a t that temperature was some point on the O'C. isothermal in the field MDCKM (polyiodide X). Now, the subsequent process of heating to 14OC.presumably must have caused the composition of the adhering mother liquor to move in the direction of X (figure 5) along a tie line connecting X with the original TABLE 3 T h e binary and ternary invariants LIWID INVARIANT
SOLID PHASE8
TEMPERATURE
I
NaI
0 0.2 47.1 60.2 74.8
Ice, iodine S a I , iodine Ice, Pl'aI.5&0 NaI , 5H20, KaI . 2 H 2 0 iYaI.2H20, S a 1
37.3 25.6 54.3 39.8 31.1 31.5 27.1 26.3 24.3
Ice, Z, NaI.5H20 Ice, Z, iodine S a I . 2 H 2 0 , Y, iYaI.5H20 Y, Z, N a I . 5 H z 0 X,Y, z X, Y, i\'aI.2He0 X, Z, iodine X, N'aI.2H20, iodine KaX, KaI.2H20, iodine
'0.
S G A I E
0 E* 113.5 E -31.5 E -12.3 U 68.1 U
Trace 99.8 0 0
B L
-33.3 E -24.7 U -13.9 U -21.8 Tj -10.1 u 2.0 G -7.0 u 16.4 U 37.6 U
22.3 42.2 9.7 27.7 47.0 45.1 53.7 60.5 66.1
H .l
K 521 C D F
0
* Invariant temperatures: E
=
eutectic, U = transition point.
point on the 0°C. isothermal-a tie line which must intersect the boundary M D (polyiodide X-XaI. 2HzO-liquid)-and accordingly one would expect the preparation to decompose into Sa1 .2Hz0 and liquid a t the temperature corresponding to this intersection. Since all temperatures on M D are lower than 16.4"C. (point D ) , it is evident that the decomposition point actually found (Le., 14°C.) and the phases resulting (Le., ?;aI.2Hz0 and liquid) are in accord with the phase diagram (Le., figure 5). Point X in figure 5 has been located on the supposition that polyiodide X has the formula 4KaI. 512.15H20. Similarly, polyiodide Y prepared a t - 15°C. with adhering mother liquor originally on the - 1 5 O C ' . isothermal in the field H M K J H should begin t o decompose (into NaI. 2H20 and liquid) at a point on the H M boundary
G
S
10
20
30
40
50
GO
P ~ E cfNriomv=
70
80
90
FIG.5. The polythermal diagram for sodium iodide-iodine-water
FIQ.6. The Janecke projection on the sodium iodide-iodine 606
plane
130
POLYIODIDES O F SODIUM.
I
607
corresponding t o a temperature below 2°C. (point M ) . I n the special experiment, it actually did so between -2" and 0°C. Polyiodide Z was removed at -15°C. from a liquid phase containing (by direct analysis) 45.0 per cent of iodine and 29.1 per cent of sodium iodide, and, by construction on figure 5 , should begin to decompose (into iodine and liquid) on the LC boundary a t a point just below C ( - 7.0"C.). It actually did so somewhere between -8" and -6.2"C. in the special experiment. Points Y and Z have been located in figure 5 on the supposition that the polyiodide formulas are 51;aI. 31z.18Hz0and 2 S a I 31z.llHzO, respectively. The interpolated isothermals The contour lines shown in figure 5 have been located in accordance with the method described in the paper on the potassium system (2). The small crosses shown along all of the 25°C. isothermal and parts of the 0" and -15°C. isothermals are the actual data which were obtained in the isothermal studies and which appear (excepting the data for 25°C.) in figures 1 and 2. The only field in which the shape of the contours is at all uncertain is the field for X a I . 5Hz0. According to figure 5, the 0°C. isothermal diagram really consists of four solubility curves, as follows: for F a I . 2 H 2 0 (not shown in figure 5 ) , for polyiodide Y (shovin in figure 5 ) , for polyiodide X (shown in figure 5 ) , and for iodine (not shown in figure 5 ) . The stable isothermal invariants thus correspond in order to S a 1. 2H20-Y-liquid, Y-X-liquid, and Xiodine-liquid. If the figure 1 be reexamined, it will be scen that the first two of theqe invariants were missed in the isothermal survey, the single metastable invariant for S a 1.2H2OPX-liquid being obtained instead. Polyiodide Y failed to appear at any time during the survey, but this compound %as found a t 0°C. by Cheesman, Duncan, and Harris in their recent work (7), and their diagram s h o w the four liquidus curves required by figure 5. The iFotherma1 for - 15°C. in figure 5 is seen to consist of five solubility curves, as follows: for XaI. 5Hz0, for polyiodide Y, for polyiodide Z, for iodine (not shown in figure 5 ) , and for ice. The stableinvariants thus correspond to XaI. 5Hz0 Y-liquid, Y-Z-liquid, Z-iodine-liquid, and iceiodine-liquid, in that order. Since the stable pentahydrate XaI .5Hz0failed to appear in the isothermal survey at - 15"C., the metastable invariant for Sa1 .2H~O-Y-liquid is found in figure 2 instead of the stable invariant for S a 1 BHzO-Y-liquid. In addition, the metastable invariants for Y-Xliquid and X-iodine-liquid were also located in the isothermal survey at - 15°C. (points B' and C' in figure 4). Two polyiodides (undoubtedly polyiodides X and Y) were obtained by Cheesman, Duncan, and Harris (7) in their work a t O'C., and, as we have just said, their isothermal diagram is in general conformance with the
608
T. R. BRIGGS, W. F. GEIGLE, AND J. L. EATON
polythermal diagram (figure 5 ) . For the sake of an easy critical comparison, we have assembled the various determinations of the isothermal invariants in table 4. It will be seen that our determinations agree very well with those of Cheesman, Duncan, and Harris except in the case of the invariant for X-Y-liquid, in which case, curiously enough, their determination is almost exactly the same as our two independent determinations of the metastable invariant for ?;a1 .2HzO-X-liquid. According to the ternary diagram (figure 5), polyiodides X and Y are isothermally congruently soluble in water over characteristic temperature TABLE 4 Comparison of the isothermal ternary invariants
8 0 L I D PHABEB
NTERPOLATED FROM T H E POLYTEERYAL DIAGRAM
- ___
Y , Z. . . . . . . . . . . . . . . . . . . .
z,
I*. . . . . . . . . . . . . . . . . . . . . Ice, Iz... . . . . . . . . . . . . . . N a I . 2 H z 0 . Y (m) . . . . . . Y, X (m). . . . . . . . . . . . . . . . X, Iz (m). . . . . . . . . . . . . . .
Reference 7
This paper
1
I
NaI
I
i
NaI
63.3
25.2
63.8
I
25.3
Not determined
37.8 45.7 55.8 43.8
38.3 33.7 26.7 34.8
Not found Kot found 56.0 26.8 43.9 35.1
38 2 38 5 43 9 34 9 55 8 26.7 Not found
12.8 38.8 48.0 35.2 8.8 47.2 51.9
51 .O 34.8 26.1 23.5 54.6 30.2 27.5
Not found 39 3 34 8 48 6 26 0 35 8 23 7 8 5 549 30 3 47 8 51 8 I 27 8
Not determined K o t determined X o t determined S o t determined Kot determined S o t determined Not determined
___ A t P5"C.: N a I . 2 H z 0 , I z , .. . . . . . . . . . . A t 0°C.: X a I , 2Hz0, Y . . . . . . . . . . . . Y , X .................... x, I * . . . . . . . . . . . . . . . . . . . ?uTaI.2H20,X (m)*. . . . . . . At -15°C.; NrtI.SHz0, Y . . . . . . . . . . . .
DIRECTLY DETERMINED B Y T E E ISOTHERMAL YETEOD
-
~
-___ I
~
Nal
~
I
~
~
* m = metastable.
ranges, while polyiodide Z is not congruently soluble a t any temperature. On the supposition that the polyiodides have the complex formulas already given (cf, however, the later discussion of this question), the ranges are approximately 1.5" to 15.5"C. (polyiodide X) and - 16.5" to 02°C. (polyiodide Y). Neither range has been tested by direct experiment. The polythermal indirect analysis After the completion of the polythermal phase diagram, a new and independent determination of the polyiodide formulas was attempted by the non-isothermal method of crystallization paths, supplemented in part with analyses of the wet solid residues (given the treatment with filter paper).
POLYIODIDES OF SODIUM.
609
I
results appear in tables 5 and 6, and in figures 7 and 8 (the determinations in table 6 were made for us by S. S. Hubard). It is unfortunate that the temperatures were not recorded in part of this work (table 5 ) , but they
. The
I
I
I
I
I
i
35,
I
I 40
a5
d 70
I 65
t 60
55
80
me ccNr
/OD/NS
FIG.7. The polythermal indirect analyses for polyiodides X and Z I
I
I
I
J
+OS
I I 1 I I 15 be 30 35 40 25/?E,
I
“5
J
50
C E N T /OD/N€
FIG.8. The polythermal indirect analysis for polyiodide Y
are not needed in the indirect analysis. The surveys are numbered in the figures to correspond with the tables; the circles on the tie lines represent the analyses of the liquid and the crosses the analyses of the wet solid, as in the previous diagrams (figures 1, 2, 3, and 4). While it is true that some of the crystallization paths are extremely
610
T. R. BRIGGS, W. F. GEIGLE, AND J. L. EATON
short, owing mainly to the shape limitations of the polyiodide fields in figure 5, the indirect analysis seems on the whole to point to fairly definite TABLE 5 Polythermal indirect an, isis LIQUID
WET EOLID
SURYEY NO.
I
NaI
I
NaI
49.31 47.58
31.23 31.80
55.18
29.40
2
54.90 52.32
28.53 28.85
57.38 57.31
28.28 28.05
3
53.40
27.74
56.66
27.96
4
58.45 58.10
27.11 26.77
58.92
27.76
20.90 14.14
47.81 50.10
32.61
43.68
2
33.92 29.40
39.98 39.53
38.18
40.49
3
39.53 38.48
36.72 34.94
40.37
38.86
4
44.00 45.23
34.26 31.93
42.10
38.52
1
53.34
26.94
57.52
25.00
2
49.68 44.36 38.34 35.86
27.98 30.02 32.34 33.32
57.67
24.82
3
48.75 45.99 43.36 39.13
26.85 27.44 28.22 29.16
57.07
24.65
4
46.92 43.01 42.61
26.45 27.21 27.42
56.00
24.71
Polyiodide X : 1
Polyiodide Y: 1
Polyiodide Z:
conclusions, which are more or less the same as those reached in the earlier isothermal surveys. Thus polyiodide X appears to be a hydrated form of
POLYIODIDES O F SODIUM.
61 1
I
the complex 4SaI.51z (Le., Xa13.6),polyiodide Y a hydrated form of the complex 5XaI.31z SaIz.z),and polyiodide Z a hydrated form of the (tetraiodide) complex 2YaI. 31z (perhaps one of the following: 2Sa14.llHzO, Sa14.6Hz0, or Sa14.5Hz0). If polyiodides X and Y really possess these complex formulas (Xa4114.14-16HzO and SasIll.17-19H~0, respectively), it may be because they are lattice compounds (cf. Grace (8)), TABLE 6 Polythermal tndzrect analysts (crystallzration paths o n l y ) TEMPERATURE
BURVEY NO.
I
NaI
55.33 54.33 52.49 50.65 49.42
28.92 28.89 29.19 29.36 29.33
57.97 57.29 56.91 56.59
27.60 27.12 26.96 26.70
-5.6 (-4r.) -11.0 -17.4
29.65 26.90 22.56
42.66 43.68 44.02
6
-6.4 (Ar.) -10.8 -15.7 -19.1
28.63 27.22 24.79 22.99
42.57 43.06 43.29 43.23
7
-2.2 (Ar.) -7.5 -12.5
42.12 42.06 42.26
35.69 34.50 33.28
". Polyiodide X : 5
10.0 ( A r . , * 7.1 1.1 -5.5 -10.0 12.9 ( A r . ) 12.5 7.9 5.0
6
0.0
Polyiodide Y: 5
~~
* Ar.
= Arrest on solubility surface.
in which the normal combining proportions are strongly modified by the fact that the sodium ion is so small and the iodide ion is so large. If this hypothesis is correct, similarly complex (and less stable) polyiodides may be expected in the system lithium iodide-iodine-water.8 8 Grace (6) has made a brief preliminary study of this system; he reports t h a t a t least one highly hydrated polyiodide is formed a t low temperatures. The formula, however, is unknown.
612
T. R. BRIGGS, W. F. GEIGLE, AND J. L. EATON
Cheesman, Duncan, and Harris, however, give different (and much simpler) formulas for polyiodides X and Y. According to them, polyiodide X-like our polyiodide Z-is a hydrated tetraiodide (Le., xa14.2H20) and polyiodide Y is a hydrated diiodide (Le., h'aIz.3Hz0). Their analyses are not very numerous, but they were carefully done by the "complete" method for iodine, sodium iodide, and water, and the evidence given in support of one of the two formulas (Le., NaIa. 2H20) is very good indeed. The formula ?;aI:!.3Hz0 given for polyiodide Y is based on very little evidence, as Cheesman, Duncan, and Harris frankly admit. Only two extrapolations through wet solid residues were made and these were not convergent. The diiodide formula, however, does possess the attraction of simplicity (the same thing is true of their tetraiodide formula for polyiodide X), but simplicity is not proof in itself, and it is evident, therefore, that still more work will have to be done on the polyiodides of sodium before this question can be finally settled. At any rate, our own work is in complete agreement with that of Cheesman, Duncan, and Harris as regards one important point,-&., polyiodide Y, which certainly contains less than three atoms of iodine to one of sodium, is a new t y p e of polyiodide for a univalent metal. In view of our own work and that of Cheesman, Duncan, and Harris, it is a t least fairly certain that polyiodide X is not the triiodide that Grace (8) supposed it to be. Grace based his formula (T\'aI3.4&0) largely upon direct analyses of the solid which still contained mother liquor, and if one examines his work critically in the light of the polythermal phase diagram (figure 5 ) , it seems very likely that his preparation was a mixture of polyiodide X and XaI. 2H20. Such a mixture of polyiodide X, S a I . 2H20,and mother liquor (on boundary MD in figure 5 ) could of course give a close approximation t o the formula h'a13. 4 H ~ 0 . ~ The complete absence of a solid triiodide in the sodium system is most surprising. Triiodides have been found in all the other systems so far investigated (Le., CsI3, RbI3, "413, S H & . 3Hz0, KI3. HzO,and KI3.2H20 in two forms). Furthermore, hlartin (10) has apparently found a triiodide (Le., ;";aIa. 2CeHbCN) in the system with benzonitrile, and Carter's measurements (6) of the solubility of iodine in dilute sodium iodide solutions are supposed t o prove that the ionized triiodide is formed. Our own work, however, as well as that of Cheesman, Duncan, and Harris, has given no indication of a solid sodium triiodide a t any temperature. 4 Grace's analysis of the solution in equilibrium with his solid preparation a t 11.6"C.lies exactly on the boundary .\ID in figure 5 . Such a solution would be saturated with both polyiodide X and KaI.2Hz0.
PdLTIODIDES OF SODIUM.
I
613
SUMMARY
1. The system sodium iodide-iodine-water has been surveyed isothermally and polythermally, and the phase diagram, restricted t o solidliquid systems in air a t approximately standard pressure, has been obtained. 2. Three different solid polyiodides have been found, and each is liydrated. On the basis of two independent sets of indirect analyses, the apparent formulas are as follows: KaaIlr.13-15H20 (i.e., 4KaI. SIz.1315H20; acicular crystals decomposing a t 16.4"C.), ?;asII1. l7-19H20 (Le., 5 S a I . 312.17-19H20; plate-like crystals decomposing a t 2"C.), and Nazis. IO-lIH20 (Le., 2SaI .31z. 10-llHzO; granular crystals decomposing a t -7°C.). The first of these compounds is Grace's polyiodide. X o solid triiodide of sodium has been found at any temperature, although triiodides of ammonium, potassium, rubidium, and cesium are readily obtained under similar conditions. 3. The recent work of Cheesman, Duncan, and Harris has been given careful consideration. They state that the first two polyiodides given above are S a r d .2H20 and (probably) KaI2. 3Hz0, respectively. All the formulas are therefore still open to question and further work will have to be done before the matter can be considered settled. REFEREK'CES (1) BRIGGS,BALLARD, ALIIICH,A N D R I K S W O :J. Phys. Chern. 44, 325 (1940). (2) BRIGGS,CLACK,BALLARD, A K D SASSAMAS: J. Phys. Chern. 44,350 (1940). (3) BRIGGS. ~ S D GEIGLE:J. Phys. Chern. 44, 373 (1940). A N D LEOSARD: J. Phys. Chem. 34, 1951 (1930). (4) BRIGGS,GREENAWALD, (5) BRIGGSA X D PATTERSOS:J. Phys. Chem. 36, 2621 (1932). (6) CAR'rER: J. Chem. S O C . 1928, 2227. A N D HARRIS:J. Chem. SOC.1940,837. (7) CHEESMAN, DUNCAN, (8)GRACE:J. Phys. Chem. 37, 347 (1933). (9) KRACEK:J. Phys. Chem. 36, 417 (1931). (10) MARTIN:J. Chem. SOC. 1932, 2641. FIALKOY, AND CHALII:Z.physik. Chern. A172, 304 (1935). (11) PLOTNIKOV, (12) STRELNIKOV: Bull. acad. sci. U. R.S.S.7, 715 (1933).
