614
T. R. BRIGGS,
c. c.
CONRAD,
c. c.
GREGG, AND W. E. REED
POLYIODIDES OF RUBIDIUM. I1 THEFREEZING POINT, SOLUBILITY, AND BOILING POINTRELATIONSHIPS IN THE SYSTEM RUBIDIUM IODIDE-IODINE-WATER AT APPROXIMATELY STANDARD PRESSURE T. R. BRIGGS, C. C. COSRAD, C. C. GREGG,
AND
W. H. R E E D
Department of Chemastry, Cornell University, Ithaca, New York Received October 30, 1940
The present communication is the second paper on polyiodides of rubidium t o appear under the direction of the senior author. The first paper-published in 1932 by Briggs and Patterson (3)-consisted of a phase rule investigation of the anhydrous binary system rubidium iodideiodine from the temperature of the eutectic to the boiling point of the saturated melt. The phase diagram was determined from thermal analyses supported by chemical analyses of the saturated melts, and it was thereby shown that rubidium forms a definite solid polyiodide (Le., the triiodide, RbI3) and that this solid is directly produced from melts consisting of rubidium iodide and iodine,-facts which have been denied, however, in a recent paper by Fialkov (lo), whose work will be examined critically later on in the course of this article (see section A, below). The only published phase rule study of the ternary system rubidium iodide-iodine-water seems to be that of Foote and Chalker (11), which appeared in 1908. -4lthough this work was confined almost entirely to the ternary invariant solutions existing a t a single temperature (Le., 25°C.) and the standard convergence method of indirect analysis was not employed, it was shown nevertheless that a solid polyiodide was formed and that this compound was undoubtedly a triiodide. S o evidence of any higher polyiodide could be obtained (cf., however, 12) and the possible existence of a hydrated triiodide (cf. 4, 5 ) apparently was not considered. EXPLORATORY ISOTHERMAL STUDIES'
One or two years prior to the beginning of the experimental work on the polythermal ternary system, preliminary surveys were made a t 25OC. and OOC. for the purpose of exploring the system and defining the solid phases, especially the polgiodides, by means of the convergence method of indirect analysis. The experimental procedure was the usual one involving solubility determinations and analyses of wet solid residues; hence we may pass directly t o the results which are presented in table 1. The solubility data give two reasonably complete and concordant isothermal diagrams. Neither of these, however, is shown here in the form 1
By C. C. Gregg and W. H. Reed.
POLYIODIDES O F RUBIDIUM.
615
I1
TABLE 1 Rubidium iodide-iodine-waterat db°C. and 0°C. TlKPERA TUBE
SERIAL NO. OR SODBCE
O'C.
EXTRAPOLATED
RbI
0
62.0
1
5.5
61.1
29.9
66.1
2 3 4 5 6 7 8 9 10 11 12 13
8.0 9.3 14.7 15.9 26.0 30.1 40.6 46.7 47.6 53.8 56.7 59.7
54.2 52.2 46.2 44.2 38.0 37.1 35.6 34.8 33.9 32.5 31.1 30.4
40.0
48.8
55.4 56.3
40.5 40.0
14
64.7
27.6
82.5
15.9
15 16 17 18 19 20 21 (13)
56.2 50.0 45.1 35.4 26.0 14.4 7.6 Trace
28.5 29.1 28.2 26.2 23.4 16.8 11.0 0
6.9 89.1 93.8 3.6 11.2 78.0 79.7 8.7 13.5 59.9 Not ai-. lyzed K o t analyzed
22
0
55.5
23
2.6
55.9
15.5
69.3
24 25 26 27 25 29
2.8 3.7 5.2 8.4 8.7 13.4
54.9 48.5 38.8 26.1 24.5 18.7
42.2 44.4 45.1
47.6 45.5 43.9
30
15.3
18.6
12.8
15.9 0
(9, 11)
25°C.
LIQUID
I
31 (13)
Trace
___
I
SOLID
RbI
RbI RbI, RbIa
RbIa
RbIa, iodine
Iodine
RbI RbI, RbIa
RbIs
RbIs, iodine N o t analyzed
I
} Iodine
616
T. R. BRIGGS, C. C. CONRAD, C. C. GREGG, AND
W. E. REED
of a separate figure, for most of the determinations on which they are based may be seen later in the ternary contour diagram (figure 3) plotted along the corresponding isothermals. The ternary invariant solutions a t 25OC. are in substantial agreement with those analyzed by Foote and Chalker, and it is also worth noting that the polyiodide (i.e., RbIa) is congruently soluble a t 25°C. but not congruently soluble at 0°C. When the convergence method of indirect analysis is applied to the data in table 1, conclusive results are obtained which show that the saturating solid phases are iodine, rubidium iodide, and rubidium triiodide at 0°C. as well as at 25°C. KOevidence suggesting a hydrated triiodide was found even at O"C., though one would be more likely to be stable a t 0°C. than a t 25°C. if it were present in the system (cf. 4,5). As to the actual results in the case of the triiodide (rubidium triiodide contains 45.55 per cent of rubidium iodide), the average of seven extrapolations to the rubidium iodide-iodine side of the triangular diagram is 45.9 =k 0.6 per cent of rubidium iodide for the 25°C. data and the average of five extrapolations is 45.2 =k 0.2 per cent rubidium iodide for the 0°C. data. The triiodide is definitely anhydrous, just as Foote and Chalker supposed it to be. THE POLYTHERMAL
SYSTEM^
A . The binary systems aodine-water and rubidium iodideiodine The article by Kracek (14) should be consulted for the iodine-water system. Later on, it will be necessary to consider certain features of this system in connection with the boiling points in the rubidium iodide-iodinewater system (section D below), a t which time we shall show the phase diagram in schematic form (figure 6 below). The diagram for the system rubidium iodide-iodine in mole percentages will be found in the paper by Briggs and Patterson (3) and will not be shown separately. After conversion of the composition data into percentages by weight for use in the present paper, the eutectic point (iodine-rubidium triiodide) becomes 803°C. and 15.1 per cent rubidium iodide, the transition point (rubidium triiodide-rubidium iodide) 188°C. and 40.6 per cent rubidium iodide, and the saturated boiling point (Le., melt, rubidium iodide, and vapor at 740750 mm.) 238OC. and 41.8 per cent rubidium iodide. In connection with the binary system rubidium iodide-iodine, however, it is necessary to consider the recent paper by Fialkov (10) to which we have already referred. In this article, in which he shows a number of iodideiodine binary diagrams based upon his own determinations, Fialkov flatly denies the correctness of the rubidium iodide-iodine diagram of Briggs and Patterson, claiming among other things that there is nothing in his own diagram to indicate the presence of any binary compound such as rubidium * B y C. C. Conrad.
POLYIODIDBS OF RUBIDIUM.
I1
617
triiodide. He also says the same thing about the senior author’s diagram for cesium iodide-iodine (1; cf. also 7 ) and refuses to concede the presence of either the triiodide, CsIs, or the tetraiodide, CSIP. ?;ow, one has only to examine Fialkov’s paper in the original in order to be able to perceive a t once the serious errors of omission which invalidate a large part of his work. For instance, he did not determine his phase diagrams in the higher ranges of temperature (this was done with great care in all of the work by the present senior author and his collaborators), and he apparently made no chemical analyses of any kind in support of his thermal analyses (Le., cooling curves). Fialkov’s diagrams are therefore quite incomplete and in general indicate little more than the position of the eutectics. The absence of chemical analyses is hard to understand, for Fialkov was admittedly familiar with the paper by Briggs and Geigle (2) on the potassium iodide-iodine system, and this paper had strongly emphasized the importance of making a t least a few direct solubility determinations as a check on the information obtained from cooling curves,-especially with systems of this nature where the melts are so heavy and opaque and the solubility curves are often so nearly independent of temperature. Incidentally, Fialkov’s article also contains the curious mistake about the solubility of sodium iodide in liquid iodine (cf. 6),-another result of failure to analyze the liquid.
B. The binary system rubidium iodide4ater The system rubidium iodide-water required a special investigation, because of the surprising lack of temperature-composition data in the published literature. The work consisted of freezing point, solubility, and boiling point determinations, all of which will be found in table 2. With three exceptions the composition values ascribed t o the present authors are direct analyses of the liquid phase, and the boiling points are corrected to 760 mm. (note that this is not the case with the ternary boiling points given later). The phase diagram-showing a few temperature arrests not given in table 2-appears in figure 1; it is of the simple eutectic type and requires no special comment. Before proceeding to the ternary system it seems desirable to say something about the rubidium iodide which mas used in the work. I t was prepared from a stock supply of the chloride by Rae’s replacement method (16); since few details about the method seem to be available, a brief summary of our own experience may not be out of place. The chloride was evaporated to dryness three times in pure constant-boiling hydriodic acid and the product thus obtained, contaminated with free iodine, was dissolved in an equal weight of water, evaporated to dryness, and heated in an oven at 120°C. The crude rubidium iodide resulting was dissolved in a minimum quantity of hot water, and absolute alcohol was added in small
618
T. R. BRIGGS,
c. c.
CONRAD,
c. c.
GREGG, AND
w. H. REED
increments as long as iodide crystals continued to be thrown down. The crystals were finally filtered off on sintered glass and dried, while other crops of crystals were recovered from the remaining mother liquor. By proceeding in this manner 73.6 g. of the iodide were ultimately obtained from 42.1 g. of the chloride-a yield of over 98 per cent-and three successive analyses of the product gave 99.84, 99.77, and 99.79 per cent of rubidium iodide, respectively. TABLE 2 Rubidium iodide-water: te ierature-composition data TIMPIRATUBE
I
I
RbI
LIouaClE
TEYPBRATUBB
Solution and ice OC.
wantby
-1.3 -3.5 -6.2 -8.4 -8.6 -12.2 -13.0 E -13.0 E
9.58 20.13 31.33 38.43 39.15 48.11 49.79 50.11
weighl
Solution and vapor (760 mm.)
101.1 102.4 104.6 108.8 110.7 113.8 116.3S 116.4 S 116.4S
15.56 30.79 46.04 60.93 65.58 71.57 75.7 t 76.0t 77.8 t
1
RbI
1
BOURCB
Solution and R b I OC.
par cant bv weight
-10.0 -2.7 0 3.2 6.9 13.9 17.4 18.0 24.3 25.0 25.0 25.6 35.6 48.5 59.4 77.2 93.0 111.5
* T h i s paper. E = eutectic point; S = saturat.-; (solid and liquid).
51.18 54.51 55.5 57.05 57.9 60.20 60.3 59.0 63.66 62.0 61.93 62.90 64.70 66.76 68.61 70.89 73.01 75.05
t = total composition
C. The condensed ternary system The methods which were employed in locating the ordinary solid-solid boundary lines in the ternary diagram need no discussion here (cf. 4, 5 ) . So serious difficulties were encountered, except in connection with the sampling of the saturated liquid phase a t temperatures above 7OOC.; hence the solubility data for the higher temperatures may be somewhat less trustworthy than those for the lower temperatures. The data for the boundaries have been assembled in tables 3 and 4, and they have also been plotted in figure 2 to produce the ternary projection diagram.
POIJYIODIDES O F RUBIDIUM.
619
I1
The ternary projection diagram, exclusive of the boiling point lines ( M N , RGI, and G2H) and the region of the conjugate liquids, is relatively simple. The solid phases are ice, rubidium iodide, rubidium triiodide, and iodine. There are no higher polyiodides and no hydrated triiodides (cj. 4, 5, 6). There is only one ternary eutectic (Le., point B : ice-rubidium iodiderubidium triiodide) and no dystectics. The triiodide melts incongruently a t point C (188'C.), where it decomposes into binary liquid and solid rubidium iodide. By a simple and obvious construction on the diagram
I
60
4
i
c
t
0.c
I
sohfion
o I
I
i
8riqqs g conrud
ofher
observers
So/r/fmn c R b l
I
ZO*
I
1
-?O.l
1
1
/v
1
1
"
'
1
' I
and interpolation of the RC and LE data of table 3 it is found that the triiodide is isothermally congruently soluble in water between 7.8" and
132°C. The method of partial composition vectors (cf. 4) has been applied to the boundaries with interesting results. For example, it is found that as the saturated liquid is cooled along the boundaries A B (ice-rubidium iodide), BL (ice-rubidium triiodide), and LE (rubidium triiodide-iodine), both solids of the saturating pair crystallize positively and departure of the liquid from the boundary is thereby precluded (cf. 4). Along practically
620
T. R. BRIGGS,
c. c.
CONRAD,
c. c.
GRECG, AND
w.
H. REED
TABLE 3 R u b i d i u m iodide-iodine-water: ternary boundary data ~~
SERIAL NO.
POSlTION O N DIAQRAM
LIQUID PEMPERATURP
I
RbI
SL
-0.5 -0.8 -1.1 -1.5 -1.8 -2.3 -2.4
2.61 3.80 5.02 6.27 9.34 11.30 12.05
15.79
L L
-2.6 -2.6
13.00 13.01
16.80 16.96
12 13 14
BL BL BL BL BL
-10.6 -6.5 -5.0 -3.9 -2.9
2.29 3.90 5.15 7.45 10.16
43.19 32.04 26.26 21.12 17.19
15 16
B B
-13.9 -13.9
1.67 1.66
50.20 50'07
17 18 19 20 21 22 23 24 25 26 27
BC BC BC BC BC BC BC BC BC BC BC
2.21 2.63 4.54 6.01 8.75 16,31 19.35 26.65 34.78 45.38 48.26
54.37 56.15 60.16 60.95 60.97 59.18 57.81 54.54 50.82 45.81 44.60
~~
I
PEASEU PRESENT
I
'C.
1 2 3 4 5 6 7 8 9
10 11
28 29 30 31 32 33 34 35 36 37 38 39
SL SL SL SL
SL SL
LE LE LE LE LE LE LE LE
LE LE LE LE
-3.7 0.1 17.6 25.4 35.6 50.6 54.2 61.4 70.8 97.3 110.1 -0.7 0.8 6.2 7.6 9.0 10.7 13.3 15.7 24.4 25.4 41.4 45.3
14.19 15.31 22.59 29.97 40.69 49.05 53.60 57.91 63.60 64.33 70.27 71.55
17.31 17.76 22.62 25.69 28.47 29.85 29.63 29.90 28.30 28.17 24.84 23.71
1
I
Ice, iodine, RbIS, liquid
Ice, RbI,, liquid
1
Ice, RbI, RbI8, liquid
'
>
RbI, RbI3, liquid
, '
RbIa, iodine, liquid
,
POLYIODIDES O F RUBIDIUM.
621
I1
TABLE 3-Concluded BERIAL NO.
POSITION ON DIAQRAM
LIQUID PEMPERATVRE
~
RbI “C.
40 41 42 43 44
73.44 76.90 78.53 79.68
22.88 21.90 20.63 19.04 18.26
60.0 65.4 68.9
45 46
98.4 98.4
21.49 83.40
15.35 10.94
47 48 49 50 51 52
94.6
23.52
38.33 41.57
16.42 18.30 19.48 20.49 22.07 22.51
74.0 72.2
53 54 55 56 57 58 59
71.3 73.0 82.0 82.7 88.1 93.8 94.4
60.10 63.17 71.54 72.10 76.08 80.26 80.79
22.06 21.30 18.05 17.49 15.78 13.47 12.96
~
60 61 62 63 64 65 66 67 68
116.4 116.6 117.0 118.1 :a.124 :a. 143
3.39 8.10 11.60 29.85 43.97 54.04
‘
1
RbIs, iodine, liquid
’
Iodine, two liquids
PEABE8 PREBENT ~~
~~~~
t Iodine, two liquids
12.98 8.89 6.15
Iodine, liquid, vapor
74.05 71.72 69.67 56.84 47.10 42.99
)RbI, liquid, vapor
all of the boundary S L (ire-iodine), however, iodine separates negatively and departure from the boundary may occur under certain circumstances (cj. 1). I n this respect, the boundary BC (rubidium iodide-rubidium triiodide) is especially interesting, for it is possible to draw from RbI3 in figure 2 a line tangent to RC a t the point marked K , and it is then found from the composition vectors that, although the triiodide crystallizes positively along the whole length of the boundary from C to B, the iodide crystallizes negatively from C t o K and positively from K to B. As a
622
T. R. BRIGGS, C. C. CONRAD, C. C. GREGO, A N D W. H. REED
TABLE 4 Rubidium iodide-iodine-water: binary and ternaw invariant points POBITION
IJOUID
ax DIAGRAM
IEUPEBATOBE
S
'C. O E -13.0 E
A E
80.8 E 188 u 98.2B 113 B* 116.0B 238 B
C R
H M
N B L GI G1
-13.9 E -2.6 U 98.4 B 98.4B
PBM-
PBICLIEPIT
I
RbI
Trace 0
84.9 59.4 0.4' 99.9' 0 58.2
0 49.9s 15.1 40.6 0 0 75.5 41.8
Ice, iodine, liquid Ice, RbI, liquid RbIs, iodine, liquid RbIs, RbI, liquid Iodine, liquid, vapor Iodine, liquid, vapor RbI, liquid, vapor RbI, liquid, vapor
1.67 13.0 21.5 83.4
50.1 16.9 15,3b 10.9
Ice, R b I , RbIa, liquid Ice, RbIa, iodine, liquid Iodine, two liquids, vapor
Temperatures: E = eutectic point; U = transition point; B 750 mm.). *Estimated from iodine-water diagram.
=i
boiling point (740-
-.-_.
P€R CENT /OO/N€
FIG.2. The system rubidium iodide-iodine-water a t 740-750 mm.
TABLE 5 Ternarv crystallization paths i n the rubidium triiodide field LIQUID P A m NO.
TEWEBATUBI
I
RbI
56.8 41.2 29.4 11.6
30.60 18.54 10.20 4.22
50.95 53.66 55,73 57.23
41.2, 29.7 18.9 4.4
18.68 10.48 5.97 2.96
53.02 54.62 55.22 56.17
41.56 30.0 19.0 4. l a
27.58 16.91 9.11 4.18
46.97 47.60 48.00 48.41
40.76 29.9 17.7 3.4 -4.9
31.84 22.09 11.06 5.00 3.09
44.57 44.13 44.01 43.50 43.30
39.25 26.3 15.05 2.16 -5.4
34.88 24.50 13.11 6.04 3.91
42.11 40.60 38.37 36.99 36.77
27.65 15.4
31.76 19.24
38.35 34,28
15.9 8.9 3.7 -1.46
24.37 16.10 11.58 8.23
30.75 26.36 24.35 22.51
2.5 -0.1 -2.4
11.29 9.46 8.25
23.54 22.21 21.77
72.0 25.5
47.47 45.15 42.34 36.41
42.20 41.11 39.18 35.73
15.0 8.2
29.47 21.95
29.38 24.52
OC.
5.. . . . . . .
