806
T. R. BRIGGS AND S. 6. HUBARD SUMMARY
The dye adsorption method of estimating specific surfaces of solids was appLed to a water-soluble crystal. The adsorption of alphazurine A in acetone solutions indicated that to saturate the surface of potassium nitrate crystals required 4.2 X lo-' g. of dye per square centimeter. This corresponds to three pairs of potassium and nitrate ions per dye molecule. REFEREXCES (1) BRAGG, W. H . , A X D BR.4GG, W. L.: The Crystalline State, Vol. I, p. 165. G. Bell and Sons, London (1933). (2) F R A N C E , W. G.: Colloid Symposium Monograph 7, 59 (1929). (3) GREEN,H.: J. Franklin Inst. 192, 637 (1921). (4) GREEN,H . : J. Franklin Inst. 204,713 (1927). (5) MARC,R.: Z . physik. Chem. 68, 104 (1909); 73,705 (1910). (6) ~ I A R C R.: , Z. physik. Chem. 79, 71 (1912). (7) MARC,R.: Z. physik. Chem. 76,710 (1911); 81, 641 (1913). J. B.: Xature 126, 814 (1930); Physics 1, 254 (1931). (8) KICHOLS, (9) PAXETH, F. : Radioelements as Indicators, Chapters VI and VII. McGraw-Hill Book Company, S e w York (1928). (10) PAULING, L.: The Nature of the Chemical Bond, p. 158. Cornell University Press, Ithaca, New York (1939). (11) PERROTT, G. ST. J., AND KIXNEY,S. P.: J. Am. Ceram. SOC.6,417 (1923). (12) ROWE,F. hl.: Colour Index, p. 183, Compound No. 714; Society of Dyers and Colourists (1924).
POLYIODIDES OF CESIUM. I11
THE FREEZING-POINT, SOLUBILITY, AND BOILING-POINT RELATIONSHIPS IN THE
SYSTEM CESIUMIODIDE-IODINE-WATER AT APPROXIMATELY STANDARD PRESSURE T. R. BRIGGS
AND
S. S. HUBARD
Department oj Chemistry, Cornell University, Ithaca, N e w York Received DecembeT 31, 1940
The present investigation is a comprehensive phase-rule study of the system cesium iodide-iodine-water in air under approximately standard pressure. It belongs with the current series of ternary studies (4, 5 , 6, 7) which, up to now, have dealt with the ammonium, potassium, sodium, and rubidium systems. Yo important changes have been made in the experimental methods. Two other studies on the polyiodides of cesium have already appeared from this laboratory. The first (8) was an isothermal study of the system sodium iodide-iodine-water at 25"C., and the second (3) was a polythermal study of the binary system cesium iodide-iodine. Two solid polyiodides
POLYIODIDES O F CESIUM.
I11
807
of cesium were found in both investigations; one proved to be the triiodide, CsIs, of Wells and Penfield (20), while the other-the higher polyiodide of Wells and Wheeler (21) and long supposed t o be Cs16-was shown to be a tetraiodide (Le., CsIJ by the convergence method of indirect analysis. The tetraiodide formula seemed surprising at the time, but it was soon confirmed by other investigators (12, 13, 17; cf. also 18). I. T H E BINARY SYSTEMS
The paper by Kracek (14) is recommended, as usual, for the iodine-water system. A schematic temperature-composition diagram of this system is shown in the paper on the rubidium iodide-iodine-water system (6) and will be found helpful later on when we come to the ternary boiling-point relationships. The diagram of the cesium iodide-iodine system will be found in the paper by Briggs (3). The solid phases are iodine, cesium iodide, cesium triiodide, and cesium tetraiodide; the two polyiodides melt incongruently when heated. The various invariant points in the system may be seen in table 3, below. The diagram, however, was based almost entirely upon thermal analysis, and inoculation with the solid phase about to appear was not resorted to while the cooling curves were being obtained. For this reason, apparently, the temperature (136°C.) originally found for the Cs14-CsIs-liquid transition point has been found to be too low and has been redetermined with the aid of the inoculation technique (cf. 4). The new tepperature is 140 f 0.5OC. and the liquid phase contains 30.5 =!= 0.2 per cent of cesium by direct analysis. This revision appears in table 3, in which all compositions are expressed in percentages by weight.'
A . The system cesium iodide-water
-4 special investigation of the cesium iodide-mater system had to be undertaken before the main ternary inv-stigation, because of the paucity of published freezing-point, solubility, and boiling-point data. The cesium iodide used in the work was prepared from the bromide by Rae's replacement method u-ith hydriodic acid (cf. 6). Five analyses of the product for iodine gave an average of 48.55 per cent, as compared with 48.86 per cent required by theory. The results of the work are given in table 1, together with the data obtained by the earlier investigators. Almost all of the compositions were determined by actual analysis of the saturated liquid phase. The phase diagram is shown in figure 1 and requires no special comment. The A few chemical analyses of melts saturated with iodine have also been made. The results (per cent of cesium iodide by weight) are: 8.5 (104.0°C.), 10.0 (1OO.IoC.), 12.6 (94.0°C.), 14.9 (S6.Z°C.), and 17.2 (79.3"C.).
808
T. R. BRIQGS AND S. 6. HUBARD
eutectic temperature is -4.OOC. and the boiling point of the saturated solution (corrected to 760 mm. in table 1) is 109.1OC. (108.6"C. at 740-750 mm.). 11. THE TERNARY SYSTEM
Foote's early work (11) on the ternary system was of considerable assistance in this part of the investigation. Foote located a ternary eutectic TABLE 1 Cesium iodide-water: temperature-composition data TEYPERATURl
j
CaI
I
8omm
PBKPRBATOBR
Solution and CsI
Solution and ice 'C.
w cent bv
-1.2 -2.7 -4.0 E -4.0 E -4.0 E
8.76 18.75 27.45 27.44 27.69
100.28 100.61 101.49 103.01 104.80 107.7 109.1 s 109.1 s 109.1 s
7.63 14.28 29.82 46.31 ' 56.97 67.84 71.38 74 t 75 t
weight
'C.
* * * * (11)
0.0 1.4 9.3 14.0 15.0 18.0 19.4 22.8 25.0 25.0 32.0 32.4 35.6 45.9 59.3 61 .O 61.3 77.7 88.0 102.8 109.1 €3
" , g i t b
30.6 31.41 36.90 39.8 40.3 41.13 43.32 47.94 46.1 46.9 49.98 50.05 51.48 55.54 60.43 60.0 60.75 65.24 67.16 70.25 71.48
* Briggs and Hubard (analyses of liquid phase); E = eutectic point; S = saturated with solid phase; B = boiling point; t = total composition (solid and liquid). for ice-CsI-CsIa-liquid a t - 4.O0C. and two ternary transition points, supposedly for ice-Cs16-iodhe-hquid and ice-CsI~-Cs16-1iquid a t -0.2' and -0.4'C., respectively. He also showed that two different sets of conjugate ternary solutions existed above about 5ZoC., one set saturated wit,h iodine and the other set saturated with what waa supposed to be cesium pentaiodide. The latter is really the tetraiodide, CsL, but, with this exception, all of Foote's work has been substantially corroborated by our own investigation.
POLYIODIDES OF CESIUM.
809
111
The main ternary projection diagram-which shows our actual analyses on the field boundaries-appears in figure 2. The data are given in tables 2 and 3. Figure 2 also shows many of Foote's data (small triangles), as well as a few crystallization paths which were specially determined in the triiodide and tetraiodide fields. Figure 3 is an enlargement of a portion of figure 2, with the addition of part of the 25°C. isothermal determined
o
x
L?Nqqs 8 uuhnrd ofher ohservers
i/
d /
SO/LIt/.O17
+csx
$'
P
0
FIG.1. The system cesium iodide-water a t 760 mm. by Briggs, Greenawald, and Leonard (8). The lines MQ, QZU, UN, RGI, GI-G2, and G2H belong to the boiling-point diagram,-i.e., they are lines along which the vapor a t 740-750 mm. is one of the coexisting phases. The other lines belong to the condensed system.
A . The condensed
system
The solid phases are ice, iodine, cesium iodide, cesium triiodide, and cesium tetraiodide. The ice field is almost invisible in figure 2, but may be seen in part in figure 3. The field of the triiodide extends over a very wide area in the projection diagram-a comparison with the triiodide fields
810
T. R. BRIGGS AND S. S. HUBARD
in the ammonium and rubidium diagrams is suggested a t this point-while a large part of the tetraiodide and iodine fields is masked by the binodal region. There are four ternary invariant points below the vapor region, viz., B a t -4.O"C. for ice-CsI-Cs13-liquid, K a t -0.5"C. for ice-CsIaCs14-liquid, L a t - 0.2"C. for ice-CsI4-iodine-liquid, and FI-F2 a t 51.5"C. for CsL-iodine-two conjugate liquids. Point B is the only ternary eutectic and there are no dystectic points on any of the field boundaries. Although the direction of rising temperature along some of the ternary boundaries has been indicated in figure 3, a complete summary here will probably assist the reader. Starting from the lowest point a t B, there is
FIG.2. The system cesium iodide-iodine-water a t 740-750 mm.
a rise along K B (ice-Cs13-liquid) in the direction of K , along LK (iceCsIa-liquid) in the direction of L, along SL (ice-iodine-liquid) in the direction of S, along BQ (CsI-CsL-liquid) in the direction of &, along A B (ice-CsI-liquid) in the direction of A (though the temperature difference between A and B v a s hardly enough to be experimentally detectable). There is also a rise (very abrupt) along UC (CsI-Cs13-liquid) in the direction of C, along K D (Cs13-Cs14-liquid) in the direction of D , along LP1 (CsI4-iodine-liquid) in the direction of F1,and along F2E (C&-iodine-liquid) in the direction of E. All of these temperature rises accord with the theorem of van Alkemade, as interpreted by Bancroft (1). As to the binodal boundaries, the temperature rises along G,Fl and G2Fz(iodine-two conjugate liquids) in the direction of G1-G2 and passes through a maximum on FIPFz (CsId-two conjugate liquids) a t P (plait point saturated with cesium tetraiodide). It is well to note that the temperature rise
POLYIODIDES OF CESIUM.
811
I11
along KD is abrupt a t first, then gradual, then abrupt again as the line nears D. The rise is also abrupt along LFI and moderately so along F2E. The condensed boundary I'C (the exact position of which in the ternary TABLE 2 Cesium aodide-aodine-water: ternary boundary data (740-Y60 mm.) I SERIAL NO. OR SOERCl
POSITION ON DIAQRAY
TEYPERATERE
LIQUID PHASE8 PRESENT
I
CSI
0.34 0.33
0.88 0.88
}
Ice, iodine, CsI,, liquid
'C.
1 2
L
L
-0.2 -0.2
3 4
LK LK
-0.3 -0.4
0.42 0.38
1.46 2.63
}
Ice, CsId, liquid
5 6
K K
-0.5 -0.5
0.32 0.30
3.52 3.42
}
Ice, CsIs, CsId, liquid
7 8 9 10
KB KB KB KB
-0.8 -2.2 -2.5 -3.2
0.21 0.08 0.07 0.05
5.30 15.07 19.26 23.60
1
Ice, G I a , liquid
11 12
B B
-4.0 -4.0
0.06 0.06
27.84 27.72
I
Ice, CsI,
13 14 15 18) 16 17 18 19 20 21 22 23 24
BQ B? B? BQ
0.09 0.16 0.29 0.38 0.64 1.22 2.05 3.06 4.62 5.69 7.19 12.16 16.23
33.51 39.39 44.14 46.4 51.23 56.52 60.41 62.66 64.90 65.07 65.43 64.76 63.40
>
CsI, CsI3, liquid
BQ
3.8 12.0 20.0 25.0 34.6 48.5 60.8 71.1 80.1 85.0 90.0 100.1 105.4
KD KD KD KD KD KD KD KD KD
7.2 9.0 25.0 25.4 44.1 54.8 57.8 62.0 69.4
0.44 0.54 1.19 1.34 2.93 5.44 5.85 8.54 12.52
4.34 4.51 7.65 7.54 13.03 18.14 18.97 22.31 26.15
>
CsIS, CsI,, liquid
BQ
I
1
BQ BQ BQ BQ BQ BQ
BQ
liquid
I
25 26 (8) 27 28 29 30 31 32
'
1
' ~
~
1 1 I
812
T. R. BRIGGS AND 8. 8. HUBARD
TABLE 2-Continued SERIAL NO. O R BOURCE
POSlTION
ON DIAGRAM
TIMPERATURE
____
I
CSI
1
‘C.
33 34 35 36 37 38 39 40 41 42 43 44 45 46 47
71.9 75.1 76.0 77.8 79.3 79.6 80.1 80.4 80.6 81 .o 82.2 84.5 88.0 95.2 103.3
15.41 18.33 19.12 23.95 29.68 34.67 36.65 40.04 44.42 48.48 51.70 53.87 56.28 59.10 61.36
29.44 30.84 31.51 33.53 35.69 37.14 37.93 38.41 38.43 38.88 38.61 38.66 38.14 37.39 36.26
48 (8) 49 50 51
6.4 25.0 27.6 36.4 48.0
0.75 1.23 1.57 2.24 3.39
1.93
52 53 54 55 56
51.8 51.3 51.4 51.5 51.3
4.16 3.96 4.15 74.38 74.28
6.99 6.90 23.30
57
60.6
77.50
21.07
58 59 60 61 62 63 64 65 66 67 68 69 70
54.6 65.0 67.3 70.9 72.3 74.4 74.9 76.7 78.2 79.5 80.0 80.4 80.6
4.55 7.69 8.93 10.01 11.80 13.90 14.78 17.31 19.16 21.34 26.49 28.50 38.22
7.78 12.36 13.87 16.37 18.09 20.71 21.25 24.16 26.38 31.27 32.61 34.31 36.92
__
1‘ CsI,, iodine, liquid
6.06
I,
1
I>1 CsI,,iodine, two liquids CsI,, iodine, liquid
POLYIODIDES OF CESIUM.
813
I11
T.4BLE 2-Concluded I SERIALNO. O R SOURCE
i
PosInoN
I
ONDIAGRAM
TEYPERATURE
LIQUID .~
I
CaI
'C.
1
PEASE9 PRESENT
'
71 72 73 74 75 76 77 78
80.6 80.0 79.5 79.4 77.0 73.9 67.0 56.4
48.18 52.85 53.69 55.77 59.47 64.48 68.17 72.90
37.98 37.27 37.22
79 80 81 82
74.1 80.8 89.6 93.4
4.15 4.35 4.18 4.55
""
1
83 84 85
74.1 81 .o 86.0
79.71 82.02 84.05
18.98 16.18 14,57
i
86 87
98.2 98.2
3.84 87.84
88 89
98.2 98.2
3.09 1.64
90 91
100.6 100.6
24.97 59.50
g::;
1)
92
109.3
9.76
66.30
!
93 94
109.8 109.7
20.50 21.09
95 96 97 98 99 100 101 102 103
108.9 108.3 108.1 107.5 107.0 106.7 106.2 106.5 122
24.57 26.13 26.55 28.83 35.38 38.24 42.42 47.65 51.00 ___
24.64
5.48 5.42
J
Iodine, t w o liquids
Iodine, two liquids
;:::i I
Iodine, liquid, vapor
Two liquids, vapor CsI, liquid, vapor
CsI, CsL, liquid, vapor 57.83 56.24 55.64 53.83 49.85
jl CSIS, liquid, vapor
49,28
46.42 45.73 46.93
,
diagram can only be surmised at present because of insufficient, data) is a continuation of BQ (CsI-CsIs-liquid), and F a is a continuation of LFl (CsI4-iodine-liquid). Under the pressure prevailing in the present in-
814
T. R. BRIGGS AND 8. S. HUBARD
vestigation (Le., 740-750 mm.), the complete boundary curve from B to C is broken into the separate parts BQ and U C because it intersects the ternary, boiling point surface a t Q (109.6"C.) and II (temperature not determined). The break in the other boundary curve, of course, is due to the fact that the single liquid phase splits into two conjugate solutions a t Fl-F2 (51.5OC.). The type of solid separation (positive or negative) taking place along the TABLE 3 Cesium iodide-iodine-water : binaru and ternary invariants POSITION
ON DIAQRAM
LIQUID TEMPERATURE
PHASES PRESENT
I
CSI
Trace 0
0 27.5 18.8 30.5 48.3 0 0 71.4 50.0
'C.
S A
E D C
R H M N B K L
{:{E Q
U
OE -4.0 E 71.5 E 140 U 211 u 98.2B 113 B 108.6 B 303 B
81.2 69.5 51.7 0.4 99.9 0 50.0
-4.0 E -0.5 u -0.2 u 51.5 51.5 98.2+ B 98.2+ B 109.6 B Unknown B
Temperatures: E (740-750 mm.).
=
Ice, iodine, liquid Ice, CsI, liquid Iodine, cSI4, liquid CsL, CsIs, liquid CSII, CsI, liquid Iodine, liquid, vapor Iodine, liquid, vapor CsI, liquid, vapor CsI, liquid, vapor
0.06
27.8 0.31 3.5 0.34 0.88 4.1 6.9 23.2 74.3 4.6 3.8 10.8 87.8 20.8 61.9 Unknown'
1 1
Ice, CsI, CsIs, liquid Ice, CsIs, CsI,, liquid Ice, CsL, iodine, liquid csI4, iodine, two liquids Iodine, two liquids, vapor CsI, CSIS,liquid, vapor CsI, CsIa, liquid, vapor
eutectic point; U = transition point; B = boiling point
various boundaries during cooling may be determined fairly easily in most instances. Positive separation of both saturating solid phases is indicated along AB, L K , K B , BQ, and F2E. Positive separation of one solid and negative separation (Le., dissolving) of the other is indicated on UC and LFl, the negatively separating solid being cesium iodide on UC and.iodine on LFI. The boundary S L is doubtful; ice certainly separates positively with perhaps a slight negative separation of iodine as the temperature approaches L. The solid phase (iodine or tetraiodide) separates positively in each of the binodal regions. The long boundary K D (CsI&sIa-liquid) is quite extraordinary. Its
POLYIODIDES OF CESIUM.
111
815
position and curvature in the composition diagram are such as to make it possible for one to draw tangents to the boundary through cesium tctraiodide and cesium triiodide meeting the boundary a t two different points, viz., X for cesium tetraiodide and Y for cesium triiodide (figure 2). One then finds (e.g., by the vector method; cf. 4) the following separations: tetraiodide positive-triiodide negative from D to X, tetraiodide positivetriiodide positive from X to Y , and tetraiodide negative-triiodide positive
FIG.3. Part of figure 2 enlarged from Y to K. It is thereforc possible for the solution to depart (cj. 4) from D X across the tetraiodide field during cooling and to depart from Y K across the triiodide field, but no departure is possible between X and Y , 1. The binodal region The binodal surface in the space model of the cesium system extends downward with respect to the temperature axis until it meets two solubility surfaces,-Le., the surface for iodine and the surface for cesium tetraiodide. Two special three-phase fields-ruled surfaces in the space model -are thereby produced in the composition diagram (figure 2), viz., GIFIFzG~Gl for iodine-two conjugate solutions and FIPFzFl for CsIa-two
816
T. R. BRIGGS AND 8. 8. HUBARD
conjugate solutions. The first of these two fields slopes downward in the space model from GI-Gz (ca. 98.2OC.), the boiling point of the two conjugate solutions saturated with iodine, while the second slopes downward from its plait point a t P (80.7OC.); the two fields meet along the isothermal tie line FIFZ(51.5'C.) to give the invariant for CsI4-iodine-two conjugate liquids. A comparison with the binodal region in the system rubidium iodide-iodine-water (6) is well worth while a t this point: the binodal surface in the rubidium system intersects only one solubility surface (Le., iodine), and no invariant corresponding to F1-Fz is produced in the ternary diagram. In order that the reader may have a somewhat better picture of the boundary lines in and near the binodal region, we have submitted in figure 4 one of our routine temperature-composition diagrams, -diagrams which we have used for testing the probable accuracy of the temperature-composition data and for purposes of interpolation. The pointr (temperature uersus per cent of iodine or per cent of cesium iodide as indicated on the diagram) are the actual experimental determinations, including those of Foote (small triangles). It is worth noting in connection with figure 4 that the curves drawn against the iodine content give a diagram which is much the same in its general form as a Janecke projection on the wateriodine plane.
2. The crystallization paths Table 4 contains the crystallization paths shown in figure 2. In the triiodide field they are almost perfectly convergent a t cesium triiodide, and while not so perfectly convergent in the other field, they are still good enough to prove that the field covered by the paths belongs to the tetraiodide. They give, in addition, further evidence (based on non-isothermal indirect analysis) in support of the tetraiodide formula. Most of the data given in table 2 for the various condensed boundaries were obtained by determining cooling curves and analyzing the liquid phase a t each temperature arrest. As the results of these surveys were plotted on the composition diagram and figure 2 began to emerge, the paths followed by the surveys were always found to be in line with the formulas Cs13 and CsT4 in those cases in which the polyiodides appeared during cooling. Since the presentation of any of these complicated surveys would involve too many details (cf., for example, 5 ) , they are merely mentioned here as a matter of record.
3. The contour diagram The contour diagram of the condensed system below the boiling points is shown in figure 5, the contours being spaced a t 10-degree intervals
7
FIQ.4. Routine graph for the liquid phase in and near the binodal region. Temperature UCTIWiodine and temperature U e m 4 . 8 cesium iodide.
TABLE 4 Cr:ry8tdli&ion pa&
-I
YOmD
L I O W
I
I
ax
11
Triiodide field %.
100
1 1 :::% I
Triiodide field (continued)
%.
26.07
51.61 52.48
;1
43.70 40.96
m
12.78
80
6.68
70
3.76
53.09 ~. 62.98
100
34.28 21.84 10.21
47. 08 44.31 41.27
82.2 80.4 79.3
42.80 29.68
38.81 37.30 36.69
n.81 1.98
37.61 22.28
88.0 79.4
58.69 55.77
37.07 36.28
0.25
21.36
Bo 80 83.2 43.2 12.0
,
II
Tetraiodide field 51.70
!
818
T. R. BRIGGS AND S. 6. HUBARD
between 10" and 12OoC., inclusive. The data are given in tables 5 , 6, and 7 . The compositions that are given to the second figure beyond the decimal point in these tables are direct solubility determinations (analyses of the liquid phase a t known temperatures), while the other data are values interpolated from figure 4 or other similar routine graphs, Table 7 gives two isothermals specially determined on the unsaturated binodal surface, but only one of these-Le., that a t 90°C.-is shown in figure 5. S o attempt has been made to draw Janecke projections (cf., however, the remark made in connection with figure 4), since figure 5 shduld give a sufficiently good picture of the space model.
FIG.5. The ternary diagram with contour isothermals Figure 5 shows that each of the cesium polyiodides is isothermally congruently soluble in water within a characteristic temperature range. In the case of the triiodide this range is estimated t o be from 80°C. (on KD) to the boiling point of the saturated solution a t a little above 106'C. (on QZU near 2 ; the rest of the congruent range is metastable at 740-750 mm.), while in the case of the tetraiodide the range is from 79.5' (on PF2) to about 89OC. (on KD). Between 51.5"C. ( F I F J and 79.5"C. (on PF,), however, the tetraiodide crystals still are not decomposed by mater into a different solid phase, but when they do dissolve in water up t o the point of saturation within this temperature range they give two (Le., conjugate) solutions instead of one. Figure 6 shows four interesting isothermal sections through the space model taken from figure 5 at 40°,60", SO", and 9O"C., respectively. The
TABLE 5 Contour data in the iodide, triiodide, and tetraiodide fields TEMPERATURE
~
LIQUID
POSITION 01 DIAQRAY
LIQUID TEIPERATURE
I
CSI
0 0.0, 0.08 0.36
30.4 30.9 3.6 0.96
0 0.1 0.6 0.9
37.4 38.0 4.8 2.2
0 0.3 1.0 1.3
43.6 44.1 6.4 2.7
0 0.5 1.5 1.7
49.0 49.4 8.8 3.6
0 0.8 2.4 2.4
53.5 53.4 11.8 4.8
0 1.3 4.1 3.73 3.8
57.4 57.0 15.6 12.95 6.6
0 2.2 3.20 4.18 7.4 6.30 6.0 71.7 77.2
10.0 25.6 21.2
I
CSI
10.0 67.0 80.7
15.6 29.2 19.0
80
0 4.6 5.99 6.68 8.94 10.21 15.29 24.09 36.5 26.8 52.6 79.5
65.6 64.7 56.15 53.09 43.33 41.27 36.31 35.83 37.6 32.5 37.5 20.5
90
!
0 7.2 9.76 12.78 21.84 33.25 49.38 57.2 78.0
67.6 65.2 57.50 52.48 44.31 42.73 40.96 37.9 22.0 69.6 64.1 57.29 53.30 51.51
100
I'
0 12.2 17.22 22.47 26.07 31.83 34.28 38.04 49,34 60.5 76.4
47.08 46.21 43.70 36.7 23.6
110
49.5 62.9 74.7
45.0 35.1 25.3
120
50.6 65.3 72.9
46.6 33.6 27.1
1 I!
'~
1~ ii 1;
3.0 3.76 6.00 8.58 13.2 12.37
63.3 62.4 52.98 38.65 32.66 27.1 25.08
1;
~! 11
,
~~
819
DIAQRAM
FIP PFz
70 (continued)
I
0
I
"
48.00
__
820
T. R. BRIGGS AND 8. 8. HUBARD
first of these sections is typical of the system below the minimum temperature for two conjugate liquids (FLFzat 51.5OC.) and is similar to the diagram determined experimentally at 25OC. by Briggs, Greenawald, and Leonard (8). The next two sections are typical of the system between the minimum temperature and the temperature of the tetraiodide-saturated TABLE
6
Conjugate liquid . . . .pairs saturated with cesium tetraiodide or iodine POLIITION ON
TEKPBRATWED
DIAGRAM
-- - DIAGRAM I CSI FIRST LIQUID
POSITION ON
'0.
80.7
P
40
37.2
80 70 60
YFi PFi PFi
26.6 10.0 6.0
32.5 15.6 10.0
52.6 67.0 71.7
37.5 29.2 25.6
51.5
FI
4.1
6.9
74.3
23.2
csI4 and iodine
60 70 80
FiGi FiGi FiGi FiGi
4.2 4.2 4.2 4.0
6.6 6.2 6.0 5.6
76.4 79.0 81.9 85.1
21.3 19.2 16.7 13.6
Iodine
GI
3.8
4.6
90 98.2
B
E
10.8 87.8 -
'
CSI,
Iodine and vapor
boiling point (740-750 mm.)
TABLE 7 Contour isothermale on the unsaturated binodal surface
-______--"C.
85
[ 11;
%.
4.2 23.60 54.14 83.6
5.8 28.93 34.63
90
1 11:
15.2
-4.0
36.06 47.44 85.1
6.6 34.01 35.74 13.6
plait point ( P a t 80.7°C.). The last section is typical of the system above the saturated plait point and below any of the boiling points. The binodal tie lines shown in these sections are more or less schematic.
B. The boiling-point diagram The complete ternary boiling-point diagram, with the direction of rising temperature shown on the longer lines, is given separately in figure 7 .
POLYIODIDES OF CESIUM.
82 1
111
I CSI
I
FIG.6. Isothermal sections through the ternary space model I
FIG 7. The ternary boiling-point diagram a t 740-750 mm.
822
T. R. BRIGGS AND 6. S. HUBARD
The lower half is similar to the lower half of the rubidium diagram (6): thus RG1 and G2H represent iodine-liquid-vapor at 740-750 mm., GlPbG2 represents two conjugate liquids-vapor a t 740-750 mm., and G1-G2 (ca. 98.2"C.) is the invariant representing iodine-two conjugate liquids-vapor a t 740-750 mm. Points R and H are respectively the "lower" and "upper" iodine-saturated boiling points in the iodine-water system. The boiling-point lines, with the exception of GlP+,GZ, also may be seen in figures 2 and 5.2 In the upper part of the diagram, the boiling-point line which begins a t M (108.6"C.) and ends a t N (303OC.) and which refers to solutions saturated with cesium iodide, is broken into the two separate parts M Q and UN by the new boiling-point line QZU, which, in turn, refers to solutions saturated with cesium triiodide (boiling solutions saturated with triiodide are not found in the rubidium system a t 740-750 mm.). Q (109.6OC.) and U (not experimentally determined) are two new invariants, both representing CsI-CsIrsolution-vapor a t 740-750 mm. ; Q itself is simply the boiling point of a solution which is saturated with cesium iodide and cesium triiodide. Its composition is given in table 2. The line MQ and the greater part of QZU have been experimentally located in the boiling-point diagram (cf. table 2), and point N is definitely known (cf. table 3). The temperatures on QZU pass through a minimum a t 2 (ca. 106.2"C.), which the experimental data place a t about 45 per cent iodine, 46 per cent cesium iodide, and 9 per cent water. U has not been experimentally determined, and the diagram near this point is more or less schematic. The reason for the invariant U is the fact that the boiling saturated liquid phase a t N (303°C.) in the binary cesium iodide-iodine system is saturated with cesium iodide and not with cesium triiodide, the latter decomposing into iodide and liquid a t 211°C. (point C in figure 2). Accordingly, the temperature of U must be below 211°C.; on the other hand, it must be above 122°C. on the basis of the data for QZU (cf. serial KO.103 in table 2). The broad field between the boiling-point lines in figure 7 is the field for single unsaturated solutions and vapor a t 740-750 mm. A few experimental boiling-point determinations in the field are shown in the figure, and several isothermal contours have been drawn in what are believed to be approximately correct positions. The boiling points in the field do not begin to rise very much until the solutions come close to the anhydrous cesium iodide-iodine system; the rise is then abrupt. 2 GJ'bGz is based upon GI and Gz (table 2) and two other experimental points as follows: 24.97 per cent iodine, 21.03 per cent cesium iodide (on GlPb a t 100.6°C.) and 59.50 per cent iodine, 28.81 per cent cesium iodide (on PbG2 a t 100.6"C.). The composition at Pb (estimated temperature 100.8°C.) is about 40 per cent iodine, 28.7 per cent cesium iodide, and 31.3per cent water (by difference).
POLYIODIDES O F CESIUM.
I11
823
An interesting isothermal section through the space model at about 107.5"C. is shown in figure 8 (obtained by superimposing figure 7 on figure 5 ) . The temperature chosen lies between 2 (106.2"C.) and Q (109.6"C.), and so gives two different boiling solutions saturated with cesium triiodide, as well as an intermediate field in which the triiodide exists alone in contact with vapor. The tie lines connecting the boiling solutions and the triiodide with the coexisting vapor are schematic, of course, since the vapor was never actually analyzed. No boiling conjugate solutions appear in c51
FIG.8. The system cesium iodide-iodine-water a t 107.5"C.and 740-750 mm.
figure 8; quite obviously, as the temperature lies above the plait point (100.8"C.). For a typical isothermal section showing two boiling conjugate solutions, the reader is referred to figure 7 in the paper on the rubidium system (6); the cesium system would give a similar section at any temperature between 98.2"C. (GIG,) and 100.8"C. (Pb). Pb
1. Phase separations along the lines MQ,Q Z U , and U N
The vector analysis (cf. 4,6) of JIQ and part of QZU is shown in figure 9, the vector arrows referring to the vapor (binary mixtures of iodine-water) or to the saturating solid (cesium iodide on MQ; cesium triiodide on Q Z l i ) , and the vector pairs being arranged so as to give a tangential resultant
824
T. R . BRIGGS AND €3. 6. HUBARD
pointing in the direction of rising temperature along the boiling-point lines. The vectors on U N and QZU near U have not been shown, owing to lack of space in that part of the diagram. According to the vectors, rising temperature along M Q (and the same is true on U N ) is accompanied by the formation of vapor and positive separation (Le., precipitation) of cesium iodide; hence M Q (and also U N ) is an ordinary boiling-point line which is followed without departure icf. 4) by ternary solutions saturated with cesium iodide, the solutions being heated in either a closed or an open system a t 740-750 mm. The same is true of the branch Q Z of the line Q Z U , except that the separating solid is cesium triiodide. When account is taken of the direction of temperature rise along M Q and ZQ, it is evident a t once that Q (109.6OC.)
10
20
30 40 FER C€NT /OD/N€
50
BO
FIQ.9. Part of figure 7, showing composition vectors is the end point of boiling (Le,, the point a t which the liquid phase completely boils away) for all solutions on or arriving at the lines M Q or ZQ, and the final product in an open system is a mixture of cesium iodide and cesium triiodide. . Point 2, presumably, is the point (cf. figure 9) where the vector arrows (for vapor V , and solid cesium triiodide) are exactly opposed; hence the point, though a temperature minimum, is the analog of a dystectic. If we make an estimate of the composition of the vapor in equilibrium with the solution a t 2, drawing a line from CsIa through Z to the iodine-water axis, we obtain a value of about 6 per cent iodine and 96 per cent water,-% result that seems reasonable in view of the difference in volatility between iodine and water. The solution a t Z boils a t constant temperature (ca. 106.2OC.) and constant composition; hence Z is a special end point of boiling, giving a residue of pure cesium triiodide.
POLYIODIDES OF CESIUM.
I11
825
The situation along the branch ZU is apparently more complex, and any analysis must be largely speculative, owing to the lack of complete experimental information. It is certain that rising temperature is accompanied by formation of vapor and positive separation of cesium triiodide for some distance beyond Z in the direction of U (cf. figure 9). Further on, however, a point T appears to be reached a t which the tie line to the vapor is tangential to ZU, and beyond this point formation of vapor occurs with negative separation (i.e., dissolving) of cesium triiodide. Assuming that the latter situation continues up to U , the whole of ZU is thus an ordinary boiling-point line along which the triiodide precipitates a t temperatures below the vapor tangency point and dissolves a t temperatures above, the latter condition leading to departure across the field of the single unsaturated solutions and vapor whenever the supply of solid triiodide is not sufficient to carry the solution to U. It is not difficult to see-especially with the aid of figure 2-that the addition of heat to the liquid phase a t the invariant U must result in the formation (positive separation) of cesium iodide and the complete or partial disappearance (hence negative separation) of cesium triiodide. When a vector analysis is made a t this point-the estimated position of which in the various figures must be approximately correct-it appears that these solid separations are accompanied by condensation of vapor. U is therefore a point a t which boiling is interrupted (cf, 6); other examples of such points (Le., on GZH) will be found in the paper on the rubidium system (6). It should be noted, however, that U is not an end point of boiling as defined above, for the liquid phase does not actually disappear at this point. Although boiling is interrupted a t U (closed or open system), there is no temperature and coniposition arrest a t U when the system is open and the vapor is steadily removed, for the obvious reason that no vapor is available for condensation under such conditions. Instead, as heat is added the liquid moves immediately along the condensed boundary UC in the direction of C, cesium iodide precipitating and cesium triiodide dissolving (note the separations given for this line in section A above). The triiodide, however, must disappear a t some point short of C; when that occurs, the solution follows a negative crystallization path in the direction of U N on the condensed solubility surface for cesium iodide (the latter dissolving as the temperature rises). This path ultimately reaches UN (either entirely by way of the solubility surface, or partly by way of that surface and partly by way of the boiling-point surface for the single unsaturated solutions), boiling begins again with precipitation of cesium iodide, and the solution moves toward N . We shall conclude the discussion with a summary of the end points of boiling for the ternary system as a whole. The solution phase in all ternary complexes of cesium iodide, iodine, and water must a t some stage
826
T. R. BRIGGS AND S. S. HUBARD
reach one of the lines MQ,ZQ, Z U , and U N , or the special point 2,a t 740-750 mm. If it reaches MQor ZQ, the end point is Q (109.6'C.), and the residue left behind a t that temperature is a mixture of cesium iodide and cesium triiodide. If it reaches ZU or U N , the end point is N (303OC.) and the residue left is cesium iodide; there is also an intermediate point of interrupted boiling at U if the solution arrives at, and remains on, Z U . Finally-and this of course is a special case-if the solution reaches 2, the latter (1062°C.) is the end point and the residue left is pure cesium triiodide. In the rubidium system (6) there is only one end point a t 740750 mm. (Le., N, 238°C.) and the residue left is always rubidium iodide; in the cesium system, however, there are three possible end points and the residue may be cesium iodide, a mixture of cesium iodide and cesium triiodide, or cesium triiodide alone, depending upon which one of the end points the solution actually reaches. 111. SUMMARY
1. The complete polythermal phase diagram of the system cesium iodideiodine-water in air under a pressure of 740-750 mm. has been determined from the lowest eutectic to the boiling points of the saturated liquids. It contains two binary compounds, cesium triiodide and cesium tetraiodide, and also a binodal region. 2. Further proof of the correctness of the tetraiodide formula for the higher polyiodide of cesium has been obtained by the method of crystallization paths. 3. The major part of the boiling-point liquidus diagram for the ternary system a t 740-750 mm. has also been obtained. It contains a region for boiling solutions saturated with cesium triiodide. The diagram has been analyzed in some detail as a special phase-rule problem involving three components, two of which are volatile. 4. A special freezing-point, solubility, and boiling-point study of the binary system cesium iodide-water forms part of the investigation, and a correction has been made in the temperature-composition diagram of the binary system cesium iodide-iodine. REFERESCES (1) BANCROFT: The Phase Rule, p. 149. Journal of Physical Chemistry, Ithaca, New York (1897). (2) BEKETOFF: Bull. Acad. St. Petersburg [41 4, 198 (1894). (3) BRIGGS: J. Phys. Chem. 34, 2260 (1930). (4) BRIGGS, BALLARD, ALRICH (MISS),AND WIESWO: J. Phys. Chem. 44,325(1940). CLACK, BALLARD, AKD SASSAMAN: J. Phys. Chem. 44, 350 (1940). (5) BRIGGS, (6) BRIQGS, CONRAD, GREGG, AND REED:J. Phys. Chem. 48,614 (1941). (7) BRIGGS, GEIGLE, AND EATON: J. Phys. Chem. 46, 595 (1941). GREENAWALD, AND LEONARD: J. Phys. Chem. 34, 1951 (1930). (8) BRIGGS, Am. J. Sci. [41 21, 32 (1906). (9) BUCHANAN:
DEFORMATION OF CELLULOSE AND O F RUBBER
(10) (11) (12) (13) (14) (15) (16) (17) (18) (19) (20) (21)
827
FIALKOV:Acta physicochim. U.R.S.S. 3, 711 (1935). FOOTE:.4m. Chem. J. 29, 203 (1903). FOOTE, BRADLEY, AND FLEISCHER: J. Phys. Chem. 37, 21 (1933). GRACE:J. Phys. Chem. 37,347 (1933). KRACEK:J. Phys. Chem. 36, 117 (1931). LASKUSO:Z. physik. Chem. Al70, 134 (1934). FAJANS AXD KARAGUNIS, from MEYERA N D DUSKEL:Z. physik. Chem., Bodenstein Festband, p. 556 (1931). RAE: J. Phys. Chem. 36, 1800 (1931). RAE: J. Chem. SOC.1831, 1578. SETTERBERG, from Gmelin's Handbuch der anorganischen Chemie, System S r . 25 (Caesium), Val. 2, p. 200. Verlag Chemie, G.m.b.H., Berlin (1938). WELLSAND PEXFIELD:Am. J. Sci. (31 43, 17 (1892). WELLSA N D WHEELER:Am. J. Sci. [31 44,43 (1892).
THE ASALOGY BETWEEK T H E MECHANISM OF DEFOR,MATIOS OF CELLULOSE AND THAT O F RUBBER P. H. HERMSNS Breda, Holland Received October 90, 1940
When we review the well-founded experimental results (2, 4, 6 to 17, 18, 24, 34,35, 36) concerning the extension of cellulose and of rubber, we must conclude that there is a striking analogy between the behavior of regenerated cellulose and that of rubber upon deformation. It seems to us important to emphasize this analogy particularly, quite apart from the fact that we have not yet succeeded in drawing up a theoretical picture of the fine structure and mechanism of molecular deformation which is capable of accounting for all the phenomena observed (1,2,4,34). The analogy in question can scarcely be accidental or merely superficial, and we are convinced that it is intimately connected with a similarity in the intrinsic structure of the two substances which has until now received insufficient attention. Like all similar analogies, this one of course has its limitations, which can best be understood when the following general features are kept in mind: (a) The lateral cohesive forces between the carbohydrate chains of the cellulose molecules are, at a given temperature, very much stronger than those between the hydrocarbon chains of the rubber molecules. (b) The influence of temperature on the behavior of rubber can to a certain extent be compared to the influence of the degree of swelling in the case of cellulose.
