Feb., 1959
VAPORPRESSURES OF B113 OVER B1-B1T3SOLUTIONS
295
THE T'APOR PRESSURES OF BiI, OVER LIQUID Bi-BiI, SOLU'TIOl\rSl BY
DANIEL
CUBICCIOTTI AND F. J. KENESHEB, JR.
Slanford Research Institute, Menlo Park, Ca1zj"ornia Received September 1.8, 1958
The vapor pressures of BiI3 over the liquid and its mixtures with Bi were determined from 410 to 470" and over the composition range 0 to 50mole % Bi. The vapor pressure of BiI3 could be expressed by the equation log p(mm.) = -4310/T 8.170. The activity of the Bi13 followed Raoult'3 law up to about 30 molc % added Bi and showed positive deviations for larger Bi concentrations.
+
Introduction As a continuation of our study of the thermodynamics of bismuth-bismuth halide solutions2 we have determined the effect of Bi additions on the vapor pressure of Bi13. The Bi-BiI, system is similar to its bromide and chloride counterparts. Phase diagram studies have shown that Bi will dissolve appreciably in liquid BiI, before a second phase separates.3 There is some evidence for the formation of solid BiI which melts incongriientlysimilar to BiBr and BiCl. The iodide system differs from the chloride and bromide in the relatively high meltiiig point of BiI3. Experimental Method.-The vapor pressures were determined by the transpiration method described in detail earlier.2 The diffusion errors and flow rate effects were essentially the same as those reported for the other systems studied. Materials.-BiIa was prepared by dissolving reagent grade Biz03in aqueous HI, evaporating to dryness and melting in a stream of dry Nz. It was then doubly distilled in a stream of dry N 1 and finally collected in the transpiration cell. BiI3 has a tendency to lose I P when heated even in an inert atmosphere. Therefore, the product collected from a distillation contained I2 in excess of stoichiometric BiI j . Analyses of two samples prepared in that way showed 35.20 and 35.33% Bi (35.44 theoretical). The BiIa thus contained a small excess of 1 2 when put into the cell. During the transpiration experiments the sample evolved 1 2 , so it may even have had a Bi excess (over stoichiometric BiI3) before any Bi was added. Therefore the exact composition of the starting material was not known precisely. However, from an estimate of the amounts of material present and I, evolved, the excess impurity (either Bi or Iz) was certainly less than one mole per cent. during the time that the vapor. pressure of ''pure" BiI3 was measured. The melting points determined by cooling curve tee$niques on four samples of Bi13 ranged from 405.9 to 406.6 . Literature values have been reported to be: 412°,33 408°,ab 405°,3a The Bi and Nz were the same as used before. Because small proportions of 0 2 in the Nz would be capable of oxidizing the iodide to 1 2 , the NZwas passed over hot Cu and tlieii Fe filings before being exposed to the sample. Analysis of Transported Vapor.-Since iodine was produced when the BiI3 was heated, the proportion of "free" iodine in the collected material mas determined in an attempt to establish the partial pressure of In over the melt. The sample collected was dissolved in a solution of HCI and III. The solution was titrated with 0.005 N thiosulfate to a starch end-point. Since iodine is produced easily by any air present, this entire procedure was performed under an NZatmosphere. Even so, some of the scatter of the iodine determinations probably was due t o air oxidation. To investigate the possibility of some gaseous species of lower valence, several samples of condensate were collected (1) This work was made possible by t h e financial support of t h e
Research Division of the United States Atomic Energy Commission. (2) For previous papers see: THIS JOGRNAL, 62, 403, 843 (1958). (3) (a) L. Marino and R. Becarelli, Atli Accad. Lincei, 2 1 ( 5 ) , 695 (1912); (b) H. S. van Klooster, Z. anorg. Chem., 80, 104 (1913); (0) G. G. Urrtzov and A I . A. Sokolova, Akad. Nauk SSSR., Inst. Obshch. Neoru. Khim., Sektor Fir.-Khim. A d . , Izusst., 26, 117 (1954).
from a melt with a gross pornposition of 70 mole % Bi. (At this composition the melt was probably a two-liquid phase system.) Analyses of those samples for Bi agreed within about one per cent. of theoretical for BiI3, SO the fraction of any low valence bismuth iodide in thc vapor was negligible for the present purposes.
Results
In each series of determinations the vapor pressure of BiI3 'was measured from 410 to 490"; then additions of Bi were made mid the pressure measured a t 410, 435 and 470". Two series of determinations were made. In one, the first Bi ndditioiis were quite small; whereas in the other the Bi additions were all relntively lnrge. 2E
\ 2 5
2 4
:\-
2 3
E' I P
2 2
D
J
21
2 0
I 9
18 1
I35
Fig. 1.-Tlie
+
140
I45
I50
lo3.
vapor pressure of BiI,.
Bi13.-The results obtained on Bi13 are summarized in Fig. 1. Tlie 1's in the figure include the results obtained on three different samples. The data a t each temperature extended over a range of about 6% in pressure. The curve drawn through the data was a straight line that could be represented by the equation
206
DANIELCUBICCIOTTI AND F. J. KENESHEA, JR.
260
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1 260
Vol. 63 I
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I
22
I
.c
E I D D
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0:
.. MOLE F R A C T I O N
e#,
Fig. 2.--The pressure of BiIa as a function of added Bi a t 410, 435 and 470'. The dashed lines represent Raoult's law. log p(mni.) = -4310/T
+ 8.170
From the slope of the line, a heat of vaporization of 19.8 f 0.5 kcal./mole was derived. The only literature value for a vapor pressure is a comment by van KloosterSbthat Bi13 boiled a t about 500". The normal boiling point derived by linear extrapolation of our data is 542'. Mixtures of Bi with Bi13.-The effect of Bi additions on the vapor pressure of BiI3 is summarized in Fig. 2. Each I represents three to five determinatJions; and the different types of 1's distinguish the different series of determinations. The curves were drawn as smoothly as possible through all three sets of points. Iodine Pressures.-The iodine pressures observed were small compared with those of the Bi13 and the values were somewhat erratic. The results obtained for Bi additions of from 0 to 6% in one series of determinations are shown in Fig. 3. For Bi additions greater than about 10% the iodine evolved was too small to measure. The equilibrium governing the 1, pressure can probably be represented by the equation BiIa(disso1ved in melt) = 3/2 12(g)
+ Bi(disso1ved in melt)
The standard free energy charge for that reaction is known,l and therefore the activity of the dissolved Bi can be calculated. Such calculations indicate that the activity of the Bi in the melts was greater than its mole fraction. The values calculated are in some doubt because the exact concentration of Bi was indefinite. (As
0
130
1
I
I
135
140
149
fx
I
io3.
Fig. 3.-The pressure of iodine over Bi-BiL mixtures: curve A, no added Bi; B, 1.7 mole yo Bi; C, 3.2 mole yo Bi; D, 6.3 mole % Bi.
stated above, the BiIa was non-stoichiometric.) The Henry's law constants determined for the Bi were of the order of five or ten. Thus, even allowing for rather large experimental errors, the constants were greater than unity.
Discussion Perhaps the most interesting feature of this system is the long range over which the Bi13 obeys Raoult's law. Thus, a t each of the temperatures studied, over the composition range from 0 to 30 mole % added Bi, the BiI, followed Raoult's law (assuming Bi and BiI3 as components). This behavior is notable in view of the very short range of compositions in which Raoult's law obtained in the bromide and chloride systems. Beyond about 30 mole yo Bi, the vapor pressure of the BiIa exhibited positive deviations which were almost as large as those observed in the chloride and bromide systems. The partial molal enthalpy of solution of Bi13was calculated from the change of its activity .xith temperature.2 The enthalpy was zero up to 30 mole yo Bi, and then rose, reaching a value of about one kcal. a t about 45 mole 7 0 Bi. Up to the highest Bi concentration (50 mole Yo) there was no evidence for a decrease in the enthalpy as observed in the chloride and bromide systems. (4) L. Brewer, et a!., "Chemistry and Rfetallurgy of hfiscellaneous Materials," N.N.E.S. IV-lSB, RlcGraw-Hlll Book Co.. Inc., New York, N.T.,1950, p. 115.
