SOLL-BILITIES I N T H E SYSTELI TT7AITER-IODInTE TO 200" BY F. C. K R A C E K I. Iodine dissolres in water to a limited extent only, but in aqueous solutions of iodides, particularly in KI, and KbI, its solubility is unusually high. This apparently abnormal solubility, taken together with other criteria, has led to a wide acceptance of the T i m that iodine combines with these salts in solution to form polyiodide.. Such conipounds, )Then they exist, should be capable of crystallizing from their solutions under the proper conditions, and hence, EI phase rule study is obviously the most direct way of attacking the problem. The system water-iodine-potassium iodide was selected, principally because potassinm "polyiodides" have figured extensively in the literature,' and secondly, because potassium iodide can be obtained in abundance, and in a state of high purity. The first measurements made have dealt with the three binary systems water-iodine, Fyater-potassium iodide and iodine-potassium iodide, which form the boundaries of the ternary system. The results for the binary system water-iodine are communicated in this article. 2. Previous measurements of the solubility ~f iodine in n-ater, sumniarized in Table I, end at 60'. The solubility i.5 small, and increases moderately
TABLLI Previous Reliable Values for the Solubility of Iodine in Water t'C 0.0
Per cent Iodine weight mol
0.0162
0 . 0 0 1I
Reference j
Jones and IIartmann: J. Am. ('hem. Soc., 37, 2 4 1 (191 j).
18.0
.o2i64
,001962
Hartley and Campbell: ,J. C'hern. Soc., 93,
2 j . o
,03394
002409
2j.o
,03403
.oozjxh
.0,3j86
,002~04
Hartley and (lampbell: op. cit. Samniet: Z. physik. (-'hem., 53, 641 (190.5). Jakovkin: Z. physik. ('hem., 18, j8j (1895). Soyes and Seidenst,icker: Z. p h y s i k . Chem.,
741 (1908j.
2
j .o
25.0
,03403
35.0
,04660 .064j2 ,09220 .10j60
002416
27, 3 j i (1898). 45.0 5j.o
60.0
,007503
Hartley and C'ampbell: op. rit. Hartley and Campbell: op. cit. Hartley and Campbell: op. cit. Sammet: op. cit.
See article by Grinnell Jones in J. Phys. Chem., 34, 673 (1930) for a summary of published work on this question; see also Briggs, Greenawald and Leonard: J. Phys. Chem., 34, 1951 (1930), which was published while this paper was in proof.
418
F. C. KRACEK
rapidly with temperature, but it is evident that the solubility curve must be of a special type in order to reach IOO per cent a t I 13.7', the melting point of iodine. A preliminary experiment confirmed the supposition that two liquid layers are formed at higher temperatures, and subsequent measurements have established the course of the solubility curve for solid iodine to I I Z . ~ ' , the quadruple S-LI-LII-S' invariant points, and the curves for the mutual miscibility of the liquid layers from 112.3' to beyond zoo'. The critical solution temperature, estimated to be about 300°, could not be reached because of the extremely high vapor pressure developed by the system.
Experimental 3. Method of Solubility Determination. Because of the volatility of both water and iodine, the usual methods of measuring solubility can not be applied to this system a t the higher temperatures. The method adopted for this work was that of rotating, in a regulated air bath, tubes in which known amounts of the constituents were sealed up, and noting the temperature a t which the last trace of the dissolving phase disappears. The temperatures were kept constant at each stopping point long enough to assure equilibrium being attained, particularly in the neighborhood of the point of disappearance of the phase which is dissolving in the solution. This equilibrium method of solubility determination is analogous with the quenching method used so extensively in this Laboratory in silicate work, and can be recommended as an excellent method for all cases in which the solubility exceeds a certain arbitrary minimum value, since the method depends upon visual detection of the last traces of the dissolving phase; in the case of crystals, the uncertainty is of the order of less than o . mg ~ of material. The refinement of the method depends upon accurate knowledge of the weights of substances sealed in the tube, and on sufficiently sensitive temperature control. With modern facilities, neither of these requirements offers any difficulties. 4 . The tubes employed were made from Pyrex tubing 11 mm outside diameter, 1 . 2 mm wall thickness. No attack on the glass was noticeable, and the mechanical strength of such tubes appears to be sufficient to withstand in excess of 40 atm internal pressure. A constriction was blown in the tubes t o facilitate sealing when filled, the open end serving as a funnel for the introduction of the materials. Iodine was weighed into a tared tube in the desired amount, water was then added and the tube sealed. The drawn-off portion was then weighed together with the sealed tube, to obtain the weight of water. A11 weighings were made to 0.1mg purely as a matter of routine. The electrically-heated air bath was regulated by a special potentiometer controller of a standard commercial type, using a five-junction copperconstantan thermocouple. The sealed tube was attached by clips to a fan, rotated a t about 40 r.p.m., which kept the air in the bath in motion. Doublewalled glass windows provided a means of observing the tube during the course of an experiment. The temperature inside the air bath was read on a
SOLUBILITIES IN THE SYSTEM WATER-IODINE
419
separate potentiometer, with a calibrated copper-constantan single junction thermocouple. The temperature control was sensitive to f0.1' a t all temperatures employed; small erratic fluctuations of temperature, due to eddies in i,he current of air in motion past the bare thermocouples, were of relatively small consequence in view of the great differences in heat capacity between the solubility tubes with their contents, and the air. j. The Method of Determinatzon of the Fized Points, namely, the melting point of iodine, and the temperature a t which solid iodine is in co-existence
FIG.I
Logarithmic graph of the solubility relations in the system water-iodine. Inset figure: Solubility relations in the iodine end of the system.
with the two liquid phases and vapor, differed from that just described. These two temperatures are only a little over I' apart, and since liquid iodine is a very dark, relatively viscous liquid, totally non-transparent, the rotating tube method can not give correct results. It was advantageous to determine these points by thermal analysis in sealed tubes, since iodine is appreciably volatile at its melting point, and the S-LI-Lrr-V temperature is above the normal boiling point of saturated iodine solutions. The tubes selected for this purpose were of the type previously employed in the study of the polymorphism of potassium nitrate,' being proIF. C. Kracek: J. Phys. Chem., 34,
225
(1930).
F. C. KRACER
420
vided with a re-entrant well for the insertion of tlhe thermocouple. To obtain good temperature distribution the tubes were supported within a heavy copper block placed in the furnace, with a uniform air space between the copper block and the tube, the heat being carried to the tube only by radiation and air convection. A reference couple was placed in a well in the copper block, and differential heating curves a t a controlled rate of ca 0 . j o per minute were taken, alternately reading the temperature of the charge in the tube, and the different,ial temperature of the charge with reference to the block. The thermocouples used were of copper-constantan, calibrated with accepted standards. 6. The Ezperimentul Results. The results obtained with solutions are collected in Table 11. TABLE I1 Determined Values of Solubilities in the System Water-Iodine Expt. SO.
Iodine
H?O
g
g
rd
A. 7 6
0.0091 ,0136
5
,0258
I1
17.2313
4 3
0.0423 ,0530 ,0757 '0733 ,0973 ,1630
rrn'
Per cent Iodine wt mol
t"C
Solid Phase: Iodine
4 , 6 5 6 1 0.00195 0.000139 3.j071 ,00388 .oooz;j 4 . j 8 0 ~ ,00563 .OOO~OO 0.0161
0.19; ,386 ,560 .093*
0.0139 ,0275
,0400
1.30%
77.1 96.0 106.1 113.
B. Tvo Liquids; Aqueous Layer
I
8 7
IO
9 15
j3I ,2764 ,I
4 . 2 9 1 j 0.00986 o.ooo;oo .OOIOZ~ ,01443 3.6732 4.0712 , 0 1 8 5 ~ ,001319 ,01986 .001409 ;, ,6907 3.0;94 ,03180 .002257 3.cji70 ,04161 ,002953 1.0224 ,04303 ,003054 ,07054 .oojoo6 3.9182
0.976 1.422
1.82; I ,947 3.082 3.995 4.125 6.j89
0.0699 ,1023 ,1317 .1.+07
126.3
,2944
143.2 155.4 1j6.4 Iij.9 187.4
,304j ,4981
206.7
,2252
188.4
C. Two Liquids; Iodine Layer 14
16.3j29
I3
zI.I'$jj I j ,1964
I2
0.0472
0.288*
.0998 .I407
,4io*
3.91* h.24*
,917" I I . 5 1 *
155. 186. >22j
grams of iodine per gram of water. mols of I2 per mol of water; H 2 0 = 18.01j& I ? = 253.86. * weight and mol per cent of water. 1
rw = rrn =
7. The melting point of iodine was determined to be 113.7', and the invariant S-LI-LiI-V temperature I I 2 .3'. The composition of the aqueous layer a t this temperature, 0 . 0 5 1 j mol per cent iodine, equivalent to 7.29 g iodine per 1000 g water, was determined with sufficient precision by noting the point at which the solubility curve for solid iodine intersects the immiscibility curve, in a large scale logarithmic plot, shown in Fig. I . The logarithmic plot is to be preferred in this case, since it separates the points on the aqueous solubility curves, and since it serves as a check on the relative
SOLUBILITIES IK T H E SYSTEM TVBTER-IODISE
42 I
accuracy of determination of the data in this region. Moreover, the slope of the solubility curves above and below the S-LI-LII-V point in the logarithmic plot is such that the point of intersection is more definite than in the usual percentage graph. It will be seen that the data obtained in this investigation form a smooth continuation of the previously known curve. 8. The composition of the iodine-rich layer a t the invariant S-LI-LII-V temperature was deduced graphically, as shown in the inset diagram in Fig. I . The immiscibility curve meets the liquidus curve of iodine a t 1.7 mol per cent H20 (corresponding to 0 . 1 2 weight per cent HzO), a t a sharp angle. The liquidus curve of iodine then rises from this point to 113.7' a t I O O per cent iodine. 9. The location of the water-iodine eutectic can be estimated from the known value of the solubility a t 0'. If we take Jones and Hartmann's (op. cit.) value of 0.000638 mols I? per liter, and 1.86" for the molar freezing point depression, we obtain the temperature of the eutectic as - 0 . 0 0 1 1 9 ~ . This is so near the melting point of ice that no experimental determination of the point can be realized, principally because iodine dissolves very slowly a t 0'. Table I11 contains a summary of the invariant points in the system.
TABLE I11 Invariant Points in System Water-Iodine Type
Ice 3 water, melting Ice solid iodine eut,ectic S-LI-LII-V, aqueous layer iodine layer Critical solution point Iodine, melting
+
Solution, mol per cent Iodine
t'C
0.0
0.0
0.0011;
-0.0012
o.oj17
112.3 112.3
98.3
-
IO0 0
ea.
300.
113 i
Discussion IO. These results are of interest from more than one viewpoint. First, and perhaps the more fundamental point of interest is, that aside from the intrinsic value of these data in establishing the previously uninvestigated formation of liquid layers in the system, we have here a plausible explanation for the anomalous behavior of polycomponent aqueous systems containing salts and iodine. I t i s a necessary consequence of the phase relations that the region of immiscibility must extend into the polycomponent systems, and since the immiscibility extends practically across the whole binary system, the effect in any polycomponent system will be large. Such solutions deviate so largely from ideal behavior that the ordinary laws of solutions lose much of their significance, and many deductions hitherto made from them for such systems must fail completely. I n a ternary system, the S-LI-LII-V surface slopes down from the horizontal line a t I 12.3' in the binary system, and meets the liquidus curve of iodine a t concentrations dependent upon the third component present. Thus, it is not surprising to find that a t z s 0 , according
42 2
F. C. KRACEK
to Parsons and Whitternore,' the aqueous solution saturated with iodine and KI contains only ca. 6 wt per cent water. 11. A somewhat minor interest attaches to the extension of solubility studies into regions above atmospheric pressure, by the use of the sealed tube method. The method is old,* but very little use has been made of it in the past for exact work, principally for lack of facilities in maintaining constant temperatures in the neighborhood of any given point within the region studied. Granted that this requirement is satisfied, the method is convenient, moderately rapid, and more accurate than any other widely applicable method suitable for investigations at temperatures above the normal boiling point in the system studied. 12. Another point of interest lies in the relation of this system to the systems water-chlorine and water-bromine, studied by Bakhuis R o o ~ e b o o m , ~ who showed that in both systems there are formed immiscible liquid layers, but that the solid phase in contact with the immiscibility region in each case is a hydrate instead of crystals of the element. The quadruple S-LI-LI1-V point in the chlorine system is a t 28.7' (m. p. of chlorine -103')~ in the bromine system it is a t 6.2" (m. p. of bromine -7.3'); in the iodine system the solid phase is pure iodine, and the temperature is 112.3'~just below the m.p. of iodine. The aqueous layer a t the invariant point contains 0.915, 0,407 and 0.05 I 7 mol per cent Cl:, Brz and Izrespectively, whereas the halogenrich layers contain 99.5 mol per cent Br, and 98.3 mol per cent 1 2 respectively (the composition is unknown for the chlorine system). The immiscibility region extends almost entirely across each of these systems. Of particular importance is the magnitude of the vapor pressures in these systems. In the chlorine system the vapor pressure of the saturated solutions is much less than the vapor pressure of chlorine; in the bromine system it exceeds the sum of the vapor pressures of bromine and water; so we may predict that in the water-iodine system the vapor pressure also exceeds the sum of the vapor pressures of the components.
Summary TYater-iodine solutions above I 12.3' form t w o liquid layers, the mutual solubility increasing with temperature. The solubility curves were determined to temperatures above 200'. Below 112.3' the solutions are saturated with solid iodine. The composition of the liquid layers at the invariant temperature is 0.0517 and 98.3 mol per cent I? respectively. Geophysical Laboratory, Carnegie Institution of Washington, J u l y , 1930.
Parsons and Whitternore' J. Am. Chem. SOC., 33, 1933 (191 I ) . Phil. Mag., 18, 105 (1884); W.Alexejeff: Ann. Physik, 28, 305 (1886); more recently used extmsively by N. V. Sidgwick and co-workers, see J. Chem. SOC. from 1911 on. a Bakhuis Roozeboom: Rec. Trav. chim., 3, 59, 7 3 (1884); 4, 69, 71 (1885); Z. physik. Chem., 2, 449 (1888). l
* F. Guthrie:
