T H E SOLUBILITY OF POTASSIUM IODIDE I N WATER TO 240" BY F. C . KRACEK I. During the progress of another investigation it became desirable to know the course of the solubility curve of K I in water at temperatures above the normal boiling point of saturated solutions. Two series of data in this region are found in the literature, namely, by Tilden and Shenstone,' and Rtard,* reaching to 175" and 213' respectively. There is considerable discrepancy between these two series; as will be seen later, the results of fitard are the more reliable, but the precision of the individual experiments is rather low. Experimental 2. The solubility measurements were made by the synthetic method in Pyrex tubes. Known amounts of twice-recrystallized K I and distilled water were introduced into the tubes which were then sealed. The tubes were rotated in a stirred air bath which could be kept a t any desired constant temperature; the presence or absence of crystals was determined by visual ob~ervation.~Particular attention was given to the determination of the disappearance of the last traces of crystals in the immediate proximity of the solubility point. 3. The bath was controlled by a five junction Cu-constantan couple actuating a commercial type of controller; the regulating junctions were bare, to minimize lag. The reading thermocouple was also Cu-constantan, enclosed in a small alundum mass, suspended immediately adjacent to the rotating solubility tube. This couple was calibrated with accepted standards, readings being made on a Wolff Feussner type potentiometer mounted in accordance with the usual practice. The control was reliable to better than o.I', and could be set and maintained a t any desired temperature. 4. The solutions acquired a light brown tint due to slight hydrolytic decomposition of KI. This phenomenon is a familiar one, and can be eliminated by small additions of KOH. It was, however, thought better not to attempt to bring about decolorization by such additions, since to do so would alter the K-I ratio in the solutions.
*
The Results The experimental data obtained in this work are recorded in Table I which gives all the necessary details. A comparison of these results with the older published determinations4 is given in Fig. I . The smooth curve is 5.
~
'Tilden and Shenatone: Proc. Roy. SOC.,35, 345 (1883); Phil. Trans., 175, 23 (1885). l l h a r d : Ann. Chim. Phys., 2, 503 (1894). a F. C.Kracek: J. Phys. Chem., 35, 417 (1931). For references see A. Seidell: "Solubilities of Inorganic and Or anic Compounds" (1919); also Bronsted's section in Int. Crit. Tables 4, 239; Scott a n d Frazier: J. Phys. Chem., 31, 459 (1927); Scott and Durham: 34, 1424 (1930).
F. C. KRACEK
948
00
150
292
200
x10
FIG.I Solubility of K I in water, in grams of KI per 100 grams of water. The small open circles from I joo upward are by Etard. The triangles at on and 2j oare by Scott and eo-workers, at 100' by Bronsted. The larger open rirclea are by Kracek. The small open circles at lower temperatures are taken at random from the literature.
passed thru Scott and Frazier's point at 2 5 ' ; it also represents quite faithfully the results of Scott and Durham, the most recent published values for the region between oo and 100'. In the region of higher temperatures the curve falls generally among the points determined by Etard; the values by Tilden and Shenstone, however, are not even approximately correct. This investigation is not properly concerned with values at low temperatures, but it may be permissible to point out that there appears to be a consistent deviation in some of the older results in the direction of unsaturation below z jo, and particularly in the determinations below 0'. If we accept Scott and Durham's and Scott and Frazier's determinations a t oo and z jo, both of which are almost directly on the curve as drawn, we must conclude
SOLUBILITY O F POTASSIUM IODIDE I N WATER
949
TABLE I Determinations of solubility of K I in HzO g KI
Number
g HzO
4
j.6805 6.2164 5.7387 4.6109
j
5.4252
6
5.3133 5.3824 j,7625 7.9992 5.8592 6.1782 6.6479
I 2
3
i
8 9 IO 11 12
* rw r,
rr*
2,9930 2,9794 2,5686 I .9685 2,1685 2.1270
I .9658
1.9717 2.7074
1.9564 2.0104 1 ,9944
Percent KI weight mol
rm'
1.8979 0.20594 ,22641 2.0865 2.2342 ,24243 ,25416 2,3423 ,27147 2 , 5018 ,27106 2.4980 .29710 2.7380 2.9226 .31 7 I3 ,32060 2.9546 ,32498 2.9949 3.073I ,33346 ,36170 3.3333
65.49 67.60 69.08 jo.08 71.44
t"C
78.1 102.4 121.6 134.9 153.7 154.3 181.7 '99.3
17.08 18.46 19.j1 20.265 21.35
71,41
21.33 73.25 22.91 74.51 24.08 7 4 . 7 1 24.28 74.97 24.53 75.45 2 5 . 0 1 76.92 26.56
201.5
206.0 213.3
236.4
KI per g of H20. mols KI per mol of H20.
= g =
TABLE I1 Solubility of K I in HzO a t even temperatures t"C
g KI per 100g
H20
129.1* 0 . 5 136.8
t"C
g KI per 100
g H20
20
144.5
150
230.3 238 5 246.7
30
152.2
160
255.0
40 50
170
I IO
159.9 167.6 175.3 183.0 190.8 198.6 206.4 214.3
263 ' 7 272.9 282.6 292'9 303.4 314.3 326.0 338.4
I20
222.3
0
IO
60 70
80 90 IO0
130
140
180 190 2 00
210 220
230 2 40
*
0.
5
that these deviations are due to failure to obtain complete saturation, a very probable error in view of the high solubility of the salt. Table I1 gives the values of the solubility of KI in H20 a t even temperatures, taken from the smooth curve of Fig. I. The table thus represents the most probable values of the solubilities, arranged to facilitate interpolation a t intermediate temperatures. Geophysical Laboratory, Carnegie Institution of Washington, October. 1950.
