1388
EVERSOLE, WAGNER, AND BAILEY
SURFACE TENSION, LIQUID DENSITY, AND VAPOR DENSITY OF SULFUR DIOXIDE SOLUTIONS OF POTASSIUhl THIOCYANATE AT loo, 15’, 20°, AND 25OC. W. G. EVERSOLE, G. H. WAGNER,
AND
G. C. BAILEY
Division of Physical Chemistry,Stale University of Iowa, Iowa City,Iowa Receieed June 8, 1941
Theories of the surface tension of electrolyte solutions have been proposed by Onsager and Samaras (13), by Dole (5), and by Bikerman (1). By taking the principal effect of the surface (solution-air interface) on the ions to be that of image forces (20) and making use of the Debye-Huckel theory and the Gibbs adsorption equation, Onsager and Samaras arrived a t a limiting law for solutions of uni-univalent electrolytes which has the form u/ao =
1 -l-(79.517/Duo)M lOg10(1.143 X 10-i3(DT)3/M)
(1)
where U/UO is the relative surface tension, D is the dielectric constant of the solvent, T is the absolute temperature, and M is the molarity of the salt solution. A characteristic property of this function is, as M 0, d(o/oo)/dM+ + m . Dole’s theory, as well as Bikerman’s, is hardly workable, since Dole’s equation relating surface tension to molarity contains three arbitrary parameters and Bikerman does not arrive a t an explicit, relationship between surface tension and concentration. On the basis of qualitative considerations, Dedrick and Hanson (4) predicted the existence of minima in the dilute range of the surface tension-concentration curves of aqueous electrolyte solutions. Subsequently, Jones and Ray (10) and Dole and Swartout (6), using different methods of measurement, observed minima a t concentrations of approximately 0.001 molar in the surface tension-concentration curves for typical electrolytes. This of course means that o/ao is less than 1 and that d(o/uo)/dM is negative, in direct contradiction to equation 1. Langmuir (12) interpreted the minimum observed by Jones and Ray on the basis of a water filni which caused the effeLtive radius of the surface tension capillary to be smaller when occupied by water than when occupied by solution. The fundamental assumptions in Langmuir’s theory have been critically examined by Jones and Frizeell (9). It appeared to the authors that this divergence between theory and observation might be peculiar to aqueous solutions and that valuable information could be gained from a study of electrolyte solutions in the nonaqueous, yet polar medium, liquid sulfur dioxide. Potassium thiocyanate was chosen as the salt for this study, because its aqueous solutions were found by Jones and Ray (lob) to give a minimum in the surface tension---f
SOLUTIONS OF POT.4SSIUM THIOCYANhTE IN SULFUR DIOXIDE
1359
concentration curve which was more pronounced and occurred a t a higher concentration than for any other electrolyte studied. A measuring microscope was employed which was capable of detecting, in the differential capillary-rise method used, a surface-tension change of 0.001 dyne per centimeter. A method was devised for measuring relative liquid densities and absolute vapor densities with a precision of O.oooO1 g. per cubic centimeter, so that any minimum in the dilute concentration range would not evade observation. Measurements were made a t IO", 15", 20°,and 25°C.over the concentration range 0 to 0.8 molal. EXPERIMENTAL
Purificction of reagents Reagent potassium thiocyanate was purified according to Kolthoff and Lingane (11)by recrystallizing it twice from conductivity water and driving off the last trace of water by heating to the melting point for 2 to 3 hr. The dried salt was pulverized in an agate mortar, redried a t 100°C.for 48 hr., and stored over phosphorus pentoxide. Weights of the weighing bottle and the salt, with and without redrying after the addition of the salt to the apparatus, were found to check to 0.3 mg. This proved that no appreciable amounts of water were adsorbed during the weighing and addition of the salt. Commercial sulfur dioxide was purified in an apparatus designed by Bond and Beach (2). Sulfur trioxide was removed by three wash bottles of concentrated sulfuric acid, which were fitted with aerators for dispersing the gas. The drying train consisted of 12 ft. of calcium chloride and 6 ft. of phosphorus pentoxide beads. The sulfur dioxide obtained in this manner was further purified by repeated distillations a t low temperatures (- -1O0C.),leaving behind a small atnount each time which was discarded. These distillations completely removed any water or sulfur trioxide because of their much higher boiling points. The chloroform used for the calibration of the helix and the capillarimeter was purified according to Timmermans (18). It had a boiling-point range of less than 0.1OC.and was finally distilled directly into the apparatus.
Apparatus The complete apparatus is shown in figure 1. It consisted of a Pyrex tube, about 30 cm. long and having an outside diameter of 2 cm., with two Pyrex capillaries (A) sealed to the outside in such a position that both capillaries were visible in the field of the measuring microscope by a slight change of focus. A calibrated quartz helix (F) and a quartz bob (G) were suspended from a glass loop (C)by means of a platinum hook (D) and platinum loop (E). The complete spring assembly was inserted through
1390
EVERSOLE, WAGNER, AND BAILEY
the lower end of the large tube before the tube was sealed. The salt and sulfur dioxide were added through B. In use, the apparatus was nearly half filled with liquid, sealed, and placed in a thermostat of 100 liters capacity. The temperature, which was maintained constant to 0.005"C., was read from a thermometer calibrated by the Bureau of Standards. From previous calibrations of the capillaries and the helix, the surface tension and vapor density could be calculated, respectively, from readings of the differential rise of the liquid in the capillaries 2nd from the length of the helix with its bob in the vapor. By inverting the apparatus and repeating these readings a check was ob-
>
e
F I ~1.. Diagram of the apparatus
tained on the surface tension, and the liquid density was obtained from the length of the helix, which was now extended upwards in the liquid. All readings were taken with a measuring microscope (21) which read to 0.00006 cni. Calibration of capillaries and helix
A 7-cm. portion of the small capillary and a 4-cm. portion of the large capillary gave a uniform variation of only one part in one thousand to R 6-mm. mercury thread. The capillaries were sealed to the large tube SO as to be parallel and to have the selected sections a t the midpoint of the apparatus. Chloroform, which has a surface tension and density very
SOLUTIONS OF POTASSIUM THIOCYANATE IN SULFUR DIOXIDE
1391
near that of sulfur dioxide, was used as the calibrating liquid for the capillaries and the helix. A correction for the rise in the large tube was made in a manner described by Richards and Carver (14). By bringing an opaque screen up tangent to the meniscus in the large tube, it was possible to duplicate individual readings of the capillary height to &0.0005 cm. The radii of the capillaries were calculated as described by Richards, Speyers, and Carver (15), using Richards and Carver’s value for the surface tension of chloroform. The radii were found to be 0.09217 cm. and 0.02367 em. The helix was calibrated by a method (22) which has been previously described. The quartz bob had a volume of 1.3676 cc. and a mass of 1.0258 g. Chloroform was used for the calibration of the helix for the liquid because the density (19) of chloroform is more accurately known than that of any other liquid which has a density near that of sulfur dioxide. The maximum pressure of the sulfur dioxide encountered in these measurements was slightly less than 4 atmospheres, and the effect of the pressure on the volume of the bob has been estimated to be less than the experimental error. Filling the apparatus After the weighed apparatus was evacuated, it was filled with dry air from a phosphorus pentoxide train. The salt was added through B in this dry atmosphere from a specially constructed weighing bottle. The outlet of the weighing bottle was a small tube which fitted through B into the large tube. B was then connected to one outlet of a three-way s t o p cock, and the apparatus was alternately flushed with sulfur dioxide and evacuated repeatedly. The apparatus was then submerged in an ice-salt mixture and filled to the desired level with sulfur dioxide by distillation, before sealing off a t B. The weight of sulfur dioxide was determined by difference. Before any measurements were made, the solution was well mixed by repeated inversions of the apparatus, and the apparatus was finally fastened to a baffle in the thermostat where it was shaken a t constant temperature for a t least 48 hr. Previous to vapor-density measurements, the helix and bob were cleansed by condensing sulfur dioxide in the top of the apparatus and allowing it to run down the helix, and were dried by immersing the lower half of the apparatus in ice water. CALCULATIONS
The values for the surface tension were calculated by the method of Richards, Speyers, and Carver (15), using the equation u =
HK(D
- d)
1392 where
EVERSOLE, WAGNER, AND BAILEY u = surface tension in dynes per centimeter, D = density of the liquid in grams per cubic centimeter, and d = density of the vapor in grams per cubic centimeter.
H = (hi
e E)
- h)+ 7 - - 0.1288 T2
+ 0.1312($- $) s
a
Here hl and hz are the observed heights of the menisci in capillaries of radii T~ and ~ 2 respectively, , above the corrected liquid level in the large tube (figure 1). K is a constant for the apparatus, which is determined by calibration. Thus, values of 6 / 0 0 determined by this method are independent, except for the small effect on H , of errors in r1 and r2. Vapor densities were calculated by means of the equation (22) d=
Lo
- L + 6.1(25 - t o ) 12660
(3)
where LO= the length of the helix in a vacuum a t 25OC.,and L = the length of the helix in a vapor of density d (grams per cubic centimeter) a t t°C. Values of L are expressed in degrees of arc on the lead screw of the measuring microscope, 1 cm. = 1700.81°. Liquid densities were calculated by means of the equation,
D
= 1.47988
- to) - Li + L' - 6.1(25 12764
(4)
where L: = the length of the helix in chloroform a t 25"C.,and L = the length of the helix in a liquid of density D (grams per cubic centimeter) a t t°C. All concentrations were corrected for the amount of sulfur dioxide in the vapor phase. One can write,
where V , = volume of vapor, V I = volume of liquid, a = V L- V , g = weight of potassium thiocyanate, and G = total weight of sulfur dioxide. By measuring the difference in the levels of the solution in the large tube (figure 1) before and after inverting, a could be calculated, the diameter of the tube being known. Since d , D, g, and G are measured quantities, equation 5 could be solved for V , and the mass of the vapor phase was given by dV,.
SOL1;TIONS OF POTASSIUM THIOCYASATE IN SULFUR DIOXIDE
1393
DISCUSSION OF RESDLTS
The results are tabulated in table 1. The measured vapor densiticw checked to i O.oooO1 g. per cubic centimeter with those calculated by assuming the vapor density proportional to pressure and using vaporpressure data obtained from measurements in progress in this laboratory. Since the vapor-preasure lowering a t the highest concentration is only about 5 cm. of mercury, this method of calculation should be valid over the entire range of concentration and was used to expedite experimentation. -4lthough the liquid densities were measured and checked to better than f O.OOO1 g. per cubic centimet,er, their accuracy was limited to this figure by the accuracy to which the density of the standard liquid was known. Since the vapor densities were practically independent of standard densities, their accuracy was limited only by the accuracy of measurement, f O.oooO1 g. per cubic centimeter. In comparison with the values listed by the Infernational Critical Tabbs (8, page 236),l our liquid densities of pure sulfur dioxide agree at all temperatures, while our vapor densities are low by approximately 0.0005 g. per cubic centimeter. It is possible that this deviation in the vapor densities could be due to an adsorbed film of sulfur dioxide on the bob and helix, but it seems unlikely in view of the met>hodof calibration. Stowe's (16) values for the surface tension of sulfur dioxide are approximately 0.6 d , p e per centimeter higher than our values. This deviation is to be expevted, since Stowe used a questionable meniscus correction, used density values which do not correspond to the saturation pressure, and had a maximum deviation of approximately 0.6 dyne per centimeter between capillaries. The authors were able to check all differential capillary heights for pure sulfur dioxide to better than f0.00015 cm. This precisian ww duplicated for the solutions only a t 20" and 25"C., where a constant temperature couid be maintained for 48 hr. or longer. Observations showed that good shaking for at least 24 hr. at constant temperature was necessary to expunge concentration gradients in the capillaries which resulted from distillation to,or fmm, the capillaries when the temperature of the thermostat was changed. This phenomenon is not observed in aqueous solutions, because of the small change in vapor density with change in temperature. Figure 2 shows a plot of the surface tension of the solutions a t lo", 15'' No, and 25°C. as a function of the pokqium thiocyanate molality. The data for the approximately 0.58 molal solution were obtained by a different worker, using a different capillarimeter calibration and a different helk assembly. Characteristic of each curve is the rapidly changing positive
* The significant figure listed in the Zntermtiotcal Cfitical Tables for the density of liquid sulfur dioxide i R only 0.001 g. per cubic centimeter.
1394
EVERSOLE, WAGNER, .4ND BAILEY
TABLE 1 Liquid density, vapor density, and surface tension of sulfu? dioxide solutions of potassium thiocyanate
1
vamapmcc.
Pure SO, 0.00837
0.04380
1.36887 1.37091 1.37576
0 04355 0 1286
1 38258 135162 1.38920 1 40123
Pure SO: 0.00835 0.04352 0.1284 0.5851 0.7957
1.39597 1,39798 1.40238 1.41421 1.46530 1.48906
kama per cc.
1
0.01070t 0.01069 0.01067
1
o.00mt Oo0908
~
'
~
I
I
1 ~
du;po;".
21.074 21.170
O.Oo906 0.00904t (0 00903)'
21 970 22.040 22 108 22.257
0.00760t 0.00760 0.00758 0 .W756t (0.00756)* 0.00748 0.00746
22.926 22.984 23.148 23.252 23.961 24.379
At 10°C. Pure Sot 0.00834 0.04350 0.1283 0.7949
1.40937 1.41102 1.41560 1.42731 1.50055
0.00634t 0.00634 0.00632 0.00630 0.00621
23.895 24.119
24.203 24.205 25.357
* Calculated from vapor-pressure data, assuming the density proportional to the pressure for these small changes in pressure.
where d = vapor P = vapor d o = vapor P o = vapor
t Measured
density over solution, pressure of solution, density over pure sulfur dioxide, and pressure of pure sulfur dioxide.
SOLUTIOKS OF POTASSIUM THIOCYASATE I K SULFUR DIOXIDE
1395
slope in the most dilute range and the constant positive slope in the more concentrated range. This increase in surface tension with molality is characteristic of electrolyte solutions. The relatively large deviations
flOLALlTY
FIG.2. Surface tension of sulfur dioxide solutions of potassium thiocyanate 1.OS
1.010
-
P 06
0
.os
.1
.15
MOLARITY
FIG.3. Relative surface tensions a t 25°C. 0, sulfur dioxide solutions of potassium thiocyanate; 0 , aqueous solutions of potassium thiocyanate (Jones and Ray (lob)).
at 10°C. in comparison with the other isotherms is evident. Thcsc d c v otions were due to the inability to maintain a constant temperature a t 10°C. for 24 hr.
1396
EVERSOLE, WAGNER, AND BAILEY
The relative surface tension of sulfur dioxide solutions a t 25OC. in the dilute range is represented graphically in figure 3 as a function of the potassium thiocyanate molarity and is compared with the data of Jones and Ray for aqueous solutions plotted on the same scale. The increase in relative surface tension with molarity is very much more pronounced in the sulfur dioxide solutions than in the aqueous solutions. This is in agreement with equation 1, sincc the surface tension and the dielectric constant of sulfur dioxide are both lower than the corresponding constants for water. Figure 3 ais0 indicates that any minimum in the dilute range would occur a t a much lower concentration than the minimum observed in the aqueous solutions. The large positive slope in the dilute range of the sulfur dioxide solutions, in contrast to the aqueous solutions, is in agreement with the OnsagerSamaras theory. An even greater slope could be expected if activities were used in place of molarities since, on the basis of TABLE 2 Properties of liquid suljur d i o d d e T 'K.
283.1 288.1 293.1 298.1
'
d w a psr OK. p4r
mgI)par cm.s
cm.2
0,1938 0.1912 0.1890
E.
I
78.760 78.011 77.366
j
* Using an average value of K
(P) K
'
Ia
2.1 2.09 2.08
'
1.9 1.9 1.9 1.86
100.95 100.96 100.98 101.00
= 2.09.
the Debye-Huckel theory, the activity coefficient of the salt in sulfur dioxide would be much less than 1, because of the low dielectric constant. Therefore, an actual test of equation 1 by means of the data listed here must await the determination of the activity coefficients of potassium thiocyanate in sulfur dioxide. Measurements for this purpose are in progress in this laboratory. The values of the surface energy (E,) and the surface entropy (S,) of sulfur dioxide are shown in table 2. Using the Ramsay-Shields equation in the form 2/a
u(;)
= R(t,
- - a) to
and :[u($r']==
-K
SOLUTIONS OF POTASSIUM THIOCYANATE IN SULFUR DIOXIDE
1397
where t, = the critical temperature (3), 157.5OC., values of K , the Eijtvos constant (2.12 for “normal” liquids), and a have been calculated and are given in table 2. Values of the parachor ( P ) are also given. The average value of ( P ) ,about 101, is in good agreement with the value of Grunmach (7), 101.5. The value of ( P ) calculated from Sudgen’s (17) atomic and bond values is 109.8. The slight deviations of a and K from their “standard” values is to be expect,ed, since sulfur dioxide is polar and these deviations are usually taken as an indication of polarity. SUMMARY
1. An apparatus and technique are described for measuring vapor density, liquid density, and surface tension on the same solution in n
closed system. 2. The surface tension, liquid density, and vapor density of sulfur dioxide solutions of potassium thiocyanate were measured over the concentration range 0 to 0.8 molal a t lo”, 15”, 20”, and 25°C. 9. The Onsager-Samaras theory agrees qualitatively with the increase in the relative surface tension with concentration, in the dilute concentration range of the sulfur dioxide solutions. REFERENCES (1) BIKERMAN, J . J.: Trans. Faraday SOC.34, 1268 (1938). (2) BOND,P. rl., A N D BEACH,H. T.: J . Am. Chem. SOC.48, 318 (1926). E., A K D SORREIVTINO, E.: J. chim. phya. 24, 77 (1927). (3) CARDOSO, (4) DEDRICK, D. S., A K D HAKSON, M.H.: J. Phys. Chem. 37, 1215 (1933). (5) DOLE,31.: J . Am. Chem. SOC.60, 904 (1938). (6) DOLE,M , , A N D SW.ARTOCT, J. A,: J . Am. Chem. SOC.62, 3039 (1940). (7) GRIXMACH, L.: .4nn. Physik [4116,401 (1904).
(8)International Critical Tables, Vol. 111. McGraw-Hill Book Company, Inc., New York (1928). (9) JONES,GRINSELL,. ~ N DFRIZZELL, L. D.: J. Chem. Phys. 8, 986 (1940). (loa) JONES,GRINSELL,A N D RAY,W. A.: J. Am. Chem. SOC.69, 187 (1937). GRINKELL, A N D RAY,W.A , : J. Am. Chem. SOC.63, 288 (1941). (lob) JONES, I. M., .4ND LISGANE,J. J.: J. .4m. Chem. sot. 67,2126 (1935). (111 KOLTHOFF, IRVING: Science 68, 430 (1938); J. Chem. Phys. 6, 894 (1938). (12) LANGMCIR, L., ASD SAMARAS, S . N. T.: J. Chem. Phys. 2, 528 (1934). (13:) ONSAGER, (14) RICHARDS, T. W.,. ~ N DCARVER, E. K . : J. Am. Chem. soc. 43, 827 (1921). (15) RICHARDS, T. W,, SPEYERS, C. L., A N D CARVER, E. K . : J . Am. Chem. sac. 46, 1196 (1924). (16) STOWE,V. 31.:J. Am. Chem. Sac. 61,410 (1929). S.: The PQTachOT and I’alency. George Routledge and Sons, London (17:i SUGDEN, (1930). (18) TIMMERMAXS, J . : Bull. SOC. chim. Belg. 24, 244 (1910). J., AKD MARTIS,F.: J. chim. phys. 23, 733 (1926); 23, 747 (1926). (19) TIMMERMANS, (20) WAGNER, CARL:Physik. Z. 26, 474 (1924). W. G.: Ind. Eng. Chem., (21) WAGFER,G. H., BAILEY,GRASTC., ASD EVERSOLE, Anal. Ed. 13, 658 (1941). (22) W.4GNER, G . H., BAILEY, GRASTC., AKD EVERSOLE, %’, G.: ’‘4 Precise Method for Determining Liquid and Vapor Densities in Closed Systems” (submitted to Ind. Eng. Chem., Anal. Ed.).
