Nov., 1954
VAPORPRESSURE OB SOLUTIONS IN LIGHT AND HEAVY WATER
997
VAPOR PRESSURE STUDIES INVOLVING SOLUTIONS IN LIGHT AND HEAVY WATERS. I. THE APPARATUS AND THE DETERMINATION OF VAPOR PRESSURES AT 30" OF SOLUTIONS OF SODIUM AND POTASSIUM CHLORIDES IN ORDINARY WATER' BY HILTON A. SMITH,ROBERTL. COMBSand JOHN M. GOOGIN Cmtributim No. 184 from the Department of Chemistry, University of Tennessee, Knoxville, Tenn. Received March 16, 1064
An ap aratus for the determination of the vapor pressures of solutions relative to the vapor pressure of water has been describe%. The vapor pressure lowerings of water caused by the presence of various concentrations of sodium chloride and potassium chloride have been determined.
Various methods for precise determination of the vapor pressure of salt solutions may be found in the literature. I n general these are differential methods based on the determination of the differences between the vapor pressure of pure water and of a salt solution, both being maintained a t the same temperature. Static methods have been employed by Frazer and Lovelace2 who utilized the principle of the Rayleigh manometer, Chandler13 and P ~ d d i n g t o n . ~Some difficulties and sources of error in the static method involve dissolved impurities, inadequate stirring of solutions, unsteadiness and unequal distribution of temperature, and change in concentration if small amounts of solution are used. The transpiration or gas saturation method was used in a simple form by Walker6 and was improved by Berkeley and Hartley,6 Washburn and Heuse,' Gibson and Adams,* Pearce and Snow9 and Bechtold and Newton.'O In general the improvements involved the gas saturator or control of the gas flow. The transpiration method as it has usually been employed has the disadvantage that a really efficient saturator will generally cause some pressure drop in the gas stream and may also cause entrainment of mist or spray. It has been shown by Menzies" that the use of fine glass wool to remove the spray causes errors due to condensation on the glass surface. While a few other methods have been used with some success, it seemed advantageous t o use the gas-saturation method for the work to be described here, and hence attention was given to improvement of earlier designs. Experimental A schematic diagram of the a paratus finally adopted is shown in Fig. 1. The general iesign is similar to that of (1) Work supported by The Atomic Energy Commission. (2) J. C. W. Frazer, B. F. Lovelace, et al., J . A m . Chem. Soc., 88, 2439 (1914); ibid., 38, 515 (1916); ibid., 42, 1793 (1920); ibid., 48, 102 (1921): ibid., 45,2930 (1923); THISJOURNAL, SO, 1669 (1926). (3) R. C. Chandler, ibid., 44, 574 (1940). (4) I. E. Puddington, Rev. Sci. Instruments, 19,577 (1948);Can. J . Research. 27B. 1 (1949). (5) J. Walker, Z . physik. Chem., 2 , 602 (1888). (6) Earl of Berkeley and E. G.J. Hartley, Proc. Rov. Xoc. (London), A77, 156 (1906); Trans. Roy. SOC.(London), A218,295 (1919). (7) E. W. Washburn and E. 0. Heuse, J . A m . Chem. SOC.,87, 309 (1915). (8) R.E. Gibson and L. H. Adams, ibid., 55,2679(1933). 81,231(1927). (9) J. N. Pearae and R. D. Snow, THISJOURNAL, (10) M. F. Bechtold and R . F. Newton, J . A m . Chem. Soc., 82, 1390 (1940). (11) A. W. C. Menzies, ibid., 42, 978 (1920).
Bechtold and Newton,lo the major differences being in the saturators and absorbers. The outstanding features of these saturators, represented by A and B in Fig. 1, were their negligible pressure drop combined with almost cpmlete gas saturation in each stage. Each saturator contained Four g l a s ~drums, 25 mm. in diameter, which dipped into the saturating liquid contained in four cells made of 38 mm. Pyrex tubing, each 7 inches long. The drums were rotated at a speed of 300400 r.p.m. by a nickel-plated rotating alnico magnet outside the saturator. This was coupled through the saturator with a second magnet sealed within one end of the drum assembly; the latter was mounted on Teflon bearings. This glass rotor exposed a fresh film of solution to the gas stream without the formation of a mist or spray, gave turbulent flow to the gas, qnd stirred the solution. Preliminary experiments established the efficiency of each unit of the cell as approximately 95% thus giving an efficiency of the four-cell saturator of better than 99.999%. Furthermore, the vaporization of water,. and hence the change in composition of the solution during a run. occurred Drimarilv in the first cell. The saturator held 100 ml. of liquid. " It was found necessary to surround the exit tube with a vacuum jacket where the tubing emerged from the water bath: otherwise, there was slight condensation at this point. A drying cycle was employed-to remove moisture from the connecting lines prior to the runs and after each run was completed. The cycles were controlled by means of mercury stopcocks (X and Y in Fig. 1). The saturators were immersed in a water-bath, the temperature of which was maintained constant to 3=0.01". The drying tubes used to absorb the water evaporated from the solutions (C and D in Fig. 1) were fashioned from thin-wall Pyrex tubing, and weighed 45-65 g. empty and 160-190 g. when filled with desiccant. The desiccant, which was placed in the U portion, was usually non-indicating Drierite covered on the exit side nnth a layer of Anhydrone, although for a few runs either Drierite or Anhydrone alone was used. The notch at the bottom of the U served to re-center the gas stream in the desiccant. In addition, the absorbers contained a water trap which was made to fit into a suitable hole in a 2-inch L-shaped copper rod. Part of the rod (inside the air bath) was insulated by means of a Dewar flask with a hole in the side to fit the water trap. The other end of the copper rod was immersed in a suitable coolant. The drying tubes and connecting lines were covered with a hood which was maintained approximately 5' above the temperature of the thermostat. I n Fig. 1, the portion of the apparatus in the hood is indicated by the heavy broken lines, and that in the thermostat by the light broken lines. The manometric fluid used to control both pressure and flow rate was a solution of o-nitrodiphenyl- and tetrabutylammonium iodide in dibutyl phthalate. It possessed low density, negligible volatility, and satisfactory electrical conductivity. These manometers as well as the thermostat were controlled by suitable electronic circuits.12Ja Oil-pumped nitrogen passed through Anhydrone was used as the carrier gas in all runs. It was passed through the apparatus at a rate of 25 to 30 liters per hour. Preliminary (12) R. H.Linnell and H. M. Haendler, Rev. X c i . Instruments, SO, 364 (1949). (13) W . P. Ratchford and M. L. Fein, ibid., 21, 188 (1950).
998
H. A. SMITH, R. L. COMBSAND J. M. GOOGIN
Vol. 58
N
Fig. 1.-Vapor pressure apparatus: A and B are the saturators; C and D are the drying tubes; F is the re dryer for the drying cycle; E is the pressure control bulb for the contacts N to control the relay G; J is the flow contro? manometer for the contacts M to control the relay H; I is the escape valve; K and L lead to flasks which serve as ballasts to minimize pressure variations; 0 and P are manometers from which may be read the pressure difference in the two saturators; Q is the stopcock used to set the pressure for a run; R and T are three-way stopcocks by which one map switch from run to drying cycle; S is a stopcock used to test for ossible leaks in the system prior to a run; U leads to a drying tube and thence to a tank of oil-pum ed nitrogen; V and are round glass stoppers which may be removed in order to obtain solution for analysis; and Z a n d Y are the inlet tubes of the drying cycle The circuit is shown open for a run; the drying section is closed. The part of the system which is in either the water-bath or the hood is surrounded by a dashed line, and that which is in the bath is surrounded by a dotted line.
&
runs showed that variation in the gas flow between 5 and 35 liters per hour gave no variance in the results. The usual length of each run was one hour. Evaporation of liquid from the saturators into the gas stream would cause cooling of the liquid, while the energy resulting from the motion of the glass drums in the saturators would cause a slight heating. Ordinarily these effects should be similar in the two saturators, so that any variations from thermostat temperature would compensate each other. Thermocouples placed in the gas stream immediately following the saturators indicated variations in temperature of less than 0.01'. The glass tubing near the thermocouples was wrapped with tinfoil to eliminate effects of radiation. The agreement of the results obtained with this ap aratus with the best data found in the literature also ingcates no appreciable differences in the temperatures of the exit gas streams. Since the saturator involves a series of four efficient saturating cells, there should be a negligible amount of evaporation from the fourth cell. Any heat generated by the rotation of the glass drums would have to be dissipated by heat exchange through the glass walls of the saturator. It is conceivable that the heat generated might not be negligible, and might not be the same for water and concentrated salt solutions. I n order t o test the magnitude of this effect, a thermocouple was set up with one junction in liquid water in the saturator and one end in the bath. With no gas flowing through the saturator, the rotors were turned at speeds of from 110 to 640 r.p.m. The thermocouple indicated no differences in temperature within exerimental error (0.01-0.02°) between the bath and the i uid saturator, even at the highest speed of rotation. %hese ex eriments were repeated using saturated sodium chloride sofhion (in contact with excess salt). Speeds of rotation greater than 530 r.p.m. were not used because of
P
difficulty caused by solid sodium chloride in the bearings. Again no indication of a rise in temperature in the salt solution was found a t any speed. It thus appears evident that errors caused by heat generated through rotation of .the drums in the saturators were negligible for the solutions studied. Sodium chloride was J. T. Baker C.P. grade, and the potassium chloride was J. T. Baker C.P.specially purified grade. The same quality distilled water waa used in both the water and solution saturators. Weights obtained for the drying tubes were reproducible to 10.0004 g. The ca ped tubes were weighed before each run; the caps were tRen removed and the drying tubes placed in the lines. Synthetic rubber gaskets were used to seal the ball and socket joints which were held together with spring clamps. After each run the tubes were allowed to cool, wiped with a damp cloth, and allowed t o e uilibrate with room conditions for a t least an hour before ?hey were weighed. The usual weighing precautions were observed. Gravimetric analyses of the salt solutions were made before and after each run. The samples for analysis were taken from the last cell of the saturator through the ground glass plug provided for this purpose.
Calculations and Results The vapor pressures of the salt solutions were calculated from the relation PO
- pi Po
mo(B
+ Apo) + po(m~- mo)- ml ( B -k Apl) ma(B f Apo) + po(m1 - mo)
where po is the saturated vapor pressure of pure water a t the temperature of the thermostat, and p l is the vapor pressure of the liquid in the saturator;
Nov., 1954 0.20
c
'
VAPORPRESSURE OF SOLUTIONS IN LIGHTAND HEAVYWATER I
I
8
I
999
TABLEI
1
VAWR PRESSURES OF SALTSOLUTIONS AT 30.01 Molality
u
80.18
Vapor pressure lowering PO - PI Po
NaCl 3.755 4.563 4.910 5.070 5.162 4.910 (25.00')
.3
1
2 0.17
E!i0.16 a h
g0.15
6
0.14 3.8
4.0
4.2
4.4 4.6 4.8 5.0 5.2 Molality. Fig. 2.-Vapor pressure lowering of sodium chloride d u tions: o, Bousfield and Bousfield (IS") (Proc. Roy. SOC. (London), A103, 429 (1923)); @, Gibson and Adams (25"); 0 , this investigation (30); 6,this investigation (25').
,
Vapor pressure lowering PO
Molality
- Pl
PO
KCI 0.139 .174 .190 .196 ,200 .188
0,754 1.081 1.665 1.869 2.629 2.824 3.957
0.0225 ,0341 .0559 .0593 .0845 ,0925 ,130
ries of runs was 0.0001 =k 0,0018. The vapor pressure lowerings of the solutions of sodium and potassium chlorides are given in Table I. The deviations (95% confidence level) for the sodium chloride solutions are 0.003-0.004 and for the potassium chloride solutions are 0.001-0.002. According to Raoult's law, a plot of (PO - p ! ) p ~ against mole fraction (or for these solutions against
0.12 4
4
5 v
0.10
M
42 0.08
0 W
P
h
0.06
a
8
2 0.04
1.0
1.4
1.8
2.2 2.6 3.0 3.4 3.8 Molality. Fig. 3.-Vapor pressure lowering of otassium chloride solutions: 0,Lovelace, Frazer and Sease (20"); (3, Pearce a n f Snow (25"); 0 , this investigation (25'1, CaS04 as drying agent; 0 , this investigation (30'); Mg(C101)t as drying agent; ,. I this investigation ( 3 0 ° ) , Cas04 with Mg(ClO& as drying agent.
mais the weight gain of the drying tube after the water saturator, ml is the weight gain of the drying tube after the solution saturator, Apo and Apl are the pressure drops across the saturator-absorber combination for the pure water and the solution due almost entirely to the absorber, and B is the barometric pressure. When water was run in both saturators, the value of (po - p l ) / p o for a se-
molality) should give a line which is essentially straight, and which is independent of temperature. Figure 2 shows such a plot for the sodium chloride solutions and Fig. 3 for those of potassium chloride. The results show that this apparatus gives excellent agreement with the best data in the literature, even though such data were obtained at other temperatures,
