4224
ROBERT R. DEWALD AND JOHN H. ROBERTS
The Conductance of Dilute Solutions of Sodium in Liquid Ammonia at -33.9, -45, and -65” by Robert R. Dewald and John H. Roberts Department of Chemistry, Tufts University,Medford, Massachusetts
02’166
(Received June 3,1068)
The conductivity of solutions of sodium in liquid ammonia at -33.9, -45.0, and -65.0’ has been measured over the concentration range 1.8 X to 8 X M . The method of Shedlovsky has been used to evaluate the data. Values of 1127, 927, and 645 for the apparent limiting equivalent conductance and 3.41 x 2.48 X and 1.88 X lova for the pairing constants, at -33.9, -45.0, and -65.0’, respectively, have been obtained. The Walden products at the three temperatures are nearly the same. The temperature coefficients of the conductivity of dilute sodium-ammonia solutions are accounted for by ion pairing.
Introduction Although metal-ammonia solutions have been studied by a wide variety of physical chemical techniques,l-a there is as yet no agreement on their exact n a t ~ r e . ~ There -~ seems to be little doubt that in very dilute solutions M ) , it is necessary to view the system in terms of metal cations and solvated electron^.^-^ As the alkali metal concentration is increased, conductance data’ require that some aggregation of anions and cations takes place. Magnetic data8)$show formation of spin-paired species, and optical data suggest that the species are independent. Kraus’ reported the conductivities of dilute solutions of sodium, potassium, and lithium in liquid ammonia. The conductance data reported by Kraus have generally been accepted as the best a ~ a i l a b l e . ~Conductance ~,~~ data have also been reported by Gibson and Phipps,14 Fristrom,16 Smith,16 and Evers and Longo.’’ Evers and Frank13 successfully reproduced the data reported by Kraus7 for the sodium-ammonia system with a conductance function that uses the mass action equations suggested by Becker, Lindquist, arid A l d e F and a modified form of the Shedlovsky function. I n their conductance study of the lithium-ammonia system Evers and Longol7 have also used this conductance function to evaluate their data. With somewhat greater accuracy, we have measured the conductance of dilute solutions of sodium in liquid ammoniaat -33.9, -45, and -65”.
Experimental Section Materials. Ammonia (Matheson 99.99%) was condensed in vacuo in a trap containing sodium metal and stored at least 1 day (600 ml). About half of the ammonia was distilled into a second trap which had been evacuated and flamed until the pressure stabilized a t less than 2 x Torr. The ammonia was then distilled back into the trap containing the sodium. The blue solution was next frozen with liquid nitrogen and The Journal of Physical Chemistry
degassed. The second trap was again evacuated and flamed. The cycle of distilling back and forth, freezing, degassing, and flaming was repeated at least five times. The ammonia was then stored in contact with sodium, distilling enough for one or two experiments into the second trap just prior to use. The ammonia was frozen and degassed twice before distilling into the apparatus. One reason for distilling the ammonia from the storage flask (which contained no sodium) was to aid in reducing carryover of nonvoIatile materials during evaporation. Using the above method for ammonia purification, we were able to obtain liquid ammonia having a specific conductance of the order of 10-7 mho or less, which is about the working range reported by other investigators.lg Also, we found20 (1) M. J. Sienko and G. Lepoutre, Ed., “,Metal-Ammonia Solutions,” W. A. Benjamin, Inc., New York, N. Y., 1964. (2) T. P. Das, Advan. Chem. Phys., 4, 303 (1962). (3) W. L. Jolly, Progr. Inorg. Chem., 1, 235 (1956). (4) S. Golden, C. Guttman, and T. R. Tuttle, Jr., J . Chem. Phys., 44, 3791 (1966). (5) E. Arnold and A. J. Patterson, Jr., ibid., 41, 3089 (1964). (6) M . Gold, W. L. Jolly, and K. S. Pitrer, J . Am. Chem. Soc., 84, 2264 (1962). (7) C. A. Kraus, ibid., 43, 749 (1921). (8) C. A. Hutohinson, Jr., and R. C. Pastor, J . Chem. Phys., 21, 7659 (1953). (9) 9. Freed and N. Sugarman, ibid., 11, 354 (1943). (10) M. Gold and W. L. Jolly, Inorg. Chem., 1, 818 (1962). (11) R. C. Douthit and J. L. Dye, J . A m . Chem. Soc., 82, 4472 (1960). (12) J. L. Dye, R. F. Sankuer, and G. E. Smith, i b i d . , 82, 4797 (1960). (13) E. C. Evers and P. W. Frank, J . Chem. Phys., 30, 61 (1959). (14) G. E. Gibson and T. E. Phipps, J . A m . Chem. Soc., 48, 313 (1926). (15) R. M. Fristrom, Ph.D. Thesis, Stanford University, 1949. (16) G. E. Smith, Ph.D. Thesis, Michigan State University, 1963. (17) E. C. Evers and F. R. Longo, J. Phys. Chem., 70, 426 (1966). (18) E. Becker, R. H. Lindquist, and B. J. Alder, J . Chem. Phys., 25, 971 (1956). (19) V. F. Hnizda and C. A. Kraus, J . A m . Chem. Soc., 71, 1565 (1949).
CONDUCTANCE OF DILUTESOLUTIONS OF SODIUM IN LIQUIDAMMONIA
9
8
Figure 1. Solution makeup vessel and conductivity cell: (1) ammonium bromide sample, ( 2 ) glass-encased magnet, (3) side arm, (4)conductivity cell, (5) calibrated bulb, ( 6 ) stopcock, ( 7 ) platinum electrodes, (8) graded seal, (9) Torr seal (Varian), (10) leads to bridge.
liquid ammonia having a specific conductance of the order of 10-lo mho could only be prepared by using the tedious and time-consuming purification scheme described by Clutter and SwiftQZ1In the present investigation, ithe conductance of the solvent was considered to be negligible even in the most dilute solutions. Sodium (United Mineral and Chemical Co.) was distilled twice in vacuo and stored in Pyrex capillaries. The amount of sodium needed to make a solution of a desired concentration could be estimated from the length of the capillary. Ammonium bromide (Fisher reagent) samples were prepared by placing the salt in a break-seal tube which had a constriction just above the sample. After evacuation to about Torr for at least 8 hr, the tubes were sealed off under vacuum. Apparatus. The apparatus (Figure 1) was constructed from borosilicate glass except for the Pyrex to lead-glass graded seals, which facilitated the glass to platinum seal. The apparatus was designed so that it could be immersed in a bath and the metal solution could be tipped on and off the electrodes. The bulb (50 ml) and stem of the apparatus were calibrated so that the volume of the blue solution could readily be determined to h0.05 ml. The conductivity cell (Figure 1) was designed to allow agitation of the solution between the electrodes during a measurement. The electrodes were platinum balls of about 0.5 mm diameter and were spaced about 1.5 cm apart. Eight different conductivity cells were used in the present study and they had cell constants ranging from 2.367 to 4.871. In some experiments, the platinum electrode
4225
surfaces were gold plated (American Electroplating Co., Cambridge, Mass.). Procedure. The cells with gold-plated electrodes were calibrated with standard KC1 solutions using the data of Jones and Bradshaw.22 Cells with brightplatinum electrodes were calibrated using a KI-I2 mixture as recommended by Kraus.' For both types of electrodes, the cell constants were found to be independent of concentration. Measurements were made at 600, 1000, 2000, and 4000 Hz. Where values of the resistance of standard solutions were found to vary with frequency, a plot of resistance us. the reciprocal of the square root of the frequency was made. From the linear plot the value of the resistance at infinite frequency was obtained. The conductivity cells were also checked for temperature dependence by first platinizing the electrodes and then determining the cell constant at 0 and 25' with 0.01 M KC1 solution. The conductance equipment used in this study was similar to that previously described.16 Before each conductivity experiment, an ammonium bromide sample and a side arm for sodium distillation were sealed to the apparatus (Figure 1). The apparatus was next put through a stringent cleaning procedure which is described elsewhere.m Cells containing gold-plated electrodes were not cleaned with the acid cleaners but were rinsed and soaked repeatedly with demineralized water. The apparatus was then connected to the high-vacuum line and the ammonia reservoir trap. A capillary containing sodium was introduced into the side arm which was then sealed. After evacuation and flaming until the pressure stabilized a t about 2 X 10-6 Torr, the apparatus was cooled b y pouring an acetone-Dry Ice mixture over it and ammonia was allowed to condense on all surfaces. T h e ammonia was removed by evacuation and the apparatus was flamed until pressures of about 2 x 10-6 Torr were obtained. The apparatus was again cooled, exposed to pure ammonia vapor, and evacuated as above. Next, the sodium was distilled into the apparatus and the side arm was sealed off. After the desired amount of ammonia was distilled into the apparatus, the blue solution was frozen with liquid nitrogen, degassed, and thawed repeatedly. The apparatus was then disconnected from the vacuum line and placed in the -33.9' bath and the initial resistance was determined. The solution was then frozen and degassed by evacuation on the high-vacuum line. Finally the apparatus was returned to the -33.9' bath and the resistance of the blue solution was recorded (20) R. V. Tsina and R. R. Dewald, unpublished results, this labor*
tory. (21) D, R. Clutter and T. J. Swift, J. Am. Chem. SOC.,90, 601 (1968). (22) G . Jones and B. C. Bradshaw, ibid., 55, 1780 (1933). (23) L. H. Feldman, R. R. Dewald, and J. L. Dye in "Solvated Electron," Advances in Chemistry Series, No. 50, American Chemical Society, Washington, D. C., 1965, p 163.
Volume 7.9, Number 1.9 November 1968
ROBERT R. DEWALD AND JOHN H. ROBERTS
4226 as a function of time for about 1hr. If a change in the resistance of the solution was observed with time, the experiment was terminated a t this point. The resistance was determined by pouring the blue solution into the conductivity cell while gently shaking the apparatus back and forth to agitate the solution between the electrodes. It was noted that, during shaking, the resistance of the solution decreased. A dynamic balance could be achieved which was only possible with continued agitation. The solution was then poured into the bulb portion of the apparatus and poured back into the conductivity cell with shaking. This was repeated until the lowest reproducible value of the resistance had been obtained. The volume of the blue solution was then noted. The above procedure was repeated every 10 min for 1 hr before making measurements a t -45 and - 65". The blue solution was then frozen and the apparatus was connected to the high-vacuum line for a final degassing. Finally, the metal concentrations were determined by addition of ammonium bromide to the metal solution which resulted in the reaction EH4+
+ Na +NH3 + Naf + 0.5H2
The total hydrogen evolved was pumped into a gas buret, and its volume and pressure were measured. The concentration of sodium was calculated from the stoichiometry of the above reaction and the measured volume of the blue solution. TempeTature Control. At -33.9" the bath consisted of a container of silicone fluid (Dow Corning No. 200) placed in an Aminco refrigerated bath. Temperature regulation was achieved with a mercury regulator which controlled, through a transistorized relay, a 250-W infrared immersion heater. A copper-constantan thermocouple was used for temperature measurements in conjunction with a AlinneapolisHoneywell portable potentiometer, Model Yo. 2746. The temperature was maintained well within the range of -33.9 0.1". At -45 and -65" the bath consisted of an insulated container of silicone fluid. The bath was cooled to the appropriate temperature with powdered Dry Ice. Also, a metal container containing a Dry Ice-acetone mixture was placed in the bath. Temperature regulation was achieved with a bimetallic regulator (American Instrument Co.) which controlled, through a relay, a 250-W immersion heater. The temperature was monitored continuously and could be maintained to within S 0 . 3 " .
Results The values of A and C are given in Table I. The equivalent conductance, A, is expressed in Kohlrausch units, and the concentration, C, in moles per liter. The values of A at -33.9 5 0.1, -45.0 * 0.3, and -65.0 0.3" are plotted us. C'/% in Figure 2. The
*
The Journal of Physical Chemistry
? OO *
'
i
I $
900-
a
g
800-
z
8 !Wz _I
9
-
'
700 -
600-
3
500-
Figure 2. Plots of A us. for solutions of sodium in liquid ammonia: circles, this study; triangles, data reported by Kraus.7
conductance data for sodium-ammonia solutions at - 34" reported by Kraus7are also shown in Figure 2
Discussion The apparent limiting values of the equivalent conductance, Ao, were evaluated by using the method of S h e d l ~ v s k y . ~Also, ~ the apparent ion-pairing dissociation constants, K I , for solutions of sodium in ammonia were evaluated at the three different temperatures. Constants used in the Shedlovsky analysis of the data are given in Table 11. The values of the dielectric constant^,^^^^^ D, and the viscosity, 27--30 11, were obtained by interpolation of available data. To compute activity coefficients, the extended Debye-Hiickel expressicp was used. A distance of closest approach of 5.5 A was used. Only concentration-conductance data below 1.3 X M were used to calculate the values of the limiting equivalent conductances and the pairing constants given in Table 11. Shedlovsky plots for data below 6 X M , which exclude only the three highest (24) T. Shedlovsky, J . Franklin Inst., 225, 738 (1938). (25) H. M. Grubb, J. F. Chittum, and H. Hunt, J . Am. Chem. Soc., 8 5 , 776 (1963). (26) D. F. Burrow and J. J. Lagowski in "Solvated Electron," Advances in Chemistry Series, No. 50, American Chemical Society, Washington, D. C., 1965, p 125. (27) H. M. Elsey, J. Am. Chem. Soc., 4 2 , 2454 (1920). (28) C. J. Planck and H. Hunt, ibid., 61, 3590 (1939). (29) G. Pinevich, Kholodil'n. Tekhn., 20, N o . 3, 30 (1948). (30) K. Fredenhagan, 2. Anorg. Allgem. Chem., 186, 1 (1931)).
CONDUCTANCIE O F
DILUTESOLUTIONS O F SODIUM
IN
LIQUIDAMMONIA
4227
Table I : Conductance-Concentration Data for Solutions of Sodium in Liquid Ammonia Run no.,
-45,00--
-33.90
electrode
lOBC
R-53, Pt R-56, AU R-57, Pt R-59, Pt R-51, P t R-49, Pt R-44, Pt R-4, Au D-33, AU R-48, P t R-8, AU D-35, AU R-29, AU D-22, Pt R-39, Pt R-25, P t R-37, Pt R-10, AU D-42, AU R-30, Pt D-46, AU R-42, P t R-41, P t
0.0793 0.1398 0.1585 0.2252 0.3245 0.4050 0.6080 0.714 0.7874 0.9484 1.188 1.300 1.502 1.718 1.950 2.416 2.944 3.564 5.361 5.861 7.864 11.92 17.67
-65.0’
103c
A
1080 1065 1063 1045 1018 1019 972 963 958 930 904 901 865 849 837 806 790 753 685 668 637 566 526
...
...
... *..
...
750
1.000
485
...
...
1.526
692
1.563
...
...
1.981
669
408
...
...
2.049
2.974
618
3,099
377
...
...
...
...
5.896
512
6.044
318
...
...
...
12.15 18.02
1127 927 645
2.88 2.77 2.79
3.41 2.48 1.88
21.8 22.7 25.1
2.558 2.992 4.320
concentration points of the data given in Table I, are shown in Figure 3. While appearing to fit the Shedlovsky function, the latter analysis yields values for the limiting equivalent conductance of about 20 Kohlrausch units higher than those given in Table I1 a t the respective temperatures. As shown in Figure 3, the experimental data for solutions of sodium in ammonia reported in the study can be reproduced by a conductance function involving the ion-pair con~ e p t . ~ ~ , ~ ~ In the higher concentration range, the results of this work are in excellent agreement with those of Kraus,’ Figure 2, but not in the dilute region (
