T H E CONDUCTIVITY OF PHOSGENE SOLUTIOTU’SOF ALUMINIUM CHLORIDE AT ZS’, 0’ AKD -45’ BY ALBERT F. 0. GERMANN
In a short paper entitled “Carbon Monoxide, a Product of Electrolysis”l, presented at the Stanford University meeting of the Pacific Division of the American Association for the Advancement of Science, the author announced that solutions of aluminium chloride in liquid phosgene conduct electricity, the solvent being decomposed in the process into carbon monoxide and chlorine. Because the production of carbon monoxide by electrolysis is unique, and because the solution of aluminium chloride in phosgene has unusual chemical properties, it became important to know something about the conductivity of this solution. Conductivity Cell. The cell used is shown diagrammatically in Figure I . The body of the cell consisted of two principal portions, A, 2 2 mm. in diameter, containing the electrodes E, and B, 30 mm. in diameter, containing the electrode supports, c, c. Surmounting B were the filling and emptying devices: F was a tube of 1 2 mm. bore through which weighed samples of aluminium chloride were introduced, and which was then sealed off; J was a flat joint by means of which the cell could be attached to the phosgene purification apparatus, and the cell evacuated, or filled with phosgene; S was a stopcock to which was adapted a brass stopcock clamp to prevent the stopper from blowing out at the highest temperature with the more dilute solutions2. D was a capillary tube extending to the bottom of the cell, sealed off at the upper end; by breaking the tip of this tube, the liquid contents of the cell could be syphoned off, and the walls washed free of solution-with the judicious use of freshly distilled phosgene. The electrodes were constructed of heavy platinum foil, 2 0 mm. x 28 mm., coated with platinum black. Electrical contact was made by means of a few drops of mercury placed in each supporting tube c, and copper wires extending down into the mercury. The procedure adopted was to begin with an amount of solution large enough to fill the cell approximately to the level at which the electrode supports were sealed in by introducing a weighed amount of aluminium chloride, sealing off the filling tube F, evacuating the cell, weighing, distilling in an excess of pure phosgene, distilling off the excess after effecting solution of the aluminium chloride, and weighing again. Conductivity measurements were then made at z s 0 , 0’ and -45’. The solution was then concentrated to approximately double the previous concentration by distilling off some of the solvent, weighing again, etc. Three or four measurements could thus be made with a single sample of aluminium chloride, after which the solution was discarded by discharging through the capillary tube D, as already indicated. Science, 60, 434 (1924). Germann and McIntyre: J. Phys. Chem. 29,
102
(1925).
CONDUCTIVITY O F PHOSGENE SOLDTIONS
1 I49
Conductivity Measurements.
The method used was the well known Kohlrausch method, using the alternating current produced by a simplified Vreeland Oscillator, except for a few of the last measurements, when a microphone hummer controlled by a tuning fork attachment was used, each capable of producing an alternating current with a frequency of 1000 cycles per second. For the highest resistances two special L. & S. boxes each containing five coils of one hundred thousand ohms each were used, and for the smaller resistances, a number of L. & N. two and four dial resistance boxes were connected in series. The bridge readings were made with the slide wire of an L. & N N o . 7651 Student's Pot,entiometer,mith the end coils in series. The cell constant and the conductivity of the pure solvent were determined on two different occasions, at the beginning of the measurements, and after half the measurements had been made by Mr. Russell Timpany on the first occasion, and by the author on the second, using fiftieth-normal solutions of thrice recrystallized potassium chloride made up with carefully prepared conductivity water. The value of the cell constant at zero was considerably lower than the value at 2 5 O , probably due to the construction of the cell. Following are the values obtained, and the value I3 assumed for the lowest temperature: at 2 5 O , C z o . 0 1 3 3 at o', C = 0.0130 at-45') C = 0 . 0 1 2 ; Purification of materials. The phosgene was purified by fractional distillation of the technical product supplied by the Chemical Warfare Service in the manner described by Germann and Gagosl and had a vapor tension at zero of 5 5 7 mm. The aluminium chloride used was distilled from a FIG.I mixture of the purest obtainable aluminium chloride and powdered aluminium, under a pressure in excess of two and one half atmospheres, in a specially constructed pyrex still. The product obtained in this way is snow white, and deliquesces in an atmosphere of phosgene vapor, while the unpurified product dissolves very slowly. Constant temperature baths. Conductivities were measured at three temperatures. A water-filled thermostat, maintaining a temperature of 2 so, constant to within O.OI', was used for the highest temperature. At zero, a large Dewar tube filled with washed cracked ice was used. For the lowest temperature, a Dewar tube filled with liquid ammonia exposed to the air, and
Y
d
i c
J. Phys. Chem., 28, 965 (1924).
1150
ALBERT F. 0.GERMANN
whose temperature was found to remain constant to within one or two degrees, was employed.
Results. Tables I, I1 and I11 contain the data obtained for the conductivity of the solutions at z5', oo and -45' respectively. Six samples of aluminium chloride were used for the measurements, numbered A, B, C, D, E, and F, weighing respectively 0.7865 gm., 3.0935 gm., 12.5100 gm., 0.0144 gm., 0.10j7 gm., and 1 . 1 9 2 2 gm. The subscripts attached to each sample number in the tables, as AI, A%,AB,etc., refer to the different diluticns obtained with the sample in question by adding or distilling off phosgene. The percentage of aluminium chloride in column 2 was calculated from these weights. Column 3 records the density of the solution, obtained for the solutions at z 5' ami at o'.'
FIG.z
from the density curves given by Germannl, and for the solution at -45' by simple extrapolation, assuming the coefficient of thermal expansion between 0' and 2 5' to hold. Column 4 gives the molecular concentration of Aluminium chloride, column j the molecular volume, or the dilution, column 6 the specific conductivity multiplied by IO^, and the last column the molecular conductiv3ty multiplied by 103. In the calculation of the molecular conductivities, following the method adopted by Franklin2 for the conductivity of sulfur dioxide solutions no correction was made for the conductivity of the solvent. Figure z contains the conductivity curves. From the smoothed curves, plotted on a large scale, the molecular conductivities have been read off at regular intervals, and set down in Table IV.
Discussion of Results. The form of conductivity curve obtained for solutions of aluminium chloride in phosgene is one that has been found in more or
2
J. Phys. Chem. 29, 138 (1925). J. Phys. Chem., 15, 675 (1911).
CONDUCTIVITY O F PHOSGENE SOLCTIONS
1151
less modified form in numerous other cases of solutions in weakly ionizing solvents, and a most exhaustive discussion of the phenomenon has been given by Franklin in his study of the cmductivity of sulfur dioxide solutions. According to this interpretation, we must consider the most concentrated solutions, which contain relatively little of the solvent,, to approach the condition of the fused salt; that is, whatever ionization there is, must be due only in very small part to the presence of the solvent, and almost entirely to the self-ionization of the solute. At the highest concentrations, however, the mobility of the ions will be considerably diminished because of the relatively high viscosity of the solution; as the solution is diluted, the viscosity will rapidly diminish, so that the ionic mobility, and therefore the molecular conductivity, will increase. This would account for the initial rise in conductivity.
TABLE I Conductivity at 2 5' Sample
70 AlCI,
Density
Molecular concentration AlCI,
Molecular volume
I ,3685
D1 DS
0.0218
12.30
9.IO
I .3692
0.245
I .3700
0.0251j
0.514
I
0.0333 0.0564 0.0706 0 . I047 0.276 0.604 0.416
0.0357 0.0763
I
E1 E2
0.123
E3
A3 F1 A1 A2
I.240
F2
Bi F3
0.007
565 0,00366 273.4 0.00783 127.7 0.01262 79.3
I . 3686
A4
~
Molecular Conduct,ance X 1c3
0.00177
0.01725
D3
Specific Conductance X I G
1.25
1,355 2.18 2.66 4.62 j.oo 6.19
,3687
I .3690
.3716
0.0528
39.75 18.9; 7.82
1,3759 I. 3760 I ,3766 I . 3813 I.3840
0.I279
0.129 0,1398 0.2258 0.276
4.43 3.624
1.80 I .81
1,3947
9.8 8.38
7.76 7.16
0.717
0.483
2.07
I ,3966
0.523
I
I .4027
0.651 I .066
1.535 0.938
1.893 1.953 3.37 5.35 6.
