Aug., 1939
CONDUCTANCE OF TETRABUTYLAMMONIUM PICRATE IN DIPHENYL ETHER [CONTRIBUTION FROM
THE
2047
RESEARCH LABORATORY OF THE GENERAL ELECTRIC COMPANY
Dependence of Conductance on Field Strength. I. Tetrabutylammonium Picrate in Diphenyl Ether at 50" BY DARWIN J. MEADWITH RAYMOND M. Fuoss
I. Introduction I n 1924, Wienl showed experimentally that the conductance of electrolytes under high field strengths was abnormal. He first ascribed the effect to a change of the temperature coefficient of conductance with field strength, but subsequent work in his Laboratory* showed that the abnormalities were due to an actual failure of Ohm's law for electrolytes.3 He showed that two effects were present, one quadratic in the field and one linear. The first Wien effect appears in strong electrolytes when the distances the ions move during their relaxation time become of the order of the diameter of the ionic atmosphere Then the retarding effects on mobility of the atmosphere are diminished, with consequent increase of conductance. In weak electrolytes, the conductance is further increased by increased dissociation in the field. The second Wien effect has been treated theoretically by O n ~ a g e rwho , ~ showed that the slopes of the conductance-field curves could be predicted from the dielectric constant and temperature of the solvent and the valence type of the electrolyte. Because the Wien effects are direct experimental evidence for the existence of the ionic atmosphere and for the pairwise association of ions under their mutual coulomb fields, and also on account of general interest in the behavior of dielectrics under high fields, it seemed worthwhile to study by bridge methods several carefully selected systems in some detail. Diphenyl ether at 50' was chosen as our solvent on account of its ease of purification, low solvent conductance and high solvent power for substituted ammonium salts, which are convenient 1-1 electrolytes for work in non-aqueous solvents. Its dielectric constant at 30' and 60 cycles is 3.53, which is low enough to give marked ion association. In it, the conductance of tetrabutylammonium picrate satisfies the simple binary association law from below millionth normal up to (1) Wien. A n n . Phys., 7 3 , 162 (1924). (2) Wien, ibid., 83, 327 (1927). (3) An excellent bibliography i s contained in a review b y Eckstrom and Schmelzer, C h e m Rev., 24, 367 (1939). (4) Onsager, J . Chern. Phys., 8, 599 (1934).
concentrations of the order of lo-* normal. Consequently the system permitted a test of the theory over a wide range of conductance and concentration. Above normal, triple ion formation6 becomes appreciable; in this range a dependence of the Wien coefficient on concentration was observed. From the initial part of the conductance-field curves, the activity coefficient of the solute can be determined.4 With our present methods, our precision in activity coefficient is not high, because i t is determined as a small difference between two large quantities, one of which depends on an extrapolation, but we can show that our experimental coefficients are in approximate agreement with the values calculated by the limiting law of Debye and Hiickel. The present results therefore demonstrate the existence of an ionic atmosphere in a solvent of quite low dielectric constant . From the linear part of the curves, we find a t low concentrations an increase in conductance of 1.6% per kilovolt, which is in good agreement with the theoretical value, 1.32%, calculated by Onsager's theory. Since this theory postulates pairwise association of ions, and explains the second Wien effect as a displacement of this equilibrium in the field, our results give experimental proof of the existence of ionic association. Further work on different types of electrolytes and on solvents of lower dielectric constant, where still higher field strengths caii be studied, is in progress. It will be interesting to see whether the shape (electrical and physical) and size of the ions influence the Wien coefficient and also to study the effect in the range of higher association than pairwise.
11. Materials, Apparatus and Method Diphenyl ether was purified by recrystallization, aftrr which it was dried first with calcium chloride, and finally with activated alumina. Our highest freezing point was 27.1'. The average solvent conductance was 0.3 >< 10-12;our best value was O.O(i X 10 - 1 2 . Tetrabutylammonium picrate was prepared as described by Fuoss and Kraus,Gm. p . 89.2-90.0" (corr.), ( 5 ) Fuoss and Kraus, THISJ O U R N A L , 56, 2387 (1933).
( 6 ) Fuoss arid Kraus, ibid.,55, 3614 (1933).
DARWIN J. MEADWITH RAYMOND M. Fuoss
2048
Conductance cells of several types were used. Some determinations were made in the cell described by Deitz and Fuoss,’ and others in the cell described by Elliott and Fuoss.s The former cell showed a pronounced Parker effect a t 1000 cycles and was only used for preliminary 60-cycle work. The second cell (cell no. 2) gave results independent of frequency (except for a normal polarization effect) and was used for about half of the work reported in this paper. For the final work, two platinum cells were used, which had two advantages over Elliott’s cell: they could be cleaned with hot nitric acid or chromesulfuric acid between runs, and they had no movable parts. Cell no. 3 is shown in Fig. 1. The high potential electrode (8.25 cm. long, 2.540 cm. inside diameter, 0.5 mm. wall thickness) was simultaneously the outer cell wall. It and the guard-test HP T combination (outside diameter 2.060 cm.) were sealed to a hollow ring of soft glass, to which were sealed filling tubes. The volume was 25 cc. A shielded lead ran to the test electrode; the shield was connected to the guard electrode, and the shieldFig. 1.-Guarded ing was unbroken from cell to bridge. conductance cell. At final balance, the guard and test electrodes are a t the same potential, so that except for an insignificant amount of fringing a t the glass joint between the guard and test electrodes, the flux is normal over the entire test electrode. The latter condition is essential, because it was necessary to calculate field strength from cell geometry and the voltage applied to the cell. Cell 4 was similar to cell 3, except that the electrode spacing was 0.491 cm., or about twice that of cell 3.
between the cells checks our calculation of field strength. Voltage Sources and Scales.-Most of our voltage work was done a t 60 cycles; examination with a cathode ray oscilloscope showed that the transformer output was free from harmonics. Some work was done a t 500-1500 cycles; here graphical integration of the oscilloscope curves showed the presence of not more than 5% harmonics. The question of harmonics is important, because the second Wien effect is linear in cell voltage and all our voltmeters read mean square voltage. The linear average V was computed from the r. m. s. average V, by the relations
whence
v
- r1)
=
V/(n-
rl)
v,
(4)
0 Cell 2
OCell 3
(1)
where V is the voltage applied to the high potential electrode. It will be seen that the agreement (7) Deitz and Fuoss, TRISJOURNAL, 60, 2394 (1938). (8) Elliott and Fuoss, ibid., 61, 294 (1939).
= (2
