RAMGOPAL AND OM NARAIN BHATNAGAR
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positive value occurs over a range in -log a,. I n this latter circumstance, the selectivity coefficient must decrease with increasing p . (b) For a given exchanger at constant composition, the selectivity coefficient will depend on the identity
of the common cation (or anion) in the external electrolyte mixture. For example, with Dowex-2X0.5 a t 2 ~ =~ 0.0, - the value of at a, fixed p will be slightly greater when Li+ is the common ion than when this is Cs+.
Studies on Solutions of High Dielectric Constant.
V1I.l
Cationic Transport
Numbers of Potassium Bromide in N-Methylacetamide at Different Temperatures
by Ram Gopal and Om Narain Bhatnagar2 Department of Chemistry, Lucknow Univwaity, Lucknow, U.P., India
(Received February 8, 1966)
The cationic transport numbers of potassium bromide in N-methylacetamide (NMA) have been determined a t different temperatures and concentrations, and the data have been used to calculate the ionic mobilities in this solvent from the available electrolytic conductance. The values of the ionic mobilities at 40°, thus obtained, compare well with those obtained by Dawson and co-workers from the usual indirect method. An estimation of the ionic solvation appears to indicate appreciable ion-solvent interaction in M I A .
Introduction
Experimental
There has been a good deal of research activity during the past few years in the field of liquids of high dielectric constant, other than water and liquid hydrogen cyanide, and appreciable experimental data, especially on t,he conductance of electrolytes in these solvents, have been reported. The question of dividing the electrolytic conductance a t infinite dilution, XO, into separate ionic mobilities in most of these solvents, remains unsolved because of the lack of experimental data on the transport number of ions. In order to make this useful data available to workers in this field, studies were undertaken in this laboratory, and the cationic transport numbers of potassium chloride in formamide at different temperatures have already been rep ~ r t e d . ~The cationic transport numbers of potassium bromide in N-methylacetamide at different temperatures are reported in the present communication.
Potassium bromide was chosen, in place of the usual potassium chloride, as the more appropriate electrolyte because of its higher solubility as compared to that of KC1 and hence the determination of the transport numbers at several significantly different concentrations is possible with KBr. A.R. grade potassium bromide, recrystallized from conductivity water and thoroughly dried, was used in making solutions. WMA of specific conductivity mho was dried over freshly ignited quicklime and twice distilled under reduced pressure. The distillate was fractionally crystallized and distilled again under reduced pressure. The process was re-
The Journal of Physical Chemistry
(1) Work supported by the Council of Scientific and Industrial Research, India. (2) Junior Research Fellow, Council of Scientific and Industrial Research. (3) R. Gopal and 0. N. Bhatnagar, J. Phye. Chem., 68, 3892 (1964).
CATIONIC TRANSPORT NUMBERS OF KBr
IN
N-METHYLACETAMIDE
peated until its conductivity fell to about 7-8 X lop7 mho. The sample melted at about 29". It was stored in dark-colored bottles in a d r y b ~ x . ~ The transport-number cell was similar to that used previously for solutioiis in f ~ r m a m i d e . ~The distance between the electrodes was now reduced to overcome the current-reducing effect of the highly viscous NMA without increasing, unduly, the voltage in the circuit. The cathode of the cell was a silver bromide electrode which was prepared as follows. h silver wire about 90 cm. long and 0.5 mm. in diameter was wound on a stout silver rod and the whole was made the anode, and a platinum wire was the cathode of an electrolytic cell containing a potassium bromide solution. h current of 10-15 ma. was passed for 4-5 hr. The anode was taken out, washed thoroughly with conductivity water, dried, and subsequently used in the transport-number cell as the cathode, the anode being a long silver wire of approximately the same dimensions as the cathode, wound in the form of a spiral of about 2.5 mm. in diameter and 2.5 cm. in length. Solutions of desired concentrations were prepared in freshly distilled arid purified samples of S M A , and all solvent transfers were done in the nitrogen drybox. The rest of the experimental procedure was the same as described p r e v i ~ u s l y ,including ~ the precautions for carrying out the experiments in a moisture-free atmosphere. The experimental results are summarized in Table I.
Table I : Transport Numbers of K in Solutions of KBr in NMA a t Different Temperatures and Concentrations +
Concn., M
350
40'
450
0.00 0.10 0.15 0.20 0.25 0.30
0.3839 0.3642 0.3606 0.3594 0.3572 0.3545
0.3900 0.3707 0.3692 0.3668 0.3638 0.3614
0.3950 0.3782 0.3767 0.3730 0.3694 0.3678
Transport no.. t+-
-
50'
0,3990 (from the graph) 0.3845 0.3797 0.3801 0.3784 0.3760
From the values of the transport numbers, t+, at different temperatures and concentrations, given in Table I, the values of the limiting transport numbers, t+O, of K + a t different temperatures were obtained by the usual graphical method using the well-known Longsworth procedure as described previously. The values of the limiting equivalent conductance, Xo,of KBr at different temperatures, needed to calculate the Longsworth function, tfo', were those given by French and Glover5 and by Dawson and co-workers.6 The re-
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quired values of the dielectric constant of XJIA have been taken as 169.7, 165.5, 137.5, and 151.8 a t 35, 40, 45, and 50", respectively.' The values of the limiting transport numbers, f + O , at different temperatures are also given in Table I.
Discussion From Table I it may be seen that the variation of the transport number, ti-, with temperature and concentration follows the same pattern of behavior as that in formamide solutio~is,~ i.e., it increases with increase in temperature and decreases with iricrease in concentration. As in formamide, the effect of temperature is consistent with the Kohlrausch's law that the faster the ion, the smaller the temperature coefficient of its mobility so that the slower moving ion I(+has a positive temperature coefficient and t+ increases almost linearly with temperature. The reverse will be the case with the faster bromide ion. The difference in the activation energies of transport of the two ions IVumber7 Julu i966
