P O T E N T I A L D I F F E R E N C E S WITH S A T U R A T E D SOLUTIONS
BY D. MCINTOSH
’
I n a paper published a number of years ago, LtitherIoffered a thermodynamic proof that changing the solvent at one electrode should have no effect on the electromotive force in a two solution cell if the solutions were already saturated with respect to the electrolyte. T h i s view was disputed by BuchereP and by Miller3 for the case of a hydrated salt and Luther4 yielded this point after a brief discussion.5 T h e objection urged was that addition of alcohol to a saturated solution of copper sulphate would cause dehydration of the salt and therefore must affect the electromotive force. Both Bucherer and Miller admit‘that there is a radical difference between cells with a hydrated salt as solid phase and cells with an anhydrous salt as solid phase. Luther’s proof, therefore, still stands with regard to the latter cells, and we are asked to consider it as proved that the electromotive force is zero in the combinatioq6 Hg
I Hg,Cl,,KCI,
water 1 alcohol, KC1, Hg,CI,
I
Hg.
If i t were not for the thermodynamical proof, no one would care whether this combination did or did not give a n electromotive force; but any general proof must be tested carefully to see whether some tacit assumption may not vitiate it. Luther’s proof is based on the assumption that the two solutions are nonmiscible. T h i s does not apply i n the case under consideration. T h e two solutions will diffuse until homogeneous and the question arises whether this diffusion has any effect on the electroZeit. phys. Chem. 19,529 (1896). Ibid. 20, 328 (1896); 22, 590 ( 1897). Jour. phys. Chem. I , 521 (1897). 4 Zeit. phys. Chem. 22, 85 (1897). Ibid. 26, 170 (1898). Zeit. Elektrochemie, 8, 403 (~goz). 6 Cf. Luther.
Potential Dayeevences with Saturated Solutions
349
motive force or not. There are several possibilities. If the cation is hydrated, a current flowing through the solution from the water to the alcohol would dilute the aqueous alcohol and would therefore tend to restoie equilibrium. Since alcohol would precipitate potassium chloride from the aqueous solution, and since more potassium chloride would dissolve in the alcoholic solution as the alcohol diffused out, it is not unreasonable to suppose that an electrolytic transfer of potassium chloride from the aqueous to the alcoholic solution would hasten diffusion. If water moves with the positive current, as a sort of electrical endosmose, this would dilute the alcoholic solution and bring about equilibrium. These are three hypotheses, any one of which calls for the existence of an electromotive force with the aqueous solution the anodic one. Other hypotheses could be formulated ; but it seemed better to determine the facts. A number of experiments have therefore been made and the apparatus used will now be described and the results given. Apparatus, etc. T h e cells were made up in tubes of various kinds, depending on the substance used. I n the case of those containing mercury, platinum wires were fused into the bottoms of the tubes, and connections between the cells made by means of nix-. row siphons or moistened yarn. T h e other cells were simple test-tubes, in which the electrodes of silver or lead were placed. T h e measurements were made by the well-known Poggendorff compensation method, using either a Rowland-D'Arsonval galvanometer or a Lippmann capillary electrometer as a zero instrument. An accumulator served as a working element, and was compared with a standard Clark cell during each series of measurements. With this apparatus results could be obtained to 0.5 millivolt, an accuracy quite sufficient for the purpose, since cells of the same composition measured a t different times often differed by several millivolts. Materials used Ethyl alcohol, dehydrated by copper sulphate and distilled, at 78" C under 75.8 cni pressure.
D.McIntosh
350
Methyl alcohol, dehydrated by baryta and distilled, at 65' C under 75.1 cm pressure. Potassium chloride, precipitated from a saturated solution of potassium chloride by hydrochloric acid, washed, and fused. Mercurous chloride, obtained from the conimercial calomel by sublimation. T h e laboratory silver nitrate, recrystallized lead chloride, and Merck's C. P. cadniiutn chloride were also used. In order to have some knowledge of the concentrations, the solubility of potassiuni chloride in the various solutions a t 25' C was determined. T h e results are given in Table I. and refer to solutions containing no mercurous chloride.
TABLE I. Solubility of KC1 in gram-mol. per liter. Ethyl
4.18 3.21 2.40 1.78 1.26
0.84 0.56
0.305
Water-alcohol solution
IPercent alcohol by weight 0 10
Methyl
4.18 3.27
20
2.46
30
1.81 1.28 0.83 0.53
40 50 60
0.125
70 80
0.042
90
0.01 I
IO0
0 -303
I34 0.087 0.052 0.
It has long been known that concentrated solutions of potassium chloride act on calomel with the formation of sruall amounts of mercuric chloride and mercury. Solutions freshly made up and ones saturated with mercurous chloride by long standing were examined and the results are given in Tables 11. and 111. In these and the following tables the current flows through the solutions in the direction of the arrow. In all cases, the aqueous solution is the anodic, and the alcoliolic solution the cathodic.
Poteiztial Dzfeveizces with Saturated Solufioizs
351
TABLE11. Mercury electrodes ; solutions saturated with mercurous aud potassium chlorides Ethyl alcohol
i
Electromotive force
-
Percent of alcohol
Percentof alcohol
I ~
F'resh solution
I IO0 IO0 IO0 IO0 IO0 IO0 IO0 IO0
0
IO 2 0
30 40
50 60
70 90
100'
i'
Saturated with Hg,CI,
I I
0.024 0.020
'I
0.018 0.016
i
0.012 0.009
0.016
1I
o.co8
0.010
0.004 0.003
0.007
1
0.029 0.023 0.OJg 0017 0.0 I 3
0.002
/'
\-
0
IO0
90
0.024 0.020
0.026
0 0
80
0.017
0.012
0 0
io
0.015 0.012
o 007
0.010
0.005
40
0.00j
0. cog
30
0.006
o 004
20 IO
0.COl
0.003 0 002
0 0 0 0 0
60 50
0.002
---\
0.017 0.006
1
Richards and Archibald have shown1 that solutions of cadiniutn chloride have no action on mercurous chloride. A few experiments were therefore made with cadmium chloride instead of potassium chloride and the results are given in Table IV. Cells i n which the mater and alcohol were saturated with lead chloride were also tried, using, of course, lead electrodes. T h e measurements gave 0 . 0 0 2 volt as the result i n the case of the water, against both ethyl and methyl alcohol.
' Zeit. phys. Cliem. 40, 385
(1902).
352
TABLE111. Mercury electrodes ; solutions saturated with mercurous and potassium chlorides Methyl alcohol
1I Percent of alcohol
Percent of alcohol IO0 IO0 IO0 IO0 IO0 IO0
IO0
Electromotive force Fresh solution
I
0
0.03 1
IO
0.029
20
0.026
30 40
0.017
SO 60
0.010
0.014 0.006
0.004 --
90
~
I
I
0.032
0.027
0.021 I
1’
0.018
0.017 0.014
0.007 0.003
0.031
0.032
E
0.017 0.014
0.021
70 50
0.007
IO0 ~
Saturated with Hg,CI,
0.022
80
IO0 IO0
,
~
40 20
30 IO
I
I
0.006
0.005
o 004 0.003 0.002
--\
1 1
0 OIj 0.014 0 012
0.009 0 006
0.004 0.003
/
I n Table V. are given some results obtained by using saturated solutions of silver nitrate and silver electrodes. While i t is clear that Luther’s conclusions in regard to these cells are unsound, it is important to see exactly where the flaw in his reasoning came in. It is to be found in one of the socalled fundamental laws of energetics : I “If two systems are in equilibrium with a third, they are in equilibrium with each other.” T h i s statement is not universally true, and’Luther has Zeit. phys. Chem. 19, 533 (1896).
,
Potential Dzferences with Satuvated Solutions
353
TABLEIV. -
Mercury electrodes ; solutions saturated with mercurous and cadmium chlorides
I Percent of alcohol IO0 IO0 IO0 IO0
Percent of alcohol 0 20
40
Electromotive force
, 1 I
0.025
0.023 0.017 0.012
~
~
80
0
IO0
0 0
80 60
0
20
Ethyl alcohol
i
1
Methyl alcohol
1
0.017'
i
0.014 0.006 0.002
0.017 0.012 0.010
0.023 0.017 0.013
TABLE V. Silver electrodes ; solutions saturated with silver nitrate Electromotive force I
Percent of alcohol IO0 IO0 IO0 IO0 IO0
Percent of alcohol
I
j
alcohol
40 60 80
0.OOj 0.003 0.002 0.002 0.or?I
IO0
0,005
0 20
80
60
40 20
0.003 0.002 0.001
1
Methyl alcohol 0.005
0.004 0.002
o.o,or 0.001
'
0.005
0.003
0.003 0.001
-
D.McI7t?tosL?
354
been so unfortunate as to make use of it in a case where it does not hold. If we take a saturated alcoholic solution of salt and a saturated aqueous solution of salt, the two solutions are each in equilibrium with solid salt but they are not in. equilibrium with each other. This law of Ostwald's is true only for nonmiscible phases. Luther makes use of it two pages later in a way which is wrong. H e inverts the statement of Konowalow and of Nernst, saying: Two solutions of the same substance are in distribution equilibrium when the partial pressure of the solute in the vapor phase is the same", or L ' ~ a t ~ ~ r asolutions ted Neither of the same substance are in distribution equilibrium of these statements is true even for partially miscible liquids. A saturated solution of succinic acid in ether is not i n equilibrium with a saturated. solution of succinic axid in water. When brought together there will be a passage of ether to the water solution, of water to the ether solution, and a change both in the relative and absolute concentrations of succinic acid i n the two phases. I n the present state of our knowledge, it does not seem possible to offer a satisfactory explanation for the direction of the current. TVhile the possibilit) of hydrated ions cannot be denied, i t seems improbable that this should be an important factor i n the case of potassilini chloride. Nothing is known as to solubility and dissociation of mercurous chloride in the mixed solvents. While the general direction of the current is what might be expected if the relative solubilities of potassium chloride were the decisive factor, the behavior of the cells when methyl alcohol is substituted for ethyl alcohol does not confirm this because then the potential difference of water against inethyl alcohol should always be less than those against ethyl alcohol. Another argument against this view is that the current flows in the same direction with mercury, silver, and lead electrodes. T h e eflect of the current is to dilute the anodic solution of potassium chloride and the cathodic solutions of silver nitrate and of lead chloride. There remains the question of electrical endosmose. It is '(
)',
Poteiztial Dzfeveizces with Satuvated Solutioizs
355
known that water does move with the positive current through porous diaphragms, but it is rather a long step from this to the cdse of mater diffusing into an alcoholic solution. While there may be a n analogy, there are no experiments which would justify such a conclusion. T h e conclusions to be drawn from this work are :( I ) I n cells of this type, the electromotive force is not zero. (2) T h e electromotive force is due to the saturated solutions not being in equilibrium. (3) I t is not possible to predict the sign or the value of the electromotive force from any property of the components. I t is 111y pleasant duty to express m y indebtedness to Professor Bancroft - at whose suggestion these experiments were made -for his kindness during their progress. McGzll University, MondP-eal, February, 1903
