I N F L U E N C E OF' THE SOLVENT I N ELECTROLYTIC CONDUCTION
BY HARRISON EASTMAN PATTEN
INTRODUCTION T h e theory of electrolytic dissociation of Arrhenius' has suggested and guided much of the research in chemistry since 1887 ; but it has also diverted attention from the study of the electrical conductivity of non-aqueous solutions and of concentrated aqueous solutions. On the basis of this theory Nernst and Thomson have attempted to show that the dielectric constant of the solvent determines its so-called electrolytic dissociating power. Bruhl attributes ((dissociatingpower ) ' to spare valences of the solvent. Dutoit and Aston claim that electrolytic conduction takes place in solutions whose solvents are associated. Beyond this no part in the process of electrolytic conduction has been assigned to the solvent by the supporters of the theory of Arrhenius and by writers of current text-books holding this hypothesis. Nevertheless a mass of experimental data exists in the literature indicating that chemical action between solvent and solute undoubtedly takes place in the production of solutions. I n 1893 D. Konowalow* published a research in which he maintained that chemical union of solvent and solute was the determining factor in electrolytic conduction. Since that time, Dutoit and Friderich,s Kahlenberg and Lincoln,4, Euler,s WaldenYfi ~~
* Zeit. phys. Chem. I, 481 (1887). Wied. Ann. 49, 733 (1893). Bull. SOC.Chim. (3) 19,321 (1898). Jour. Phys. Chem. 3, 12 (1899) ; also Trans. Wis. Acad. Sciences, Arts a. Letters, 12, 395; and Jour. Phys. Chem. 3, 457 (1899). 6 Zeit. phys. Chem. 28, 619 (1899). 6 Zeit. anorg. Chem. 2 5 , 213 1g00.
Solvent
ilz
Electrolytic Conduction
555
Werner,' Franklin and K r a q 2 Kahlenberg,3 Walden,4 and Smits,s working upon various solutions by the electrical conductivity method, and in some cases using cryoscopic and ebullioscopic methods as a check, have established facts which render the theory of electrolytic dissociation untenable. More recently I,. Kahlenberg6 has shown that instantaneous precipitation takes place as well in solutions which are non-conductors of electricity as in the best electrolytes, I n another paper Professor Kahlenbergs takes the position that mutual attraction between the solvent and solute is the essence of the so-called osmotic pressure and is the cause of the process of solution. T h e influence of the solvent on the change in concentration a t the electrodes in an electrolytic cell is clearly shown in the work of H. Schlundt.8 Moreover, A. Ponsotg has proved that van 't Hoff's relation PV = i R T is on a weak basis, theoretically as well as experimentally, and G. Q2uinckerohas called into question the efficiency of the semipermeable membranes used by Pfeffer" and others in deterinining the osmotic pressure of solutions. Nernst, Garrard and Opperinannxzfind that diffusion is a complex process in which the solvent takes a part; and J. W. Bruhl's has shown that the solvent influences the change of isomers in solution. Lastly Hans Wolf14 has studied the influence of several neutral solvents upon the conductivity of a few aqueous electrolytes, but without accomplishing any marked advance. Zeit. anorg. Chem. 15,I (1897). B~ill.SOC.Chim. (3) 19,334 (1898). Bull. Univ. Wis. No. 47 (Science Series), Vol. 2, No, j,297 (1901). *Bull. de ',I Acad. Imp. des Sciences d e St. Petersbourg ; V' series, Tome XV, No. I , (1901). j Zeit. phys. Chem. 39, 385 (1902). Jour. Phys. Chem. 6, I (1902). Ibid. Jour. Phys. Chem. March (1902). Coniptes rendus, 434 (1894-95). lo Wied. Ann. 3, 631 (1902). l1 Osmotische Untersuchungen, 8, (Leipzig, 1877). l 2 Gessell. Wiss. Giittingen, Nachr., Math-Phys. Klasse, I , 68 (1900). l 3 Zeit. phys. Chem. 30, I (~Sgg). l4 Zeit. Elektrochemie, 8, 117 (1902); Zeit. phys Chem. 4 0 , 2 , 222 (~goz),
556
Harrison Eastma n Pat ten
T h e main purpose of this investigation is to establish further facts concerning the influence of the solvent upon the process of conduction in electrolytes, which may be of use i n framing a more adequate theory of solution and of electrolytic conduction. Method and apparatus T h e problem was attacked experimentally in two ways : ( I ) I t was determined what relation exists between the amounts of various solvents required to alter the specific electrical conductivity of a given solution-liquid in liquid or solid in liquidto a stated degree. (2) I t was determined how the specific electrical conductivity of solutions of two homogeneous liquids, which are miscible in all proportions and are known to form compounds according to the law of definite proportions, varies with their percentage composition. T h e Kohlrausch method was used for measuring the electrical conductivity,’ an Arrhenius cell with a tightly fitting cover being employed. T h e temperature of the cell was kept at 2 5 O C to within one-tenth of a degree by an Ostwald thermostat. In preparing the solutions the cell was weighed, approximately to the fourth place, absolutely to the third place, a quantity of liquid sufficient to cover the electrodes was then introduced and the cell again weighed. After making the conductivity measurements, a small portion of the second liquid was run in from a burette and a third weighing of the cell made. T h e liquids were well mixed by moving the electrodes up and down, and after the temperature had been adjusted the conductivity was measured. By repeating this process the specific conductivity of solutions varying from rather dilute to exceedingly concentrated was determined. I n general, molecular conductivities were not determined : (I) because the concept of molecular conductivity is based on the assumption that the solute alone-or its parts, the “ions” of See Ostwald’s Hand u. Hilfsbuch f. physico-chemische Messungen.
557
Solvent in Electrolyiic Conduciion
Arrhenius -conduct the electric current through a solution ; and as stated i n the introduction, this assumption is not warranted by the experimental facts ; and (2) because in the one case calculated (Fig. IO, curve I, pyridine and acetic acid) the molecular conductivity does not approach a maximum at infinite dilution. Similar exceptions to the theory of electrolytic dissociation are abundant in the literature, so i t is deemed more profitable to work along the lines that will be presented.I Sources of error I n order to ascertain the extent to which the conductivity of an electrolyte in an Arrhenius cell is changed by varying the depth of the liquid above the upper electrode, the following measurements were made in both the cells employed in this investigation, using an aqueous %/sopotassium chloride solution, the conductivity of which was 0.00277 reciprocal ohms. Cell No.
0.2
0.9 4.5
60 60
60
I1
I
49.71 49.70 49.75
0.3 3.0 6.0
Cell No.
2
60
!
60
60
1
47.20 47-25 47.20
The specific gravities of the liquids worked with are all known, so any one wishing to do so can easily calculate the approximate molecular conductivity.
558
Harrison Eastman Pallen
T h e error due to variation in temperature is great. A 13.07 percent' solution of aniline in acetic acid showed a specific conductivity of 3.107 X 10-3 at 24.9', and of 3.121 X 10-3 at 25.1' - a change of 0 . 2 2 percent per 0.1 degree. A 23.5 percent solution of acetic acid in pyridine gave a specific conductivity of 7.250 X 10-5 at 25.1', and of 7.300 X 10-5at 25.2' --variation of 0.69 percent for 0.1 degree. T o control this error the thermostat was delicately adjusted ; the thermometer, which was near the cell, was closely watched, and check readings on the conductivity were made. T h e error due to bubbles of gas collecting on the electrodes was avoided by moving the electrodes about till the bubbles were dislodged. I n the solvents studied, bubbles are not so prone to form as in aqueous solutions. In the most viscous solutions there is undoubtedly a small error of this sort, since i t is well nigh impossible to get the last bubbles off the electrodes without allowing the solution to stand for a considerable time - too long, in fact, to be permissible. From the slight differences in the readings when the electrodes were still visibly coated with bubbles, when only a few bubbles remained, and finally when, after long standing, all the bubbles had cleared away, I consider the negIect of this error perfectly justified. T h e pyridine used probably contained a trace of picolene. With this exception the solvents employed were in a high state of purity. T h e solutions studied conld be left ten hours.over a waterbath in the cells used without a readable change in the conductivity, showing that the covers of the cells fitted securely. But the curves with a great number of points nevertheless show a slight influence of the presence of moisture, since it was necessary to continue the readings through several days in order to have the curve continuous. T h e necessary addition and abstraction of material to get the different percentages renders some exposure to the air unavoidable. I have assured myself In this research the percentage strength of solution always means the number of grams of solute contained in IOO grams of the solution.
Solvent in Electrolytic Conductioiz
559
that the error thus incurred was not sufficient, however, to necessitate greater refinement of method for the present purpose. In a few cases the conductivity did not a t once become constant on mixing the liquids because of a measurable speed of chemical reaction between the substances. This will be discussed further below. Consideration of the foregoing sources of error shows that i t is unnecessary to compute the specific conductivity to more than three digits. Accordingly the calculations were made with a slide-rule, using Ostwald's tables for Wheatstone's bridge.
Materials Two liters of Kahlbaum's C. P. acetic acid containing 99.41 percent acid (by a potassium hydrate test) was cooled till about one-half had crystallized. T h e liquid was poured off, the crystals melted and again frozen and the remaining liquid poured off. These last crystals were melted and dehydrated with a little phosphorus pentoxide. T h e acid was poured off from the phosphorus pentoxide and distilled. T h e fraction amounting to some 700 cc, which came over a t 116'-117' under a pressure of 738.7 mm, was used. Its melting-point was 16.59' ; its specific conductivity was less than 2 X IO-^ ; and its specific gravity was 1.0463a t 25' as compared with water a t the same temperature. T h e pyridine, quinoline, amylamine, isobutylamine, aniline, dimethylaniline, and xylidine used were dried in each case over fused potassium hydrate for a t least a week, and then allowed to.stand over freshly broken potassium hydrate. If the sharp corners of the caustic showed signs of rounding off, the drying of the liquid was continued until no such action was observed on freshly broken pieces of the caustic. T h e pyridine boiled between 114' and 117' at 738.5 min pressure ; the quinoline (syi thetic according to Skraup) a t 232' under 746 mm ; the amy, amine a t 96" under 738 m m ; the isobutylamine at 67' to 69' under 745 mm ; the aniline a t 117' under 88 mm ; the dimethyi aniline a t 123' under 87 mm ; and the xylidine a t 214' under 744.5 mm. T h e latter substance had a specific gravity of 0.9747 a t 25' as compared with water of the same temperature.
560
Harrison Eustm a vi Pa l'kn
T h e following preparations used were of the best C. P. varieties of standard manufacturers - Kahlbaum, Schuchardt, or Merck. A few of the compounds were prepared in this laboratory. All the substances were thoroughly dried with dehydrating agents, phosphorus pentoxide being used in case of benzene, toluene and xylene, and fused calcium chloride in all the other cases except the alcohols, which were dried with lime, distilled, and finally redistilled after adding some metallic sodium. T h e acetone was dried for months over anhydrous copper sulphate. T h e boiling-points of these liquids were as follows : benzene 79.4" to 79.5" at 746.9 mm ; toluene 108"to 110" a t 744.5 mm ; xylene 134.4" to 135.5" at 743 mm ; cymene 167" to 169" a t 740.2 mni ; amylene 40" at 724 mm ; methyl alcohol 65" to 66" a t 749 mm ; ethyl alcohol 77.6" to 77.8" at 738.2 mni ; acetone 55.5" to 56" at 738.2 mni ; methyl acetate 54" at 742.3 min ; methyl nitrate 64.4" at 730 m m ; isobutyl nitrate 120" to 124" a t 728 m m ; chloroform 60" a t 747.9 mni ; carbon tetrachloride 75.2" to 75.7" at 740.4 inm ; bromoform 144" to 146.2" a t 740.6 mm ; ethyl bromide 37.4" to 41 " at 732.4 mm ; propyl bromide 69" to 71" at 740.1 m m ; amyl bromide 118.5" to 119" a t 742 mm ; ethyletie bromide 128.5" to 128.7" at 741.2 mm ; butylene bromide 150" to 152" at 741.2 mm ; methyl iodide 41" to 43" at 742.7 mm ; ethyl iodide 71" to 71.1" at 747 m m ; arnyliodide 127" to 128" at 740.3 mm ; benzonitrile 187" to 188" a t 737.9nim ; nitrobenzene 205" to 205.5" at 743.8 nim ; benzaldehyde 173.4' to 173.7' at 729.9 mm ; amylsulphydrate 115" a t 742.8 rnm ; ethyl xanthogenate 194" to 196" at 742.8 mm. T h e naphthalene used was resublimed ; and the distilled water employed was carefully prepared i n this laboratory. Results
T h e first series presents the experimental results showing low the specific electrical conductivity of a solution consisting of 17 percent pyridine and 83 percent acetic acid is changed by diluting the solution with various solvents. A mixture of pyridine in acetic acid was chosen because such a solution was found
Solvent iiz Elect yo lytic Coizdtiction
561
to be miscible with a relatively large number of solvents. T h e solution was made to contain 17 percent pyridine and 83 percent acetic acid because this particular strength was found to have the highest specific conductivity of any iiiixture of these two liquids. A solution containing 19.303 g pyridine to 93.654 g acetic acid was made up, corresponding to 17.07 percent pyridine to 82.93 percent acetic acid. T h e specific conductivity of this solution was 8.70 x 10-3 a t 25". A second solution containing 18.412 g pyridine to 89.892 g acetic acid was made up, corresponding to 17 percent pyridine to 83 percent acetic acid. T h e specific conductivity of this solution a t 25" was 8.70 x 10-3. T h e bridge readingwasexactly i the same as for the first solution. f A third solution containing 17,425 g pyridine to 85.085 g acetic acid was prepared corresponding to 16.998 percei7t pyridine to 83.002 percent acetic acid. T h e pyridine used w2;s not the same sample as in the first two solutions ; neither wds the acetic acid. T h e pyridine distilled at 115' to 117' u n d r l 745.3 mni. T h e acetic acid came over at 117' under 739 mni. T h e specific conductivity of this solution was 8.66 X 10-3 2.5' C. T h e solution containing 17 percent pyridine to 83 percer t acetic acid was diluted with different amounts of various so)vents and the specific electrical conductivity of each resulti ' solution was measured a t 25". T h e results thus obtained E given in Table I. which follows. T h e headings of the table self-explanatory. Each of the pure solvents used had a spec conductivity of less than 2 X IO-^, except water (4 X I O - - ? , methyl alcohol (5.2 X 10-5); acetone (8.22 X IO-^); methyl nitrate (4.52 X IO^) ; benzonitrile (9.40 X xo4) ; and benzaldehyde (1.00 X IO-^). To facilitate a comparison of the numerous results given in Table I., these are presented in graphic form in Figs. I to 5 following the table.
I
Harrison Eastmaiz Patten TABLEI 1
Specific conductivity of solution containing I 7 percent pyridine 1 83 percent acetic acid on dilution with various solvents. (Percents indicate number of grams of dilutant in IOO grams of mlting solution. Specific conductivities are multiplied by IO^.) I.
Benzene
Sp. cond.
Percent
5.92 17.60 32.06 37.02 45.75
/
44.6 9.02
4. Cymene --
1'
0.0
!
16.0 29.0 37.6
I
''
I
'
1
a
0.0
0.42 0.84 1.25 1.74 2.15 4.07 5.99 IO.
0.0
18.0 30.5 48.6
1
i
1
870.0 410.0 93.5 29.2
Toluene
Percent
/I
5 . Naphthalene
(1
38.8 41 -4
Sp. cond.
866.0 685.0 495.0 297.0 52.3 28.4 18.4
6. Amylene 0.0 22.2
870.0 204.0
581.0
38.0
121.0
-
-
0.0
870.0
5.94 11.60
719.0
-
-
-
-
1
8. Methyl alcohol ~; 9. Ethyl alcohol
1150.0
--
870.0 !555.f3 338.0 136.0
22.12
35.5
1
-
il
5.79 11.51
1
1020.0
Acetone
0.00
I
1
1 1260.0 I/
3. Xylene Percent
~
870.0 0.0 898.0 7.2 923.0 [ 24.7 955.0 1 52.0 994.0 68.6
1 1,
Sp. cond.
866.0 3-42 769.0 13.06 1 479.0 27.80 140.0 39.42 26.7 46.75 6.55
-
1I
I
)I
0.00
12.2
7. Water
f
~
2.
I
-
