STUDIES OF THE ELECTRICAL CONDUCTANCE O F NON-AQUEOUS SOLUTIONS BY LEON IRWI?; SHAW
I n the study of aqueous solutions, their conductance has received considerable attention, b u t similar work on the conductance of non-aqueous solutions is entirely disproportionate t o their importance. A further study of the latter will aid materially in forming an explanation of the results obtained in the study of both types of solutions. As stated by Kahlenberg and Lincoln' conductance was first studied in aqueous solutions, then in solutions in which the solvent was a mixture of water and some other solvent, and lastly in solutions t h a t contain no water. They give a bibliography of the work done up to that time. The earlier theories have been taken up by Kahlenberg and Lincoln and by Lincoln2 in their articles. Some of these will also be considered in this paper after the experimental data have been presented. Lincoln shows t h a t several of the theories presented to explain conductance are untenable. In the present investigation other examples have been found where these theories n-ill not explain the observed facts. These d l be considered in detail later. Kahlenberg and Lincoln3 made qualitative measurements of the conductivities of four salts (ferric chloride, antimony trichloride, bismuth trichloride, and mercuric chloride) in seventy-two organic solL-ents of very varying types. The quantitatix-e work recorded in the first article was done in solutions of ferric chloride, antimony trichloride, arsenic trichloride, phosphorus trichloride, cuprous chloride, stannous chloride, and bismuth trichloride The solvents employed in preparing the solutions n ere methyl alcohol, ethyl alcohol, acetone, ethyl acetate, ethyl acetoacetate, benzaldehyde, and Jour Phys Chcm
' I b l d , 3, -+ji (1899) Loc cit
j, I Z
(1899)
Studies o j LITon-AqueousSolutions
I 63
nitrobenzene. I n the second article qualitative experiments on the conductance of eighteen salts were made in twenty-nine solvents, and then quantitative measurements were made using six different salts and eighteen different solvents. However, measurements were not made for each of six different s d t s in all of the eighteen different solvents, but only for certain salts in certain solvents. As a result of this work they showed among other things tlhat the electrolytic dissociation theory as promulgated t o explain the electrical conductance in aqueous solutions apparently can not be applied in its present form to explain tlhe conductance of non-aqueous solutions. Paul Walden has carried on the most extensive research work on conductance in non-aqueous solutions since that of Kahlenberg and Lincoln. The results of his investigations appeared in ten articles from 1903 to 1908.' In the first two of these articles he considers the relation between the dielectric constant of the solvent and the dissociiating power of the latter These first two articles will be the only ones considered, because the others deal n-ith the relation of other properties to the dissociative power. Walden's ork consisted of the study of the dissociative power of fortynine solvents with one solute, tetraethyl ammonium iodide (in a feiv exceptional cases other solutes were used), and on these observations he generalized, and concluded that the dissociative pon-er of :t solvent is directly parallel to the dielectric constant, in harmony with the Sernst-'I'homsen rule To get the degree of dissociation, it !vas necesary for him, in most cases, to calculate the value of the condiictance ai infinite dillition, as no limiting 1-alue could he obtained evperimentally. He disagrees with Dutoit and -lstonL that only those solvents, that are associated, yield conducting solutions. By making his conductivity measurements at different temperatures, he also shows that the product of the __
-
~
-~
__
Z t i t phys C h e m , 46, 103 ( I C I O ~ ) ,54, 131, 55, 207, 5 8 , 479 59, 192, 385, 60, 87 ( I O O i ) , 61, 633 (1908) - Comptes r m d u s , 125, 240 (189;)
2 h 1 , 1183
(190i>)
Leon Irwin Shaw
164
conductivity a t infinite dilution (calculated) multiplied by the temperature coefficient is nearly constant Many other points are also taken up -4 bibliography of the work on non-aqueous and mixed solvents is given by Walden in his first article, “Organic Solvents and Ionizing hIedia.”’ In the next ten articles he has kept the bibliography up-to-date. The solvents used by JValden represent thirteen different types of compounds, namely, alcohols, aldehydes, ketones, acids, acid anhydrides, chlorides, bromides, amides, esters, nitriles. thiocyanides, thiocarbamides. and nitro compounds He purified the solvents by distillation and other means, until a product of minimum conductance \\-as obtained He then concluded t h a t the solvent u a s pure. This point will he taken up after the presentation of the experimental data As before stated, a‘alden, in most of his researches, used only one solute, namely, tetraethyl ammonium iodide, in a number of solvents. He arranged these solvents in a list according to their decreasing dielectric constants, to see if there was any regular agreement betn-een the ditlectric constant and the dissociative power, and found i o ) th:s o ~ i c sclutc., that i t was always the case, that the greater the dielectric constant the greater the dissociative pon-er The purpose of the present investigation was t o find out if other solutes would act in an analogous manner to tetraethyl ammonium iodide; t h a t is to say, if we took another salt, and dissolved i t in the same solvents, if the molecular conduction of this given salt, in the different solvents, a t a given dilution, would be highest in the solvent of the highest, and lowest in the solvent of the lowest dielectric constant Three salts were therefore taken, and the conductance of their solutions in nine solvents was measured. The solvents used in the present investigation are as follows : ( I ) methyl alcohol; ( 2 ) salicylic aldehyde and LOC.cit
Studies of LVo~iA queous Solutio 11s
16j
benzaldehyde; ( 3 ) acetic acid anhydride; 13) acetyl chloride ; (;j 1 ethyl acetate and dimethyl sulphate; (6) benzo-nitrile; ( 7 ) acetone; 18) aniline; ((9) epichlorhydrin. I t will be noted t h a t they represent nine different types of organic compounds. The solutes used in the quantitative work were lithium chloride, mercuric chloride, and iodine. In the qualitative tests, cadmium iodide, cobalt carbonate, cuproiis iodide, ferric chloride, mercuric iodide, mercurous chloride, nickel carbonate, antimony trichloride, cuprous chloride, zinc acetate and lead sulphate mere also tried. The solvents used were all of c. P. grade. prepared by Jlerck, Kahlbaum, or Schuchardt, except the ethyl acetate, which was a pure sample that had been prepared by Professor Kahlenberg. They were each first dried by an appropriate drying agent, and then distilled. Thus a fraction of very constant boiling point was obtained. This distillate was thien further treated and distilled from a flask, to lvhich a hard glass condenser w a s connected by means of a ground glass joint. In exceptional cases a block tin condenser or a platinum still and condenser was used. Thus a liquid of less conductance could sometimes be obtained. The distillate was always protected from the water and carbon dioxide of the air by means of soda-lime and calcium chloride. The distillation was always I-epeated in a given apparatus, until a minimum conductance vias obtained for the substance. In some cases this was less than that found by other recent investigators. If the first distillate was condensed in glass, a tin or platinum condenser was then substituted, and thus in some cases a distillate of constant and lower conductance wits obtained. All condensers, flasks, etc., used in this work were cleaned thoroughly with the solvents best adapted to remove the substance last used therein, then they were rinsed with distilled water, and dried in a current of pure air. The method of purification of each of the solvents will now be taken up. The numbers given as the conductance of the purified solvents indicate the number of divisions of the defection of the galvanometer needle. For guidance as to
Leon Irwin Shnx!
I 66
what these scale divisions represent approximately in reciprocal ohms the following table is appended :
TABLEI ____.
~
Spaces
Reciprocal olims
0 j0
0 .j
0 75 I .OO
0.5 0 5
s
00
X IO-^
x x
0 j X
1o-O 10-5
IO-^
The cell used was one that will be referred to as the qualitative cell. I t \vas a platinum crucible with a tightly fitting cover and a polished platinum electrode inside ;1 direct current was used and the conductance was measured by noting the initial deflection of the galvanometer needle. Xethyl alcohol hlerck's C. P.product was used. Xfter standing three weeks over anhydrous copper sulphate it was distilled off, and condensed in glass. The portion boiling a t 63.5' and 714 mm was saved. This n-as treated with sodium and distilled ; then n-ith anhydrous copper sulphate; then again with sodium and distilled, each time rejecting the first and last eighth. This then exhibited a constant conductance represented by j .o divisions, in making the qualitative tests. Salicylic aldehyde (Kahlbaum's C. P.) was dried by means of anhydrous copper sulphate, and distilled from it three times condensing in glass. The observed conductance was 0 .j division. ,Acetic acid anhydride n-as allowed to stand over fused sodium acetate for three weeks, and then distilled from this four times, condensing in glass. The conductance was 1.0 divisions. Acetyl chloride. Merck's C. P. product was distilled twice and condensed in glass. Its conductance was 1 . 5 divisions. Ethyl acetate from Professor Kahlenberg was distilled three times. It had a very constant boiling point, 75' a t 744 mm, and showed a conductance 0.5 division.
Studies o j A170n-dqueousSolutioiis
16j
Dimethyl sulphate. Schuchardt's C. P. \vas distilled being condensed in a block tin condenser. The boiling point \vas constant a t 188" and T j I mm. The distillation was repeated seven times. The substance would blacken each time on heating. constant conductance of 2 j . 0 divisions rras finally secured. On analysis for the sulphate radical, it was found t o contain a large percentage of sulphuric acid, and therefore i t n-as a constant boiling mixture rather than a definite compound. Benzonitril. Schuchardt's C. P. article \vas dried for three weeks over anhydrous copper sulphate, and distilled, using a glass condenser. I t had a very constant boiling point. After four distillations the conductance was constant, the galvanometer deflection being 3 .o divisions. ,ketone. Merck's highest purity was kept over potassium hydroxide for 2 days. It turned bron-n. It \vas distilled oft', and the portion boiling between 51" and 56' was treated Jyith fused calcium chloride and heated on the steam bath for S weeks! a reflux condenser being attached. It was then distilled, using a tin condenser. The conductance w a s constant I .j divisions after three distillations. The liquid was then treated with anhydrous copper sulphate, and condensed in glass. when, afi:er three distillations, the conductance w.s constant a t 0 . ; j divisions. ,hiline. hlerck's highest purity product ivas treated with potassium hydroxide for 2 weeks and distilled, when a strawcolored distillate v-as obtained. -1fter five distillations from anhydrous copper sulphate the distillate vas colorless, rmd had a conductance so low t h a t no mo\-ement of the plI.i-anometer needle could be detected. Epichlorhydrin. Krthlbaum's C. P. product !vas treated il.itli anhydrous copper sulphate for 2 lreeks, and tiistilled, being condensed in block tin. It had a constant boiling point of 1 1 2 ' a t 744 mm. -1fter distilling three times, the distillate IWS colorless, and had a constant conductance, the gals-aiioint w r indicating 0 . 5 division. This could not be changed 11y
I 68
Leon Irwin Shaw
repeated treatments and distillations, using either a glass or a platinum still Benzaldehyde Kahlbaum's C. P. product was distilled three times in a current of carbon dioxide. I t was immediately stoppered with the flask full of gas On standing a few days autooxidation would decompose the product, giving crystals of benzoic acid, for which reason it was not used in this investigation. The solutes used were all of the anhydrous C. P. variety of standard makes For the qualitative tests they were dried to constant weight, in an air bath, a t as high a temperature as they would stand without decomposition. For the quantitative work the solutes \\-ere purified as follows. Xercuric chloride, Schuchardt's C. P. product, was sublimed tn-ice, and the fine crystalline sublimate n as used Lithium chloride, llerck's recrystallized C. P. preparation. m-as heated in a mortar, in an air bath, for 3 hours, to 2 1 0 ' ~ when i t was removed and pulverized while hot. This was repeated four times During the last two treatments there n a s no caking of the salt. On dissolving in water, the salt showed neither alkaline nor acid properties ton ard phenolphthalein Iodine. -4solution of Kahlbaum's C 1'. potassium iodide was decomposed by chlorine. The iodine thus obtained was washed and dried, after mhich it x a s sublimed three times with an admixture of potassium iodide. It was then allowed to stand in ;1 desiccator over sulphuric acid for 2 days during n-hich time it lost no weight. Qualitative tests were made on the saturated solutions to see if the various solutes dissolved in the different solvents, and yielded conducting solutions I n these tests the qualitative cell was used As a result of these tests, the three substances before mentioned were selected as the most suitable for the work in this investigation Lithium chloride and mercuric chloride represent true salts, while the iodine is an element. The results of these experiments are given below together with other data. -111 three of the solutes did not dissolve in each of the nine
Studies o? S o n - i l queous SolutiotiJ
169
:;ol\-ents, but in every case where solution resulted, the quam -titative measurements of the conductance of the solutions were made. The measurements were all made a t 2 j O C, in a cell of the Xrrhenius type, by the Kohlrausch method. :I resistance box of a msximum of 33,330 ohms was available, and thus small conductances could be measured with accuracy. To insure constant temperature, the cell was immersed in a thermostat bath regulated to within o.oojo. The soluitions were made by n-ei;ghinginto a weighing bottle an amount of solute a little less than that xvhich Tvould dissolve in I O cc of the solvent. This amount had to be ascertained by an independent pre1imina::y experiment. The solvent iat 2 , j O 1 ',vas then put into the n-eighing bottle, and the latter vas immersed to the neck in the thermostat, To insure accuracy. the pipettes Ivere kept in a glass cylinder immersed in the bath, atid stoppered to keep out moisture. Fil-e cubic centimeters of the solution vi-ere then transferred to the cell and the c o ~ i cltictance measured. A n equal \-olurne of pure sol\-ent \vas then introduced, and the conductance measured again. Five cubic centimeters of the solution were then R-ithdran-n. an equal volume of pure solvent n-asadded, and the conductance measured again. This process was repeated till accurate measiirernents could no longer be made. The plates of the cell \\-ere coated \\-ith platinum black. They were 15 mm in diameter and 2 mm apart. The cell, betn-een each different solution, was lvashed irith the proper solvent, then n-ith carbon dioxide free water, after Ivhich it was dried in pure air. The air was purified . b y drawing it through the following train : -Acidified permanganate, sulphuric acid, soda-lime, calcium chloride, a heated tube, sodalime and calcium chloride. The constant of the cell was redetermined with S I O O KC1 after each solvent had been used, and was not found to change appreciably. The resistance of each solution was measured a t least three times, with different resistances in the rheostat. I n only a few cases was there any disagreement, and then a large number of measurements were made and the average taken.
Leon Irwin Shaw
170
The results of the qualitative measurements will now be given. I n the following Tables, I1 and 111, giving the qualitative results, s stands for soluble, ss for slightly soluble, d for decomposed by solvent, os for conductivity too large t o measure, h for conductance of the solution is no greater than t h a t of the pure solvent, p for conductance of pure solvent, being the scale divisions of the galvanometer, DC for dielectric constant of pure solvent. The values of the scale divisions of the galvanometer in reciprocal ohms are givcn in Table I. TABLEI1 Solvent
Epichlor hy drin 0.5 9 h Dimethyl sulphate 2 5 . 0 Acetone 0 . 7 5 os j.0 0s illethyl alcohol I1 Benzonitril 3.0 2 1 . j Acetyl chloride Iz I .0 Acetic anhydride I 0.0 Aniline 20 Ethyl acetate 0 5 Salicylicaldeh y de 0.5 3
Epichlorhydrin Dimethyl sulphate Acetone 3Iethl;l alcohol Renzonitril hcetyl chloride -icetic anhydride *hiline Ethyl acetate Salicylicaldehyde
1j.0
I? I8
4
h 8
35 h j 20
h h 1i
h h
9
1 h
6 15 h 13 h 0
8
h
s
os
os 3.0
or
21 .O
6 9 5 4 h
32.5 26.0
6 .j 13.9
15
5
I0
j
os os
12
4
8
IO
3
ox
6
4
5
d
d
IO
11
os os
I1
h 9
lz 3
12
Ij
I?
6 os 30
it
IO
11
S
I1 IO
I2
oc
3
11
!l
Ij
0
5
0.5
OC
4 6
IS
os
2
Ij
IO
6 17 3
h
20
26.0
12
s
OS 0s I, i
OS
I j .j
5.5
3 h
25 it
IO 20
OS 20
os
IZ
Ij
18
2 j
I2
0s
2
30 18 h h
15
I
1-i
6
h
20
11 I
20 IO
Studies oj S o ~ - ; queous l Solutioxs
171
The values of the dielectric constant here given are taken from the compilation in IT-alden's second article' except those for aniline and ethyl acetate which are Tereschin's figures. Looking over the results in Tables I1 and 111, it appears that methyl alcohol and acetone yield the greatest number of well conducting solutions Acetyl chloride and acetic anhydride are the next best for this purpose lvith the salts investigated. This might be expected, for methyl alcohol has a high dielectric constant. Holvever, both epichlorhydrin and benzonitril have a higher dielectric constant than acetone, and still do not show as qreat "dissociating porter'' as exemplified in forming conducting solutions. -hiline n-hich has a coefficient of association of I oj according to Rarnsay and Shields' gives solutions of higher resistance than ethyl acetate or acetic anhydride in case of all of the fourteen salts investigated. Ethyl acetate and acetic anhydride each have coefficients of association of 0.99 according to Ramsay and Shields.' Therefore it can not be held with Dutoit and &ton3 t h a t only those solvents t h a t are polymerized yield conducting solutions. This has been shown before by Kahlenberg and Lincoln' and by Iialden.l Xlany of the solvents t h a t are saturated compounds, yield better conducting solutions than aniline which is surely unsaturated Therefore the theory of Bruh14 that only those solvents t h a t are unsaturated yield conducting solutions, is untenable This was also shown by Kahlenberg and Lincoln.' The results of the quantitative measurements will now be given I n the tables x represents the specific conductance In reciprocal ohms, z' the volume in liters in which I gram Loc. cit. Jour. Chem. Soc., 6 3 , 1089 ((1893). Comptes rendus, 125, 240 11897). 'Zeit. phys. Chem., 18, 514 ( 1 8 9 j ) ; 27, 317 (1898), Ber. chem. Ges. 13erlin, 30, 163 (1897).
Leon Irwin Shaw
172
molecule is contained, and u the molecular conductance.
Solutions in Epichlorhydrin The solvent was purified as before stated. It had a specific conductance of 0.61 x IO-’ reciprocal ohms.
TABLEIT llercuric chloride -
X
‘I
-
4s x 97 x 39 x o 95 x o 69 X 2 I I
.o
Io-4
2
Io-4 IO-^ IO-^ IO-^
4.0 6.0 12.0
24.0
TABLEY Iodine X
_U
d
~
~
x x 3.01 x 2.14 x I .39 x 1.15 x 0.93 x 0.80 x 5.35
Io-6
2 7
0
j .21
10-6
5 4
0 0255
IO-^ IO-^ 10-6
IO
8
6 43 3 21
10-0
86 j
IO-fi
I73 4 346 8 693 7
Io-6
0 . 7 6 X IO-^
1045
o 0326 0 0464 o 0603 0 0997 0.1613 0 2775 o 2813
Lithium chloride was insoluble in epichlorhydrin and so could not be tested.
Solutions in Acetone The solvent was purified as before stated. It had a specific conductance of 0.37 x IO-^ reciprocal ohms.
Studies o] -Yon-.-Iqueous SoLutioizs
173
TABLE171 Uercuric chloride V
X
1L
0
0
47 95
I 90 3 SI
7 63
15 26 30 5 hl
0
I22 0
I
244
0
4SS
0
977 0 ,950 0
T.IIN,E1-11 Iodine U
I
s2
3 64 7 2s
I t is to be noted that in 110 case does the molectilar con ductance shon- a limiting 1-alue 011 dilution, biit increases steadily el-en a t very high dilutions Therefore it is impossible to-determine the cond lctivity at infinite dilution Consequently in these cases the degree of dissociation," a t any given dilution, can not be found by di\-iding the conductance a1 any given dilution, by the conductance a t infinite dilution *Is another examFle which shows that Ostn-ald's or Rudolphi's dilution lan- 'does not always hold, is cited the case of lithium chloride in acetone. Here the molecular conductance first increases, than decreases, and then increases again, as the solution is diluted farther. "
Leon Irwin Shaw
I74
6 . 6 x IO-^ 5 . 1 X IO-^ 3 . 9 x IO-^ 2 . 9 x IO-^ 2.0 x IO-^ I . 4 x IO-^ 0 . 9 4 x IO-^ 0 . 6 5 x IO-^ 0 . 4 1 X IO-^ 0 . 2 6 X IO-^ 0 . 1 5 x IO-^ 0 . 7 1 x IO-5 0 . 3 0 x IO-' 0 . 2 6 X IO-' 0 . 1 8 X IO-' 0 . 1 5 x IO-5
3.5 7.1 14.2 28.5
57.1 114.0 228.0 456.0 913.0 1,830.0 31650.0 7,310.0 14,600.o 29,200. o 58,400. o I 17,000. o
2.36 3.66 5.63 8.29 11.59 15.76 21.47 29.69 37.46 47.51 54.83 51.89 43.96 76.02 105.25 175.00
Walden in his work' subjected a solvent to one and the same definite treatment repeatedly, until a definite minimum conductance was obtained, and then concluded t h a t the substance was pure. He said that the conductance was due to ionization of the solvent. I have gbtained both acetone and epichlorhydrin of less conductance than that obtained by Walden. In the case of the acetone a constant minimum conductance was obtained by one given treatment, but when another treatment was employed, a still lower conductance was obtained. These and other cases point to the probability not of '' self dissociation" but of constant impurities. ,According to JTalden, the solvent power for tetraethyl ammonium iodide diminishes as the dielectric constant increases. That this is not the case for all solutes is shown by the fact t h a t lithium chloride dissolves readily in acetone (dielectric constant Z I . ~ ) , but not in acetyl chloride idielectric constant 15.5). Looking over the results in the tables of the quantitative LOC.cit.
Studies o j A-on-Aqueous Solutz'om
I75
measurements, it will be noted t h a t a t a given dilution, say I gram molecule in two liters, iodine shows a higher conductance in acetone than in epichlorhydrin. The dielectric constant of the first, hon.ever, is lower than that of the second. On the other hand, for mercuric chloride, in the same solvents, a t the same dilutions, the reverse is true. I n order to compare the results more easily 1-he following tables are given. (See Table IX.)
TABLE1 1 Xercuric chloride -
ZL
in acetone
II
in epichlorhydrin ~
435 o jIS o 607 o 689 0
4 S
16
-~~
0,496 0.788 0,900 I .200
Iodine 2'
u i n acetone
2
24 42
4 S 16
6;
-I 3-
ZL
i n epichlorhyclrin 0 013 0 023 0 02s 0
('39
The dielectric constmt of acetone equals 2 1 . 0 , that of epichlorhydrin 26.0 111 Table IX only approximate values are given for u a t the stated dilutions. As before mentioned, mercuric chloride shon-s a higher conductance a t a given dilution in epichlorhydrir than in acetone, n-hile of iodine the (See Table 1 1 . ) Consequently, if in this case re\-erse is true n e arrange our solvents in the order of their decreasing di electric constants as ITalden did, n-e find t h a t in the case of mercuric chloride, the greater the dielectric constant of the solvent. the greater the conductance. This is parallel with ITalden's results with tetraethyl ammonium iodide However, in the case of iodine, in these same tn-o solvents, the re\-erse is true. That is, iodine shon-s a much higher coil-
I 76
Leon Ivwin Shaw
ductance a t the same dilution in acetone, than in epichlorhydrin. Therefore, while in the case of one of these substances it can be said with Walden, that the greater the dielectric constant of the solvent the greater the dissociative power,” in the case of the other it cannot. It cannot be held, therefore, as an infallible rule that the ‘‘ dissociative pon-er” of a solvent stands in direct parallelism to the dielectric constant. This investigation was carried out under the supervision of Professor Louis Kahlenberg, to whom I am under many obligations for his valuable suggestions, and I take this means of acknowledging the same. ‘I
Cheiiixal Lahoiatoi 1 C izziersztj o j TI tscoizst~z .\ladzsoii
