A STUDY OF T H E ELECTROLYTIC DISSOCIATION OF SOME SALTS I N FURFURAT, BY FREDERICK H. GETMAN
According to the familiar Nernst-Thomson' hypothesis, the dissociabing power of a solvent is largely determined by the magnitude of its dielectric constant. I n general, those solvents which possess high dielectric constants cause greater dissociation of a given solute at a given concentration than solvents whose dielectric constants are low. Furfural, C5H1O2,has long been known to possess a relatively high dielectric constant, its mean value at 25' being 38. If the more common solvents are arranged in the order of their dielectric constants, furfural is found to stand above such well-known dissociating agents as methyl alcohol, ethyl alcohol, acetonitrile and acetone. Notwithstanding this fact, a search of the literature has failed to reveal sufficient experimental data to warrant any conclusion being drawn as to whether furfural solutions do or do not conform to the Nernst-Thomson generalization. A few measurements of the conductance of solutions of ferric chloride in furfural were made by Lincoln2 in the course of a comprehensive study of the conductance of non-aqueous solutions, while Walden3 has measured the conductance of solutions of his so-called "normal" electrolyte, tetraethylammonAside from these two series of measureium iodide, in furfural a t '0 and 25'. menls, there appears to have been little or no attention given to the dissociating power of this solvent. With a view to contributing something toward this hiatus in the literature of non-aqueous solutions the present investigation was undertaken. Preparation of Materiads. The furfural used in this investigation was obtained from the manufacturer admixed with a relatively small quantity of water. Its purification was effected by a method essentially the same as that described by M a i d . A 500 cc. portion was first distilled with the addition of I gram of sodium carbonate to neutralize any pyromucic acid which might have been formed as a result of oxidation of the aldehyde. The portion boiling between 159' and 161' was collected and redistilled; the second distillate, consisting of the portion coming over between 160' and 162', generally possessed some color. After adding granular calcium chloride, the distillate was stored in tightly stoppered flasks in a dark room until required for use. In the final distillation only that portion boiling between 161' and 162' was collected. The mean boiling point, corrected to 760 mm. was 161.7'. The refractive index of the purified solvent was determined and found to be n, = Nernst: Z. phys. Chem., 13, 531 (1894); Thomson: Phil. Mag., (5) 36, 320 (1893). *Lincoln :J. Phys. Chem., 3, 464 (1899). a Wladen: Z. phys. Chem., 54, rgo (1905). Mainn: Chem. Met. Eng. 26, 779 (1922).
ELECTROLYTIC DISSOCIATION O F SOME SALTS I N FURFURAL
213
1.52717 at 20°, a value in close agreement with that found by Bruh1.l Because of the rapidity with which the substance polymerizes, a fresh portion was distilled in the preparation of each solution, care being taken to protect the receiver from the light. Notwithstanding the precautions taken in its preparation, the specific conductance of the different portions of furfural was found to vary from 1.38X10-~to 4,77X10-~reciprocal ohms. The mean values of the specific conductance of the furfural used by Lincoln and Walden,2 respectively, were as follows: 25.6 X IO-^ and 1.45 X IO-^ reciprocal ohms. The solutes studied in this investigation were the iodides of lithium, ammonium, sodium, potassium and rubidium. The salts were obtained in chemically pure condition and were carefully recrystallized and dried according to the uwal prescribed methods. After purification, the salts were preserved in desiccators over sulphuric acid until required for use, when portions were removed and dried to constant weight before dissolving in the solvent and diluting to a definite volume. Apparatus. The conductance of the solutions was measured in the usual manner by means of t8he Kohlrausch method. A drum-wound slide-wire bridge with extension coils was used in measuring the conductance of the solutions, an air condenser being employed to balance the effect capacity in the circuit. The resistance boxes were used in seriep; one a three-dial box ranging in capacity from I to 999 ohms and having an accuracy of 0.1 per cent, and the other a two-dial box consisting of Curtis-wound coils ranging in capacity from 1000to 110000 ohnis and having an accuracy of 0.04 per cent. An audio-oecillator giving a frequency of 1000cycles was used as the source of current together while a specially tuned set of telephone receivers served to determine the position of the tone minimum on the bridge. The conductance cells were of the: Arrhenius type with tight-fitting stopper? and sealed-in electrodes. Owing to the ease with which furfural undergoes oxidation, the electrodes were not only not platinized but also were rendered as smooth as possible by polishing with fine emery cloth. The cell constants were determined by means of a 0.01 N potassium chloride solution the specific This conductance of which was taken to be 0.001412 reciprocal ohms at 25'. solution was prepared according to the directions laid down by Krausand Parkers. The values of the cell constants were frequently checked throughout the entire investigation. All measurements were made at 2 so, an electrically controlled thermostat serving to maintain this temperature constant to within 0.01'. All volumetric apparatus was carefully calibrated for use a t prevailing room temperature. Method and Experiwiental Results. The mother solutions were prepared by direct weighing, and never more than a single dilution was made from the same mother solution. This procedure was followed in order to minimize Bruhl: Ann. 235,
* LOC.cit. a
I
(1886).
Kraus and Parker: J. Am. Chem. SOC., 44,2422
(1922).
2 I4
FREDERICK H. GETMAN
errors due to polymerization or oxidation of the solvent. In viewof the p m i bility of the solvent undergoing gradual alteration, the conductance of one or more of the solutions was redetermined after an interval of several hours, but. in no case was any appreciable change detected. In fact, judging by the gradual change in the color of the pure solvent, on the one hand, and that of the iodide solutions on the other, one is inclined to suspect that the presence of the dissolved substance exerts a stabilizing influence on the solvent. No difficulty was experienced in checking any of the conductance measurements, notwithstanding the comparatively unstable character of the solvent. The smoothed values of the conductances of the different solutions at even concentrations, as derived from the mean experimental data, are given in the subjoined tables. In these tables C denotes the concentration of the solute in mols per liter of solution, v the corresponding volume, A the equivalent conductance, A. the limiting conductance, and a the conductance ratio, A/&. For comparison, Walden's data for tetraet.hylamnionium iodide' are also tabulated..
TABLE I Conductance of Lithium Iodide in Furfural
C
A
V
0.1 0.05
IO
22.95
20
26.25
0.02
50
0.01
IO0
0,005 0.002 0.001 0.0005
200
29.65 31.40 32.68 33.67 34.10 34.40 (35.24)
0.0
500 1000 2 000 (x
a=A/Ao 0.651 0,745
0.841 0.891 0.927 0.955 0.968 0.976 I .oc3
TABLE I1 Conductance of Sodium Iodide in Furfural
C
V
0.06 0.05
16.67 20
0.02
50
=A/A,
0.765
2 000
0.786 0.876 0.927 0.962 0.981 0.990 0.944
c/3
I .@OO
0.01
IO0
0.005 0.002 0.001 0.0005 0.0
2 00
500 1000
Walden: Z. phys. Chem. 54, 150 (1905).
ELECTROLYTIC DISSOCIATION O F SOME SALTS I N FURFURAL
215
TABLE I11 C 0.1 0.05
0.02 0.01 0.005 0.002 0.001 0.0005 0.0
C 0.07 0.06 0.05
0.02 0.01 0.005 0.002 0.001 0.0005 0.0
Conductance of Potassium Iodide in Furfural V A IO 30.50 20 33.81 SO 37.60 I00 39.99 200 41.43 42.20 500 1000 42.40 2000 42 . s o M. (43.101
Q!
=A/Ao
0.708 0.784 0.872 0.928 0.961 0.979 0.984 0.986 I .ooo
TABLE IV Conductance of Rubidium Iodide in Furfural 2, A 14.29 33.33 16.67 34.10 20 34.95 50 39.00 IO0 41.IO 200 42.40 500 43 ’ 50 1000 44.00 2000 44.30 00 (45.oo)
TABLE V
C 0.07 0.06 0.05 0.02 0.01 0.OOj 0.002 G.001 0.0005 0.0
Conductance of Ammonium Iodide in Furfural 2) A 14.29 26.47 16.67 27.74 29.22 20 35.43 50 IO0 38.70 200 40.85 42.67 500 1000 43.65 2000 44.30 OL (46.10)
Q!
=ALL 0.573 0.602 0.632 0.769 0.840 0.886 0.923
0.047 0.961 I .000
TABLE VI Conductance of Tetraethylammonium Iodide in Furfural C V A ~i=h/Ro 0.005 41.40 0.856 200 0.003 43.25 0.892 333.3 0.002 44.23 0.914 500 0.001 1000 45.47 0.940 0.0005 2000 46.22 0.955 0. o CT. (48 40) I .ooo ’
216
FREDERICK H. GETMAN
The difficulty of securing satisfactory readings a t concentrations below with the ipcrease in the magnitude of the errors introduced in corrections for the specific conductance of the solvent, rendered inexpedient any attempts to secure accurate measurements at greater dilut,ions. The value of the limiting conductance was determined by means of the extrapolation formula recently proposed by Walden'. He has furnished a large number of examples of non-aqueous solutions for which the limiting conductance can be satisfactorily calculated by means of the formula0.0005 N , together
in which AI and A, are the equivalent conductances at the dilutions vl and v2, respectively. The values of A, for the solutions of the iodides in furfural, calculated by the above formula, were found to be in almost perfect agreement with the values of A, determined by the gra.phic method proposed by Noyes and Falkz in which the reciprocal of the equivalent conductance, A, is plotted against (CA)" , m being assigned such a value as shall cause the experimental data to fall on a straight line, The data of the foregoing tables are also presented in graphic form in Fig. I in which the values of the equivalent conductance are plotted against the cube roots of
FIG.I I. Lithium iodide. 11. Ammonium iodide. 111. Sodium iodide. IV. Potassium iodide. V. Rubidium iodide. VI. Tetraethylammonium iodide Walden: Z. anorg. Chem. 115, 57 (1921). J. Am. Chem. SOC.34,454 (1912).
* Noyes and Fdk:
ELECTROLYTIC DISSOCIATION O F SOME SALTS I N FURFURAL
217
Discussion of Results. It will be observed that the curves shown in Fig. tend to arrange themselves in two distinct groups according to the rate at which the conductance changes with the concentration. The first group comprises the iodides of lithium, sodium, potassium and rubidium, while the second includes the iodides of the ammonium radical. This difference in the slope of the conductance curves of the alkaline iodides, on the one hand, and of the ammonium iodides, on the other, is noteworthy. In aqueous and alcoholic solutione, the form of the conductance curves of ammonium salts, in general, bears a close resemblance to that of the salts of the alkali metals. If tetraethylammonium iodide, Walden's typical binary electrolyte, behaves in furfural in a perfectly normal manner, then it follows that the iodidesof the alkali metals are abnormal in their behavior in this solvent. On the other hand, the difference in the form of the two groups of conductance curves may be attributed to the tendency of aldehydes to form complexes with the ammonium radical. It is also of interest to point out that none of the six salts under consideration conform to an interesting relation recently discovered by Walden', involving the dielectric constant and the viecosity of the solvent. Walden has shown that for a large number of dilute solutions, the product of the viscosity, 9, and the dielectric constant, D, of the solvent, multiplied by the dilulion, ( v ) ~ . and ~ ~ , the corresponding value of Ao-Au=dv is equal to a constant, 51.4;i.e., K = qD (v)0*4bdv = 5 I .4. (2) I
The values of K for dilute solutions of the salts included in this investigation are given in Table VII.
TABLE VI1 Values of K = D ( V ) O . ' ~ for ~ ~Furfural Solutions ~250=0.0149, D25.=38. Dilution LiI NaI KI Rbi "41 (CzHd4NI 38.70 31.92 31.84 8.35 14.57 7.14 500 8.87 12.68 14.45 5.20 31.06 37.15 1000 8.66 12.13 14.55 4.16 31.18 37.76 2000 (14.52) ( 5 . 5 0 ) (8.63) (12.91) (31.36) (37.87) Although the values of K for the individual salts are approximately constant they differ widely from each other and from the mean value 51.4,found by Walden for several binary electrolytes in sixteen different solvents. I t would be of interest to know why furfural solutions fail to conform to this relationship. The fact that the values of the conductance of the iodides of lithium, sodium, potassium and rubidium in furfural stand in the inverse order of the molecular weights of the solutes may be regarded as an indication of ionic solvation. The degree of ionic solvation may be calculated, according to Walden2, from a lrnowledge of the limiting conductance, A,, of the solute, its 1 2
Walden: 2. anorg. Chem. 115, 73 (1921). Walden: Z. Elektrochem. 26, 65 (1920).
2
18
FREDERICK H.GETMAN
molecular weight, M, the viscosity of the solvent, 7, and the molecular weight, M,, of the latter. Walden showed that the value of the constant in the equation 11070 = K', (3) is dependent upon the molecular weight of the solute; and that for solutions of different solutes in various solvents, the product of the limiting value of the conductance, the viscosity of the solvent and the square root of the molecular weight of the solute remains constant at 25'; i.e., (4)
11.15
When the ions of the solute are solvated, its molecular weight will obvioudy be increased. Under these conditions, if the solute is not polymerized, the value of the constant in equation (4) will be less than 11.15, and the weight of solvent associated with the ions will be given by the equation
Hence, the number of mols of solvent associated with the ions will be W/M,. Walden states that the iodide ion is not solvated in ethyl alcohol, methyl alcohol, acetone or nitrobenzene; therefore, it may be assumed that its tendency to undergo solvat,ion in furfural is negligible. On this assumption the degree of solvation of the cationp of the six salts included in this investigation has been calculated as shown in the following table.
TABLE VI11 Degree of Solvation of Cations in Furfural Solute
LiI
W/M0
3.30
NaI 3.52
KI
RbI
1.44
0.67
NHJ 1.24
(C2Hb)dNI 0
On the assumption that the chloride ion is not hydrated, the calculated values of the degree of hydration of some of the above ions in water is as follows:I.i, 4 . 7 , Na z .o, and K I .3.' These figures, it will be seen, are of the same order of magnitude as those calculated for the same ions in furfural as a solvent. The variation of the conductance of the iodide solutions in furfural with concentration is found to be accurately expressed by the equation derived by Storch2, who showed that the functional relation between conductance and Concentration can be expressed by means of the equation
A, -A = KA"C"-' (6) The exponent n is varied as required by the experimental data. I n the case of aqueous solutions, this equation has been found to apply over a comparatively wide range of concentrations. The Storch equation hae the advantage that it Washburn and Millard: J. Am. Chem. SOC.,37,694(1915). Storch: Z. phye. Chem., 19, 1.3 (1896).
ELECTROLYTIC DISSOCIATION OF SOME SALTS I N FURFURAL
219
expresses the concentrations of both the dissociated and undissociated portions of the solute as a function of each other. This becomes apparent when equation (6) ie written in the form C (110 -11) = K (-4C)n, (7) which is obviously equivalent to C(I-a) =ConstantX(Ca)”, (8) where a is the conductance ratio, A/A,. On plotting the values of I/A against those of (Ch)m,where m = n - I , and assigning successive numerics! values to the exponent until the points fall as nearly as possible on R straight line, the three arbitrary constants, n, K and A, can be evaluated. The subjoined table gives the values of n thus obtained for the six different salts in furfural solution.
TABLE IX Values of nit] the Function C(h,-A)
=K(Ch)n at 25’.
Solute LiI NaI KI RbI NHJ (C2Hs)hNI I . 70 1.80 1.75 1.75 1.90 n 1.85 The mean value of n for aqueous solutions of these same salts is approximately 1.50. As has already been mentioned, the values of A, as determined by the graphic method described above are in remarkably close agreement with those obtained by means of Walden’s extrapolation formula. In conclusion attention should be directed to the fact that the values of the conductance ratio, 11/11,, in Tables I-VI, are relatively high. If this ratio affords a true measure of the degree of electrolytic dissociation of these salts when dissolved in furfural, it follows that the latter is to be ranked among the best known dissociating agents, and offers additional confirmation of the validity of the Nernet -Thornson relation. Summary. ( I ) The conductance of solutions of six different binary electrolytes (LiI, NaI, KI, RbI, NHJ and (C2H6)4NI)in furfural has been studied. The value of the limiting conductance, A,, has been determined by extrapolation and the values of the conductance ratio, A/Ao, at concentrations ranging from 0.1 N to 0.0005 N have been calculated. These ratios are found tto be relatively high, from which it follows that furfural is to be regarded as a solvent possessing high dissociating power, provided that the conductance ratio affords a true measure of the degree of electrolytic dissociation. ( 2 ) When the conductances are plotted against the cube roots of the concentrations, it is found that the values of the conductance of solutions of ammonium iodide and tetraethylammonium iodide increase more rapidly with dilution than do the corresponding values of the conductance of solutions of the iodides of lithium, sodium, potassium and rubidium. This difference between the behavior of the ammonium salts on the one hand, and that of the alkali iodides on the other, may be due to the tendency of the ammonium radical to react with the aldehydic solvent, or it may be ascribed to some abnormality in the dissociation of the alkali iodides in furfural.
FREDERICK H. GETMAN
22 0
(3) None of the furfural solutions studied in this investigation conform to the relation deduced by Walden involving the viscosity and the dielectric constant of the solvent, viz., K=~,D(v)O.~~dv=j1.4 (4) The degree of solvation of the cations of the different salts has been calculated and found to be approximately of the same order of magnitude as in aqueoue solutions. ( 5 ) The Storch equation has been shown to express the functional relation between conductance and concentration of the solutions of the different salts with considerable accuracy, the value of the exponent, n, in the equat,ion
C (A -A,) = K(Ch”) being greater than in the case of the corresponding aqueous solutions. Hillside Laboratory Stamford, C o m . June 20,192s.
