422
D.4WSON, LEADER, KEELY, .4ND ZIMMERMAN
T H E LOW-TEMPERATURE CONDUCTANCE AND VISCOSITY OF CONCENTRATED SOLUTIONS OF AMMONIUM FORMATE AND AMMONIUM ACETATE I N SOME MIXED SOLVENTS L. R. DAWSON, C. R. LEADER, W. M. KEELY, A N D H. K. ZIMMERMAN, JR.
Department o j Chemialry, University o j Kentucky, Lexington, Kentucky Received April S, 1060
.
Although the Debye-Huckel and Onsager theories have gone far toward the elucidation of the conducting properties of electrolytes, especially in aqueous solutions in the very low concentration range, the complications encountered when the solutions become much more concentrated than about 0.1 formal have conspired to leave this phase of conductivity studies in a considerable state of obscurity. Of recent years, however, the field of conductance in concentrated systems has become more active, with the result that a sizeable amount of experimental information is becoming available. The work reported here is intended as a contribution to this stock of experimental data rather than as an attempt to cast theoretical light upon the problem. The data which have been gathered will serve to pose new questions and emphasize old ones. rather than to answer them. The work of Walden (3), which culminated in the statement of the rule that, in the very dilute range, the equivalent conductance is directly proportional to the fluidity of the system, together with the extensive elaboration of this rule by H. C. Jones and his coworkers (2), indicates that a logical starting point in an investigation of this sort lies in the study of the relationship between conductance and viscosity. It is well known that Walden’s rule does not hold in the concentrated range, but it is probable that information of value may be derived from the nature of the deviations which are observed. For this reason, it is considered that the data presented herein may prove to he of some value. EXPERIMENTAL
Ammonium formate was prepared by passing ammonia through 88 per cent formic acid, evaporating under vacuum on the water bath, and drying the crystals between layers of filter paper. Ammonium acetate of reagent grade was used without further purification. Absolute methanol, C.P. grade, was dried over copper sulfate and distilled through an efficient column. Dimethylformamide was purified by fractional distillation through a helixpacked column. Densities were determined by means of a Sprengel pycnometer. An Ostwald-Fenske type viscometer was used, the standardizations being accomplished through the known viscosities of water and acetone. Conductances were determined with a Wheatstone bridge, using alternating current supplied from a stable oscillator. Solutions were contained in a Washburn conductivity cell.
TABLE
1
Density, viscosity, and specific Conductivity of a 60:60 weight per cent tnizture of methanol and dimethylformamide
.C.
25 20 10 0
- 10 - 20 -30
-40 - 50
DENSITY
VISCOSITY
K.W. 0.8658 0.8706 0.8807 0.8900 0.8999 0.9098 0.9196 0.9281 0.9388
crnfipoirrr
obm-lcm.-: X 1 0
0.638 0.703 0.855 1.012 1.321 1.596 1.991 2.400 2.912
20.8 20.0 16.7 15.4 12.4 10.i 9.06 7.72 5.57
TABLE 2 Density, viscosity, and specific conductivity of solutions of ammonium formate i n a 60:60 weight per cent mzzture of methanol and dimethyl formamide a s functions of temperature and concentration
1
TEYPEMTURE
DENSITY
~
VISCOSITY
,,
8PLCIlIC COhmuCIIvITY
0.178 Molal T. 25 20 10
0
- 10 -20
-30 -40 -50
K . / d
cenlipoircr
0.8705 0.8760 0.8872 0.8980 0.9090 0.9200 0.9310 0.9420 0.9530
0.719 0.779 0.937 1.201 1.451 1.720 2.145 2.705 3.220
6.91 5.66 5.00 4.00 3.13 2.19 1.99 1.45
0,799 0.840 1.277 1.710 2.610 2.999 3.500 4.501 4.903
11.0 9.27 8.50 6.89 5.82 4.95 4.25 3.09
0.532 Molal
25
20 10 0 10 -20 -30 -40 -50
0.8973 0.9099 0.9230 0.9350 0.9480 0.9610
-
0.996 Molal 25 20
10 0
- 10 -20 -30
-40 -50
I I
0.8861 0.8920 0.9053 0.9180 0.9300 0.9430 0.9560 0.9680 0.9800
I
0.957
1.ooO 1.555
2.051 3.127 4.088 4.976 6.022 7.077 423
14.5 12.7 11.3 8.64
424
DAWBON, LEADER, KEELY, AND 2IMMERM.W
Temperature control to f0.5"C. was obtained with a manually operated thermostatic bath of dry ice and acetone contained in a 1-gallon Dewar flask. All solutions were prepared on a weight basis, transfers of material being carried out in a dry-box in order to prevent contamination by moisture. The density determinations permitted conversion of concentrations to molar and normal scales. RESULT8
I n order to establish the influence of the added electrolyte upon the system, determinations of the density, viscosity, and specific conductivity of the solvent
TABLE 3 The conductivity and viscosity of anhydrous ammonium acetate solutions in methanol IEYPIMTUPE
I
mgEET&
VISCOSITY
0.191 Molal 'C.
ccnlirlohss
20 10 0 -10 -20 -30 -40
0.76 0.85 1.11 1.30 1.72 2.09 2.78 3.88
-50
20 10 0
- 10
0.82 0.91 1.17 1.36
ohm-L%-l
X
3.78 1.96 1.02 0.96 0.90 0.84 0.70 0.42
1 I
I
7.29 5.01 4.11 3.15
I
TIMPLPATUPK
~
co~~&.y
VISCOSITY
0.380Molal 'C.
-30 -40 -50
j
3.10 4.09
i
j
1.06 0.89
0.759 Molal
- 10 -20 -30 -40 -50
1.54 2.01 2.45 3.33 4.51
50.0 26.8 22.2 16.4 13.7 11.4 10.0 5.38
(methanol-dimethylformamide) as a function of temperature were made (see table 1). The same properties were then determined at three different concentrations over the temperature range from 25°C. to -50°C. for ammonium formate in this solvent system. These data are given in table 2. Comparison of the last columns of tables 1 and 2 shows that the intrinsic conductivity of the solvent combination is negligible compared to that contributed by the solute. For this reason, the data given in table 2 are uncorrected for the conductivity of the solvent mixture. Because of the reducing action of ammonium formate, it was thought to be advisable also to explore the behavior of the more stable ammonium acetate in methanol itself and in a mixed solvent. For this purpose, a mixture of 75 per cent methanol and 25 per cent water by weight was chosen as one which con-
4%
CONDUCTANCE AND VISCOSITY OF CONCENTRATED SOLUTIONS
veniently combines low freezing point with relatively high solute solubility. In table 3 are given the viscosities and conductivities for three different concentrations of ammonium acetate in anhydrous methanol, while table 4 gives the viscosities and conductivities of four different concentrations of ammonium acetate in the aqueous methanol. It should be pointed out that the viscosities given in tables 3 and 4 are kinematic viscosities rather than absolute viscosities, as are those given in tables TABLE 4 The conductivity and
ViSCOSity
of ammonium acetate solutions i n a 76:M weight per cent
mizture of methanol and water
*C.
ccdslokcr
m
0.794 0.953 1.24 1.42 1.94 2.45 3.27 4.80
10 0 10
-m -30 -40 -50
ohwrln.-l
X lo1
3.00 2.62 2.33 1.96 1.65 1.31 1.05 0.816
'C.
20 10 0 - 10 - 20 -30 -40 -50
0.336 Molal
m
-
10 0 10
-m -30 -40 -50
ccnlirlokss
ohm-lcm.-l X 101
1.43 2.33 3.83 5.10 7.11 9.36 12.50 17.39
11.3 9.06 7.02 5.41 4.64 3.91 2.58 1.47
3.13 Molal
1.02 1.54 2.11 2.59 3.51 4.26 5.51 7.34
6.37 4.99 3.92 2.90 2.15 1.94 1.04 0.971
20 10 0
-10 -20 -30 -40
- ,-a
1.95 3.19 5.24 6.97 9.72 12.8 17.1 23.8
22.7 17.3 14.0 10.3 7.91 5.26 3.76 2.65
1 and 2. The former, therefore, CaMOt be treated theoretically by means of the Anhenius-type plots and must be regarded simply as a means of estimating qualitatively the relative viscosity of the systems involved. DISCUSSION
The systems studied in this investigation represent cases in which a high degree of molecular association is to be expected. Figure 1, which is constructed from the data of tables 1 and 2, bears out such a prediction. I n that figure, the logarithm of the viscosity coefficient is plotted against the reciprocal of the absolute temperature. The Arrhenius equation,
In 7
=
In B
+ AE,i./RT
42G
DAWSON, LEADER, KEELY, AND ZIMMERMAN
FIQ.1. The logarithm of viscosity as a function of reciprocal temperature for ammonium formate in a mixed solvent consisting of 50 per cent methanol and 50 per cent dimethylformamide by weight. The symbols are as follows: 0 , solvent mixture only; . 0.178 , molal in solute; 0 , 0.532 molal; A,0.996molal. TABLE 5 The temperature and concentration dependence of equivalent conductance i n solutions of amnionium formate in 60:60 weight per cent miztures of methanol and dimethylformamide llQmVALEmCUNDVCTANCL AT TEE CONCLN’IEAIIONS
IWEMTVPE
0.178 Molal
0.532
Mok
0.996 Mold
*C.
20 10 0 - 10 -20 - 30 -40 -50
44.9 36.3 31.6 25.0 19.3 13.4 12.0 8.64
24.2 20.1 18.2 14.5 12.1 10.2 8.59 6.16
17.3 15.0 13.1
9.90
which has received ample theoretical verification (l),predicts that, in ideal or nearly ideal liquid systems, such a plot aa figure 1 should be linear with a slope which is proportional to the activation energy for viscous flow.I n the present
427
CONDUCTANCE AND VISCOSITY OF CONCENTR4TED BOLUTIONS
case it may be observed that, although the curves appear to be linear in the lower range of temperatures, the departure from linearity at temperatures above -1O’C. is very marked. The fact that the solvent mixture itself exhibits this behavior indicates that the effect in the solutions is a real one. However, the solute seems to enhance this phenomenon, since one observes that the departure from linearity becomes progressively greater as the formate concentration is increased. Therefore, this phenomenon must be recognized as a major contributor to the fact that, as the concentration of the solute rises, the obedience of the conductivities and viscosities to Walden’s rule becomes systematically poorer. In table 5 are given the equivalent conductivities derived from the data of table 2, while table 6 shows the “constants” to which the data lead. I t will be noted that, although the conductance-viscosity products at 0.178 molal seem to vary rather widely from any simple curve, a definite trend toward a curve TABLE 6 The temperature and concentration dependence of the Walden’s rule constant, Aq, i n rolutiow of ammonium formate i n 60:60 weight per cent miztures of methanol and dimethylformamide Aq A 1 TEE CONCENIXATIONS
0.170 Mold
0.532 Molal
35.0 34.0 38.0 36.3 33.2
20.3
28.8
35.5 38.6
1
0.996 Molal
__
%.
20
10
0
-10 -20 -30 -40 -50
32.4 27.8
25.6 31.1 37.9 36.3
17.3 23.3 26.8
31.0
30.2
having a maximum at about 0°C. is to be observed. The presumption that such a curve represents a real phenomenon rather than an illusory one is supported by the fact that these products at the other two concentrations seem to show precisely the same type of temperature dependence, the maximum shifting to progressively lower temperatures aa the concentration rises. I t is possible that the cause of this behavior may be traceable theoretically to the same type of phenomenon observed in the viscosities themselves, but we have been unable to do this. I n this connection it appears significant, however, that the positions of the maxima just mentioned seem to coincide rather closely with the temperatures at which the departurw from linearity first appear in figure 1. In addition it should be noted that, although the data of tables 3 and 1 do not lend themselves to so detailed an analysis, still, if one can consider the product of specific conductance by kinematic viscosity at any given molal solute concentration ae a valid basis for a rough estimate of the system, it is observed that in the system with the simple methanol solvent this product is very approxi-
428
DAWSON, LEADER, KEELY, AND ZIMMERMAN
mately constant, while in the system with the mixed methanol-water solvent it once again shows a maximum value, which this time gradually shifts to lower temperatures as the concentration is lowered. It seems evident, therefor., that the nature of the solvent is responsible for the behavior of this product in mixed solvents, and it is suggested that the observed trends reflect changes in the kind of solvation which the solute undergoes in such solvents, and that this solvation is very strongly temperature-dependent. SUMMARY
Densities, viscosities, and conductivities have been determined as functions of temperature and concentration for solutions of ammonium formate in a solvent mixture consisting of 50 per cent methanol and 50 per cent dimethylformamide by weight. Conductivities and kinematic viscosities as functions of temperature and concentration have been determined for solutions of ammonium acetate both in anhydrous methanol and in mixtures of 75 per cent methanol and 25 per cent water by weight. In the former system, it was found that the viscosities fail to obey the Arrhenius equation, illustrating a type of nonideality which seems characteristic of the solvent, but which is also enhanced by the presence of the solute. The conductance-viscosity products in this system are not constant; rather, at a given concentration they seem to display a maximum when considered as functions of temperature. The general features of these observations are repeated in the systems involving ammonium acetate. REFERENCES (1) EWELL AND EYRINQ: J. Chem. Phys. 5, 726-36 (1937). (2) JONESAND CARROLL: h.Chem. J. 32, 521-83 (1904). (3) WALDEN: Bull. scad. imp. sci. (Petrogrsd) lWS, 559-82.
