ELECTROLYTIC CONDUCTANCE IN ALCOHOL-WATER MIXTURES
2165
Studies of Electrolytic Conductance in Alcohol-Water Mixtures. I. Hydrochloric Acid, Sodium Chloride, and Sodium Acetate at 0, 25, and 35" in Ethanol-Water Mixtures'
by H. Olin Spivey and Theodore Shedlovsky Rockefeller University, New York, New York
(Received December 0, 1066)
Conductance data on dilute solutions of hydrochloric acid, sodium chloride, and sodium acetate in ethanol-water mixtures at 0, 25, and 35" are presented. Measurements were made in approximately 10,20,40,60, and 80 wt % ethanol, but a t 40% ethanol, potassium instead of sodium salts were used. These and supplementary data from the literature were examined in terms of the 1965 theoretical Fuoss-Onsager equation. It describes the experimental results within the limits of experimental accuracy and with reasonable values for the ion-size parameters. No detectable ion association was found for these electrolytes below 80 wt % ethanol. Variations in the Walden products with solvent composition and temperature are reported and discussed with respect to current theories concerning solvent structure and ion-solvent interactions.
Studies of electrolytic conductance in ethanolwater mixtures were initiated to provide experimental data on the behavior of strong and weak electrolytes in such systems. In this paper, we limit ourselves to results obtained for hydrochloric acid, sodium chloride, and sodium acetate;2 results on acetic acid will be re ported in a following papei. Interest in alcohol-water mixtures stems, in part, from the similar amphiprotic properties of alcohol and water molecules, both involved in ion solvation and proton binding, but to different extents. Variations of these interactions may thus be studied by measurements over the composition range of the binary solvent system. Numerous properties of alcoholwater mixtures which,.m-e a function of solvent composition exhibit extrema which may well be reflections of increased solvent structure in this region of solvent composition. Appropriate conductance measurements provide useful and sensitive indications of ion-solvent interaction, proton-anion and proton-solvent association, and solvent structure. Conductance studies over the complete range of solvent composition have been previously published only for the methanolwater system a t 250j4although a few measurements on
selected water-ethanol compositions are also available.6 As demonstrated by our present work, the variation of conductance with temperature provides further useful insights into these phenomena.
Experimental Section The resistance measurements were made with alternating current bridges which had been carefully calib~-ated.~JOil thermostats at 25 and 35" were regulated to within *0.005", and temperatures were observed with periodically calibrated Beckmann differential thermometers. A dewar flask containing water and crushed ice served as the thermostat at 0". A (1) This research was supported by the National Science Foundation through Grant 21385. (2) The potassium salts were used for the measurements in 40% ethanol. (3) F. Franks and D. J. G. Ives, Quart. Rev. (London), 20, 1 (1966). (4) T. Shedlovsky and R. L. Kay, J. Phys. Chem., 60, 151 (1956). (6) (a) B. L. Murr and V. J. Shiner, J . Am. Chem. Soc., 84, 4672 (1962); (b) J. L. Hawes and R. L. Kay, J. Phye. Chem., 69, 2420 (1965); (0) I. I. Bezman and F. H. Verhoek, J. Am. Chem. SOC.,67, 1330 (1945). (6) J. G. Jan2 and G. D. E. McIntyre, J. Electrochem. SOC.,108,272 (1961). (7) T. Shedlovsky, J. Am. Chem. SOC.,5 2 , 1793 (1930).
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2166
quartz flask cell8 (cell constant 0.43184) with unplatinized electrodes and one of Pyrex (cell constant 6.4832) with lightly platinized electrodesg were used for all the measurements at 25” and some of the measurements at 0 and 35’. Jena glass pipet cells1° were used for the remaining experiments. Electrode polarization errors were eliminated by taking readings at several frequencies and extrapolating to infinite frequency. l1 Readings a t 5 and 10 kHz were usually found to be sufficient for this purpose in subsequent measurements. In contrast to flask cells, no significant current shunt errors were found with the pipet cells when used in the icewater bath, within the frequency range employed in the measurements. Cell constants were determined to within *0.01% precision a t various times by calibration with aqueous potassium chloride or hydrochloric acid solutions,12 and by means of cell intercomparisons with acetic acid solutions. Previous experience and simple calculation indicates that cell constants are not significantly altered within the temperature range of our experiments.13 Conductance water was obtained from a Barnstead High Purity conductivity water still which was fed through a deionizing column. The specific conductance of this water was routinely between 1 and 4 X mho/cm. Ethanol was prepared by distillation of commercial grade absolute ethanol through a vacuum-jacketed column after refluxing 12 hr with magnesium ethoxide. l4 Nitrogen gas which passed through an oxygen removing and drying train was bubbled through the boiling ethanol and the middle liter of a 1.8-1. distillate was collected in a nitrogenfilled flask a8ndused for conductance solutions. The specific conductance of the azeotrope was about 2 X mho/cm. The composition of ethanol-water mixtures was computed from duplicate density measurements by interpolation from known density data.16 Reagent grade sodium chloride was carefully purified by recrystallization. Sodium acetate solutions were prepared fresh by the addition of pure acetic acid16 to standardized, carbon dioxide free sodium hydroxide in ethanol. Standardizations and solution iransfers were made with weight burets and approximately l% excess acetic acid m as used to suppress hydrolysis. and ‘Odium The genera‘ procedure for ‘Odium EVXtate experiments was essentially the Same as had been previously described.s Argon gas was used to remove atmospheric impurities from the solvents and in the conductance Of this Or Other inert gases through the for hours did not change - the conductance of any of the electrolvte sohacid* Increments Of stock solutions (approximately 0.1 M in electrolyte) The Journal of Phyaical Chemietry
H. OLINSPIVEYAND THEODORE SHEDLOVSKY
were added to the conductance cell from weight burets to give various concentrations in the range of 0.001 to 0.015 M . For each electrolyte and solvent system, at least two separate series of measurements were performed at 25”. However, successive series of measurements on hydrochloric acid solutions failed to show sufficient reproducibility for the precision we were seeking. This was judged by the fact that marked negative deviations, which increased with dilution, were observed in the or A‘18 plots and that these deviations were not sufficiently reproducible from experiment to experiment by amounts as large as 1%. This could be due to some loss of acid through adsorption or ion exchangelg a t the walls of the cells. To reduce such possible acid losses, the following procedures were adopted. A previously cleaned and steamed cell was soaked several hours with approximately 1 M hydrochloric acid in the solvent to be used. It was then rinsed four times with fresh solvent before starting a series of measurements. This procedure did not result in significant changes in the conductances of the solvents, mhich varied from 2 X in 99% to 2.5 X mho/cm in 10% ethanol.*O In addition, measurements were also made on successive portions of individual solutions of definite concentrations, prepared and stored in a Pyrex flask.12b These modifications in the experimental procedure improved reproducibility in the dilute range about tenfold, namely, to within 0.1%. To check on possible errors arising from esterifica(8) T. Shedlovsky, J. Am. Chem. SOC.,54, 1411 (1932). (9) G.Jones and G. M. Bollinger, ibid., 53, 411 (1931). (10) T. Shedlovsky in “Technique of Organic Chemistry,” 3rd ed, Interscience Publishers, Inc., New York, N. Y., 1963,p 3031. (11) R. A. Robinson and R. H. Stokes, “Electrolyte Solutions,” 2nd ed, Butterworth and Co. Ltd., London, 1955,p 92. (12) (a) J. E. Lind, J. J. Zwolenik, and R. M. Fuoss, J . Am. Chem. Soc., 81, 1557 (1959); (b) R. H.Stokes, J. Phy8. Chem., 65, 1242 (1961). (13) See ref 11, p 97. (14) L. F. Fieser, “Experiments in Organic Chemistry,” 3rd ed, D. C. Heath and Co.,Boston, Mass., 1955,p 285. (15)The d04 values were taken from A. V. Rakovski, Chem. Abetr., 23, 26248 (1929); values of d% are from N. 5. Osborne, E. C. McKelvy, and H. W. Bearce, Bull. Bur. Std., 9, 424 (1913); and values of d% are from N. S. Osborne, E. C. McKelvy, and H. W. Bearce, J. wash. Acad. Sci., 2, 95 (1912). (16) The glacial acetic acid was a special cut obtained from the Niacet Co. and judged to be 99.94%. _ -Dure on the basis of its freezinn point (see ref4). (17) T*ShedlovskYv J - Am- C h m . S0C.r 54,1405 (1932). (18) R. M. Fuoss and F. Accascina, “Electrolyte Conductance,” Interscience Publishers, Inc., New York, N. Y., 1959,p 197. (19) J. E. Prue, J. Phye. Chem., 67, 1152 (1963); E. Grunwald, et d.,J . Am. Chem. SOC.,84,4664 (1962);H. 0.spivey, unpublished studies. (20) Solvent composition is always specged on a wei&t per basis.
ELECTROLYTIC CONDUCTANCE IN ALCOHOGWATER MIXTURES
2167
Table I : Properties of Water-Ethanol Mixtures .Wt % CzHaOH
35" 0"
20
10
0
0.99987 0.99708 0.99406 88.03 78.54 74.78 17.921 8.949 7.225
0.98477 0.98038 0.97685 82.32 72.8 69.2 33.11 13.28 10.06
0.97567 0.96640 0.96134 76.80 66.99 63.5 53.19 18.08 13.32
tion in hydrochloric acid solutions, conductance experiments with 60% ethanol solvent were performed using first aqueous stock solutions, then stock solutions in 60% ethanol-water solvent. The ethanol content of the cell was readjusted with ethanol after each addition of aqueous stock solution. No difference in results was obtained in these experiments. Since, in addition, the conductance remained stable with time, esterification errors of importance are unlikely. The conductance data for 0 and 35" were obtained by comparihg measurements a t these temperatures with values a t 25". Resistance ratios, R Q / Rand ~ ~R3b/ R2b,were determined, usually in pipet cells, for at least three concentrations for each electrolyte and solvent. Solvent background conductance was sufficiently small to have no disturbing effect on the resistance ratios. Rinsing and filling of the cells were made in an argon atmosphere and solution resistance was measured first at 25", then at 0 or 35", and again at 25". The values were considered to be acceptable only if the resistance returned to its original value at 25" to within 0.1%. In a few instances in which the pipet cell measurements did not meet this criterion, satisfactory results were obtained by using flask cells.
Results Thedensities, d, 16dielectricconstants, D, at Oo,21 25°,22 and 35°,22and viscosities, q,2a used in the calculation of our results are from the literature and are summarized in Table I. Our conductance data were analyzed with the Fuoss-Onsager 1961j2' equations : (1) for associated electrolytes, and (2) for unassociated electrolytes A = AQ - Sc'/'yl/'
+ E'cy In (6Eflcy) + LCY- KACTAP (1)
- Sc"'
+ E'c In (6E'lc) + Lc
(2) In these equations, A is the equivalent conductance at the concentration c (equivalents per liter of solution) A = A.
40
0.94941 0.93151 0.92385 63.72 55.02 52.0 71.4 23.74 17.2
60
80
100
0.90726 0.88700 0.87851 49.87 43.40 41.0 57.50 22.32 16.6
0.86035 0.83909 0.83209 37.92 32.84 31.0 36.90 17.38 13.55
0.80627 0.78507 0.77641 28.3 24.3 22.8 17.7 11.01 9.14
and A. is its limiting value, y is the degree of dissociation, K A is the association constant, and f is the activity coefficient. The constants S and E', but not E f l , depend on Ao, as well as on the temperature T (Kelvin), D,and 7, but are not otherwise adjustable parameters. The coefficient of the linear concentration term, L, is an adjustable parameter. It is a function of the "ion size," a"(L), the values for which are thus obtained from the data. Numerical values for all the theoretical coefficients involved in the equations depend of course on fundamental constants such as electronic charge, Avogadro's number, etc. In our computations we had adopted the values given by FUOSS, et Using the computer program of Skinner and F U O S S , ~ ~ the data were analyzed with eq 1. However, if the value obtained for the association constant, KA, was less than 10, recomputation with eq 2 was made. Data from separate experiments on a given electrolyte and solvent were combined for purposes of the above analysis. The values for the parameters in the equations are listed in Table 11. The experimental results are thus given in implied form for reasons of compactness of presentation of the large number of measurements. The degree of agreement between experimental values and the corresponding ones given by the equations is indicated in the columns, u, the standard deviation, while the columns, N , list the number of experimental measurements in each case. The results are given for only 25", at which temperature the experimental accuracy was somewhat greater. This is because the conductance values at 0 and 35" were calculated from resistance ratios26relative to those at (21)J. Wyman, Jr., J. Am. Chem. SOC.,53, 3292 (1931). (22) G.Akerlof, ibid., 54, 4125 (1932). (23) E. C. Bingham and R. F. Jackson, Bull. Bur. Std., 14, 59 ( 1918-1919). (24) R. M.Fuoss, L. Onsager, and J. T. Skinner, J . Phye. Chem., 69, 2581 (1965). (25) We are indebted to Drs. Fuoss and Skinner for making their
computer program available to us.
Volunte 71, Number 7 June 1867
H.OLIN SPIVEYAND THEODORE SHEDLOVSKY
2168
r
5 m
ab
M M M
w m w
w
ag 0 0
M
0
8
m
v)
0
0
>In N
b
,
0
m
v)
0
N 10
0
0
5 m
0
v) N
0 0
&t 0 0 0
0 0 0
0
M M M
*mm
0 0 0
0 0 0
s h
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ELECTROLYTIC CONDUCTANCE IN ALCOHOL-WATER MIXTURES
Table III : Walden Products in Water-Ethanol Mixtures at 25'" Wt %
7
0
IO*
20
40
60
80
100
0.658 0.448 3.130 0.683 0.366
0.72 0.51 3.6 0.71 0.38
0.753 0.546 3.76 0.727 0.387
0.676
0.509 0.435 1.710 0.536 0.369
0.373 0.319 0.744 0.390 0.298
0.259 0.224 0.690 0.241
2.97 0.699 0.391
Values of ho are from ref 28. * Values of ho in 10 wt % ethanol are estimated from data at 0, 20, 40, etc., wt % ethanol and are subject to considerable uncertainty. See text.
The ion-size parameters and association constants 25". However, except in a few cases, thea"(L) and K A calculated from eq 1 or 2 and listed in Table I1 are values a t 0 and 35" did not differ substantially from reasonable and consistent with data on other electhose a t 25". trolytes in water-ethanol solvents.5b Thus, no sigDue to systematic errors in connection with the hynificant ion association (KA < 10) is indicated below drochloric acid solution conductances and the elaborate 80% ethanol. Unfortunately, the physical interpreprocedure for preparing sodium acetate solutions pretation of these parameters is too uncertain to draw viously discussed, such results may be less accurate further conclusions, a situation which is aggravated than may appear from the u values. For data at 25", by the relatively small deviations of these data from we estimate uncertainties of *0.3% in AO for hydroOnsager's limiting law.24 However, the fact that chloric acid and sodium acetate and f0.05% for soreasonable values for these parameters are obtained dium chloride. At 0 and 35", the uncertainties in in each case is significant. AO values for each electrolyte may be as great as 0.5%, due in part to the smaller number of experimental Because a different transport mechanism is involved points. Adjustment of A. data to round weight per for the proton than for normal ions, one might suppose that the concentration dependence of hydrochloric cent solvent composition involved only small correcacid conductance might show differences from that tions and involved no loss in accuracy of the data. of salts in the mixed dipolar solvents. However, this Other parameters of the conductance equation are not significantly altered by these small changes, as can be does not appear to be the case. The assumption that verified by calculation. the nature of the transport mechanism is immaterial For comparing the behavior of individual ions in the for the Onsager theoryz9is consistent with our results. mixed solvents, we examined limiting ionic Walden As shown in Figure 1, limiting equiva!ent conductproducts, Xor]o. which are listed in Table 111. The inances of each electrolyte at 25" decrease smoothly with dividual ionic conductances were computed with the solvent composition throughout most of the compositransference number data at 25" of Gordon, et 1 2 1 . ~ ~ ~tion range, despite the large maximum in viscosity in water and in pure ethanol, and those of Fratiello28 near 50y0 ethanol. Conductance curves at 0 and 35" at 20.10, 39.94, 60.27, and 79.37% ethanol. Because are similar. In this respect, the A, profiles resemble of uncertainty in the graphical interpolation between those observed in methanol,' although the viscosity 0 and 20.1% for the value we chose for 10% ethanol, maximum in water-ethanol mixtures is approximately the listings in Table I11 for this solvent composition twice as large. Obviously, ion mobilities are not premay be in error by as much as 10%. dominantly controlled by the bulk viscosity. This is more clearly demonstrated by the Walden products, Discussion which are normalized to their value in water and shown The only data available for comparison with ours in Figure 2 as a function of solvent composition. If were those of Murr and Shiner58 for hydrochloric acid conductance in water-ethanol mixtures at 25". From (26) At/Ara = (Rzs/Rt)( d W d ' r ) . their AO values at 60, 70, and 80 volume % ethanol, (27) (a) G. C. Benson and A. R. Gordon, J . Chem. Phys., 13, 473 we estimate a limiting conductance of 100.8 in 60 (1945); (b) J. R. Graham, G. S. Kell, and A. R. Gordon, J . Am. Chem. SOC.,79, 2352 (1957). weight % ethanol. This value is in very good agree(28) A. Fratiello, University Microfilms, Ann Arbor, Mich., Order ment with our experimental value of 101.0 f 0.2 based No. 63-1019; DiS8eTtQ.tiOn Abstr., 23, 2338 (1963). Transference on more than ten separate experiments in this solvent number data at 0 and 35O were not available. with the various experimental procedures we have used. (29) See ref 11, p 371. Volume 71, Number 7 June 1867
H. OLINSPIVEY AND THEODORE SHEDLOVSKY
2170
A0 450r
400h
* From table I1 A HCI data from Bezmon and Verhoek + Viscosity (rl0)
350
90
1.2
I .o
103q
-+-+-
0.8
0.6
0.4
0.2
0 0 0
IO
20
30
40
50
60
70
90
80
ion mobility depended on bulk viscosity in accord with Stokes' law, the Walden products would be proportional to the reciprocal of the ion si~es.~OThe variations in Walden products with solvent composition (Table I11 and Figure 2) and temperature (Table IV) might then be explained through changes in ion Table IV : Temperature Dependence of Walden Products, ( A o d a d (A o ~ o ' ~
NaCl HC1 NaAc
W t 5% ethanol 40
20
eo
80
-
0.930 0.8273 0.8133 0.9026 0.9461 0.741 0.6463 0.6208 0.7084 0.8271 0.9528 0.8708 0.8568 0.9406 0.9530
a Ratios of 25 to 0" and 35 to 25' show similar trends with solvent composition; all exhibit minima near 20 wt % ethanol, for example.
solvation alone. Such an explanation, however, requires increasing ion solvation with increasing temperature, ignores other data indicating structurebreaking effects of ions (particularly potassium and chloride ions*'), and does not explain the behavior of the Walden product for the hydrogen ion. An alThe Jourmal of Phyeical Chemistrzl
I
I
60
80
I 100
Figure 2. Variation of ionic "Walden products," normalized to water with solvent composition a t 25".
Figure 1. Variation of limiting equivalent conductances (Ao) and of solvent viscosity ( q o ) with solvent composition a t 25".
10
I
40
Weight per cent ethanol
100
Weight per cent EtOH
0
I
20
ternative and more satisfactory interpretation of the Walden products is based on the views summarized by Franks and Ivesa concerning the structure of ethanolwater mixtures and the effect of ions on this structure. A variety of thermodynamic, kinetic, ultrasonic, and transport dataa indicate that addition of simple alcohols to water initially enhances the structure of the solvent. This structure enhancement appears to reach a maximum near 20 to 30% alcohol in the case of ethanol; at higher ethanol compositions, the water structure is progressively reduced. The sharp maximum in Xoqo for the hydrogen ion supports this conclusion since its excess mobility depends on proton transfers among associated solvent dipolar molecules. For other ions, the large Walden products in water relative to their values in pure alcohol are thought to be due to the local structure-breaking effect of these ions on water. The maxima in the Walden products near 20% ethanol may thus be a consequence of the greater disruption of water structure in the vicinity of the ions for solvent of this composition. This viewpoint also predicts that Walden products should decrease with increased temperature because of reduction in solvent structure at higher temperatures. Moreover, one might expect the temperature effect on the Walden product to be largest near 20% ethanol cor(30) See ref 11, p 130. (31) R. L. Kay and D.F. Evans, J . Phye. Chem., 70, 2326 (1966).
2171
STUDIES OF ELECTROLYTIC CONDUCTANCE IN ALCOHOL-WATER MIXTURES
responding to maximum structure. Both these predictions are in agreement with the ratios of Walden products as listed in Table IV. We observe that the concentration dependence of ionic mobilities in alcohol-water mixtures is in accord with theories based largely on ionic electrostatic forces and on a continuum model for the solvents. However, mobilities themselves and their temperature dependence are predominantly affected by ion-solvent interactions which are not easily explained in terms of ionic solvation alone. The conductance behavior of the salts in
this study is consistent with their known structurealtering effects in water and current views concerning the structure of alcohol-water mixtures. Conductance and transference number measurements on other salts at different temperatures in alcohol-water solvents would be helpful in elucidating the competing ionsolvent and solvent-solvent interactions that occur.
Acknowledgment. We are indebted to Miss Lorraine Seher for her valuable technical assistance in our experimental studies.
Studies of Electrolytic Conductance in Alcohol-Water Mixtures.
11. The Ionization Constant of Acetic Acid in Ethanol-Water Mixtures at 0, 25, and 3S01
by H. Olin Spivey and Theodore Shedlovsky Rockefeller University, New York, New York 100.$1
(Received February S,1967)
Measurements are reported on the electrical conductance of dilute solutions of acetic acid at 0, 25, and 35" in ethanol-water mixtures over the whole range of solvent composition. From these data and those on hydrochloric acid, sodium chloride, and sodium acetate solutions in the same solvent systems previously reported by us, values for the dissociation constants for acetic acid have been computed for these temperatures over the entire solvent composition range. The results of these studies are discussed briefly from theoretical considerations.
Introduction Studies of electrolytic conductance of the typical weak acid, acetic acid, in alcohol-water mixtures over the entire range of solvent composition provide a convenient and accurate means for determining the ionization constant. Not only do such determinations provide useful data for the theoretical understanding of the ionization process in such systems in which two different dipoles, water and alcohol, as well as the anion compete for the proton, but they can also serve the
practical purpose of establishing pH scales in these systems. In this paper we shall present the results of conductance measurements on dilute solutions of acetic acid in ethanol-water mixtures a t 0, 25, and 35" over practically the entire range of solvent composition. The corresponding ionization constants were obtained in a (1) This research w&s supported by the National Science Foundathrough Grant No. 21385.
Volume 7 1 , Number 7 June 1067
