3878
D. EVANS,C. ZAWOYSKI,AND R. KAY
The Conductance of the Symmetrical Tetraalkylammonium Halides and Picrates in Acetonitrile at 2S01
by D. Fennel1 Evans, C. Zawoyski, and Robert L. Kay Mellon Institute, Pittsburgh, Pennsylvania
16219
(Received June 1 , 1966)
Conductance measurements are reported for Me4NBr, Me4NI, Me4NPi, Pr4NBr, Pr4NI, Bu4NBr,Bu4N1, and Bu4NPi at 25' in acetonitrile. The data, after analysis by the FuossOnsager theory, produced a constant ion size parameter of (3.6 0.2)d for almost all of the salts. Only the tetramethylammonium halides were found to be associated to any appreciable extent in keeping with the behavior of these salts in nitromethane. The association behavior of electrolytes in acetonitrile, nitromethane, and nitrobenzene, solvents of almost identical dielectric constant, and acetone, a solvent of lower dielectric constant (20.5), are compared, after recalculation to bring the data into conformity with the Fuoss-Onsager theory. The association in nitrobenzene solutions is higher than expected, but for all of the solvents the association constants increase with decreasing crystallographic size in contrast to what is found in hydrogen-bonded solvents. K I in acetone is the one exception to this rule.
*
Introduction This work is part of a systematic study of the transport properties of the symmetrical tetraalkylammonium salts in dilute and concentrated solutions. We are particularly interested in the interaction of the hydrophobic side chains of these electrolytes with water. Before such interactions can be characterized, it is necessary to establish the behavior of these electrolytes under a variety of conditions. Here, we report the association behavior Of these electrolytes in a nonhydrogen-bonded solvent of lower dielectric constant. This work supplements investigations in methanol,2 D20,3 and aqueous solution^,^ which are to be reported in subsequent papers. There has been no systematic investigation with the required precision for a detailed analysis of the concentration dependence for the symmetrical tetraalkylammonium halides in acetonitrile. Much of the early work6J lacked the required precision, and other investigators measured only a limited number of salt^.^^^ Popov and co-workers have reported data for the tetramethylammonium halides and a number of polyhalides.9 Other conductance investigations of this solvent have been concerned primarily with determining individual ion conductances from measurements The Journal of Physical Chmiatry
on salts composed of two large ions, such as tetrabutylammonium tetraphenylboride'O or tetraisoamyl~ ~ m o n i utetraisoamYlb0ride.'' m Experimental Section The Pyrex conductance cells were of the Erlenmeyer type as described by Daggett, Bair, and Kraus,lZ contained 500 ml. of solution, and had cell constants (1) Presented in part at the 147th National Meeting of the American Chemical Society, Chicago, Ill., Sept. 1964. (2) R. L. Kay, C. Zawoyski, and D. F. Evans, to be published. (3) R. L. Kay and D. F. Evans, to be published. (4) D. F. Evans and R. L. Kay, to be published. (5) P. Walden and E. J. Birr,2.physik. Chem. (Leizpig), 144A, 269 (1929). (6) J. C. Philip and H. R. Courtman, J. Chem. SOC.,97, 1261 (1910). (7) C. M. French and D. F. Muggleton, ibid., 2131 (1957). ( 8 ) G. Korttim, 8. D. Gokhale, and H. Wilski, 2. physik. Chem. (Frankfurt), 4,86 (1955). (9) A. I. Popov and N. E. Skelly, J. Am. Chem. SOC.,76, 5309 (1954); A. I. Popov, R. H. Rygg, and N. E. Skelly, ibid., 78, 5740 (1956). (10) D. S. Berns and R. M. Fuoss, ibid., 82,5585 (1960). (11) J. F. Coetzee and G. P. Cunningham, ibid., 86, 3403 (1964); 87, 2529 (1965). (12) H. M. Daggett, E. J. Bair, and C. A. Kraus, ibid., 73, 799 (1951).
CONDUCTANCE OF SYMMETRICAL TETRAALKYLAMMONIUM HALIDES AND PICRATES
of about 1.3 cm.-l. The platinum electrodes were lightly ~1atinized.l~The cells were constructed by E. Sexton, University of Indiana, Bloomington, Ind. The conductance cell of Hawes and Kay14 containing the guarded electrode was not suitable for these measurements, owing to its rather small cell constant, and construction of a second guarded electrode assembly was too formidable a task at this time. The cells were cleaned with boiling nitric acid, rinsed with conductivity water, and steamed before each run. Acetonitrile was delivered directly to the cell from the distillation receiver through an all-glass closed system under Nz pressure after the cell had been thoroughly rinsed with acetonitrile to remove the last traces of water. I n the cell calibration runs, water was added to the cell directly from an ion-exchange column through polyethylene tubing and a Pyrex filling device fitted with appropriate stopcocks. The cell was fitted with the Hawes-Kay14 salt cup dispensing device which permitted salt samples in 11-mm. Pyrex cups to be added to the solution successively without exposing the cell contents to the atmosphere. Both the cell and the dispensing device contained stopcocks which permitted the cells to be thoroughly swept with argon before addition of solvent, during the addition of solvent, and while the salt cup dispensing device containing the salt samples was placed on the cell. The use of the heavy inert argon and a completely closed system once filled resulted in solvents of specific conductance as low as 1.4-3 X lo-' ohm-' cm.-l for water and 3-9 X ohm-' cm.-l for acetonitrile that underwent almost no detectable conductance change once temperature equilibrium had been attained. Temperature equilibrium was hastened and maintained during a measurement by rapid magnetic stirring of the solution using a seamless Teflon-coated magnet. The room temperature was maintained above that of the bath to avoid condensation of the solvent in the cup dispensing device. All resistance measurements were carried out using a calibrated Dikes-Jones bridge with an oscilloscope as detector. The constant-temperature oil bath was maintained within 0.002' by means of a mercuryin-glass thermoregulator. The absolute temperature was determined by a calibrated platinum resistance thermometer and a Mueller bridge. The cell constants were determined at 25" by measuring the conductance of aqueous KCl over the concentration range 1-10 X M and using the averaged conductance equation of Lind, Zwolenik, and F u o s P to obtain the absolute value. The maximum spread in the cell constants over the whole concentration range studied
3879
was 0.02%. The measured resistances were corrected for the usual small frequency dependence. All solutions were prepared by weight, vacuum corrected, and the concentration was calculated on a volume basis by means of solution densities determined from d = do i A% where 6 is the concentration in moles of solute per kilogram of solution. A solvent density do = 0.7767 was measured, and A was determined from density measurements on the solution remaining after the conductance runs. Salt densities ranging from 1.6 for Me4NI and 1.1 for BurNBr were used in the vacuum correction. Conductivity water was prepared by passing distilled water through a 1.53-m. column of mixed-bed ionexchange resin. Conductivity water was collected from the column only after a thorough rinsing of the resin. Acetonitrile (Fisher Scientific Co.) was purified by the procedure described by CoetzeelGwith the addition of a final fractional distillation under nitrogen from calcium hydride through a 1.22-m. Stedman column. The physical properties at 25" obtained for the purified solvent were 0.7767 g. ml.-l for the density, 0.003448 poise (see below) for the viscosity, 36.02 for the dielectric constant, and 3-9 X low8 ohm-' cm.-l for the specific conductance. A single capillary pycnometer was used for the density measurements, and our value agrees well wit>h literature v a l u e ~ 1 ~ Jwith ~ , 1 two ~ exceptions.18,19 The viscosity was first determined in a Ubbelohde suspended-level-type viscometer (flow time 540 sec.) which was calibrated at 25" with water (0.008903 poise) .20,21 The viscosity reported here for acetonitrile is in good agreement with the generally accepted value~~~J~+'~ that center around 0.00344 poise. However, a recent calibration with 20% aqueous sucrose,23 n - d e ~ a n eand , ~ ~n-hexaneZ4resulted in a value of 0.00341 (13) G. Jones:and D.M. Bollinger, J . Am. Chem. SOC.,57,280 (1935). (14) J. L. Hawes and R. L. Kay, J. Phys. Chem., 69,2420 (1965). (15) J. E. Lind, Jr., J. J. Zwolenik, and R. 31.Fuoss, J . Am. Chem. Soc., 81, 1557 (1959). (16) J. F. Coetzee, G. P. Cunningham, D. K. McGuire, and G. R. Padmanabhan, Anal. Chem., 34, 1139 (1962). (17) C. J. Janz, A. E. Marcinkowsky, and I. Ahmad, Electrochim, A d a , 9, 1887 (1964). (18) F.Aocascina, 5. Petrucci, and R. M. Fuoss, J . A m . Chem. SOC., 81, 1301 (1959). (19)D. F.-T. Tuan and R. M. Fuoss, J . Phys. Chem., 67, 1343 ( 1963). (20) J. F. Swindells, J. R. Coe, Jr., and T. B. Godfrey, J . Res. Natl. Bur. Std., 48, 1 (1952). (21) J. R. Coe, Jr., and T. B. Godfrey, J . Appl. Phys., 15, 625 (1944). (22) R. A. Robinson and R. H. Stokes, "Electrolyte Solutions," 2nd Ed., Butterworth and Co., Ltd., London, 1959.
Volume 60, Number 11
November 1966
3880
D. EVANS,C. ZAWOYSHI,AND R.
poise. We have attributed the difference to a viscometer surface tension effect and suspect that this lower value is more nearly correct. We have used 0.003448 in all of our calculations since a change in the viscosity of as much as 3% was found to produce a negligible difference in the conductance parameters. The dielectric constant was measured in a cell designed for absolute measurementsz6 in which the cell constant was determined by direct capacitance measurements on the cell filled with dry nitrogen ( E 1.0005), using a General Radio Type 1615A capacitanceconductance bridge. The cell constant was approximately 2.2 pf. and reproducible to 0.05% after repeated cleanings with chromic acid solution. Our value of the dielectric constant of anhydrous acetonitrile agrees well with that reported by Fuoss and co-workerslOJs but not with the previously accepted value of 36.7.26 All of the halide salts were commercial products (Eastman Kodak) whereas the picrates were prepared by neutralization of the appropriate tetraalkylammonium hydroxide with picric acid. All salts were dissolved in the appropriate solvents, filtered, and recrystallized three times. The solvents used for the recrystallizations were acetoneether mixtures for Bu4NBr and BU4x"I and methanol-ether mixture for Pr4NBr and Pr4NI. Great care was taken to ensure that the ether was peroxide free. The salts were dissolved in a minimum amount of acetone or methanol, and ether was added until precipitation commenced, at which point the solution was cooled and the resulting crystals were filtered in a fritted-glass funnel. lMe4NBr and Me4NPi were recrystallized from a 1 : l mixture of methanol-water, N e 4 N from a 1:4mixture of methanolwater, and BurNPi from a 1 : l acetone-methanol mixture, all on a volume basis. After a preliminary drying, the salts were finely ground in an agate mortar and dried in a vacuum oven fitted with a liquid Nz trap at the following temperatures: butyl salts and MerNPi, 56"; Pr4NI, 70'; Pr4NBr, 31e4NBr, and Me4N1, 90". The salts were stored in glass bottles in darkened desiccators over CaS04.
Results The measured eauivalent conductances. the corresponding electrolyte concentration ( M ) , the solvent conductances, and the density increments, A , are shown in Table I. The data were analyzed by the Fuoss-Onsager conductance theoryz7 which can be expressed as
* = Ao - sc"' +
log
for unassociated electrolytes and as The J O U Tof~Phyeical chemistry
+ ( J - Fh)c
(l)
A =
A0
- S(Cr)"*
KAY
+ ECr log Cy +
(J - FAo)Cr - K A C T A ~ ' (2)
for associated electrolytes. The symbols used here have the usual and accepted meaning.*' The computer programs used for the analyses have been described in detail elsewhere. 14,28 The iterated (e obtained from the J coefficierit was used to estimate the mean ion activity coefficient $. The computation used unweighted values of A since the measurements were carried out at approximately equal increments in
C. It is necessary to make some kind of viscosity correction since these large electrolytes affect the solution viscosity to a significant extent. Although it is not altogether clear as to what form this viscosity correction should take,29we decided to set the coefficient F in eq. 1 and 2 equal to the viscosity B coefficient as first suggested by Fuos~.~OIn any case, the correction has no effect on BO nor on the association constant and only results in small changes in J and (e. I n the most serious case (BurNPi), d is increased by 0.5, and in the case of Me4NI,by only O Z 3 1 The values of the viscosity B coefficient^'^ used were: Me4NBr, 0.58; Pr4NBr, 0.77; BurNBr, 0.93; R4e4NI, 0.52; Pr4NI, 0.71; Bu4NI, 0.87; MerNPi, 0.78; BupNPi, 1.13. The parameters obtained from an analysis of the conductance data are given in Table 11. The results for more than one run are included for some salts, but in each run the first entry in Table I1 gives the parameters obtained from eq. 1 and the second entry, the parameters obtained from eq. 2. Included are the standard deviations in each parameter and the standard deviation of the individual points, uA. It should be noted that only the result from eq. 1 is listed for Bu4NPi owing to the fact that eq. 2 gave a very ~
~
~~
(23) J. F. Swindells, C. F. Snyder, R. C. Hardy, and P. E. Golden, National Bureau of Standards Circular 440 Supplement, U. 5. Government Printing Office, Washington, D. C., 1968. (24) Cannon Instrument Co., State College, Pa. (25) G. A. Vidulich and R. L. Kay, J. Phgs. Chem., 6, 383 (1962). (26) A. A. Maryott and E. R. Smith, National Bureau of Standards Circular 614, U. S. Government Printing Office, Washington, D. C., 1951. (27) R. M. Fuoss and F. Accascina, "Electrolytic Conductance,'' Interscience Publishers, Inc., New York, N. Y., 1959. (28) R* J * Am* chm* 82* 2099 (lgeo). (29) Possibly the correct form for this correction would result from measuring the conductanceof these solutions with an inert substance present to give solutions of varying viscosity as suggested by E. R. Nightingale (private communication, 1964). Extrapolation to a viscosity equal to that of the solvent for each salt concentration would presumably give the corrected conductance. (30) R. M. Fuoss, J . Am. C h m . SOC.,79, 3301 (1957). (3f) This calculation required a value of &J/&which here depends pnmarily on ho and varies between 340 for Bu4NPi and 470 for MerNI.
CONDUCTANCE OF SYMMETRICAL TETRAALKYLAMMONIUM HALIDES AND PICRATES
3881
Table I: Measured Equivalent Conductances in Acetonitrile at 25" 104c
A
MerNBr
A
10% = 8.7 8.255 15.481 22.632 32.075 38.205 46.490 56.619
=
0.074
178.991 171.884 166.402 160.491 157.243 153.363 149.287
10'C
A
BQNBr 108~0= 8.8 A = 0.096 11.410 18.228 26.947 31.952 38.643 44.486 50.194
149.707 146.504 143.295 141,739 139.879 138,415 137.105
104c
A
BQNI
A
108Ko = 5.9 4.208 8.578 13,916 19.997 25.805 31.537 38.926 45.488
=
0.133
156.446 153.135 150.136 147.516 145.377 143.523 141.430 139.803
1 0 ' 1 ~= 6.2 PrdNBr
1 0 %= ~ 5.1
1 0 8 ~= 6.2
A = 0.086
3.725 9.715 15.575 22.243 29.839 37.652 44.949 52.267
163.728 158.978 155.750 152.859 150.104 147.695 145.699 143.913
lOSK0
=
157.964 154.370 151.680 148.536 145.698 143.268 141.336
5.9
4.449 9.901 17.271 25.399 33.685 41.946 47.199 56.450
154.394 150.526 146.968 144.666 142.422 140.775 138.743 137.700
162.972 158.889 154.994 151.681 148.901 146.536 145.166 142,984
4.058 10,345 14.937 19.395 25.521 34.077 42.878 48,591
156.620 152.114 149.743 147.813 145.549 142.875 140.531 139.173
MQNI
A
1 0 8 ~= 4.8
108x0 = 7.4 11.825 19.124 25.948 35.508 45.739 55.914 65.336
4.422 9.847 17.061 22.953 29.779 35.344 43,194 47.610
=
0.109
185.330 178,436 173,347 170.039 166.152 162.831 158.949 155.631
6.042 14.177 22.446 28.962 37.902 46.698 58.491 70.491
MerNPi lO?q = 4.7 A = 0.116 3.720 10.405 17.177 23.587 29,580 34.664 40.881 48.335
164.666 159.817 156.504 154.049 152.083 150.608 148.988 147,231
Pr'NI 10%.0 = 5.4
A = 0.121
BulNPi 1 0 8 ~= 5.1
4.184 9.869 17.383 23.832 30.825 38.904 47.283 53.297
165.038 160.734 156.734 154.121 151.713 149.308 147,123 145.701
2.594 8.073 13.688 19.758 25,835 34.631 41.806
A
=
0.124
133.860 129,971 127.132 124.866 122.933 120.712 119.110
1 0 ' 1 ~= 9.2 5.019 10.809 16.661 24,081 31.219 39.025 50.087 60.068
small and negative association constant, indicating a negligible amount of association.
164.319 160.099 157.065 153.984 151.526 149.199 146.359 144.184
The detailed results given in Table I1 are summarized in Table [I1 where the results of multiple runs on the Volume 69, Number I 1 November 1966
D. EVANS, C. ZAWOYSKI,AND R. KAY
3882
Table I1 : Conductance Parameters for Acetonitrile Solutions at 25" Salt
Ao
a
MerNBr
191.3 f 0.7 195.22 f 0.07
0.21 f 0.05 4.4 i 0.2
170.84 f 0.06 171.09 f 0.05 170.87 f 0.05 171.19 f 0.04 170.82 f 0.04 171.01 f 0.03
2.72f 0.04 3.7 f 0.2 2.83 f 0.03 3.3 f 0.1 2.75 f 0.03 3.4 f 0.1
162.01 f 0.02 162.09 & 0.04 162.01 f 0.01 162.10 f 0.01
3.20 f 0.02 3.5 zt 0.2 3,lQf 0.01 3.45 f 0.04
PrrNBr
BurNBr
MerNI
195.03 f 0.3 1.2 f 0.1 196.73 f 0.05 3.5 f 0.1
PrrNI
172.63 i 0.05 172.86 f 0.05 172.60 f 0.07 172.90 0.06
2.87 f 0.04 3.7 i 0.2 2.86 i 0.04 3.8 f 0.2
163.92 i 0.03 164.04 f 0.04 163.95 f 0.02 164.06 f 0.02
3.09 f 0.03 3.7 f 0.2 3.14 f 0.02 3.6 f 0.1
171.78 i 0.02 171.84 f 0.03
3.46 & 0.01 3.7 f 0.1
*
BurNI
MerNPi BurNPi
J
.A
i1
2430
0.8 0.03
5.4 k 0.9
1880
3.3 f 0.6
1725
0.03 0.05 0.02
3.6
0.5
1750
0.06
1.8 & 0.8
1730
1.4 f 0.2
1700
0.03 0.02 0.01 0.004
19.3 f 0.5
2050
0.5 0.03
KA
46
0.08
0.08 0.03 0.1 0.04
4.5 f 0.9 1895 f l
1920
3.1 f 0.9
1810
2.4 f 0.5
1770
0.04 0.02 0.03 0.01
1.2 f 0.6
1890
0.02 0.02
I690
0.07
5
139.40 i 0.04 4.05 f 0.05
Table I11 : Averaged Conductance Parameters for Acetonitrile Solutions Sdt
Ao
Me4NBr Pr4NBr BudNBr
a
KA
195.2 171.1 162.1
4.4 3.4 3.5
46 4 2
MeaNI Pr4NI BuaNI
196.7 172.9 164 0
3.5 3.8 3.6
19 5 3
MerNPi Bu4NPi
171.8 139.4
3.7 4.0
1 0
I
same salt have been averaged by weighting each parameter by its standard deviation. The values of CY, B, El, and E2 for acetonitrile are 0.7374, 230.9, 5.474, and 251.0, respectively, where S and E in eq. 1 and 2 are given by S = aAo B and E = El&- Ez.
+
Discussion A calculation of the conductance differences at infinite dilution between bromides and iodides and between tetramethyl- and tetrabutylammonium salts indicates an uncertainty in A. of 0.2 conductance units The J O U Tof~Physical ChernbtTy
or 0.1%. In comparison to other results in nonaqueous solvents, this is quite satisfactory but somewhat greater than the experimental error expected. The cause cannot be attributed to salt impurities since the same halide salts were found to give excellent agreement in the ion conductances for aqueous4 and methano12 solutions where transference data are available. We attribute this deviation in additivity of ionic conductance a t infinite dilution to very slight amounts of impurities, possibly water, in the acetonitrile. Our differencesbetween the tetrapropyl and tetrabutyl salts of 8.9 f 0.1 and between the tetramethyl and tetrabutyl salts of 32.8 0.2 compare favorably with the values 9.0 and 32.8 obtained by Berns and Fuoss'O from a series of tetraphenylboride salts. Also, our average difference of 24.8 =t 0.1 between the iodides and the picrates with the same cation can be compared to the 25.0 reported by Coplan and F u o s P obtained from triisoamylbutylammonium salts. A direct comparison of our A. with those reported by other workers is not nearly so favorable. Before such a comparison can be made, it is necessary to recompute the data from the literature so as to bring them into conformity with the Fuoss-Onsager conductance equations. The results of such an analysis for all of the literature data for acetonitrile solutions that have the necessary precision are given in Table IV. A comparison of these results with those in Table I1 shows that our values of A. for hIerKBr, l\le4NI, Pr4NI, and Me4NPi are considerably higher (1-3 conductance units) than those measured by Popov and Skellys and by Walden and BirrJ6whereas our value for Bur NPi is two conductance units Ion-er than that calculated from the measurements of French and hhggleton.' The fact that our precision is considerably better and the fact that the ion conductances obtained from our salts in methanol and water are in agreement with transference data add considerable weight to the reliability of our data. An inspection of Table I11 shows that, with the exception of I~!te&Br,~~ d values obtained for all of our salts are almost identical and equal to 3.6 0.2. The same behavior was observed in the similar solvent, nitromethane,34where an d of 3.9 f 0.3 was obtained for the tetramethyl- through tetrabutylammonium chlorides and bromides if the viscosity correction, F , is
*
*
(32) M. A. Coplan and R. M. Fuoss, J . Phya. Chem., 68, 1181 (19134). (33) The high value of 4.4 for MetNBr could be the result of a poor split between the term linear in C in eq. 2 and the association term. If K A for this salt was reduced by only 3 units to 43, an 6 of 3.6
would result. This illustrates the precision required for the accurate determination of L. (34) R. L. Kay, S. C. Blum, and H. I. Schiff, J . Phys. Chem., 67, 1223 (1963).
CONDUCTANCE OF SYMMETRICAL TETRAALKYLAMMONIUM HALIDESAND PICRATES
Table IV : Conductance Parameters for Acetonitrile Solutions at 25" Salt
Me4NCl Me4NBr Me4Nl EtrNI Pr4NI
AQ
192.9 193.2 194.6 187.29 169.9 172.0
f0.4 f0.4 f0.3 f 0.06 f 0.1" f 0.3
1.9 3 1.3 3.3 6.7 3.3
f 0.6 f 0.3 f 0.5" f 0.5
Me4NPi Et4NPi PrdNPi Bu4NPi AqNPi
170.81 164.62 147.2 141.28 135.1
f 0.09 f 0.07 f 0.1" f 0.06 zk 0.1
5 5.0 5.1 4 8
f1 f 0.9 f 0.2" f2 f 2
MerNBPh4 EtrNBPL Pr4NBPh4 BurNBPh4
152.28 f 0.02" 142.77 f 0.03" 128.41 f 0.006" 119.49 f 0.04"
4.6 4.9 5.3 5.3
f 0.1"
LiC104 NaClO4 KC104 RbC104 csc104
182.1 191.2 205.7 201.4 204.5
3.1 3.5 2.4 8 7
& 1.6
KI
186.6 f 0.1'
f0.4 f 0.4 f0.5 f0.8 f0.8
f 0.8 f1
Ref.
RA
C i
56 f 36f 5 f 5 f
6 6 4 2
3 f 3 8 f 4 10 f 4 13f 6 39 f 7
2.27 f 0.08"
5
7 5 7 7 10 10 10 10
f 0.1" f 0.1" f 0.1" f2.0 f 1.6 f9 f5
9 9 9 8 5 9
24 23 33 55 63
f 8 f 9 f 9 f 31 f 18
36 36 36 36 36
17
' Parameters obtained from eq. 1; eq. 2 gives negative K A .
assumed to be the same as that for acetonitrile solutions. There appears to be no relationship between d and the contact distance between ions since the ion sizes in the salts involved here vary considerably. It should be noted in Table I1 that it is d and not J that is constant for each salt. Thus, since J is a function of A. and d, it appears only necessary to remove A. in order to obtain a relaxation effect that is independent of the particular salt under consideration. However, it should be noted that higher d values are obtained from the data for the tetraalkylammonium tetraphenylborides. There is an indication of a decrease in d with decreasing cation size, but this could be a reflection of a small amount of association as the cation size decreases. It can be seen in Table I1 that, with the exception of the picrates, C T ~ is lower for the results from eq. 2 than those obtained from eq. 1. This is the best criterion for association available at present. Only the tetramethylammonium salts show any appreciable amount of association, but all salts follow the same association pattern; namely, association decreases as the ion size increases. We do not consider the small increase in K A for the tetrapropyl- and tetrabutylammonium iodides over that for the bromides to be
3883
real but rather the result of experimental error. This association behavior has been confirmed in a number of other investigations. Salts involving very large ions, such as the tetraalkylammonium tetraphenylborides'" and B U ~ N P Fhave ~ , ~ been ~ found to be unassociated, whereas Me4NPFP shows signs of very slight association in acetonitrile. The same behavior appears to be followed by all of the data found in the literature as recorded in Table IV with the exception of those of French and Muggleton' for the picrates, which are in direct contrast to our results. We found no association for Bu4NPi in acetonitrile. The data of Popov and co-workersg for the tetramethylammonium halides show appreciable association, but similar data for the trihalides show no signs of association. The association constants for the alkali perchlorates in acetonitrile, as reported by NIinc and Werblan,36 increase as the cation size increases, but a recalculation of their data on the basis of eq. 2, as given in Table IV, indicates that, although these salts appear to be associated to some extent, the precision of the measurements does not permit much to be said about the change in K A with cation size. The K A values for RbC104 and CsC104 appear to be somewhat higher than the others, but this is undoubtedly caused by a poor separation between the association term and the term linear in C in eq. 2. If d is set equal to about 3.5 for these salts, association constants of about 40 f 30 and 50 f 20 result, making the KA values for all the perchlorates identical within the precision of the measurements. Owing to the large uncertainty in these results for the alkali perchlorates, the question of the association behavior of the alkali metal salts in acetonitrile is still an open question. The same association behavior is observed in nitromethane solutions34 in that ?\Ie4SC1 and Me4NBr have association constants of 45 f 1 and 31 f 1, respectively, Et4NC1 and Et4NBr are only slightly associated, K A 5 2, and the halides of the larger quaternary cations show no signs of association. The conductance data of Taylor and Kraus3' and Witschonke and Kraus3* for nitrobenzene solutions have been recalculated to conform to the FuossOnsager theory and are given in Table V. It should be noted that for both the symmetrical tetraalkylammonium and the alkali picrates, the association constant increases with decreasing ion size in keeping (35)J. Eliassaf, R. M. Fuoss, and J. E. Lind, Jr., J. Phys. Chem., 67, 1941 (1963). (36)9.Minc and L. Werblan, Electrochim. Acta, 7,57 (1962). (37) E. G. Taylor and C. A. Kraus, J. Am. Chem. Soc., 69, 1731 (1947). (38) C. R. Witsohonke and C. A. Kraus, ibid., 69, 2472 (1947).
Volume 69,Number 11
November 1966
3884
D. EVANS,C. ZAWOYSKI,AND R. KAY
with our results for acetonitrile. This is in contrast It is of interest to point out the higher association to what is found for the alkali metal salts in hydrogenof salts in nitrobenzene as compared to those for the bonded solvents like the alcohols.28 same salts in acetonitrile and n i t r ~ m e t h a n e solvents ,~~ The association pattern found for acetonitrile and with approximately the same dielectric constant. It nitrobenzene can be confirmed by comparison with the is difficult to explain this result without resorting to results for a similar solvent of lower dielectric constant some kind of specific solvation effects. For example, such as acetoneagrMas shown in Table V. Here, associathe high association constant for B u ~ N inB nitro~ ~ ~ ~ ~ ~ tion is much more extensive, and the decrease in asbenzene has been attributed to the formation of a sociation as the ion size increases can be clearly seen. nitrobenzene ion pair complex. However, it is difThere is one notable exception: K I is less associated ficult to attribute the extreme association of the alkali than potassium picrate by a considerable amount. metal picrates to the formation of such complexes. This is confirmed by Walden's41 result for NaI as We feel that a more likely explanation lies in assuming compared to Kraus' value for NaPi. This inversion that an almost complete lack of solvation of the small in association behavior has been noted by K r a u ~ , ~alkali ~ metal and halide ions in nitrobenzene accounts but he gave no explanation. It is difficult to attribute for the large association. Of course, the possibility the result to an error in measurement since the precision exists that nitrobenzene was the only anhydrous of the measurements involved appears to be quite solvent studied and that in acetonitrile and nitroadequate. We attribute the small association of K I methane the hydrated ions were being investigated. to small amounts of water in the acetone, a solvent A 0.01 wt. % concentration of water would result in that is extremely difficult to obtain in a completely an equimolar mixture of salt and water at the highest concentrations studied. anhydrous state. It is appropriate at this point to bring up the question of small association constants, many of which were Table V : Conductance Parameters for Nitrobenzene and obtained in this investigation. I t is possible to Acetone Solutions at 25' separate out the association term only with data of the Ref. Salt Ao a KA highest precision if K A is less than 5. It is possible that these so-called association constants are mere Nitrobenzene artifacts of the conductance theory. However, the 38 81 f 6 38.54 f 0.03 7 f2 EkNCl consistency of the association pattern reported here 38 56 f 3 33.46 * 0.02 4.9 f 0.7 BurNBr for the more precise data and the fact that the associa38 6.9 X 1 O P NHrPi 23.6 f 0.9 37 33.34 f 0.01 3.6 f 0.2 McNPi tion changes as predicted for a decrease in the dielectric 7.4 f 0.4 37 3.89 f 0.08 32.43 * 0.003 EtNPi constant add considerable weight to the reliability of 3 f l 37 29.48 f 0.007 3.1 f 0.2 PrtNPi 2.6 f 0.9 37 27.86 f 0.004 3.3 f 0.2 ButNPi these numbers as being at least proportional to association constants. It was hoped that by studying as1.7 x 10'" 38 LiPi 3.6 x 104" 38 NaPi sociation in mixed solvents a t lower dielectric constants 1.46 X lo* 38 KPi it would be possible to extrapolate back and determine Acetone the association constants for solvents of higher di0.8 f 0.5 1140 f 40 39 182.7 0.3 MetNF electric constant like acetonitrile. However, log K , 16 f 2 41 370 f 20 194.2 f 0.2 EttNCl us. 1/e plots are generally not linear but instead are 5.2 f 0 . 6 162 f 7 40 190.72 f 0.03 PrtNI 5.4 f 0 . 2 264 f 5 183.2 f 0.1 39 ButNBr curved, owing possibly to specific solvent effects, 6.1 f 0.3 143 f 6 39 180.3 f 0.2 BurNI with the result that the extrapolation of solvents 7 f l 220 f 20 40 174.7 f 0 . 1 AmrNBr of higher dielectric constant can be ambiguous. 183.4 f 0.1 5.8 f 0.1 67 f 1 40 McNPi Very recently Harkness and Daggett4* have pub45 * 2 39 6.3 f 0.1 176.65 f 0.03 EtrNPi 41 27 f 8 5.2 f 0.4 156.2 f 0.1 PrrNPi lished their work on the conductance of the tetraalkyl152.34 f 0.08 17 f 5 4.6 f 0.2 39 BurNPi ammonium halides in acetonitrile at 25". Before 80 f 4 5.8 f 0.2 39 182.8 f 0.1 ButNClOt BUNNO:
187.11 f 0.007
4.96 f 0.02
143.1 f
LiPi NaPi KPi
157.7 f 0.2 163.5 f 0.2 166.0 f 0.1
1.5 f O . 1 5.4 f 0.3 5.0 f 0.2
819 680 244
KI
192.9 f 0.2 184.8 f 0.1
5.2 f 0.2 7.1 f 0.8
98 110
f
* 10
5
39 41
NaI
183.6 f 0.2
177
f 15
41
14
f1
" From eq. 2 with E, J,and B set equal to zero.
The Joumol of Physiccrl Chemietry
0.4
f 9
f 9 f 5
39 39 39 39
(39) M. B. Reynolds and C.A. Kraus, J. Am. C h a . Soc., 70, 1709 (1948). (40) M. J. McDowell and C. A. Kraus, ibid., 73, 3293 (1951). (41) P. Walden, H. Ulich, and G . Busoh, 2. physik. Chem., 123, 429 (1926). (42) J. B. Hyne, J. Am. C h a . SOC.,85,304 (1963). (43) R.L.Kay and D. F. Evans, ibid., 86,2748 (1964). (44) A. C. Harkness and H. M. Daggett, Jr., Can. J. Chem., 43, 1215 (1965).
CONDUCTANCE OF SYMMETRICAL TETRAALKYLAMMONIUM HALIDESAND PICRATES
these results could be compared to those reported here, it was necessary to recompute their data to bring the conductance parameters into conformity with the Fuoss-Onsager theory. Owing to the large proportion of data for extremely dilute solutions, a better fit was obtained by weighting the conductance values by C. The results are given in Table VI. Their A. values are within 0.2 unit of our values except for Bu4NBr, Me4"I, and Bu4NI, in which case their values are considerably higher. Their differences in AO between iodide and bromide average 1.7 f 0.1 if their very high result of 2.7 for the tetramethylammonium salts is ignored. This is in excellent agreement with the value 1.7 f 0.2 calculated from our results in Table 111. This would indicate that our value of 196.7 is the more nearly correct AOfor Me4NI. Also, the check with the data of Coplan and Fuoss as described above makes our AO values for the tetrabutylammonium salts the preferred values. The d and Ka values reported in Table VI show such large uncertainties, owing to lack of sufficient data for higher concentrations, that a r'gor )us interpretation is not possible. However, our conclusion that the larger the ions the less the degree of association in acetonitrile
3885
Table VI : Conductance Parameters of Harkneee and Daggett for Acetonitrile Solutions at 25" salt
AQ
z
Me4NBr EtrNBr Pr4NBr BQNBr AmNBr
195.0 f 0 . 5 185.3 f 0 . 1 171.2 f 0 . 1 162.9 f 0.04 156.8 f 0 . 2
4 f1 4 f 0 . 5 11 f 3 4 . 4 f 0.02 11 f 2
MerNI EtrNI Pr4NI Bu~NI AwNI
197.7 i 0 . 2 186.9 f 0 . 2 173.1 f 0 . 2 164.0 f 0 . 1 158.4 f 0 . 3
5.0 3.6 4 5 8
f0.5 f0.6
f2 f l f1
KA
40 1 0 33 6.6 23
f5
26 5 6 8 20
f 2 f3
1 4 f5 f0.1 f5
f 8 f 5 f5
solutions is verified by the tetramethylammonium halides. The abnormally high K A value obtained for PrrNBr, Am4NBr, and Am4NI can be attributed to a poor split between the last two terms of eq. 2 as indicated by the exceptionally large values of d.
Acknowledgment. This work was supported by the Office of Saline Water, U. S. Department of the Interior, under Contract 14-01-0001-359.
Volume 69, Number 11 Nmember 1986
