4208
R. KAY,C. ZAWOYSKI,AND D. EVANS
The Conductance of the Symmetrical Tetsaalkylammonium Halides
and Picrates in Methanol at 25 and 10’’
by Robert L. Kay, C. Zawoyski, and D. Fennel1 Evans Mellon Institute, Pittsburgh, Pennsylvania
(Received June 88, 1966)
Conductance measurements are reported for Me4NCI, BQNCI, Me4NBr, Et4NBr, PrdNBr, BQNBr, n-AmdNBr, Me4NI, Pr4NI, BudNI, n-Am4NI, Me4NPi, and Bu4NPi in methanol solutions at 25’ and the bromides and iodides at 10”. The ion-size parameter d had a constant value of 3.6 k 0.2 at 25” and 3.5 & 0.2 at 10’ for all the salts. Only the tetramethyl- and tetraethylammonium halides are significantly associated among the bromides and chlorides, but all of the picrates and iodides and even the large tetraphenylborides are definitely associated in methanol solutions. A decrease in temperature has little effect on the association of the iodides but decreases the association of the bromides in proportion to the ETproduct. Dielectric constants of 38.01, 35.70, and 32.62 are reported for methanol at 0, 10, and 2 5 O , respectively.
Introduction The conductance of the tetraalkylammonium halides in aqueous solution has been found to be abnormal2 in that the conductance decrease with concentration becomes greater as the anion size increases, in contrast t o the predictions of the Fuoss-Onsager theory.8 It is possible to explain this abnormal conductance decrease by an appeal to ion association that increases as the anion size increases. The same type of abnormal behavior was found by Lindenbaum and Boyd4 for the activity coefficients of these salts in aqueous solution, which they explained by ion association. However, they postulated that ionic association was enhanced to a considerable extent by water-structural effects rather than by simple coulombic interaction, In this paper, we report the association behavior of these salts in methanol solutions as obtained from conductance measurements at two temperatures. Methanol is a structured solvent with a dielectric constant in the intermediate range (32.6) that is not too different from that of acetonitrile (36.0). We have determined the association behavior of these salts in acetonitrile5 and found the association to depend primarily on ion size, increasing as the crystallographic size of both anion and cation decreases. The reverse behavior is reported here for the same salts in methanol sohtions; that is, the association behavior is in agreement The Journal of Physical chemistry
with that required to explain the results for aqueous solutions. A considerable amount of data from the literature for methanol and other alcohol solutions has been recalculated to bring it into conformity with the Fuoss-Onsager theory and is reported here for comparison purposes.
Experimental Section The resistance measurements and weighings were carried out with a precision of better than 0.02%. The details of the conductance bridge and cells have been described elsewhere,6r6 as has the procedure followed in making the measurements, with the exceptions as noted below. The cells were of the Erleameyer type and contained 500 ml, of solution, to which the weighed salt wm added in Pyrex cups from the dispensing device in a closed system.6 All resistance measurements were corrected for the usual small frequency dependence. (1) Presented in part at the 147th National Meeting of the American Chemical Society, Chicago, Ill., Sept. 1964. (2) D. E’. Evans and R. L.Kay, J . Phys. Chem., in press. (3) R. M. Fuoss and F. Accascina, “Electrolytic Conductance,” Interscience Publishers, Inc., New York, N. Y., 1959. (4) S. Lindenbaum and G. E. Boyd, J . Phys. Chem., 68, 911 (1964). (5) D.F. Evans, C. Zawoyski, and R. L. Kay, ibid., 69,3878 (1965). (0) J. L. Hawes and R. L. Kay, $bid.,69, 2420 (1965).
CONDUCTANCE O F SYMMETRICAL TETRAALKYLAMMONITJM HALIDES AND
All salts used for the measurements at 25", with the exception of the chlorides, were the same samples as were used for the acetonitrile6 and aqueous2 solutions. The measurements at 10" were carried out with freshly prepared salts purified as previously describeda6 The chlorides were prepared by metathesis of the corresponding purified iodides by AgCl in methanol, the absence of iodide being determined by the starch test, The resulting AgI was removed by a fine, fritted-glass filter. Rlle4NC1 was recrystallized twice from methanol by the addition of peroxide-free ether and dried at 140" in a vacuum oven, while Bu4NC1 was recrystallized twice from acetone by the addition of ether and dried at 56" in a vacuum oven. Owing to the extreme hygroscopic nature of these chlorides, extreme care was taken to avoid contact of these salts with water vapor. All manipulations involving the chlorides were carried out in a, drybox with appropriate precautions to avoid explosive concentrations of ether and acetone in the drybox atmosphere. We are indebted to Dr. J. Gordon for samples of his highly purified n-Am4NBr and n-Am4NI. All solutions were prepared by weight and vacuum corrected. Owing to the hygroscopic nature of the chlorides, the usual 11-mm. Pyrex cups containing these salts were weighed in capped, soft-glass, 7-ml. standard weighing bottles that had been reduced in size by a factor of 3. All salt transfers involving the chlorides were carried out in a drybox. The salt cup dispensing device was of considerable help during the measurements on these hygroscopic salts. Extremely high solvent conductances were encountered initially during measurements at 10". This was found to be due to the contraction of the solvent on cooling which pulled air into the cells. The problem was overcome by maintaining positive pressure of Nz in the cells during the cooling process. Reagent grade methanol was purified by passage through reagent grade, mixed-bed, ion-exchange resin, followed by a fractional distillation in a 1.2-m. Stedman column. The ion-exchange resin had been previously treated with methanol until all water had been removed. Titration with Karl Fischer reagent indicated no more than 0.01 wt. % water in the final product, which had a specific conductance of 5-12 X at 25". The measured density was 0.78658 at 25" and 0.80073 g. ml.--l at 10". The viscosity at 10" was found to be 0.672 cp. as measured with a Ubbelohde-type viscometer, using as standard the methanol viscosity at 25" of 0.5445 C P . ~ More recent measurements, carried out in this laboratory using a different viscometer of the same type, indicate this value to be low and 0.676 cp. to be more nearly correct at 10". All calculations
PICRATES
4209
reported here were carried out using the lower value since it was found that this small change in the solvent viscosity had a negligible effect on the conductance parameters. The absolute dielectric constant of methanol was measured in the completely guarded three-terminal dielectric cells of Vidulich and Kay,s using the General Radio Type 1615A transformer bridge. The cell constant of approximately 2.2 pf. was determined by direct measurement and corrected to vacuum. All measurements were carried out at 10 kHz. since there was a negligible frequency effect. The values of E obtained at 0, 10, and 25" were 38.01, 35.70, and 32.62, respectively, Our result at 25" is in excellent agreement with that of Gosting and Albrights and of Jones and Davies,1° but at every temperature our values are about 1% higher than those reported by Koizumi and Hanai. l1 The absolute temperature at 10" was determined within 10.002" by means of a calibrated platinum resistance thermometer and a Mueller bridge, as was the case at 25". The cell constant at 10" was determined by measuring the conductance of aqueous KC1 solutions and comparing the A, with that predicted by the conductance equation for the temperature dependence of the conductance of KC1 between 5 and 55", as given by Harned and 0wen.l2 The resulting change in the cell constant between 25 and 10" was less than O.Ol~oand in good agreement with the change calculated from cell geometry and coefficients of expansion as given by Stokes and Robinson.13
Results The equivalent conductances and concentrations given in Table I for 25" and in Table I1 for 10". Included with each run is the solvent specific conductance, m, in ohm+ cm.-1. The values of the density increments A are given for each salt. These were obtained at 25" from density measurements on the most concentrated solution studied in the conductance with the assumption that solution densities follow
(M)are
(7) G.Jonea and H. J. Fornwalt, J . Am. Chem. Soc., 60, 1683 (1938). (8) G.A. Vidulich and R. L. Kay, J . Phvs. Chem., 66, 383 (1962). (9) P. S. Albright and L. J. Gosting, J . Am. Chem. Soc., 68, 1061 (1946). (10) T.T. Jones and R. M. Davies, Phil. Mug., 28, 307 (1939). (11) N. Koizumi and T. Hanai, Bull. Inst. Chem. Res., Kyoto Univ., 33, 14 (1955). (12) H. S. Harned and B. B. Owen, "The Physical Chemistry of Electrolytic Solutions," 3rd Ed., Reinhold Publishing Corp., New York, N. Y.,1958, p. 233. (13) R. A. Robinson and R. H. Stokes, "Electrolyte Solutions," 2nd Ed., Butterworth and Co. Ltd., London, 1959, p. 97.
Volume 69, Number 12 December 1966
42 10
R. KAY,C. ZAWOYSKI,AND D. EVANS
Table I: Equivalent Conductances in Methanol a t 25" 104c
A
104c
-Me4NC1-A
108Ko = 7.6
0.031
10.316 20,240 30.756 46 883 54.731 65.237 75.502 86.136
111.395 107.463 104.419 100.865 99.442 97.758 96.289 94.915
I
=
-Bu4NC1A =
~ O * K O= 4.6
0.050
9.249 18.099 26.945 36 267 47.003 56,495 65.192
84.127 81.538 79.582 77.981 76.441 75.264 74.311
I
--MedNBr108KO
A
=
5.2
=
0.062
4.905 118.394 14.737 112.903 23.854 109.495 34.779 106.320 45.787 103.730 56.648 101.555
69.611 79.155
104c
A
99.354 97.911
-Et4NBr1O8Ko
A
7.3
0.078
2.980 10 329 17.025 25.154 31.763 39.165 46.246 56.429
112.094 107.619 104.908 102,400 100.700 99.056 97.661 95.909
I
-Pr4NBrA
108K0 = 5.7
0.083
2.848 7.987 15.165 21.093 27.080 33.060 40 228 49.851
98.171 95.101 92.268 90,469 88.962 87.645 86.254 84.638
I
=
--BupNBr-108~0= A = 7.2
0,088
3.962 9.633
90.416 87.641
15.659 21.157 26.883 32.638 38.487 43,995
A
104~
85.528 83.984 82.630 81.441 80.360 79.432
1 O * ~ o= 7.3
A =
3.307 8.084 12.526 17,185 23.041 28.310 33.398 40.387 46.689
90.882 88.285 86.552 85.052 83.504 82.306 81.269 80.017 79.004
0.088
---n-Am4NBrlo*Ko =
9.1 3.485 8.719 14.996 21.704 28.395 35.277 41.581 50.592
A = 0.104 86.851 84.179 81,973 80.111 78.598 77.264 76.190 74.821
+
the linear relationship d = do A@zwhere @z is the concentration in moles per kilogram of solution. The values of A at 10" were assumed to be the same as those at 25', and measurement on a few salts indicated that the difference in A at the two temperatures was no greater than 0.005. The conductance parametern for both 25 and lo", obtained from least-squares analyses6 of the conductance data using a computer and the Fuoss-Onsagers equations in the form A =
A0
- L ~ C ' '+~ EC log C + (J - FA0)C
(1)
or, if the electrolytes were associated, in the form A =
A0
- S(Cr)*"
+ EC? log Cy + (J - FA0)Cy - KACrAf'
(2)
are given in Table 111. Included in Table 111 is uA, the standard deviation of the individual points. With two exceptions, it was found that treating the salts as The Journal of Physical Chemistry
A
104~
---Me4NI108Ko = 8.6
A
=
0.076
5.992 123.367 12.902 119.178 20.601 115.765 28.750 112.903 37.408 110.358 47.709 107.822 59.591 105.307 72.009 103.043
-PreNI~ O * K O= 5.1
A = 0 * 091
A
a
0.091
3.887 103.226 8.307 100.419 13.849 97.834 19.862 95.578 25 424 93.856 I
1040
91.769 90.411 88.745
-Bu~NI-A =
108Ko = 5.4
0.100
3.830 8.292 14.557 20.478 27.594 33 297 40.665 47.272
96.347 93.622 90.854 88 802 86.757 85,343 83.732 82.457
A = 0.107 110.867 107.892 105.414 103.270 101.346 99.726 97.741 96 167 I
I
--n-Arn4NI--losKo =
fi =
8.0
0.108
4.620 10.142 15.663 22.633 30.654 37.199 44.033 52,144 63.656 75.496 85.661 97.002 107.414
A
--Me4NPi108Ko = 9.6 3 272 7.621 12.934 18.394 24.598 30.699 39.336 46.763 I
I
2.949 104.108 8.956 100.041 14.576 97.467 20.591 95.285 25.988 93.632 33.069 91.773 38.882 90.426 48.875 88.414 108K0 G 6.0
33.319 39.190 47.330
A
91.602 88,593 86.346 84.106 81.978 80.497 79.119 77.661 75.942 74,303 73.067 71.765 70.683
108Ko = 7.0 4.255 9.522 14.640 20.254 26.319 33.076 41.405 49.332
A
=
0.107 109.880 106.797 104.553 102.571 100.785 99.064 97.224 95.704
---BuhNPi-A
108Ko =
11.2 2.848 8.135 14.507 20.985 27.245 34.023 42.093 50.460
0.117 82.021 79.120 76.768 74.831 73.331 71.929 70.473 69 167 I
unassociated electrolytes resulted in uAthat were from 2 to 14 times larger than those resulting from assuming association and using eq. 2, We have considered this a reliable criterion for association and have not included the results for the analyses with eq. 1 except for Bu4NC1and BurNBr at 10'. The constants a,p, El, and E#had the values 0.8545, 153.59, 7.350, and 103.5 at 25" and 0,8206, 122,76, 6.778, and 148.5 at lo", respectively, where, in eq. I and 2, X = a& f p and E = E& - Ez. The quantity F in eq. 1 and 2 corrects for the increase in the solution viscosity due to the addition of salt. For the large salts involved here, the increase in viscosity is not negligible, although not nearly so large as in aqueous solutions. We have followed Fuossa.14 in setting F equal to the viscosity B coefficient. It should be remembered that only J and, (14) R. M. Fuoss, J . Am. Chem. Soc., 79, 3301 (1967).
CONDUCTANCE OF SYMMETRICAL TETRAALKYLAXIMONXJM HALIDESAND PICRATES
Table 11: Equivalent Conductances in Methanol a t 10' 10'C
A
104c
A
104c
A
---LM~~NB~108x0 = 10.5
1 0 8 K o = 5.5
-Pr+NI----. 1 O 8 ~ 0= 11.3
7.529 16,632 26.810 41.456 55.743 70.024 80.714 96.549
3.371 9.017 15.065 22.761 31.049 40.763 47.778 55.722
3.963 8.885 15,578 23.224 30.623 38.866 47.583 55.635
95.165 91.627 88.803 85.732 83.364 81.254 79.978 78.305
--BurNBr73.348 71.066 69.421 67.902 66.218 64.778 63.865 62.955
10'~o = 6.0
1 0 8 K o = 5.7
1 O s ~ 0=
8.705 19.461 30.507 39.969 50.342 60.620 74.463 91.411
4.341 8.995 114.853 21.146 29.087 36.743 46.135 57.330
4.020 10.670 19.000 26.058 44.175 55.400 62.337
94.594 90.758 87.939 86.008 84.189 82.637 80.819 78.925
-Et4NBr108KO = 5.7 5.319 13.953 22.662 37.351 44.942 53.767 62.351 72.437
89.727 86.426 84.062 81.155 79.955 78.674 77.606 76.422
72.892 71.107 69.409 67.976 66.590 65.324 64.064 62.767
3.525 13.198 113.846 24.658 32.001 38.540 44.508 51.991
3.923 9.019 15.201 22.972 30.285 38.889 48.272 58.348
73.321 69.887 68.526 67.339 66.088 65.114 64.314 63.390
--Me4NI--9.6
l o s K o = 12.1
6.820 100.165 14.074 96.850 20.297 94.683 28.390 92.381 38.507 90.006 48.278 88.061 67.462 86.474 71.812 84.363
=i
I
83.617 80.519 77.839 76.064 72.595 70.914 69.995
------BU~NI-10'KO = 10.2
lOS~o 78.484 76 237 73.844 72.278 70.693 68.971 67.831 66 603
5.2
10%0 = 4.6
-PnNB4.948 10.982 19.684 27.340 36.689 48.239 57.756 69.405
83.514 81.160 78.901 76.679 75.012 73.434 71.972 70.781
78.044 75.632 73,539 71.439 69.838 68.239 66.818 65.409
108~0= 5 . 0 4.049 9.270 15.256 22.209 29.516 37.538 44.437 50.891
77.658 75.204 73.214 71.329 69.722 68.216 67.063 66.115
I
consequently, the ion-size parameter d are affected by the particuIar choice for F. The viscosity B coefficients used are given in Ta,ble IV. They are based on the values for Me4NBr and Bu4NBr at Tuan and FUOSP 25O, on our own measurements for Bu~NI,Bu4NHr, Me4NI, and Me4NBr at 25 and lo", on Jones and Fornwalt's'e difference in B for KC1 and KBr of 0.024, and on the assumptions that interpolated values for the tetraethyl- and tetrapropylammonium salts are valid and that B for the picrate ion in methanol is close to the value obtained for acetonitrile solutions.l6 In any case, the total change in zi due to inclusion of the
4211
F term is small, amounting to ordy 0.15 A. for MerNBr and 0.25 8. for Bu4NBr at both temperatures. The B coefficients are based on a molar concentration scale. The results listed in Table 111 are reduced to a more concise form in Table V, where the conductance parameters resulting from multiple runs have been averaged after weighting each value of the parameter by its standard deviation.
Discussion Ion Conductances. The limiting ionic conductances for the quaternary ammonium ions in methanol solutions at 25" can be calculated from our results since transference data for IiCI,l7 NaCl,I7 and LiCI1* and precise conductance data for most of the alkali hali d e are ~ available. ~ ~ ~ ~From ~ these data, the average values of 52.36, 56.45, and 62.78 were obtained for Xo for the chloride, bromide, and iodide ions, respectively, after recomputation by the Fuoss-Onsager theory.21 When these values are subtracted from the corresponding A, for the quaternary ammonium halides given in Table V, the limiting cation conductances in Table VI result. The agreement is as good as can be expected for the bromides and iodides, with the exception of the n-Am4N+ ion. The values obtained from the chlorides are not in so good agreement as we would hope, but we attribute this to the experimental difficulties resulting from their extremely hygroscopic nature. The agreement with the halides reported by HartleyZ2and eo-workers is within the precision of their data after recomputation to bring them into agreement with eq. 1. Knox and 33ve1-s~~ measured the quaternary ammonium picrates in methanol at 25", and their value of ho(RPi) = 99.71 (after recomputation with eq. 1) results in Xo(Pi-) = 47.27. Their resulting X o f values for the quaternary ammonium ions are given in Table VI and are somewhat lower than our values. We get Xo(Pi-) = 47.07 from the Me4N+ salt and 47.20 from the BulN+ salt (average value Xo (Pi-) = 47.14 f 0.07). If our value of X,,(Pi-) is (15) D. F.-T. Tuan and R. M. Fuoss, J . Phys. Chem., 67, 1343 (1963). (16) G. Jones and H. J. Fornwalt, J . Am. Chem. SOC.,57, 2041
(1935). (17) J. A. Davies, R. L.Kay, and A. R. Gordon, J. Chem. Phys., 19, 749 (1951). (18) G. A. Vidulich and R. L. Kay, t o be published. (19) J. P. Butler, H. I. Schiff, and A. R. Gordon, J . Chem. Phys., 19, 752 (1951). (20) R. E. Jervis, D. R. Muir, J. P. Butler, and A. R. Gordon, J . Am. Chem. SOC.,75, 2865 (1953). (21) R. IJ Kay, ibid., 82, 2099 (1960). (22) See footnote a of Table VI. (23) See footnote b of Table VI.
Volume 69, Number 1% December 1966
4212
R. KAY,C. ZAWOYSKI,AND D. EVANS
Table 111: Conductance Parameters for Methanol Solutions a t 25 and 10' Salt
J
uA
MeaNCl Bu4NC1
120.82f0.06 91.38f0.04"
3.28 f 0.09 3.94 i 0.04a
7.3 i 0 . 8
1557 1385
0.04 0.05
Me4NBr Et4NBr PrrNBr BurNBr
125.16i0.04 116.95f0.03 102.55fO.01 95.37 f 0.02 95.42 f 0.01 91.41f0.01
3.50 f 0.09 3.81 f 0.11 3.72 f 0.07 3.49 f 0.09 3.68 f 0.05 3.51 f 0.06
14.0 f 0.7 9 . 8 f0 . 7 6.3 i 0.4 2 . 6 f 0.6 4 . 1 i0 . 3 2.5 f 0.4
1689 1697 1470 1307 1365 1267
0.03 0.02 0.01 0.01 0.007 0.009
131.35f0.05 108.99 f 0 . 0 4 108.84f0.01 101.72f0.006 97.42 I 0 . 0 2
3.51 f 0.10 4.52 f 0.20 3.83 f 0.08 3.76 f 0.03 3.70 f 0.08
18.0 f 0.8 21.4 f 1 16.3 i0 . 5 15.6 f 0 . 2 15.7 f 0.6
1769 1807 1591 1473 1399
0.03 0.03 0.01 0.004 0.01
115.99 f 0.07 115.76f0.02 86.14f0.03
3.81 f 0.32 3.78 f 0.09 3.4 f 0 . 2
11 1 2 1 0 . 5 f 0.6 7 f l
1678 1664 1172
0.05 0.01 0.02
101.78 f 0.07 101.77 f 0.03 94.82f0.04 83.07i0.05 76.90 i:0.01" 76.90 z!= 0.02" 76.92 & 0.01"
10" 3.1 f O . l 3.28 f 0.06 3.4 1 0 . 1 3.3 -40.2 2.97 c t 0 . 0 2 " 2.95 i 0.03a 2.96 zk 0.02"
11 f 1 12.1 i 0 . 5 6 . 7 i:0.9 5 f l
1125 1174 1121 992 840 837 837
0.05 0.02 0.03 0.03 0.02 0.03 0.02
106.95f0.02 88.02 5 0.07 88.11fO.01 82.3640.03 82.00f0.02
3.61 f 0.06 3.4 1 0 . 4 3.88 f 0.06 3.9 f 0 . 2 3.7 r t 0 . l
19.2 f 0 . 4 15 f 2 17.5 rt 0.4 18 Z!C 1 16.5 =k 0.9
1328 1059 1177
0.01 0.05 0.008 0.02 0.01
n-AmdNBr Me4NI Pr4NI Bu4NI n-AmrNI Me4NPi BuaNPi 7
Me4NBr EtrNBr Pr4NBr Bu4NBr
Me4NI
Pr4NI BuNI a
KA
d
Ao
1124
1069
Evaluated from eq. 1.
Table IV: Viscosity B Coefficients for Methanol Solutions at 10 and 25" Salt
Me$l"r EtcNBr PraNBr Bu4NBr Me4NL Pr4NI BUN
100
26'
0.42 0.58 0.73 0.89 0.38
0.43 0,56 0.70 0.84 0.38 0.66 0.80
0.70 0.85
aa1t
250
MeaNC1
0.45
BU4NC1
0.86
n-Am4NBr n-AmdNI Me4NPi BurNPi
0.98 0.94 0.78 1.13
used with the Knox and Evers data for the tetraaJky1ammonium picrates, their resulting XO+ values for the tetraalkylammonium ions agree well with ours, except for the EtBNf ion. On the other hand, Fuoss and Copland24report an average value XO(Pi-) = 46.86 0.02 from very precise and multiple measurements
*
The Journal of Physical Chemistry
OR NaPi and KPi. Since their Ao(Bu4NPi) = 86.12 is in excellent agreement with our value of 86.14, the discrepancy in Xo(Pi-) could be explained by our A0 for the halidea being about 0.34 too high or Fuosa and Copland's value of A. for NaPi and KFi being low by the same amount. We prefer the latter conclusion since Fuoss and Copland have measured i-AmaBuNI, which gives Xo(i-Am3BuNf) = 36.61. If this is subtracted from their value of Ao(i-Am3BuNPi) = 83.69, a Xo(Pi-) = 47.08 results, which is in better agreement with our value of 47.14 than with their value of 46.86 obtained from the alkali picrates. Furthermore, if we combine our value for the picrate ion with their AO(i-Am3BuNPi),we obtain ho(i-Am8BuN+) = 36.55, in excellent agreement with the value 36.61 obtained from the iodide (see above) and in fair agreement with the value 36.78, which results from the measurements
(24) See footnote c oi Table VI.
4213
CONDUCTANCE O F SYMMETRICAL TETRAALXYLAMMOXIUIVI HALIDES 24ND PICRATES
Table V : Summary of Averaged Conductance Parametere for Methanol Solutions a t 25 and 10" Salt
d
A0
XA
-~
3.3 3.9
7
Bu~N CI
25" 120.82 91.38
Me4NBr EtrNBr Pr4NBr BulNBr n-Am4NBr
125.16 116.95 102.55 95.39 91.41
3.5 3.8 3.7 3.6 3.5
14
MedNI Pr4NI BuaNI n-Am*NI
131,35 108.85 101.72 97.42
3.5 3.8 3.8 3.7
Me4NPi Bu4NPi
115.80 86.14
3.8 3.4
c
MehNBr Et4NBr Pr4NBr BmNBr
-loo 101.77 94.82 83.07 76.90
3.2 3.4 3.3 3.0
MeaNI PnNI Bu~NI
106.95 88.11 82.12
3.6 3.8 3.7
r
Me4NC1
_.
0 10
6 3 2 18
17 16 16 11
7
-
12
7 5 0 19
17 17
-
unity than that quoted by Copland and Fuoss. This confirms their contention that i-Am8BuNBPh4 should be an excellent reference electrolyte for the determination of single-ion conductances in methanol solutions since, by assuming an equal split in hofor both ions in other solvents, it could be invaluable in extending our knowledge of single-ion conductances without the arduous chore of measuring transference numbers. Unfortunately, the lack of transference data for methanol solutions at 10" prevents a calculation of limiting ion conductances. As a matter of fact, conductance measurenients at any temperature other than 26" appear not to exist for methanol solutions, except for the data reported here for 10". The consistency of the A, obtained can be checked by comparing the differences in A, for the bromides and iodides with a common cation. These differences amount to 5.18, 5.04, and 5.22 from the Me4N+, Pr4N+, and Bu4N+ salts, respectively. The Walden product can be used to calculate limiting ion conductances at 10" from the corresponding values at 25", and the results can be compared to the measured no. Such calculations result in AO values that are 0.4 unit too low for the Me4N+salts, about correct for the Pr4N+salts, and 0.3 unit too high for the Bu4W+salts, Ian-Xixe Parameters. An inspection of the d values
Table VI : Cation-Limiting Conductances for Methanol Solutions a t 25" c1-
68.46 68.7" 61.1"
Br-
68.71 68.9" 60.50 60.7" 46.08
I-
Pi -
68.75 69 1"
68.6b
I
60.ga 46.07
Best values
xo +.go
68.73
0.3742
60.5
0.3294
46.08
0.2509
38 94 I
0,2120
36.6 35.4 34.8
0.1993 0.1928 0.1895
6O.lb 45.96
Bu~N *
39 02 I
&AmsBuN i-Am4N n-AmaN +
38.94
38 94 I
36.61c
f
+
34.96
34.64
38. 36.55' 35.4b
" T. H. Mead, 0. L. Hughes, and H. Hartley, J . Chem. Xoc., 1207 (1933); A. Unmack, E. Bullock, D. M. Murray-Rust, and H. Hartley, Proc. Roy. SOC.(London), A132,427 (1931). E. C. Eyers and A. G. Knox, J. Am. Chem. Soc., 73,1739 (1951). ' M.A. Copland and R.M.FUOSS, J. Phys, Chern., 68, 1177 (1964). c
of Kunze and Fuoss on the alkali tetraphenylborides,26 that give Xo(BPh4-) -- 36.50, and from the Copland and FuossZ4value of Ao(.i-Am3BuNBPh4) = 73.28. We have given this latter value little weight in the "best values" quoted in the last column of Table VI. It is interesting to note that the ratio Xo(.GAm3BuN+)/ Xo(BPhr-) = 36.6/36.50 = 1.003 is wen c h e r LO
in Table V shows that they are surprisingly constant, as was the case for the tetraalkylammonium salts in nitromethane and acetonitrile solutions. An average value of 3.6 f 0.2 at 25" and 3.5 f 0.2 at 10" for B is obtained for all the salts in methanol solutions. (25) R.W.Kunze and R. M. FUOSS, J. Phys. Chem., 67,385 (1963).
Volume 69, Number 12 December 1966
42 14
R. KAY,C. ZAWOYSKI,AND D. EVANS
Table VII: Conductance Parameters of Other Workers for Methanol Solutions at 25' Salt
i-AmaBuNI MedNPi
EtrNPi PraNPi BurNPi i-Am,BuNPi GAmNPi Bu4NBPh4 i-AmaBuNBPhd
AO
a
KA
uA
Ref,
99.38 k 0 . 0 4 115.89 k 0 . 0 2 116.0 z k 0 . l 107.31f0.02 107.63 zk0.06 93.12 f 0.01 86.12 i 0.01 86.04&0.02 83.68i0.01 82.54f0.02 75.99 f 0.01 73.3 f O . 1
4 . 5 =t0 . 5 4.2 i 0.4 4 . 0 zk 0 . 8 4.2 3~ 0.6 4 zk1 6.4 & 0.3 3.7 f 0 . 1 4.5 & 0.2 4.2 i 0 . 1 4 . 3 f0 . 8 5.1 f 0 . 4 5.3 f 0.6
18 k 3 13 f 2 13 & 6 18 f 3 13 5 21 k 3 10 f 1 13 f 1 12 i 1 15 i 3 37 & 4 32 f 3
0.01 0.01 0.03 0.02 0.02 0.006 0.002 0.01 0.002 0.007 0.006 0.004
a
See footnote c of Table VI. See footnote b of Table VI. Accascina and L. Antonucci, ibid., 29, 1391 (1959). a
' F. Accascina
This is identical with the d values found for the alkali halides in methanol, 21 in excellent agreement with the 3.6 =t 0.2 obtained for these salts in acetonitrile,& and in good agreement with the value 3.9 + 0.3 obtained for some of these salts in nitromethane soluIt would appear that the tetraalkylammonium halides and picrates, salts that involve a considerable variation in crystallographic radii, produce the same electrophoretic and relaxation effects in these three solvents once differences in A0, r, and E have been taken into account. Thus, the continuum theory seems totally adequate to describe the concentration dependence of the conductance of the free ions in these three solvents ; however, the size dependence in the Fuoss-Onsager theory appears to need revision. On the other hand, it should be noted that d for the larger tetraphenylborides listed in Table VI1 are greater than those reported for the halides and picrates in agreement with what was found in acetonitrile solution,6 where an (2 of 5.3 was also reported for BuqNBPh4, The significance of the trends in d in these solvents will be clearer when the electrophoretic effect has been obtained independently from the concentration dependence of transference numbers. Transference datal7118have already indicated that the electrophoretic effect as c a l ~ u l a t e dfrom ~ ~ the Fuoss-Onsager theorya is essentially correct for the alkali halides in methanol solutions. A project to obtain this information for the tetraalkylammonium halides is under way. Association Constants. The results of an analysis of all of the data in the literature for the symmetrical tetraalkylammonium salts in methanol solutions with the required precision for an application of eq. 2 are given in Table VII. The viscosity B coefficient for The Journal of Physical Chemistry
and S. Petrucci,
b C
b C
b a
b a
b a a
Ric. Sci. Suppl., 29, 1383 (1959); F.
the two tetraphenylborides was estimated to be l.2.15 It can be seen that Knox and Evers data for Me4NPi are in good agreement with our own (Table V), whereas the data of Copland and Fuoss agree better with our data for Bu4NPi. It would appear, from the results in Table VII, that all of the tetraalkylammonium picrates are associated to the same degree.30 The most outstanding feature of the association constants given in Tables V and VI1 is the fact that salts containing large ions show a considerable amount of association. This can perhaps be seen better in Figure 1, where log Ka is plotted us. the reciprocal of the sum of the estimated ion radii.31 The data of Fuoss and C ~ p l a n dfor ~ ~Bu,NBPh4, i-AmsBuNBPh4, and GAmaBuNI have been included; and, for ease of representation, the picrates and tetraphenylborides were assumed to have the same size as the corresponding iodides. The dotted line in this plot represents the slope which was computed from the coulombic term e2/ekT = 7.44. It can be seen in Figure 1 that, (26) R. L. Kay, S. C. Blum, and H. I. Schiff, J. Phue. Chem., 67, 1223 (1963). (27) A small correction due to the solution visoosity has been added to the average value of 3.7 obtained from the C% reported by Kay, Blum, and Schiff26 after correction of the misprint for BunNBr from 3.19 to 3.91. Since Viscosity data are not available for nitromethane solutions, the F in eq. 1 was assumed to be the same as found for acetonitrile solutions which in turn are almost identical with those used here for methanol solutions.16 (28) D. 8 . Berns and R. M. Fuoss, J. Am. Chem. SOC.,82, 6685 (1960). (29) R. L. Kay and J. L. Dye, Proc. Natl. Acad. Sci. U.S., 49, 5 (1963). (30) The somewhat high K A of 21 and the high & of 6.4 for PrrNPi can be attributed to a poor split in the last two terms gf eq. 2. If i is changed t o agree with our average value of 3.6 A., a KA of about 12 results since b J / & = 260. This is in much better accord with the data for the other picrates. (31) See ref. 13, p. 126.
4215
CONDUCTANCE OF SYMMETRICAL TETRAALKYLAMMONIUM HALIDES AND PICRATES
C H30H
v cl-
Table VI11 : Conductance Parameters for Ethanol and 1-Propanol Solutions a t 25"
m
Pic-Q BPh,
0 Br'
Salt
A0
MerNC1 MekNBr EtrNBr
51.87f0.04 54.03 1 0 . 0 2 53.54f0.03 66.5 f 0 . 6 B5.03f0.05 54.2 f O . l
d
KA
Ref.
4.2f 0 . 4 5 fl 3.9f0.5 4 f 1 6 f4 3.7f0.2
141 f 8 164f 3 96f 8 130 f 13 110 f 4 0 69f 6
a
n-CaH70H 27.07f0.02 5.2f0.4 6.4 f0.5 28.52 f 0.02 24.43 f 0.06 5.0 f 1 . 6 4 . 6 f 0.7 25.80 f 0.03
393f 9 503 f 10 311 f 35 418 f 17
d d d d
CzHsOH
EtD"
0.5
t
P(P' I
14
I
I .I6
I
I
I
I8
i/i?X
Figure I. Association constants for the tetraalkylammonium halides, picrates, and tetraphenylborides as a function of crystallographic ion sifie at 25' (open symbols) and 10' (filled symbols). The picrates and tetraphenylborides are assumed to have the same size as the iodides for comparison purposes. All data are from this research except for the tetraphenylborides and i-AmaBuNI which are from ref. 24. The numbers next to the points on the plot identify the cations: (1) MebN+, (2) Et4N+, (3) Pr4N+, ( 4 ) Bu4N+, (5) n-AmqNf, (6) i-Am3BuN+.
as the anion size increases, the association increases, and the increase appears to be greater in the case of the large cations than of +,hesmall cations. Of particular importance is the fact that the iodides are more associated than the corresponding bromides since the conductance data for aqueous solutions of these salts can be explained best by assuming that the iodides are associated to a much greater extent than the bromides. The results shown in Table VI11 for ethanol (E 24.3) and 1-propanol ( 6 20.4) show the same variation of KA with size in that association increases as the halide ion increases in size and, owing to the lower dielectric constant, the effect is much more pronounced. The large association constants shown here for salts of such large ions as tetrabutylamnionium iodide and tetrabutylammonium tetraphenylboride are completely unexpected since these salts show very little, if any, association in acetonitrile.6 I n acetonitrile, the KA for these salts was less than 5. The same behavior is found for the tetraalkylammonium picrates. Generally, association of large ions is attributed to the llack of solvation of the ions. The known association behavior for methanol solutions could be explained on this basis alone since the highly solvated alkali halides18J9~21 have been found to be completely dissociated except for the larger cesium chloride, which has an association constant of about 9.32 However, although the lack of solvation would explain the high degree
Me4NPi Et4NPi EtaNBr EtaNI Pr4NBr
PrdNI
a b b
a c
'
a See footnote a of Table VI. M. Barak and H. Hartley, 2. physilc. Chem. (Leipzig), A165, 273 (1933). R. Whortcm and E. S. Amis, 2. physik. Chhem. (Frankfurt), 17, 300 (1958). T. A. Grover and P. G. Sears, J. Phys. Chem., 60, 330 (1956).
of association of salts containing large ions in methanol solution, the lack of association of these salts in acetonitrile solutions would have to be attributed to a considerable increase in solvation energy in that solvent over that found in methanol solutions. We believe that this is unlikely and that a more plausible explanation rests in assuming that some factor is stabilizing the formation of ion pairs of large ions in methanol solutions. It is possible that structural features brought on by the hydrogen-bonding character of the alcohols plays a part in the association process. I n an attempt to study the possible effect of solvent structure on the association behavior of these large electrolytes, we extended our measurements to 10". The association constants are represented in Figure 1 by the filled-in symbols, as compared to the open symbols for the measurements at 25". The small decrease in liT~for the bromides is of about the correct magnitude t o be attributed to the change in the eT product due to the decrease in temperature. The slight increase in K A for the iodides, after taking into account the change in the ET product, could be an indication that the increased amount of methanol chains present a t the lower temperature stabilizes the ion pairs. Data for much lower temperatures will be required before this hypothesis can be tested. Acknowledgment. This work was supported by Contract 14-01-0001-359 with the Office of Saline Water, U. S. Department of the Interior. (32)
R.L. Kay and J. L. Hawes, J. Phys. Chem., 69,2787 (1965).
Volume 69. Number 12 December 1966
