THOMAS B. HOOVER
876
phthalocyanine crystals or a chemical or physical ( e . g . , lattice imperfection) impurity. The generation of free radical resonances on exposure to oxygen has been noted in experiments on biological materials by Miyagawa, Gordy, Watabe, and Wilbur. I n that work the oxygen was also reversibly absorbed. Even though the free radical resonances described in ref. 16 are somewhat asymmetrical whereas the resonances generated by oxygen in Pc are symmetrical and approximately half the width of the biological resonances, it may be that the centers that cause the oxygen to be absorbed in that material are electronically similar to the absorbing centers in Pc. Samples of fine HzPc powder were irradiated with y- and neutron rays. Preliminary studies of the ir-
radiated samples point out that the resulting e.s.r. spectra indicate that the free radical, as well as the phthalocyanine molecule, is stable under irradiation. The stability of phthalocyanines with regard to ionizing radiation may, as in the case of porphyrins,17 be due to the very high resonance energy of the compound.
Acknowledgments. The authors wish to acknowledge helpful discussions with Dr. X.E. Wolff and the assistance of L. Korsakoff in supplying pure metal-free phthalocyanine powder. (16) I. Miyagawa, W. Gordy, N. Watabe, and K. M. Wilbur, Natl. Aead. Sci., 44, 613 (1958). (17) B. Pullman and 9. Pullman, N a t u r e , 196, 1137 (1962).
Conductance of Potassium Chloride in Highly Purified N-Methylpropionamide from 20 to 40''
by Thomas B. Hoover 12'ational Bureau of Standards, Washington, D . C .
(Received Sovember 7 , 1968)
The conductance of potassium chloride in N-methylpropionamide was measured a t 5 O intervals from 20 to 40' and in the concentration range of 5 X to 3 x lo+ N . The Fuoss-Onsager conductance equation represents the data with only a small contribution from the term linear in concentration. The ion size parameter d J increases from 0.6 to 1.5 with increasing temperature, while the mean hydrodynamic (Stokes) radius is 3.1 8.
K-Methylpropionamide (YRIP) is of interest as a solvent for electrolytes because of its unusually high dielectric constant (176 a t 25 "), Dawson, Graves, and Sears2 have shown that a Kohlrausch plot represents the conductance data for potassium chloride fairly satisfactorily to much higher concentrations in N X P than in water. They found, however, that the slope of the plot differed by a fe\\T per cent from the theelimiting Of the Onsager equation' The present study was undertaken in order to obtain data The J o u r n a l of Physical Chem,istry
of sufficient precision to permit application of the extended conductance equation of Fuoss and O n ~ a g e r . ~
(1) Presented before the Division of Physical Chemistry a t the 145th National Meeting of the American Chemical New York, N Y., September 9-13. 1963. (2) L. R. Dawson, R . H. Graves, and P. G. Sears, J . Am Chem. Soc.; 79, 298 (1957). (3) R. M. Fuoss and F. Accascina, "Electrolytic Conductance," Interscience Publishers, Inc., New York. N. Y.. 1959, p. 195.
CONDUCTANCE
OF 1’oTASSIUM
CHLORIDE 1N N-METHYLPROPIONAMIDE
Experimental The Jones-Dike conductance bridge4 used in this work was found, by comparison with precision a.c. coils, to introduce a frequency-dependent error. In order to keep the error from this source negligible, frequencies were restricted to 2 kc.p.s. when the cell resistance was greater than 2000 ohms, and to 4 kc.p.s. a t lower resistances. Conductance cell I, with constant of 0.010747, had concentric cylinders of bright platinum as electrodes. The outer electrode was the longer and was constricted a t the ends to provide partial shielding of the inner cylinder, which was always connected to the high potential corner of the bridge. Cell 11, with a constant of 0.10684, was of the Jones and Bollinger5 design and was calibrated against potassium chlorides6 Concentrations of 0.01, O.OOFj, and 0.001 N revealed no Parker effect greater than 0.05%. Cell I was calibrated by intercomparison with I1 by means of and lo-* M solutions of sodium bicarbonate7 and 0.1 M boric acid. Lead resistances were determined by measuring the conductance of constant-boiling hydrochloric acid in each cell in series with a precision 1000ohm resistor. The automatically regulated oil bath was stable t o j=0.002° for several hours. Temperatures were measured with a calibrated platinum resistance thermometer. Solutions were prepared by weight in 125-ml. flasks and were transferred to the conductance cells under dry nitrogen. Magnetic stirrers were used to dissolve the salt; the stirring bars were enclosed in glass or polytetrafluoroethylene. Within the range of molalities (m) and temperatures covered by this investigation, the densities of the solutions could be represented by the equation P = PO(1
+ km)
(1)
where k has the value 0.042 f 0.002. Molarities (c) were calculated from molalities by the equation
c/m
= po(l
4-km)/(l
+ 0.07456m)
(2)
Measured viscosities were fitted by least squares to an equation of the form 9/70
- S,,C’/~ = 1
+ BC
877
Table I : Viscosity Coefficients for Potassium Chloride Solutions in N-Methylpropionamide 1
20
B
1.39
25 1.37
30 1.35
35 1.33
40 1.31
is resistance and f is frequency, usually showed a slight curvature, resistances were extrapolated to infinite frequency by the equation
+
R, = R, (4) which can be fitted exactly to three points for which the frequencies lie in geometric progression. Thus
R,
=
(RiR3 - Rz2)/(Ri
+ R3 - 2Rz) (forfz2 = fIf3)
(5)
At least five such frequencies in the range 0.5 to 4 kc.p.s. were used and the equation was fitted to the widest range consistent with the graph of all the points. The exponent b in eq. 4 varied from 0.76 to 1.10, being higher in the more concentrated solutions. The maximum difference between the extrapolated resistance and that measured a t 2 kc.p.s. was 0.4%. The conductance of the solvent was subtracted from that of the solution. Dielectric Constant. The substitution methodg was used to obtain the dielectric constant of NMP a t audio frequencies with the same bridge and cell as were used for the conductance measurements. Calibration of cell I with chlorobenzene, 1,2-dichloroethane, and water yielded an extrapolated vacuum capacitance of 8.64 pf. (the conductance cell constant corresponds to 7.41 pf.). The results for X’MP, given in Table 111, have an estimated error of j=0.5yo. Literature valueslO are about 2% lower. The effect of moisture was investigated and the discrepancy, which is slightly greater than the combined estimates of error, could be accounted for by less than 0.2y0 water in the sample used by Leader and Gormley. N-Meth&ropionamide. The solvent was prepared by the procedure of Leader and Gormley’O and was purified by repeated fractional distillation a t 5 mm. in a 1000 X 4 cm. column packed with 3-mm. glass helices.
(3)
where S,, the Falkenhagen coefficient, was estimated from A. on the assumption of equal ionic mobilities.8 Values of B are given in Table I. The conductance measurements, summarized in Table 11, covered the concentration range of 5 X to 3.3 X M . Each solution was prepared independently and each was measured a t three or more temperatures. Because plots of R us. f-l, where R
(4) P. H. Dike,
Ra.. Sci. Instr., 2, 379 (1931). (5) G. Jones and G. M. Bollinger, J . Am. Chem. Soc., 5 3 , 411 (1931). (6) J. E. Lind, Jr., J. J. Zwolenik, and R. M. Fuoss, ibid., 81, 1557 (1959). (7) J. E. Lind, Jr., and R. M. Fuoss, J . Phus. Chem., 6 5 , 999 (1961). (8) See ref. 3, p. 234. (9) C. P. Srnyth, “Dielectric Behavior and Structure,” McGrawHill Book Co., Inc., New York, N. Y.. 1955, p . 212. (10) G. R. Leader and J. F. Gormley, J . A m . Chem. SOC..7 3 , 5731 (1951).
Volume 68, Number 4
April, 1964
THOMAS B. HOOVER
878
Table I1 : Conductance of Potassium Chloride in N-Methylpropionamide ----ZO
0
___ A
104~
6.76 14.66 47.48 81.67 155.63 204.25 204.25 312.88 7--
14,60 47.28 81.32 154.97 203.38 311.55
8.906 8.891 8.882 8.894 8,907 8.915 8.913 8.927 250----------.
10.240 10.227 10.244 10.257 10.269 10.290
-
---30°104~
A
0.53 3.92 4.07 6.71 8.62 14.54 14.54 19.88 35.61 41.14 66.49 111,10 154.30 187.06 238.83 310.21
11,621 11.723 11.681 11.702 11,693 11.711 11,720 11.697 11,711 11.685 11,699 11,717 11,744 11.754 11,764 11.783
Table 111 : Properties of N-Methylpropionamide t
P
D
100,
20 25 30 35 40
0.9347 ,9308 ,9268 ,9228 ,9188
185 176 167 159 151
6.06 5.25 4.58 4.03 3.56
1.94 2.23 2.53 2.85 3.15
-----350104~
A
3.91 4.05 8.58 19.80 35.46 41 .OO 66.21 80.62 110.62 186.25 237.80 308.87
13,326 13.270 13.278 13,289 13.307 13.274 13.301 13,317 13.326 13.360 13.382 13.406
?-----400-10'0
A
3.89 4.04 6.65 8.54 19.71 35.31 40.79 46.67 65.92 110.14 185.45 200.77 236.77 307.54
15.042 14.979 15.062 14.996 15.022 15.031 15.006 15.018 15.026 15.052 15.105 15.108 15.126 15.155
crystallized once from conductivity water to free it from soluble resin. After drying a t room temperature, the salt was fused under nitrogen. Spectrographic analysis showed less than 10 p.p.m. each of A1 and Si, less than 1 p.p.m. of Ca and Mg, and no other detectable impurities. The pH of a 3.5 N solution in carbon dioxide-free water (pH 7.1) was 6.4.
Results and Discussion The function A,,', defined by
A conductivity cell in the line to the receiver was used to monitor the purity of the product. The NMP was stored and transferred under dry nitrogen. Gas chromatography of a sample 8 months after its preparation showed 0.1% water and an unidentified peak corresponding to a substance present in the amount of about 0.02%. This substance was eluted just before the water. The specific conductance of different batches of solvent fell within 2% of the values given in Table 111, which are an order of magnitude lower than those of the previous studyS2 The freezing point, -30.9", was 12" higher than the literature valuell; other physical properties are given in Table 111. Potassium Chloride. Reagent grade potassium chloride was further purified by ion-exchange treatment with the use of Dowex 50 resin. A 0.1 N aqueous solution of the salt was passed through the column and eluted with the same concentration of hydrochloric acid prepared from bromide-free potassium chloride. l 2 The middle half of the eluate was put on a second column and eluted with 30% hydrochloric acid, discarding the first and last quarters of the eluate. The salt obtained by evaporation of the eluate was reThe Journal of Physical Chemistry
A,' = (q/qo)A
+ Sc"'
- EC log c
= A0
+ Jc
(6)
was accurately linear in concentration. The viscosity correction to the observed equivalent conductance, A, consisted of the right side of eq. 3 (1 Bc) without inclusion of the Falkenhagen term8; X and E are theoretical terms whose values are given in Table IV. A least-squares fitting of eq. 6 gave A. and J , which are given in Table IV along with the estimates of their standard deviations. In order to reduce the e&ct of experimental scatter at the lower concentrations, the points were weighed by c. Consequently, the derived parameters depend chiefly on the four or five measurements obbained a t concentrations greater than 0.01 N . Values for A, at 30 and 40' may be compared with 11.6 and 14.9, respectively, reported by Dawson, Graves, and Seam2 These authors also called attention to the fact that the Walden product (last column of Table JV) varies somewhat with changes of temperature.
+
(11) G. F. D'AlelioandE. E. Reid. J.A m . Chem. Soc., 5 9 , 109 (1937). (12) G. D. Pinching and 311 (1946).
R. G. Bates, J. Res. Natl. Bur. Std., 37,
CONDUCTANCE OF POTASSIUM CHLORIDE IN N-METHYLPROPIONAMIDE
879
Table IV : Conductance Parameters and Constants for Eq. 6 t
S
-E
20 25 30 35 40
6.4340 7.5584 8.8420 10.2685 11.8374
0.1828 ,2109 ,2408 ,2669 ,2926
Std.
A0
Std. dev.
J
dev.
AOVO
dr
8.884 10.225 11,691 13.280 15.002
0.003 ,003 .004 ,005 ,005
1.4 2.1 3.0 4.2 5.1
0.1 .1 .2 .2 .3
0.5377 ,5368 ,5354 ,5347 ,5341
0.65 0.92 1.18 1.43 1.52
The J coefficients provide a measure of the ion size parameter, d J , if short-range interionic attractions are neglected and if the proper allowance has been made for the viscosity of the medium. The resulting values for &, shown in Table IV, are impossibly small. Some other estimates of the ion sizes may be made if it is assumed that both ions have the same radius and mobility. The Walden product, a t 30°, corresponds to a hydrodynamic (Stokes) radius of 3.06 A. Inclusion of the Sutherland correction' gives 4.59 A. If the large values of the viscosity coefficient B in Table I are interpreted as a purely hydrodynamic phenomenon due to the Einstein volume e!ectl3 they correspond to a mean ionic radius of 4.74 A. However, the electrostatic interaction of the ions with solvent dipoles constitutes a far more plausible explanation of the viscosity effect. Zwanzig14has recently derived an equation for the contribution of this effect to conductance, but the dielectric relaxation time of NMP, which would permit a test of this theory, is not known. The apparent molar volume of potassium chloride a t 30" is 36 cc., corresponding to an ionic diameter of 3.85 A. The small values calculated for dJ indicate that this parameter has been forced to include other short range effects. The most likely of these is ionic association, although it is also possible that an undercorrection was made for viscosity. If a plausibl! value for dJ is used in the calculation of J , e.g., 3.33 A. (the
value found for potassium chloride in water') then an ion pair association constant of 0.5 is required, to account for the slope of the A,,' vs. c curve at 30". It is clear that the sphere-in-continuum model is unsatisfactory for potassium chloride in N-niethylpropionaniide. Indeed, this model could scarcely be expected to apply strictly as it is well-e~tablished~~ that the structure of N-substituted amides is that of long, polymeric chains held together by hydrogen bonds. There is evidence, however, of ion-solven t interactions. These effectively increase the viscosity and decrease the dielectric constant near an ion, as indicated by the large apparent hydrodynamic radii and by the existence of ion pairs despite the very high bulk dielectric constant.
Acknowledgments. The author wishes to thank Dr. W. G. Borduin for supplying density and viscosity data and Mr. C . G. Malmberg for many helpful suggestions, particularly regarding the dielectric constant determinations. He is indebted to Mr. E. L. Weise for the gas chromatographic examination of the solvent and to Mr. J. M. Cameron for the statistical analysis of the conductance data. (13) See ref. 3, p. 63. (14) R. W. Zwanzig, J. Chem. Phys., 3 8 , 1603 (1963). (15) R. Lin and W. Dannhauser, J . Phyls. Chem., 67, 1805 (1963).
Volume 68, Number Q
April, 1964
