KOTES
1724
129.5'). Measurements were made with a Cary spec= trophotometer. o-Nitrophenol, in alkaline solution at 25 O, has maximum absorption a t 414 nip, eZ 4,630; this is slightly temperature dependent', for a t 5" the maximum occurs a t 412 mp and eZ 4,57:. At 60°, it occurs a t 420 mp with €2 4,600. The acid solution has maximum absorption a t 350 mp with el 3% 150 a t so, 3,090 at 25O, and 2,950 a t 60'. The corresponding data for the cresol are: in alkaline solution, 5', 393 m p , c2 16,400; 25', 396 mp, c2 16,700; 60') 401 mp, EZ 16,800; in acid solution, 5', 315 m p , €1 7,760; 25', 315 mp; el 7,560; 60°, 315 nip, el 7,180. For pK measurements the solutions were buffered with equimolal mixtures of HHzP04 and Sa2HP04, (-log fx"c1) values of which have recentslybeen recomp ~ t e d . Measurements ~ were made a t a number of wave lengths and in buffer solutions of different total ionic strength ( I ) . Some typical results are given for 25' in Table I. The pK values have been corrected, as described p r e v i ~ u s l yfor , ~ the effect of the phenol on the ) of the buffer. -log ( a w y c ~value TABLEI" IONIZATION CONSTANTS AT 25' o-Kitrophenol, 2 X M ; 1 cm. cells; 410 mp, DI = 0.087, D Z= 0.913; 417 mp, D 1= 0.054, Dz = 0.927 -log
---410
I
(5HYCI)
D
0.10
6.974 7.013 7.040 7.080
.06 .04
.02
mp--a
7-417
pK 0 , 3 8 2 0.357 7.229 .398 .377 7.232 ,408 ,389 7.237 ,429 ,407 7.244
D
mp-01
0.364 0 . 3 5 5 ,380 ,373 ,392 ,387 ,407 ,404
pK 7.233 7.238 7.239 7.248 Mean
pK (cor.) 7.228 7.230 7.231 7.232 7.230
4-Xitro-~rn-cresol,5.5 X low6M ; 1 em. cells; 390 mp, D1 = 0.037, 0 2 = 0.891; 400 mp, D I= 0.015, Dz = 0.909 -10s I 0.10 .06 .04
.02 .01
(5HyC1)
6.974 7.013 7.040 7.080 7 . iia
7-390
D
mp--01
0.265 0.267 ,281 ,288 ,295 ,302 ,308 ,317 ,320 ,331
-400 mppK D 7.413 0.259 0.267 ,286 7.411 ,271 ,302 7.404 ,285 ,319 7.413 ,300 7.416 ,312 ,332
PK
pK 7.413 7.410 7.405 7.410 7.414 Mean
(cor.) 7.412 7.410 7.403 7.409
7.409 7.409
I is the total ionic strength of the equimolal phosphate buffer; a! = ( D - 0 1 ) / ( 0 2 - 01) is the degree of dissociation of the phenol. D, = optical density of acid solut'ion (phenol completely in the undissociated form); D2 = optical density in alkaline solution (phenol completely dissociated); D = optical density in phosphate buffer.
The data a t other temperatures are summarized in Table I1 where, for brevity, we record only the degree of dissociation (a)in the phosphate buffer, with I = 0.1, and the average, corrected pK. Our value for o-nitrophenol at 25', p K 7.230, is in good agreement with p K 7.234 found by Judson and Kilpatrickj and pK 7.229 by Dippy, Hughes, and Laxt o n 6 It is higher, however, than the figure, 7.210, given by Biggs.' The results from 5 to 60' can be represented by the equationa pK = Ai/T - At -I-A3T where T ( = "C.
+ 273.15) is the temperature in de-
(3) R. G. Bates and R. Gary, J . Res. Natl. Bur. Std., 66A,495 (1961). (4) R. A. Robinson and A . K. Kiang, Trans. Faraduy Soc., 61, 1398 (1955). (5) C. 31.Judson and M . Kilpatrick. J. A m . Chem. Soe., 71, 3110 (1949). (6) J. F. J. Dippy, S. R. C. Hughes, and J. W. Laxton, J . Chem. Sac., 2995 (1966). (7) A. I. Biggs, Trans. Faraday Soc., 62, 35 (1556). (8) K. S. Harned and R. A. Robinson, ibid., 86, 973 (1940).
Vol. 67 TABLEI1 IONIZATION COKSTANTS FROM 5 TO 60'
o-Xi trophenol 10'
50 Ly
pK(cor.)
350 Ly
pK(cor.)
15'
20'
25'
30°
0.269 0.288 0.311 0.331 0.356 0.379 7.499 7.424 7.353 7.293 7.230 7.180 40'
45O
50'
55O
60°
0.398 0.421 0.443 0.471 0.490 0.509 7.135 7.085 7.043 6.993 6.966 6.931 4-Ni tro-rn-cresol 5'
a!
pK(cor.)
10'
35' CY
pK(cor.)
16O
20'
25O
30'
0.194 0.210 0.232 0.247 0.267 0.289 7.671 7.596 7.525 7.466 7.409 7.354 40°
45'
50°
55O
60°
0.308 0.328 0.349 0.370 0.392 0.415 7.303 7.257 7.215 7.176 7.133 7.099
grees Kelvin. For o-nitrophenol we find A, = 2223.12, Az = 4.3092, As = 0.013709; for 4-nitro-m-cresol AL = 2075.02, Az = 3.1531, Aa = 0.012082. We have calculated the free energy, enthalpy,and entropy changes a t 25O and compared them with the corresponding changes for p-nitrophenol, as follows ASQ,
PK,
p-Nitrophenol o-Xitrophenol 4-Nitro-m-cresol
25'
kj. mole-'
AHO, kj. mole-'
7.156 7.230 7 ,409
40.85 41 ,29 42.29
19.73 19.23 19.16
AGQ,
j. deg.?
-71 - 74 - 78
mole-'
Thus, three substituted phenols with similar ionieation constants have similar enthalpy and entropy changes on ionization. CONDUCTAXCE I N DIRSETHYLSULFOLA3'E BYJEHUDAH ELIASSAF,' RAYMOND M. FUOSS, AND JOHN E. LIND,JR. Contribution No. 1736 from the Sterling Chemistry Laboratoru of Yale Uniuersity, New Haven, Connecticut Received April .4! 1965
Burwell and Langford2 reported that the equivalent conductances of tetraphenylarsonium chloride and of trimethylphenyJammonium iodide in sulfolane (tetramethylene sulfone) are "independent of concentration to within 1%" at low concentrations, and that contact distances (it) of the order of hundreds of khgstrom u1iit.s would be needed to fit the data a t higher concentrations. There seems to be no a priori reason to expect such peculiar behavior in these systems; we therefore reconsidered the sulfolane data and also made some exploratory measurements in 2,4-dimethylsulfolane, which has a lower dielectric constant. Our conclusion is that the conductance is normal in both solvents; the theoretical slope in sulfolane, however, is so small that, over a narrow concentration range, the theoretical tangent appears to be horizontal. Hence points near the tangent seem to be independent of concentration. Sulfolane has a dielectric constant of 44 at 30' and a viscosity of 0.0987 poise. The limiting slope becomes S = 0.54& 7.3. For limiting conductances of the order of 10, S is about 13. If one considers an unas-
+
(1) Grateful acknowledgment is made for a postdoctoral fellowship from a research grant made t o Yale University by the California Research Corporation. (2) R. L. Burwell, Jr., a n d C. H. Langford, J . A m . Chem. Sac., 81, 3799 (1959).
NOTES
August, 1963
1.725
iodide (curve 2) has a larger limiting conductance, because the cation is smaller, and, for the same reason, shows a moderate amount of association (K.4 = 40). As shown in Fig. 1, the curve now approaches the limiting tangent from below. I n the case of potassium thiocyanate (curve 1 ) both ions are small; the expected consequences appear. The limiting conductance is large, and the phoreogram drops rapidly away from the limiting tangent as concentration increases, showing considerably more association ( K A= 80) than the quaternary salt.
THE ADSORPTION OF METHANE AND NITROGEN ON SILICA GEL, SYNTHETIC ZEOLITE, AND CHARCOAL BYA . J. KIDNAY AND $1. J. HIZA Cryogenzc Engzneei zng Laboratory, Natzonal Bureau of Standards, Boulder, Colorado Received December 89, 1968
c
0.05
0.00 Fig. 1.-Phoreograms
0.15
in dimethylsulfolane: MerPhNI; 3, Ph4AsI.
1, KCKS;
2,
sociated electrolyte in sulfolane over the coiicentration range 0.0010--0.0036 (that shown for PhdAsCI in ref. 2, excluding the lowest point), the conductance should change by about 0.37 A unit. If Burwell and Langford's point a t the lowest concentration is disregarded, a line of theoretical slope m7ill fit their other data within several per cent; their A scale is so compressed that the theoretical tan.gent becomes almost horizont,al. Experimental 2,4-Dimethylsulfolane was distilled at 100" and about 1 mm. from sodium hydroxide, using nitrogen t o sweep air out of the The vissystem3; the solvent conductance was 2-4 X cosity a t 25' is 0.0904. The dielectric constant is 29.5 and the density4 is 1.1314. Tetraphenylarsonium iodide was prepared as described by Lyon and Mann6 and recrystallized twice from water. Trimethylphenylammonium iodide (Eastman) and potassium thiocyanate were recrystallized from ethanol. Conductances were measured a t 25.00", in a cell with constant equal to 0.1245. Electrical equipment and techniques have already been described.6 Typical conductance data are summarized in Table I. The accuracy is only about 1%.
TABLE I CONDUCTANCE IN DIMETHYLSULFOLANE KCNS
108,
A
13.02 10.01 7.70 5.39 2.65 0.00
6.44 6.90 7.29 7.83 8.80 11.47
PhNMesI 103~ A
5.22 3.66 2.73 1.93 0.82 0.00
7.17 7.51 7.76 8.02 8.56 9.40
PhaAsI 103~ A
2.55 2.03 1.323 0.941 ,480 ,000
7.35 7.46 7.61 7.70 7.85 8.22
The phoreograms are shown in Fig. 1. The conductance curve for tetraphenylarsonium iodide (curve 3) lies above the limiting tangent, similar to the curves for the alkali halides in water. The positive deviations from the Onsager tangent can be described by the Fuoss-Ontiager equation
+
+
ii = A, - SCli2 EC log c JC (1) using a contact distance LE. of 5-6 A. Trimethylphenylammonium (3) We are grateful to Professor Bururell for suggesting this method of purification. (4) E. Hirsch and R. A I . Fuoss, J . Am. Chenz. SOC.,77, 6115 (1955). ( 5 ) D. R. Lyon and F. G. Mann, J . Chem. Soc., 666 (1942). (6) J. E. Lind, Jr., and R. M. Fuoss, J . Phys. Chem., 66, 999 (1961).
Nitrogen adsorption Bas been studied extensively a t liquid nitrogen temperatures in conjunction with surface area measurements, but very little work appears to have been done on methane in this temperature region. The available methane data are limited to adsorption measurements on sodium chloride,l sodium bromide,2 and graphitea and heat capacity measurements on rutile.4+ I n earlier work,' isotherms of methane on silica gel were determined at 76 and 88.5"K. These isotherms exhibited unusual behavior in that, when the amoimt of methane adsorbed is plotted against the relative pressure ( p / p o ) , the higher temperature isotherm exhibits the greater capacity over the entire pressure region studied. The work reported here was undertaken to obtain more information regarding the temperature dependence of methane adsorptioii and the adsorption isotherms of methane and nitrogen on commercial adsorbents. Experimental The adsorption apparatus used in this work was of a standard volumetric design and consisted of a calibrated gas buret, constant volume manometer, adsorption bulb, and vacuum system. For pressures greater than 10 mm., mercury was used as the manometric fluid; for lower pressures, a low vapor pressure oil (specific gravity 0.9827) was used. All pressure readings were made with a cathetometer, to a precision of 0.02 mm. of fluid. Liquid nitrogen was the refrigerating bath for all of the runs. The bath pressure could be held above or below normal atmospheric pressure, thus allowing the temperature to be controlled from 70 to 82°K. A two junction copper-constantan therniocouple, calibrated with a platinum resistance thermometer, was used for the temperature measurements. The three adsorbents used were silica gel,*" synthetic zeolite, and coconut shell charcoal.8c (1) S. Ross and H. Clark, J . Am. Chem. Soc., 7 6 , 4291 (1954). ( 2 ) B. B. Fisher and W.G. McMillan, J . Chem. Phys., 2 8 , 549 (1958). (3) L. Bonnetain, X. Duval, and M. Letort, Compt. rend., 254, 1363 (1958). (4) K. S. Dennis, E. L. Pace, and C. S. Baughman, J . A m . Chem. Soc., 76, 3269 (1953). ( 5 ) E. L. Pace, E. L. Heric, and K . S. Dennis, J . Chem. P h y s . , 21, 1225 (1953). (6) E . L. Pace, D. J. Sasmor, and E. L. Herio, J. Am. Chem. Sue., 74, 44:13 (1952). (7) M. J. H i m and A. J. Kidnay, "The Adsorption of Methane on Silroa. Gel a t Low Temperatures," Advances in Cryogenic Engineering, Val. 6 , Plenum Press, Inc., New York, N. Y., 1961. (8) (a) Davison Chemical Co., "High Capacity" grade, 6-10 mesh; (b) Linde Molecular Sieves 5A, 1/16-in. pellets; (c) Barnebey-Cheney Co., Type IG-1, 8-10 mesh.
