2. The cobalt carbide in the preparation used \vas either paramagnetic or very weakly ferromagnetic. 3. I n the range from about 80 to 25 per cent cobalt carbide, the decomposition-time curves \\-c.re linear and hcnce the reaction \vas of apparent zero order in that range. 4. An empirical activation energy cf 54.3 lrcwl. per mole n.as found for the range of apparent zero order. 5. The details of a magnetic method are dewibed, by means of which the complete course of a reaction in thc solid state can he follon-ed under certain condit inns. The authors wish to thank W. E. Dieter for the ainalyses for cobalt. REFEIIESCES (1) BAHR,H. A . , A X D JESSEN, V.: Ber. 63, 2226 (1030). (2) EXAIETT, P.H., A N D SHULTZ, J . F . : J . Am. Chcm. Soc. 51, 3249 (1929). (3) H A U LR , . , ASD SCHOOS, T.: Z. Elektrochcm. 45, 663 (1939); Chem. Abstracts 34, 298. (4) MENDRICKS,B., JEFFERSON, 11.E., ASD SHULTZ, J. F.: Z.Krist. 73, 376 (1930). ( 5 ) HOFER,L. J. E., A N D PEERLEE, R.C . : J. Am. Chem. Soc. 69,893 (1947). W.C.: J. Am. Chem. SOC.69, 2497 (1947). (6) HOFER,L. J. E., ASD PEEBLES, ( 7 ) H O N D AIturated. The rate of removal of water seems to be governed by a slow process. Our explanation of the slow process is that the molecules must diffuse to the spot of capture rather than travel on the surface. Experimentally it is found that hours are required to equilibrate a charcoal bed with water vapor a t 80 per cent relative humidity. whereas other vapors are removed practically instantaneously. When charcoal is immersed in a liquid its apparent density is near that of graphite, 2.26, but is found to vary somewhat from one liquid to anothrr This effect is shon-n in table I . 11hicbh gives data of Harkins and Ewing (4). OtherF have reported similar data. In all cases it is found that water gives a wmeu har lon-er apparent demity for chai-coal than do other liquids, i.e., \I-ater molecules cwmpletely This 1s commonly attritmted t o the inpenetrate the pores 1
CAPILL.IRITT I N D S U R F A C E AREA O F CHSRCOAL
679
ability of water to penetrate the smaller pores:. -in alternat,ive explanat,iori which we prefer is that n-atein does not fill the larger pores as completely as do other liquids. Since the surface forces betiyeen water and carbon are small, the induced polarization from t8hewalls will not extend t o as \vide a meniscus foi. \rater as for others; therefore pores \vi11 not be as completely filled. It is usually found in the determinations of isotherms that V , for n-ater is somewhat less than for cai~bophilic adsoi%ates. This too attribute to incomplete filling of p o ~ r s :rather , than t o selectivcn screening by sniall pows. Were the latter thti case one \\-ould expect the adsorption of u x t e r to he more rather than less than that of othci, wdsorbatrs, since thr. \\atcr niolecule is quite small.
Heats of adsorptioth The heats of adsorption by c h a r m i l indicate that the forces between the solid and thcl adsiorhate :we l i t r g ~ rthan those for graphite. and that the heat of adsorption varies \\-ith t tic pot*e cliamrtc.t,. 111 L: pi,wioiis p:ipri. (0) \vc hive given
IPPAKt\l
DEXSITI
Mercury . . . . . . . . . . . . . . . . . . . . . . . . \Titter. . . . . . . . . . . . . . . . . . . . . . Propyl alcohol . . . . . . . . . . . . . . . . Chloroform . . . . . . . . . . . . . . . . . . . 1 Benzenr. .. p-Xylene. . . .. .'
0.865 1.843
1.960 1.992 2.006 2.018
[I
l'etroleuiii e t h c r . . . . . . . . . . .
!I
Acetone. . . . . . . . . . . . . . . . . . . . . Ether. . . . . . . . . . . . . . . . . . . . . . . . . . Perit~ic.. ..
, Carboii disulfide. .
'1
. . . . . . . . .
2,042 2 U5i 2.112 2 120 2.129
680
C. PIERCE, J. W. WILEY AND R. N. SMITH
-78°C. is not good below about 0.01 po, we could not determine the isosteric heats for the first portions adsorbed, when presumably they are highest. Our calculated values are shown in figure 5. It is seen that the heat of adsorption remains high, right up to near saturation, that for the region investigated it is less than the calorimetric values of Lamb and Coolidge and Pearce and Reed, and that a higher heat is indicated for slightly activated SGOOH than for highly activated S84. All this is in keeping with our proposed model.
Retentivity of vapors b y charcoal Comparison of isotherms for SGOOH and 584 shows an interesting relation between the extent of activation, or thc pore widths, and the ability to adsorb vapor a t low relative pressure. The low-pressure regions of the 0°C. isotherms are plotted on enlarged scale in figure 6. It lis seen that SGOOH, whose saturaI
I
I
I
I r
:>
9 CK4
$ 0
- 200
100
.02
.04
.06
.08
.IO
FIG.6 . Low-pressure adsorption by charcoals tion volume is 110 cc. of ethyl chloride per gram, holds niuch more ethyl chloride at low relative pressures than does S84, for n-hich V , is about 3i.5 cc. per gram. In fact, a t a relative pressure of 0.01 SGOOH adsorbs 80/110 its saturation value. The greater retentivity of SGOOH, as compared with S84, appears to be due to the greater number of narrow pores in the slightly activated sample. The smaller the meniscus on which adsorption occurs, the lower t8hevapor pressure of adsorbate.
Adsorption oaf ammonia On carbon surfaces ammonia is intermediate in behavior between water and most other adsorbates. In previous work TI-e have shown that ammonia on graphite gives a Type I11 isotherm but that it is much more highly adsorbed than water. Since charcoal isotherms for water show that adsorption proceeds a t much lower relative pressure than on the plane siirface of graphite, we have
68 1
CAPILLARITY AND SCRFACE AREA O F CHARCOAL
compared the adsorption of ammonia on charcoal and on graphite. Isotherms are shown in figure 7, both a t -78°C. I t is seen that on charcoal the isotherm is Type I, very much like the ethyl chloride isotherm. The liquid volume of ammonia a t saturation is about 0.30 cc., which is almost identical with that of ethyl chloride. Thus it appears that the narrow capillaries can condense ammonia a t relative pressures far less than are required to cover a plane surface with a monolayer.
560
-2
I
I
I
I
-- I4
kz
32
480-g~
- 12
400-
- IO
-]
8
Kelvin equatioir Sjnce the shape of water isotherms (and in particular the hysteresis on desorption) suggests capillary condensation, the Tcelrin equation ( I I ) has frequently been used t o compute capillary widths. The equation is
rvhere c is surface tension, 1’ is molar volume, 0 the angle of n-etting betx-een dsorbate and surface, and r the capillary diameter. This equation is a statenent of the fact that vapor pressures in a capillary meniscus are lower than in mlk liquid a t the same temperature. The cause for this lowering is the induced ’orces extending from the n-alls into the liquid layer. On the basis of the prolosed model we believe the vapor pressure lowering t o be a function of the force if attraction between adsorbent atoms and adsorbate, the temperature, the vidth of the capillary, perhaps the shape of the pore, and the polarizability of he adsorbates. These effects are all taken into nccount in the Kelvin equation
682
C. PIERCE, J. 11~. TVILET .\SD K. S . SMITH
by tlie terms u and 8. IJut \\-hen one wnsiders u meniscus of molecular diniensions t'lw meaning of surface tension and angle of wet'ting, d i i c h are macro scale phenommn., tend to disappear. We therefore do not believe the Kelvin equation to be applicable to the determination of pore sizes. If water and ethyl chloride are filling essentially the same capillaries, the equation should be equally applicable t'o both, but it has usually been applied only to water because the pore diameters coniputed from other adsorbates are impossibly small in the lowpressuiv region \There most of the adsorption occurs.
True area of charcoal. If our conclusions are correct it appears that one cannot determine the real surface urea of i t charcoal from its adsorption isotherm. The area as determined is too large by a factor which is the average number of molecules on each wall a t the point taken as V m . We believe that this average number of molecules can be quit'e large, since the shapes of the isotherms are not greatly changed when one activates it sample so as t o increase the capillary widths threefold, as \vas donc in going from SriOOH t o $84. ST'e would place the uveragr rvicith of the capillaries in S(i0OH at least 4-5 molecular diameters, since t,his charcoal holds the same liquid volume of ethyl chloride at 0" and -78"C., and of ammonia at -/8"('. 'rhus, on activation the minimum width should be three times as much or 12-15 molecular diameters (since V , increases threefold). I t is not at all unrcwonitble to assume that surface forces can ext'end through LI meniscus of that \\-idth, since the isotherm of ethyl chloride on graphit,e shon.s that multilayers can be built up to an average film thickness of 10-20 molecules and it is probable that the actual film thickness over the mo3t artive sites is even much great el,. On tlie basis of this argument ive n-ould cstirnate tlie surface area as from one-fourth to one-twentieth the value computed from the point B V,,&value m d \\-e doubt that areas as large as commonly reported, of the order of 1000 sy. m. pet' gram, arc' ever real for poroiis solicls. s u MMAHT
Evidence is prwcnted in support of the \*ien- that adsorption 1,y charcoal is due chiefljr t o capillarj. (widensation rather than to format ion of a surface layer, as in nun-porous aclsohent s. 7'1~.forces cxiising ctipillary condensatiun at lurv pressiii*rsart' of t h e h ; i n i c l t y p ~;is those \vhicli caiisc multilayer adsorption on :I plalnc s ~ i r f : i ( ~[ ),u t 1)ccaus:c oi thv c ~ f t ' w sof uppositti \\-ztllsthey are stronger at :I distance o f sevcrul molecii!ai, diameters frum the substratr tlim is found for a plane surface. If wpillttry contlrrisatiuii w r u r s , it is not possible t o determine tho ,surfact, i u w t uf L: porous solid froin :III acl$:orpt ion isot,hei.m and the commonly accepted \ x l ~ i e sw e much too large. I n capillaries :i fe\v molecular diameters in ividth thc KvIi-in rquation is not ;lpplic~itble. Retentivity is gwater for slightly :ictiI-iitccl sntnplcs than foix ~ i i o i ' t sliighI>. ; i r t i \ x t d ones.
DIFFTSIOX CURREST O F YTTERBIUM
683
and >Ira Ern-in S. Smith in preparation of samples and equipment for these experiments, and RIr. Wayne Tenolia for determination of the ammonia isotherm of charcoal. REFERENCES (1) BRUNAUER, DEMING, DEMISG,A N D TELLER: J. Am. Chem. SOC.62, 1723 (1940). (2) EMMETT A X D BRUNAUER: J. Am. Chem. SOC. 69, 1553 (1937). A N D DEITZ:J. Research Natl. Bur. Standards 36, 285 (1945). (3) GLEYSTPEN (4) HARKIXS ASD EWING:J. Am. Chem. SOC.43, 1787 (1921). (5) KLOTZ:Chem. Revs. 39, 211 (1946). (6) LAMBAND COOLIDGE: J. Am. Chem. SOC.42, 1146 (1920). (7) MCBAIS:Sorption of Gases and Vapours by Solids. Geo. Rutledge and Sons, Ltd.. London (1932). (8) PEARCE A S D RhED: J. Phys. Chem. 39,293 (1935). (9) PIERCE A X D SXITH:J . Phys Colloid Chem. 62, 1111 (1948). (10) SMITH A N D PIERCE:J. Phys. Colloid Chem. 62, 1115 (1948). Phil. Mag. [4] 42, 448 (1871). (11) THO~UPSOX: (12) WILKINS:Proc. Roy. SOC.(London) A164, 496 (1938).
D E P E S D E X C E O F THE DIFFUSION CL-RRENT OF YTTERBIUM ON SI!PPORTI?;G ELECTROLYTE =IND ~ € 1 ’ GERALD B. B.4RTOK
AR’D
J . D. KURRATOV
Department of Chemistry, The Ohio Stat< 1-niversity, Columbus 10, Ohio
ISTRODU CTIOS
It has heen ,ihunn by Xoddacl; and Bruckl (6) and by Laitinen and T a ~ b e l
(4)that YII-~exhibits a polarographic wive with a half-rave potential of approximately - 1.4 v. against a saturated calomel electrode (S.C.E.). Europium is the only rare earth shon-ing a lower (more positive) reduction potential. Laitinen (3) and TTalters and Pearce (S) by measurement of the YbA3--Ybi2 couple found that the wave at - 1.4 v. is due t o the reduction of Tb-t3 to I-h+? Laitinen and Taebel (4) have shon-n that the IlkoviE relation is obeyed at concentrations from 0.623 to 3.110 millimoles per liter of ytterbium in 0.1 -1ammonium chloride at presumably constant pH. The present n-ork covers the concentration range from 0.032 to 1.41 m1111mole. per liter and the pH range from 3.5 to 7.5. The purpose of this study was to find some indication of the aggregation of ions ;it hydroxyl-ion concentrations bclov, that at n-hich coagulation occurs and to observe thc influence of tliffercnt wpportiiig electrolytes on the difluqion current and cmgulation. 1 Presented before the Division of Physical and Inorganic Chemistry a t the 113th National Meeting of t h e American Chemical Societv, Chicago, I l l i n n i
