Adsorption of Nitrogen on Carbon Adsorbents at Low Pressures between 69 and 90 K

Transcription

1 Vl. 5 5, N.!, July Jurnal f Research f the Natina l Bureau f Standards Resea rch Pape r 26 Adsrptin f Nitrgen n Carbn Adsrbents at Lw Pressures between 69 and 9 K Juan de Dis Lpez-Gnzalez,! Frank G. Carpenter,! and Victr R. Deitz X i t r gen ad s rp ti n i,therm s \\'er e dete rmin ed n t hree car b n ad s r ben t s (tw bn e cha r s and an ac t i vated cc n u t shell ch ar cal) a t t hree te m per atu r es bet w een 69 and 9 K and at r el at i ve pre:isurcs frm 1-7 t.9. A cr.v st a t was uscd t b tain tc m perat u res belw t he nrm al b iling pint f l i q uid ni t r gen. Th e a t tainmen t f steady pressure r eadi ng s a t v cr y lw p ressu r es r equired cnsid er ab l y m r e t im e t han a t h ig her pr essu r es. Cn ect i ns fr t her m m lecul ar diffu si n \\'ere fun d t b e ap p reciab l e at p r essures b el \\' ne m illim eter f mer cury. A t r elati ve p r ess ures b el w 1- ', t he am un t f ni t r gen ad s rb ed d epended upn t he c nd i t i n f t he surf ace in reg ar d t p r ev iu s ad s r pti n f ni trge n. A tec h ni q ue i n v l v i ng a seri es f c nsec u t i \' e ad s r p t in s at l w te mper atures and u tass i n g s a t r m te mperat ur e led t a rep rd ucib le is t herm a t lw p ressures. T hese d ata c u ld b e r ep resen ted b y a F r c u ncl li ch equ ati n. Th e Brun a uer, Emmett, T ell er su r f ace ar eas f t he ca rb n a cl s rbc nt s wer e als dct'e r min ecl. 1. ntrductin 2. Experimental Prcedure Durin g the las t decade the m a in emph as is f l'esea reh in gas adsrpli n has been n th e evalualin f surface area. T he success a Lla in ed h as bee n d ue la rgel y t Lile develpm ent f t ile Brun a uer, E mm et t, T eller (B.E.T.) t\ter.\' f muh imlee ular adsrptin and t the cns t rucl ive eritieal wrk tha t fllwed. '' ll(' fac t LhaL Lltc H.E. T. eq uat in was inappli ca ble t dat a a L rel at ive press ures 2 belw.5 was j' i Ltlc cnscq uence beca use adsrplin measllrcmc nts in this rcg in werc nl need cd in the smfaee-al'ea dete rmin a ti ns. Exp crimental d ifficul ties h avc b ec n an add it inal causc fr t lte scarcily f da ta n phy sical ads rpti n at lw relative press ures. Adsrp tin dat a at vcr.v lw prcss ures ar c necessary fr t he calculatin f t hc1'm cl,v namie fu nctins f the adsrb ed phase. Th ese fun ctins are f spccial in terest in t he ra nge f ver y lw s u d ace-cverage [1].3 The in Lerest in s uch adsrp tin ist herms ha" been furt her st imul ated b y t he pssibili ty f b tain ing detailed infrm atin cncernin g t he hetergeneity f a surface. Th e recen t p apers f Hill [2], H alse,\' [3 ], Rh din [4], Halsey and T aylr [5], Sips [6], and C k, P ack, and Oblad [7] indicate sm e prmisin g a ppraches. t is believed t hat s urface hetergen eit ies a nd im pur it ies ar e t he m r e cmmn cause f t he vari a ble prper ties f cmmercial carbn adsrbents. N i trgen-adsr plin m easurem en ts at relative pressures abve 1-2 were r epr ted by D eit z and Gleysteen [8] fr a numb er f carbn adsrb en ts. As t hese samples wer e still available, the ppr tuni ty was taken t exte nd t he data t relativc pressures f a but 1-7 T hi s col'l'espnds t abslute pressurcs f abu t n e-ten t h micrn f H g. A [ur-s tagc McL ed gagc,4 s hwn in fi gure, was add ed t a c nventina l ad rp ti ll apparatus. The calibratin was m ade gr avimet ricall y wit hm ercll ry a ll d b.\' pressu re-v Lume data, using h el ium. Pressurcs dw n t a bu t.1 mm H g c uld be m cas urcd wi th an acc uracy f abut 1 pcl'cent. An accuracy f t Ac knwledgment is made t \ V. V. Lebcnf' tr in f t he Surface Ch emistr y Sectin fr t he design a nd fabri catin f this gage. HGH VACUU M X 1 X 1 REFR GERANT OR CRYO STAT R esearch Assciate f the Nati na l B ureau f Sta nda rds re presenting the Bne C har Research Prject, nc., R ela ti ve p ressure is th e rat i f t he act ual pressuf e f nitrgen t its eq uilibrium va pr pressur C'. s " igur C's in bracke ts ind icnlc t he li tera t ure references at the end f Lhis paper...-- TO MERCURY LEVE L NG BULBS F[(l U RE 1. Diagrammatic sketch f the gas adsrptin system.

2 2 percent was attained fr pl'essures dwn t 1-2 mm Hg, 3 percen t fr pressures t 1-3 mm, and 5 percent r mre fr pressures as lw as 1O - mm, the lwest pressure attempted. The accuracy f the high-pressure (abve 1 mm) readings was abut O.5mmHg. Nitrgen frm a cylinder f cmpressed gas (waterpumped) was purified by passage ver cpper at 35 C fr the remval f residual xygen and then thrugh Ascarite and phsphrus pentxide n glass cttn t remve carbn dixide and water vapr. The temperature f the bath surrunding the adsrptin chamber was determined with either a nitrgen r xygen vapof-pressure thermmeter. The temperatures 9. and 77.7 K were btained with liquid xygen and liquid nitrgen, respectively, biling at at,mspheric pressurc. The temperatures 72.2 and 69.5 K were btained with a special crystat described in the fllwing paragraph. n all cases the temperat ure f the adsrptin chamber was cnstant t within abut.5 K Crystat The crystat, see figure 2, cnsisted f a widemuthed -liter Dewar fitted with a rubber stpper and a 3.8-cm inside diameter brass tube clsed at ne end, that reached nearly t the bttm. Liquid nitrgen placed in the annular space between t he brass tube and the Dewar was biled und er a fixed reduced pressure t btain the desired temperature. n peratin, air cnde nsed inside the brass tube t frm all excellen t heat-transfer medium between the adsrptin t ube and brass tube. The vaprized nitrgen was drawn ut f the crystat thrugh tw 8-mm inside diameter tubes t a surge tank fr smthing pressure flu ctuatin s. The pm",ure cntrller was,imil.r t that u"d ij vacuum distillatins and cnsisted f a shrt mercury manmeter with electrdes, as shwn in figure 2. T start peratins th e stpcck n the cntrller was pened t the vacuum. When the desired perating pressure was reached, the stpcck was clsed and the cntrller thereafter maintained this pressure. As a cnsiderable vlume f ni trgen was vaprized, the piping, slenid valve, and the vacuum pump were adjusted t prvide an adequate capacity. n peratin, the slenid valve cycled abut nce every 2 t 5 sees. Each pening f the valve brught a sudden pressure decrease, but with a sufficiencly large surge tank these fluctuatins resulted in variatins f nly abut 5-mm Hg pressure ver the liquid nitrgen in the crystat. The slpe f the vaprpressure curve in the perating regin is abut 5 mm/deg, h ence, the pressure variatin crrespnded t changes f nly abut.1 deg C. These fluctuatins were smthed by the heat capacity f the apparatus, and variatins f nly ±.5 cleg K were indicated by the vapr-pressure thermmeter. As a cnstant temperature was maintained ver a number f hurs, it was necessary t replenish the s upply f liquid nitrgen in the crystat. A valve designed t perate when immersed in liquid nitrgen is shwn in figure 2. t cnsisted f a Pyrex ball jint withut grease, with the tub ing t the ball jint sealed ff and extended ut f the liquid, where it served as a handle. n the ff-psitin the ball was almst disengaged frm the scket. n this psitin the pening in the scket was effectively clsed by the sh ulder f the ball. T pen the valve, the handle was twisted slightly t further disengage the jint. Despite minr leaks and infrequent clgging with ice crystals, th e peratin was quite satisfactry. The entire crystat was lagged except fr a small windw t bserve the level f liquid nitrgen. Abut 3 liters f liquid nitrgen was cnsumed per hur in maintaining a cnstant temperature f abut 7 e. PRESSURE CONTROLLER 2.2. Crrectins fr Thermmlecular Diffusin As there is an appreciable temperature difference between the manmeter and the adsrb en t, there is the pssibility f intrducing appreciable errr frm thermmlecular diffusin when measurements are made at very lw abslute pressures. A careful review f the available literature was made [9,1,11, 12], and the fllwing equatin f Liang [11] was cnsidered the mst apprpriate fr calculating the thermal transpiratin factrs: "'''LV'! HANDLE. :U88ER STOPPE" HtGH CAPACTY VACUUM PUMP CONDENSED f.ter OE... 1t NTltOGEN lollng UNOE" VACUUM...us TUllE SAMPLE OF ADSORBENT FG U RE 2. AR, HEAT (XCHANGE MED UM DEWAR ahe (<gx?+f3he (<gx) + R", ahe (lpgx)2 + f3he (<gx) +1 ' VAPO't PRESSUrtE THERMOMfTffl -;_w;j where P 2 is the bserved pressure, P the crrected pressure, and R the crrectin factr. X is the prduct f P 2 by d, where d is the diameter f the tube cnnecting the cld t the warm part f the apparatus. Rm = (TdT2)1 /2, where T=temperaturef the Diagrammatic sketch f the liquid-nitrgen crystat and cntrls. 12 ' (1)

3 (' ld end f the c nn ective tube (adsrbent temperature) and T 2 = temperature f th e warm end f the c n neetive tu be (rm temperature). The terms a H e and /3He arc empirical cnstants: (1) ll!he= 2.52 independent f temp erat ure a nd (2) /3He = 7.68 (J - R m ), the press ure being m easured in millimeters f mercury and the tube diameter in millimeters. The factr <Pg is th e empirical relative "pressureshifting factol"" and is dependent upn the gas bu t nt the temperature. t is related t the cllisin diameter f the gas mlecules. With the abve listed values f ll! and /3, <P He= 1 and <PN2 = t is ev ident that as the pressure appraches zer (X--? O); then R appraches its theretical minimum valu e, Rm- Als, fr high pressures (large values f X ), R ppraches unity. n t he present wrk, the cnnectmg tube was l.o-mm diam, and nitrgen was the gas used at lw pressures. n figure 3 are SbO'VJl the variatins f B with pressure fr th e present case in. t.he in terval frm.1 t 1 mm Hg fr t he fur dfferent temperatures. The values f R fr pressures abve 1 mm Hg are very clse t unity. mm..,_loo p....j 2.3. Materials Th e t hree carbn adsrbents were amng thse previusly investigated [8J. C har 2 was a n ew bne char btained frm the manufacturer. C har 27 had b.een empl?yed in a cane-sugar refinery fr a c nsderable tme and had been remved frm t h e cbar stck because f its high specific density (1.3 gjml cmpared t.63 gjml f char 2). The sample f char 27 used in this wrk differed frm that previusly reprted [8] in tha.t freign substances visually detectable were mechamcally remved. C har 1 an activated ccnut-sh ell charcal, was identical,ith that previusly reprted [8J. 3. Experimental Prcedure The three adsrb en ts were simultan eusly u tgassed at 4 C fr 16 h' at the b eginnin g f the experimen ts. Th e pressure ver the freshly utgassed adsrbent befre any additin f nitrgen was always f t he rder f 1-6 mm Hg. n all cases, t he first add itin f nitrgen was very small s that equilibrium was attained at a very l, pressure. fhen, mre mtrge n was added, and the lum es adsrbed w er btained at increasing equilbrium pressures. After sever al pints had be en determined, t be samples were utgassed fr J 6 h1' at rm temperature. This prccss f adsrptin and utgas in g at rm temperature was repeated several tmes fr each sample. B ecause f the areat experimcntal diffi cul ty in btaining desrpti n bda ta in the lw press ure ran ge, all f the res ul ts reprted here arc fr adsrptin. <n 4. Results C...J 4.1. Attainment f Steady-State Pressure z :E ::...J lop r-----t-. 7-ft------r :: '" :E '"ex::: <n <n '" ex: a..... ex:... p 69.,-1( <n CD " " ci '"...,..:....1 P ''-;.4: :_l: 'L,-----L L-----.J R= P CORRECTED POBSERVE FGU RE 3. Th ermal-transpiratin fa ctrs fr nitrgen in a 1.-mm t1.be. Th e time required t reach a steady state was fund t b e a functin f tbe final pressure f the adsrbed nitrgen, and d e pend ed als UpOl1 the stage f utgassing peratins. After t be first utaassin a b b at. 49 C, t J1e s.tea d. 1. state at t he lwest pressure readmg was attamed t 2 h', but in the m easm'ements after the final utgassings at rm temperature the attainm ent f a steady state required a much ln ger time, as is illustrated in figure 4. n all cases, t he attainment f a steady state after the intrductin f the first prtin f nitrgen required th e ln gest time. Curve A in figure 4 was bserved fr the lwest pint f the istherm fr char 27 at 9 K (see fig. 6) and a steady-state pressure f.4 J.L was realized after 1 hr. n several ther similar cases it was bserved that such a steady state persisted after 13 additinal hr. Curve B' f fiaure 4 illustrates that nly 3 h' was required whn the steady-state pressure was abut 2 J.L. When the pressure was in excess f 1 mm, the steady state was reached in abut 2 min, in agreement with earlier exp erien ce [13J (curves C, D, E, and F, fig. 4). The. influen.ce f cnsecutive utgassings n the quantlty f mtrgen adsrbed was f particular interest. Figure 5 cn tains the lw-pressure adsrp

4 L 7mm '" ":: -" <.) OOmm Omm - :: mm <.) UJ "'. UJ :: <f 1 i: 8 UJ <f CD 1 \xx,,..! [< E c!, 1 B \ \ j. A.- j TARLE 1. Adsrptin,.,f nitrgen n char 2 9. K n,7 K 69.5 K pl' F pl' V pl' X lo- 7 rnl rnl ml 3. ()(). 28 X X lo X JO , 2 X XO-' X JO X O-' X 1-' X 1-' XlO- ' ll, X lo-' , 22X lo- ' X -' X lo X O-' 15. n X O- ' , 18XO ()() X 1-' X O-' 27.96, X 1-' X O X 1-' X X X -' 26., X X l ',,,= ',,= V'/= '=2,698 mm R g 1'= mll H g 1'= 27.3 mm Hg 11 expressed at K and 1 atm TME, HOURS Attainment f steady state at varius final pressw"es. FGURE 4. Curyes A and B: char 27 at 9 K ; C, D, E, and F: char 27 at 72.2 K. 12 r---t_----_i _------_ , , , = , $ 'E! 8--f_ _i _------_ ! :- 4 H :l,.l= O L_--- FGURE 5. XO-:3 2XO-3 3 XO- 3 4 XO- 3 5XO-3 RELATVE PRESSURE, p/ p Lw-pressure istherms f nitrgen n char 2 at 9 K. T he infiucnce f cnsecutive utgassings with the adsrbent at rm tern perat.ure is shwn; init ial utgassin g a t 4 C. tin data fr char 2 at 9 K, and it m ay be seen frm this example that t he amun t adsrbed increased with cnsecutive utgassings. By in terplatin g each curve t an equilibrium relative pressure f 2 X 1- " the vlume f nitrgen (milliliter STP) adsrbed was.4 ml aftcr the initial utgassing at 4 C and.65, 1.6, 4.3, 9.1, 9.1, and 9.1 ml, respectively, after each f six cnsecutive lltgassings TARLE 2. Adsrpti n f nitl'gen n cha?o K 7i.i K i2.2 K: 1'11' T pi p F 1'11' V ml ml m l 8. 5 X X O X lo X 1-' X O ,53X O-' , X 1-' X O X lo- ' X 1-' XO-' X 1-3 5, 1. llx O- ' X 1O- ' X -' X -' 3. 1, X lo-' X -' 3. 68, X -' , 23X-' 4, 31, X 1-' , X l-' 4.23, 362 5, , , ;: , , , , , , ",,=6. 28 ',,=6.6D V ",= 6.95 P= 2,698 mm H g 1'= 796,5 mll Hg P=397,3 mm H g at rm temperature. The istherms becm e cinciden t abve a r elative pressure f abut 1-2, in which regin this effect was nt eviden t. n the first adsrptin experimen ts with chars 2 and 27 after the first utgassing at 4 C, it small increase in pressure was bserved i.n the system when t he first prtin f nitrgen at very lw pressure was intrduced int t he adsrptin chamber. This increase was bserved nly during the first few minutes 14

5 , 1 TABLE 3. Adsrptin j nitrgen n char 1 9. K pip, pip, ' X - ' 1. 5XO-' 5.7X -' 1.74X- 3.5 X X 1- ' u.8x O- ' 2.7 X -' 5.2X O-' 7.9X 1-' S _ _._- rnl :33(j :]4: : :)54.87 :],\6.: : : X - ' 2.95X 1-' 9.26X 1-' 2.3X - ' 4.J9X 1-' 8.67X 1- ' 1.5X 1-' 2.4X 1-' 2.79X 1-' 7.12 X - ' :1:32 ii:] (i rnl JOG J , M \ '.. =378 P= 2,698 mm f g 1'.. =397 P= i96.5 mm H -'! f cntact, and then the pressure decreascd in a nrmal way tward equilibrium. When t his effect was bserved, the final values f the relative pressures were ntably high. A pssible explanatin is presented in a later paragraph. The Limes required t realize a steady state fr char 27 were nly abut half thse required fr chars 2 and 1. at the sam e final pressure. Perids f abu t 5 hl' were necessary t attain equilibrium in chars 1 and 2 at t he lwest pressures btained. The vlumes f nitrgen adsrbed fr char 2 ( , 9. K ), char 27 (72.2, 77.7, 9. K ), a d char 1 (77.7, 9. K ) are given in tables ], 2, and 3, respectively. The data recrded are th final steady-state values bserved after a number f cnsecutive utgassings at rm tempera Lures sufficien t t yield reprducible values fr t he vlumes f nitrgen adsrbed Calculatin f stherm Equatin An istherm equatin was sught, valid fr t he regin f the lwest measurable pres ures, which culd then be used with sm e cnfidence t extraplate t the vlume adsrbed at pressures t lw t measul'e. Such an extraplatin is necessary in t he calculatin f thermdynamic functins f adsrbed mlecules [14]. Plts f the vlume adsrbed as a functin f relative pressure fr chars 2, 27, and 1 yielded curves that became sleeper as the pressure was reduced, as shwn in figure 6 fr char 27. A lin ear behavir was fund, hwever, n a Freundlich plt fr all temperatures and all chars frm the lwest relative pressul'es up t abut 1-4 Typical plts are shwn in fi gure 7. The istherms bserved at the lwer temperatures were linear up t a higher value f relative press ure. Fr example, in char 27 at 72.2 K a straight line was bserved up t p /P 3 X 1-2 ; at 77.7 K the straight line was valid up L 5 X lo- \ and at 9. K up t 4 X O- 4 A similar behavir was bserved in chars 1 a nd 2. Parameters n f Lhe Freundlich equatin w('re determined, and these are given in Lablc 4. T AH LB 4. C har 'T'C'lll penlture stherm cnstants Jr adsrptin j nil?'gen Freundlich cnstant, fl B.E.T. parameters \ '.. c /,'. Surface area { / ( ml m'l(f b 4() L. 4.3S { { b () i ,) 4.38 \ _ R9 11 Packing cnstant based n density f liquid nitrgen fr cnverti ng V", t surface area. b Calculated frm t he Lang muir istherm. 8 c gj <f) w ":l "', c; '. XQ K 77.7 K 9. K 1 2XK) 3XO - 4 XO - RELATVE PRESSURE, p/p 1 l i 5XO Fw URE 6. Nitrgen adsrptin istherms n char 27 at 72.2, 77.6, and 9 K. 15

6 ... '; 1 :: f-!!!- = E - - "''" '" Ul 72.Z 1( ( 9. "1( ( «"' : :..J RELATVE PRESSURE. p/p 7. Freundlich istherms fr the nitrgen adsrptin data at lw pressure n char 27 at 72.2, 77.6, and 9 K. FG URE Bth scales arc lgarithmic Surface Area Calculatins CO 2, CO, and Hz by cntinued thermal treatment. Sme f this cmplex culd be remved during the utgassing at 4 C, but an appreciable prtin always remained in the adsrbent. The presence f this residual cmplex culd have a majr influen ce n the bserved steady-state pressure in the range f very lw pressures. The experimental results suggest that ne f the factrs respnsible fr the behavir bserved at lw pressures may be directly related t the residual cmplex. The first prtin f nitrgen intrduced was able t displace sme cnstituent f the cmplex, and, as a result, the liberated decmpsitin prducts, such as CO, CO 2, and H 2, cntributed t the bserved pressure. Unfrtunately, means were nt available t analyze the prducts f utgassing at the lw pressures emplyed, but this wuld be a desirable future experiment. The cnsecutive utgassings at rm temperature, tgether with intrductin f mre nitrgen, served t remve the cmplex until a stage f r elative stability was reached, as shwn by the final reprducible values. The particular prgram f utgassing is bviusly ne f a pssible large number. Any alteratin in the technique f utgassing, such as a secnd heating t 4 C, wuld n dubt have had an additinal influence n the number f adsrptin sites shared between the surface cmplex and the adsrbed nitrg-en. Anther factr f influence is assciated with the diffusin f gas mlecules at lw pressures. Because f the intricate structure f the prus material, mre time is required fr the nitrgen t lcate the adsrptin sites f minimum ptential energy. A similar argument may b e valid in cnnectin with surfacediffusin phenmena. Sme such cnsideratin is required t explain the fact that even relatively well utgassed adsrbents (4 C fr 16 h') required lng perids t attain equilibrium in the lw-pressure range [15]. Other things b eing equal, relatively shrter perids were required t attain a steady state fr char 27 cmpared t char 2. t has been shwn elsewhere [16] that the crystalline structure f basic calcium phsphate (abut 9 percent in these materials) was The adsrptin data fr chars 2 and 27 were fund t give lineal' B. E. T. plts in the range f relative pressures between.5 and.35. The values f Vm and c were evaluated frm the B. E. T. equatin fr a free surface (2) where V (reprted at STP) is the vlume adsrbed at pressure p, and P is the saturatin pressure. The values f the cnstants V rn and c are given in table 4. The crrespnding surface areas were based n a packing factr calculated frm the density f liquid nitrgen [8]. The values given in table 4 were calculated frm the adsrptin data recrded in tables 1, 2, and 3. V rn, als calculated frm the data btained in the first adsrptin run after utgassing at 4 C, was fund t be 26.5 cmpared t 25.5 fr the steady-state value. Despi te the large difference between the first prtin f this istherm and the crrespnding part f the istherm btained after several utgassings at rm temperature, there was clse agreement in the values f V m btained frm the tw sets f data. The adsrptin data fr char 1 did nt give a linear B. E. T. plt. The crrect values f Vm fr char 1 are given by Langmuir plts, which were fund valid in the relative pressure range between.2 and _8. The values f Vm at 77.7 and 9. K are als given in table Discussin 5.1. Attainment f Steady-State Pressures As the carbn adsrbents investigated were essentially pyrlysis residues f bne r ccnut shell and nt subj ect t a defined state f decmpsitin, it is highly prbable that sme surface cmplex cntaining carbn, hydrgen, and xygen remained with the adsrbent. This culd decmpse int 16 '

7 ;, "w, mre highly develped in char 27. n this case the cntributin f the vids in t he prus structure t the ttal adsrbing surface was less. n additin, the carbn aceus r esidue in char 27 bad a gr eater stability as a r esul t f many heatings t which it had been subj ected in t he cmmercial r ev ivificatin prcess Surface Area When th e rati V /V m was pltted (see fig. 8) as a fun ctin f p/p frm 1-7 t 1- \ i t, was fund t hat the curves fr chars 2 and 27 were in cincidence at 9 K. n t hi s r egin n mre t han 4 pet'ce n t f the surface was cvered. This agreed with previus wrk in th e range.1 t. 3 [8], where t he entire surface was cvered. Hwever, at t he lwer temperatures, t he curves fr differen t adsrben ts were separated t a sign ifi cant exten t, which implies a different interactin f t he nitrgen ml ecules with t he surface. Fr t he case f a given adsrbent at 2 r 3 temperatures, t he curv es were separated t greater exte nts than indicated by the crres pnding chan ges in t he densities f liquid nitrgen. The values f V /V m ar e largest at the lwest temperature fr a given valu e f p/p' The surface areas given in t able 4 ind icate an in crease with decrease in t he temp er ature. t is nt knwn wh eth er r nt this is within t he experim ental errr f t he Vm determinat in. t may als be related either with unknwn changes in density f t he adsrbed nitrgen p hase r with t he pssibili ty t hat t he effective van del' Waal's diametcr f the adsrb ed nitrgen mlecule at t he differ ent temperatu res is cri t ical with t he dimensins f the v id. Therefrc, t he s urface a vailable t a ni trge n mlecule, especially in prus m ateri a ls, mi ght be c ttespnd i n gl' greater Validity f stherm at Very Lw Pressures Th e exp erimental data shw that a Freundlich ist herm is valid in the m easurable range at very lw press ures. When a Langmuir istherm is valid at lw pressures, H enry's law, V = ax, is autmatically btain ed as the limiting case. Hill [17] has shwn frm statistical mechanical-tb ermdyn amic cnsideratins t hat all adsrp tin istherms must apprach H enry 's law. Hence, in t he present case wher e Freundlich istherms a re bserved at the lwest pressures, it is n ecessary t pstulate a tran si. tin range belw which ne mu st r eali ze t he lin eal' depend ence f adsrbed vlume n pressure. t is nt knwn wh ere t hi s range exists fr the adso l'bents investigated. Fr H enry's law t b e beyed, the slpe f the plts in figure 7 must be unity. t m ay b e seen frm these gr aphs that t he first bserved pints at t he lwest pressure fr the isth erms at 9. and 77.7 K are displaced in the crrect directin. Hwever, it will be n ecessary t btain additinal data at even lwer pressure t be sure f t his behavir Cnclusins.5 The attainment f equilibrium at very lw press ures is a cmplica ted prcess, in whieh t he nat ure f the s urface and the impurit ies asscia ted with t he slid surface pl ay a n imprlan t rl e. Variatins in the utgassing techniques can chan ge this par t f the istherm t a nta ble ex tent. As th e nat ure f the pred min a nt frces respnsible fr adsrptin cba nges signifi cantly with s urface cverage, n knwn is t herm equatin ean be expected t be valid frm x = O t x = 1. The Freu ndlich equatin appears t be valid at very lw pressures in a grea t many cases, but it is nt valid when the surface cverage appraches a mnlayer wher e x is apprximately.1. The B.E.T. equatin appears t be frequently valid betwee n the pin t wh ere th e Freundli ch r elatinship fails and relative press ures f abut.35. n this regin (.5 t. 35) the factrs that arc nt taken int accunt in the B.E.T. mdel wuld nt cn tribute appreciably t the vlum e adsrbed. The beh avir abve.35 in relative press ure has at present n adequate th eretical basis. n general, ne can say th at the nature f the predminant frces invlved in an y adsrptin prcess is a func t in n t nly f the nature f th e adsrbate, adsrbent, a nd temperature, but als f the pressure range, whi ch determines the fractin f t he surface cvered. r , , , , , G9C."" E ' 9C.",,ft \.2.1 lf j j F rcu RE 8. rl1he ra ti, (l1, Lw-pressure adsrpti n data f r char 27 at 72.2, 77.7, and 9 K. is shwn a.s a fun ctin f. t where x= rclativc pressure, a nd V= vlum e f nitrgen adsrbed (S'rp ). 17

8 References {] T. L. Hill, P. H. Emmett, and L. G. Jyn er, J. Am. Chem Sc. 73, 512 (1951). [2] T. L. Hill, J. Ch ern. Phys. 17, 762 (1949). [3] G. D. Halsey, J. Am. Chem. Sc. 73, 2693 (1951). [4] T. N. Rhdin, J. Am. Chem. Sc. 72, 5692 (195). [5] G. D. Halsey and H. S. Taylr, J. Chern. Phys. 15, 624 (1947). [6] R. Sips, J. Chern. Phys. 16, 49 (1948); 18, 124 (195). [7] M. H. Ck, E. H. Pack, a nd A. G. Oblad, J. Chem. Phys. 19, 267 (1951 ). [8] V. R. Deitz and L. F. GJeysteen, J. R esearch N BS 29, 191 (1942) RP1496. [9] F. C. Thmpkins and D. E. Wheeler, Trans. Faraday Sc. 29, 1248 (1933). [1] S. Weber, W. H. <eesm, and G. Schmidt, R ep. V Cngres nt. Frid 2, 45, 61, 74 (1936). [11] S. Chu Liang, J. App!. Phys. 22, 148 (1951); J. Phys. Chem. 56, 66 (1952); 57, 91 (1953). [12] W. V. Lbenst ein a nd V. R. D eitz, J. R esearch NBS <i6, 51 (1951) RP2174. [13] V. R. Deitz and L. F. GJ eyst een, J. R esearch N BS (1945) RP1674. ' [14] T. L. Hill, J. Chem. Phys. 17, 52 (1 949). [15] S. Chu Liang, J. Phys. Chem. 57, 84 (1953). [16] R eprt fr the Five Y ear P erid f the Bne Char R esearch Prject. nc. [17] T. L. Hill, J. Ch em. Phys. 18, 246 (195). 'VASHNG'l'ON, D ecember 3,