I ~ I S S O C I L ~ T I OSTUDIES, X I
BT; TTILDEK D. I3.ISCKOFT
In an unfinished series of papers on the equilibria of stereoisoinersI there has been gil-eii a discnssion of some of the phenonietia to be espectecl \vlien one snbstance changes more or less completelj. iiito another in the liquid plinse at a moderately low rate of speed. In this paper it is proposed to outline some of tlie plieiioiiieiia to lie expected n-lien oiie siibstaiice changes slon.1~-into t n o different substances in tlie liqiiicl or vapor pliase ; it is proposed to stnclg. the dissociation of conipoiiiids froin the point of Tiel\- of Duheni’s theor>-. T h e paper is also 3 continuatioii of a preceding one 011 the pressui-e-teinperature cliagraiiis for t\vo-component s:.steim2 i n Fig, I is given tlie scliematic
,d
t
A
C
Fig.
I
concentratioii temperature diagram for a binar!. systeni in wliicli a compound esists that does iiot clissociate iiieasixab1~-into tlie components in solution. Siicli a system could be realized with acetic anhydrid and n-ater, since acetic a n h j d r i d must freeze at some temperature, and it is verj- improbable that acetic acid aiid acetic anhydrid form t\vo liquid layers at any teniperatnre.
I n the diagram, Fig. I . the point -\ represents the iiieltiiigpoiiit of the component -1, the point n tlie ineltiiig-point of tlie coiiipoiieiit H , aiicl the point C the iiielting-point o f the coiiipoiiiid C. T h e curve & I D is the houiiclarj. ciirve for component -\, soliitioii and vapor, DC E is the Iiouiidarj- curl-e for coiiipoiiiid, solution aiid vapor, svhile ER is the correspoiidiiig ciirvtl for the component R,:iolutioli aiid vapor. If tlie coiiipoiiiid doe5 not dissociate a t all, DC aiicl EC are t\vo cm-1-es meeting at ai1 angle : otherwise the curve has a triie, roiuided teiiipernturr iiiasimiiiii. The diagraiii lins been so drann that tlie coiiipoi~nd iiielts a t a higher teiiiperatnre tliaii either of the conipoiieiits. I t is not kiion.ii Tvliether this is iiecessarilj- the case; but i t is certainlj- the iiiost coiiiiiioii state of things. It is to be noticed tliat the concentrations, as 4 i o n n in the diagi-ani, are the aiialjticnl concentrations and that the aiiioiuit of the coiiipo~tiidpresent i a the solution is not gi\-ttii. If the compouiid be supposecl to dissociate iii solution, tlie dissociation products n-ill lon-er the partial pressure ol tlie coiiipouiid and will therefore l o n e r its freezing-point. Since t h e aiiioiiiit of dissociation is fised for each teiiiperatiire, pro\.itletl the vapor phase be infiiiitesiiiia! or coiistaiit iii anionlit, tlie freezing-point will be lon-ered onlj. to a perfectlj- tlefiiiite aiiioiint. There \{.ill thus be a stable ineltiiig-point corresponding to the stable triple-point in tlie case of stereoisomers Iii t h e dingrani, Fig. I , this point is marked CI. Otlier things being eqiial, the teiiiperature clifference between C and C, Jvill 1m-j. n-itli tlie atiiount of dissociation. If the coiiipoiiiid he snfficieiitl!. dissociated, the poiiit C1ma>-fall helolr the coiltinuation of 13E, in which case a stable iiieltiiig-point is no longer possible. If the point Cr be lon-ered still farther, tlie whole cur\-e rion. iiiarked D C E iiiaj- lie helovi tlie extension of the curves .ID and BE, in which case the coiiipiiiid caiiiiot exist i n equilibriiiiii \\-it11 solution. T h i s is apparentlj- the case n-itli the double salt,I
2SLj.
Cf. van t k r \\-ads.
' Bauer.
Iion. .ikaci. ~ ~ e t e l i s c Terslag, h. .iinstrrtlatn, 1897,
Zeit. plij-s. Cliem. 18, rSo ( 1 S 9 j ) .
11i'l0'c.i- D. B n 71 c7,ift
7.1
HgIz2,4gI, and with the racemate forin of inethyl inannosid.* If the rate of dissociation in solution he sufficieiitlj- low, tlie coniponnd ma!. be iiiade to melt at an!- temperature hetween C aiid C,,tlie highest temperature being reached by rapid heating. If \ye start with niistures coiitaiiiiiig the compound and ai1 excess of one of the components. we sliall get melting-points l!.ing on the ciirve D C E if we lieat rapidly and melting-points Ij-ing 011 a curve passing througli CIin case we lieat very slowl!.. Theoreticall>-this ne\v curve will cut a new curve AII)Ia t DI at a temperature lower than that of DI and will cut a new curve REI a t EI a t a temperature Ion-er than that of EI. I t is probable however that, in the large majorit!- of cases, no serious error xi11 be introduced by assnining that the same curves A D and B E are obtained u-hether n - e lieat slon-ly or rapidly and that D, coincides with L) aiid EIwith E. TYe should then have tlie ciirve I)CIE for the stable freezinx-point curve aiid D C E as the limiting instable freezing-point cur\-e, nliile solid, liquid aiid vapor c:in be realized temporaril!- in an!- part of the field CDCIEC provided of course that the curve D C E can he realized esperiineii tal 1J-. Froin the iiiiblislied data plithalic anlijdrid and water appeared to lie a s!-steni coining tinder this head. Phthalic acid is said t)j- Lossen* to iiielt a t IS^" ; by Aclorj to melt a t 203' when pulverizecl, aiid at 213' when in large crj-stals;. Crraehel sa!.s that x l i e n not heated too rapidlj- the melting-point is 190' 19j". Howej appears to liax-e been misled bj. these discrepancies into assuming the existence of two isonieric forms of phthalic acid, one iiieltiiig at 184' aiid the other a t 203' or higher. T h i s error has been corrected by TTheeler6 who attributes the 1-ariation of the melting-point, as Keinsen7 had ~
I
IJ
Fischer and Reeiisch. Ber. clietn. Ges. lierliii, z g , 2927 Liehig's .~ntialeti,144,7s (1S6; j . Ihid. 164, z j o (1872 1 . A1m.Clietn. Jour. 18, 390 (1S96). Itlid. 18, S 2 S (1S96). Ibitl. 3, j o ( I S S I ) .
~
1S9;).
done before Iiiin, to tlie formation of anliydrid aiid water. TTleeler, howeyer> practicall!- cleiiies tlie existence of any point at about 18.4". He sa!.s : ' ' I t is \vt.ll-kno\m that the ineltiiigpoint of ortlioplitlialic acid is reall!. its decomposition point, i. e. the acid loses water and passes into die anhydridt., the nieltingpoiiit of the anli!dride being about 128". I t would be expected tliat this decomposition niiglit be prodiiced a t different teiiiperatures according to the length of time the snhstance is liented, according to the quaiitit!. of tlie substance taken and according to the condition, botli pli!.sical and chemical, of the saiiiple. Sucli indeed has been foiind tlie case. I t is t1it:refore not siirprisiiig that, if phthalic acid is heated abol-e IzS', differelit 111el ti iig-poi i i t s a re ob t ai ne tl 11 11der (1i fferen t coiid i ti cms. ' ' T 11is s t a t e i n a t is i:ot in accord Lvitli the theor!. as del-eloped in these pages. The discrepant!. is due to the fact that Tylieeler is (lisciissiiig the changes that occiir during continiious distiilatioii froiii an open melting-point tube, for lie says esplicitlj- that the acid loses water". ;It the elid of the esprriiiieiit TTlieeler Iins oiil!, phthalic a n l i ~ d r i t land 110 plitlialic acid Rarriiig tlie oyerlooking of a possiblt. eutectic point with phthalic acid and plitlialic aiili!-drid as solid phases,' this statement of TYlieeler's is entirelJ- correct ; l x i r it applies oiily to :i ca:e of fractional distillatioii and therefore tliron-s 110 light 011 the question of a stable iiieltiiig-poiiit for p h t l i ~ ~ lacid. ic I t occurred to iiie tllat the ilieltiiig-point of 184' obtaiiiecl 11~-Hone was prohnbl!. the stalllr iiielting-Foiiit of phtlialic acid, corresponding to tlie point Cz in the diagraiii. - i t iii!. req:iest l i r . < ). IT.I. deteriiiiiiing the coiiceiitrations on the cur\-e I X E correspoiiding to a temperatnre of 184". T h e first coiiditioii n-as iiot fulfilled in the experinieiits of 3Ir. Ih-owi and the second is iiot f~rlfilled at all, so that for the present we are niiahle to estimate the amount of dissociation ~
' 'rllis - KIEBK. TT‘heii the phases are solid A)solution aiicl \.apor the partial pressures are given bj- & I D and A’Dr ; when the phases are solid coinpotild, solution and \-apor, tlie partial pressures are given b>. DEI and DIE: ; when the phases are solid solution and vapor, tlie partial pressures are given by Ern’ and E:P, n.hile D H I aiid DIX,, EIJII and E K l are the pairs of curves for the s!.stem solid A, solid coinpoutid aiid 1-apor. and for the systeni, solid P,. solid coinpoutid and vapor respecti\-el!.. the first-mentioned curve giving tlirongliont the partial pressures of aiid the second the partial pressures of P,. T h i s cliagrain could he realized viitli sodium sulfate aiid water. I t is to he noticed that the curve:; DEr mid JIIE, meet a t an angle because
Fig. S
the>- refer to different univariaiit systems. I n the case of a salt and n a t e r the curve B ’ E I will have a distinct pressure maxiiiiiiiii ; but that is not necessarily the case for a11 s>.steins of this class. I n all cases the metastable prolongation of B’EI will lie below DE,. T h i s point has alreadj- been discussed in a precec-1iiig paper1 so that a reference here will suffice. I t is to he noticed that a coinpound instable a t its melting-point caiiiiot ha\-e a
LVildLv D. Bnizcroft
92
m a s i m u m sublimation temperature of the first type, though i t may have one of the second. Before closing this rather long paper it will be necessary to consider a curious phenomenon discovered by Ramsay and Young. T h e equilibrium pressures of phthalic acid and still more of succinic acid, as determined by the baronietric method, vary wit11 the amount of acid taken. I quote what Ramsaj- and Y011ng say about the pressures of dissociation of succinic acid in a barometric tube.’ -1very large number of experiments \\-ere made with this substance. I n the first three series the numbers obtained gave, when graphically represented, curves with double flexure ; and so long as the same amount of substance was employed recombination at any gix-en temperature reached the same point as regards pressure as did decomposition. T h e curves obtained from these series were fairly concordant. I3nt, on increasing the ainount of substance, the pressures for gi\ en temperatures were mucli higher, and the double flexure tended to disappear. IYith phthalic acid it is possible that the vertical’ trend of the curve iiiiglit have been clue to a n insufficient amount of substance ; but this cannot have been the case with succinic acid. ;Is the pressure of dissociation of succinic acid appears to depend on the amount of substance present, and is capable of being altered a t will>and as the number of experiments made on this subject comprise several thonsand readings, and lasted over six weeks, we content ourselves n it11 reproducing the results i n the form of curves. It may be generally stated that the larger the amount of mbstatice the higher the pressure a t which the double flesure appears. It niaj possibly be that succinic acid has the power of forming intermediate anhydrides of some such and that its abnormal type as COOH.C2H4.C0.0.C0.C2H4.COOH, behavior is to be accounted for on the supposition of their existence, but this is merely a conjectiire.” From the plotted curves i t appears that for a given temperature, differences in pressure of as much as IOO mm are to ((
Phil. Trans. 177,I. 103 (1S86). Ramsay and Young measure pressures along the abscissas instead of along the ordinates a5 is usually done.
Dissociation Stzidies
93
be observed, the higher pressures corresponding to the larger amounts of succinic ac.id. These resnlts are so very surprising that they must be explained. W e may reject the hypothesis of a n intermediate anhydrid a t once. K h i l e we cannot prove that sach a componnd is not formed, the assumptioil does not help 11s in any way and may therefore be discarded as superfluous so far as the facts before 11s are concerned. T h e facts to be e s plained are that a t the same temperature different pressures are obtained by varying the amount of substance taken. Since v e are dealing with a two-component system there can only be t u o phases because such a phenomenon is impossible n-ith three phases. T h e vapor phase is certainly one of the two, and as the discrepancies are noticed a t temperatures a5 l o v az Soo, tlie other phase cannot be a liquid phase. T h e conclusion is therefore forced iipoii 11s that there is only one solid phase. Since the solid mass contains both succinic acid and succinic anh) drid, these two substances must form solid solutions. I t has ahead). been pointed out that 3Ir. Brown’s data for the freezingpoints of plithalic anhydrid and phthalic acid point to the formation of a series of solid solutions. Since Ramsay and \70u~ig find the same phenomena with phthalic acid as with succinic acid, there seems no reason to doubt that succinic acid and m c cinic anliydrid do c r j stallize together.’ espe-iall? as no other hvpothesis is possible. -issunling tlie formation of solid solutions, i t becomes a simple matter to explain the phenoinena. T h e vapor given off by siiccinic acid in the barometer tube con,sists chiefly of water. If we have a lapor phase of constant 1-01lime a certain amount of succinic acid will be dissociated. T h e residual amoiint of a n h drid ~ will be a sinaller percentage of the total iolid, t h e more succinic acid is taken. n’ith large amount> rlf succinic acid we should therefore get dilute solutions of anli)-drid in the acid and therefore a higher i-apor-pressare. T h i s is exactly what was found by Rainsaj and Young. Of course, if the vapor phase is not kept coilstant in amount, there will be minor fluctnations d a e to this. T h e smaller the vapor phase,
’
There is nothing in the crystalline forms of succinic acid am1 succinic ariliydrid to make this hJ-pothesis improhahle.
the less water nil1 ex-aporate from the solid mass and the more dilute will he the solid solution formed. ;lis RainsaJ-and Young giye no numerical data, it is impossible to tell whether this variation was a n important factor in their experiments or \vhetlier the discrepant results are to be referred solel!. to the variation in the iiiass of the solid phase. T h a t . however, is a question of no real importance. Tlie double flexure in the ciin-es iiiidoubtedl!liegins at the temperature at which the solid solntion is entirely inelted. One ciiriow feature about tlie cnrves is that the clouhle flexures occur between the temperatiires of 180‘ arid I 84” nliile succinic acid melts, according to Kainsay and Young, at I S I ’, ;It first sight this seems to negative tlie explanation of tlie doiible flexures just offered : b u t this difficult!. is not serious. Tlie inelting-point of ISI is undoubtedl!-, though not so stated, tlie stable melting-point of succinic acid with no yapor phase or ; I relativel!. sinal1 one. Under these circmnstances, the dissociation products are preseiit in the inelt in eqiii\-nleiit quantities. If we increase the 7.apor pliase, water vaporizes atid if there were no further dissociation i n the solntion it n.ould 11econiemore tliliite and the freezing-point ~ ~ o urise. l d Of course, as the [vater 1-aporizes more acid n-ill dissociate; but it is quite conceiT-ahle that a t first tlie diliiting due to the vaporization might more than coiiiiterlialance tlie concentrating of the solution clue to the further dissociation of the acid. If the nieasiirenients of Kainsaj- aiicl Yoiiiix are to lie taken as accurate, this must he the case. T h e main features of this paper iiiaT- lie re-stated as follon-s : I . There has been a geiiernl discussion of pressure-teiiiperatiire diagraiiis foi- tvio-coinpoiieiit s!,stenis in which a conipoiiiicl is forniecl. 2 . 1)ulieni’s theor!. of ‘ pernianent changes ’ has been shown to be of great value in stucl!.iiig these systeiiis. 3 , Explanations have lieen founcl for tlie interesting pheiioniena recorclecl 1 2 ~ . Ranisay a i d l-oiing in a paper entitled on E\-aporatioii aiid Ilissociation, Part I. Coi.mII L7ui;,cnifj,,I ) c < c I ~ z ~icq9%C cI.,
