Cao Co2 System

8/6/2019 Cao Co2 System

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M a y, 1023 SYSTEM, CALCIUM OXIDB-CARBON DIOXIDE 1167work of refining the crude material which was carried on by one of USin summer.

S u m m a r yTh is paper recoun ts a determ inatio n of t h e atomic weight of galliumby mean s of th e ana lysis of gallium chloride. T h e gallium wh ich form edthe starting point was very carefully purified, converted into chlorideby th e action of pu re chlorine, and fractionated in this form. T h e frac-tiona tion was effected by distillation an d su blim ation in chlorine, in nitro -gen, and in a vacuum. T h e sal t was analyzed b y essential ly th e samemethod as th a t employed in th e case of alum inum brom ide, an d yieldedthe result G a = 60.716, if silver is 107.88 and chlorine is 35.458.CAMBRIDGE8, MASSACHUSETTS

[CONTRIBUTIONRO M TH G GEOPHYSICALABORATORY,ARNGCIGNSTITUTIONFWASIIINCTONT H E SYS T E M , CAL CIUM OXIDE -CARBON DIOXIDE

B Y F. HASTINGSMYTHN D LEASON . ADAMSReceived February 10, 1023

Previous Invest igat ions and Purpose of t he P re se n t W orkThe reaction between calcium oxide and carbon dioxide has long beenthe subject of investigation. Co mp aratively few workers, however, havesoug ht the behavior of calcium carbona te when i t is subjected t o conditionsof relatively high tem pe ratu re an d pressure. T h e reason for this isdoubtless found in the fact t h a t difficulties of experimentation a t pressuresin th e neighborhood of a thousand m egabars and a t temp eratures abov e1000O are great. Nevertheless, a knowledge of th e pressure -tempe raturerelations of this s imple system is necessary before further exact knowledgecan readily b e ob tained of system s which contain m ixtures of calciumcarbon ate with other carbon ates or with silica. From systems, either withor w ithou t t h e presence of w ater, wou ld crystallize min erals such as th edolomites and silicate carbon ate compo unds of which spu rrite and can-crinite are examples. W e hope th a t th e present work may serve as a basisfor the further investigation of the natural condit ions which have givenrise to th e formation of m any complex minerals containing calcium car-bonate as one constituent.In th e following pages are described a n app aratu s which has proved rea-sonably satisfactory for such experimentation, an d th e results which havebeen obtained in the 2-componen t system, calcium oxide-carbon dioxide,the reac tion being represented by the equat ion, C a C 0 3 = C a 0 + COZ.Of th e equilibria da ta already o btained a t lower pressures an d tem -peratures, which may b e controlled in app aratu s of thin-walled tubes, only


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1168 F. HASTINGS SMYTH AND LEASON H. ADAMS Vol. 45th e resul ts of Johns ton will be cited,l since these conta in w ithin themselves,or supersede, th e results of earlier investigations. W e shall show later th a tthese d at a are consistent with th e results of th e present work.

At high pressures the most interesting, and probably the first, efforts to determinethe conditions under which calcium carbonate may be fused were made by Sir JamesHall in the opening years of the nineteenth century, and these early experiments haverecently been described by Flett* in all necessary detail. Hall heated various chargesof ground natural calcite and chalk, and of precipitated calcium carbonate in containersmade of sawed-off gun barrels. These containers were closed by welding a t one endand by a fusible plug at th e other. They were heated in an oil muffle furnace, the endwith the plug being allowed to protrude from the furnace in such a way tha t i t could becooled to prevent fusion of the plug. In the absence of pressure gages, a blow-off valveheld down by known weights was provided, by means of which the final pressure beforeth e opening of the valve could be recorded.With this apparatus Hall found, after 6 or 7 years of careful work, that 52 atmos-pheres was the lowest pressure at which calcium carbonate might be fused, or a t any ratesintered together, and th at a complete marble was formed at a pressure of 86 atmos-pheres, and carbonate of lime absolutely fused under a pressure of 17 3 atmospheres.It may not be out of place to pay tribute again t o the skill with which these ex-periments were conducted with apparatus which we would now call crude. The re-sults obtained are remarkably close to those which may be had with the use of theelaborate equipment of the modern laboratory.Other earlier attempts to heat calcium carbonate under high pressures were madewith similar apparatus. Le Chateliei3 and Joannis4 used a closed bomb made of heavyiron pipe which was heated in a gas furnace. Pressures were developed in these bombsby the partial dissociation of the initial charge of calcium carbonate.The difficulties inherent in these procedures are t hat the iron container is so muchweakened at high temperatures and that it is impossible to develop really high pressures,good temperature distribution throughout th e charge is not obtainable and the tempera-tures within the charge cannot be accurately measured. Purthermore, the meltingpoint of pure calcium carbonate cannot be obtained, for the charge is partialty decom-posed during the operation. Le Chatelier has concluded tha t the melting point of cal-cium carbonate lies at about 1020, under a pressure of about 10 megabars.Great improvement over these methods has been effected by H. E. B ~ e k e , ~ho hasconstructed a thick-walled bomb enclosing a small platinum-wound furnace. In thewalls of this container, which was capable of holding pressures up to 150 megabars, wereplaced insulating plugs carrying the furnace and therme16 eads. An opening for addingand withdrawing carbon dioxide was also provided. The principle of this apparatus isevidently correct. The bomb walls remain cool while the temperatures are generatedin a small space surrounding the material i o be heated, and can be measured accurately.A t the same time the pressure in the system can be controlled without dependence on thecarbonate charge as a source of carbon dioxide.For purposes of reference some of the results of Boekes work are tabulated in TableI. These results do not cover a sufficient range of pressure and temperature to determine

Johnston, THISJOURNAL,32, 938 (1910).Flett, Sci. Monthly, 13, 308 (1921).Le Chatelier, Compt. rend., 115, 817, 1009 (1892).Joannis, ib id . , 11.5, 934 (1892).Boeke, Neues Jahrb. Mineral. Geol., 1, 91 (1912).e Abbreviation for thermo-element, proposed by W . P. White.


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May, 1923 SYSTEM, CALCIUM OXIDE-CARBON DIOXIDE 1169the direction of a curve, and it will develop presently tha t they are somewhat discordantamong themselves, though evidently more nearly correct than any previously obtained.By a method of interpretation of his results which is obscure, Boeke places the melting

TABLEABOVE TH E EUTECTICOINT

t Pressure Pressure t Pressure Pressurec. megabars mm. O c. megabars mm.1275 146 109400 1280 117 874001282 124 92700 1271 104 782801274 142 106400 1250 74 554801272 139 104120 1250 81 608001276 10 1 76000

point of pure undissociated calcium carbonate a t 1289", under a minimum pressure ofcarbon dioxide of 110 megabars. He finds a eutectic between calcium carbonate and cal-cium oxide fusing a t 1218'. In addition, by using a heating-curve method, Boeke hasfound a reversible transformation of one form of calcium carbonate into another withaccompanying absorption of heat at about 970'.

SOME OF BOEKE'S ALUES FOR EQUILIBRIUMRESSURES FOR CALCIUM CARBONATE:

Theoretical OutlineFrom a theoret ical standpoint th e 2-component system, calciumoxide-carbon dioxide, m ay be tr ea ted as entirely similar to th e system ,cuprou s oxide-oxygen. Since a theoretical discussion of t h e la tte r sys tem

has been presented with considerable detail in other places, ' nothing beyon da general sta tem en t of t h e behavior and results to be expected in th e systemunde r discussion will be given here.According to t h e conditions of pressure and tem pera ture prevailingin the system, we may have the fol lowing react ions. (1) Solid calciumcarbonate dissociates to give solid calcium oxide and carbon dioxide gas.(2 ) A t and above the tem pera ture a t which ca lc ium carbonate a nd ca l-cium oxide fuse t o form a eute ctic mixture, dissociation of solid calciumcarb on ate to form calcium oxide is immediately followed by th e fusion ofth e calcium oxide formed with so me of th e remaining calcium carb onate t ogive a liquid ins tea d of a second solid phase. (3 ) If the solid calciumcarbonate disappears entirely, further dissociation at a given temp eraturemu st be from th e l iquid phase a nd is accompanied b y p recipitation of cal-cium oxide, since th e remova l of calcium c arbo nate from t h e solution wouldotherw ise increase th e con cen tration of calcium oxide, which is impossibleif th e tem per ature remains unchanged.Th ese three possibilities of a ction within th e sys tem m ay be summ arizedas follows.CaCOa (solid) CaO (solid) f COa (gas) (1)CaCOa (solid)+xCaCO3 (so1id)eCaO (liquid solution in xCaCO8) + COa Igas)(2)CaCOs (liquid solution in xCaO) * CaO(so1id) +- xCaO (solid) + COa (gas) (3 )

7 (a) Smyth and Roberts, THISOURNAL, 42,2582 (1920). (b) Roberts and Smyth,iln'd., 43, 1061 (1921).


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1170 1 HASTINGS SMYTH A N D LlCASON 11. A D I \ M S VOl. 45T h e foregoing cases exclude th e for m atio n of solid solutions, indic ationsof which hav e not been found.The pressure-temperature rclations for all three reactions may befollowed by means of the Clapeyron equat ion, dp/dT= A I I / T A V , inwhich AH represents the heat absorbed in each reaction when 1 mole ofcarbon dioxide gas is produced. I n Reaction 2 this includes the heatwhich would be absorbed if no fusion took place, plus a n additional hea tabsorption du e to th e fusion of th e x mols of calcium carb on ate with th e

1 mol of calcium oxide produ ced. T h e nu m ber of mols of calcium car-bo na te which fuse w ith on e mol of calcium oxide varies with th e pressureand temperature, but for given equilibrium conditions remains constant.In th e case of Re action 3, on th e other hand, the h eat which would beabsorbed b y th e reaction if th e calcium oxide and calcium c arbona te re-mained b oth as solid phases, is diminished by th e hc at evolved when thcrecrystallize from th e solution 1mol of ca lcium oxide produc ed a nd x mols ofcalcium oxide corresponding to th e calcium carbon ate lost in th e reaction.I n general , A H 1 is a positive quantity, AH2 is always greater than A H Iand AH3 is always algebraically less th a n AHl.Fo r Reaction 1 his equation ma y be integrated w ith a close approxim a-t ion to the t ru th b y assuming tha t the to ta l volume change, A V , is equal

to th e volume of carbon dioxide formed which, in tu rn , m ay be evaluatedin term s of pressure an d tem pera ture by th e assum ption of th e ideal gaslaws. A H m ay b e taken as co nstant , and with these assumptions th e inte-grate d equa tion is th a t of a stra igh t line w ith log I , and 1/T s t he variables.Below the eutectic point our experimental data for Reaction 1 nearlyjustify these assumptions. It is convenient to choose these variables topresent th e da ta throug hout the tem pera ture range of t he experiments, andth e equation of the actu al graph of th e da ta deviates bu t sl ight ly from t h atof a straight l ine. This actual equation can be determined from th e da ta,and from th is and an ac curate equation of st at e for carbon dioxide, AHfor Reaction 1 ma y be calculated a t any temperature.In spi te of the fact th a t AH is far from c on stan t after fusion takes placeto form a solution of compo sition varying w ith th e tem pera ture, it is stillconvenient to retain log p and l / T as variables to present th e da ta in theform of a plot, a nd this ha s accordingly been d one in Fig. 3 .Equilibrium pressure values for Reaction 1 have been summarized andaccurately checked b y Johnston, ' up to a tem perature of 894" and a pres-su re of 7lG mm . of mercury. T h e points determined from these d at a byplotting th e reciprocals of th e absolute tem pera ture against th e logarithmsof th e corresponding pressure also lie along a cu rvc which de viates b u t littlefrom a straigh t l ine.However, as th e pressure within t h e sys tem is raised, the correspondingtemperature reaches a point such that the calcium oxide formed by dis-


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May, 1923 SYSTGM, CALCIUM OXIDE-CARBON D IO X ID E 1171sociation no longer remains solid, b u t fuses with p a rt of th e remainingcalcium carbonate. This is the eutectic point , and here and a t highertemperatures Reaction 2 take s place. As long as solid calcium ca rbon ateremains, there are three phases and th e system is monovariant. T h epressure-temperature curve, however, assumes a steeper slope above theeutec tic poin t, because of increased hea t abso rption in th e fusion, a nd t h elogarithmic plot no longer approxim ates a straig ht line. As th e pressurean d temp era tur e continue t o rise, larger quan tities of calcium carbo nateare carried into solution by each mol of calcium oxide formed, un til th ecomposition of th e liquid approaches th a t of pure calcium carbona te.Only a t this l imiting pressure and tem pera ture may we speak of th e meltingpoint of pur e calcium carbonate. We shall see presently th a t this point hasbeen approached by us in temperature though perhaps not in pressure,and t h a t i t lies considerably above th e "melting point" as given by B oeke.It is in this respect th a t Boeke's points are discordant among themselves.Above t h e eutectic an increase in pressure mu st always be followed by a risein th e tem pera ture a t which carbon dioxide gas, solid calcium carbo nate a n dliquid co ntaining calcium carbonate and calcium oxide are in equilibrium.A glance a t Table I will show t h at this is no t uniformly th e case with th epoints here recorded. F or . example, a t 76000 mm. pressure, a meltingpoin t tem pera ture of 1276" is obta ined , while a t th e higher pressure109400 m m ., th e slightly lower tem pe ratu re of 1275" is foun d.As in th e general case, when a dissociation p rodu ct, h ere calcium oxide,remains th e solid phase, a nd calcium carbonate is present only in th e liquid,further dissociation is accompanied by a precipitation of more calciumoxide, and Reaction 3 takes place. T h e he at effect here is less th an t h a tof Reaction 1 and th e slope of th e pressure-temperature curve is less th a nt h a t of th e curve below the eutectic point. Along a section of this curve,possibly along th e m ajor p ortion, t h e equilibrium pressure falls with risingtempe rature, T h e general form of this curve has also been discussedelsewhere faand ,since no actua l points on i t have ye t been found, a furthertheoretical treatm ent will no t be given here. Its terminal point lies a t th ehigh tem pera ture an d correspondingly low pressure of th e melt ing point ofpure calcium oxide.T h e eutectic point is the q uadruple po int of th e system, and here calciumcarbonate, calcium oxide, liquid and calcium oxide gas can co-exist inequilibrium.I n connection with th e equil ibrium curve for React ion 1 i t remains tobe noted t h at if there be a t ransformation point as given by Boeke8of oneform of calcium carbonate ( P form) into another ( a form) with absorption

Wilhelm Eitel (from advance proof sheets of "Neues Jahrb. Mineral. Geol."furnished by the author), using Boeke's apparatus, has recently obtained thermal evi-dence similar to tha t of Boeke th at a transformation PCaC03E aCaCO3 takes placeat970" +5".


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1172 P. HASTINGSSMYTHAND LBASON H. ADAMS Vol. 45of hea t, there will be a change in slope of the curve a t th is poin t, and th eslope of the logarithmic plot between 970 and the eutectic point will be

I 10 Cms I

less than th a t of th e line de-fined by lower temperaturesand pressures.Apparatus

The apparatus used con-sists essentially of a heavywalled steel container showndiagrammatically in Fig. 1,inside of which is placed asmall platinum-wound resis-tance furnace.The body of the containeror bomb is cylindrical, open atboth ends, with an internal di-ameter of 10.5cm. and a n externaldiameter of 23.5 cm., which givesa wall thickness of 6.5 cm. Thetop and bottom pieces are separatecircular plates, seating against

copper gaskets when the bomb isclosed.The cylindrical shell is madein two parts, an outer shell shrunkon an inner cylinder. Around theouter wall of the inner cylinder iscut a spiral groove through whichcooling water may be circulated.The top and bottom pieces are alsoin two parts with a groove be-tween them for the same purpose.Through the bottom plate pass 2holes for carrying furnace leads,one of which is shown in th e figure.These leads are made of silver rod3 mm. in diameter, which will eas-

3 ly carry a current of 35 amperesP without overheating. Similar,though smaller, holes through thetop carry 3 thermel leads 0.4 mm.in diameter.A very satisfactory packingfor all these leads is made by ram-Fig. 1.-Diagram of apparatus for high pressure ming into place around each wire,work, showing section through steel bomb with fur- at a pressure of about 2000 kg., anace in place and plan of top and bottom pieces of layer of lithographers stone, a softbomb. rubber washer, and on top of these


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May, 1923 SYSTEM, ALCIUM OXIDE-CARBON DIOXIDE 1173a layer of talc. The rubber and talc prevent leakage of gas, and the lithographer'sstone, while sufficiently soft to crush into place, holds firmly enough to prevent leak-age and blow-outs a t high pressures. The container and pipe lines, previous to use withgas, were filled with oil and tested up to pressures of 2000 megabars.The therniel used is of platinum and platinum with 10% rhodium alloy, calibratedat the quartz and gold points.

The resistance furnace is of platinum wire 0.6 m m . in diameter, wound on alundumtubes. The inner tube is about 13 m m . in diameter and the outer tube is chosen to fitclosely over its winding. Both tubes are wound over a length of 12.5 cm. and to within2 cm. of t he top of the bomb. The windings are connected in parallel, a useful arrange-ment a t high pressures when large amounts of energy are necessary to at tain the highertemperatures.

The principal hcat losses a t high pressures, with carbon dioxide a t least, occurthrough convection. Therefore, cvcrything possible is done to cu t down this effect.The furnace tubes do not reach to the bottom of the bomb and are closed at the lowerend and protected from circulating gas by a silica glass beaker embedded in the insulating_material which fills the bomb cavity.Within the furnace the small plati-num crucible containing the charge issuspcnded on the thermel leads andis enclosed in a silica glass test-tubewhich reaches to the top of the fur-nacc. Thus the crucible, though elec-trically sufficiently well insulatcd, ispractically in solid contact with theinner furnace tube and the heatingcoils. For the sake of simplicity thethermel junctions only are shown inthe diagram. Details of the conncc-tion and of the construction of thecrucible used are shown in Fig. 2, Aand B. The lower junction is withinthe charge, and the upper junction im-mediately above the crucible, whichgives the temperature outside of andabove the charge. Th e thermel leads

A B

Pig. Z.--Platinum crucibles used for heatingand fusing CaCOJ. A . Covered crucible, thecover itself being part of thermel. B. Dividedcrucible with differential thermel used to detectsmall heat effects.-pass through alundum disks (not shown in the diagram) which fill the remaining spacein the silica test-tube from a point immediately above the crucible to the top of thefurnace. A n alundum plate screwed t o the inner surface of the bomb top protects thesteel from the action of the hot gases which leak up through the furnace and aroundthe bafRe plates.The entire cavity of the bomb together with the lower part of the furnace is filledwith powdered alundum ground to 60 mesh.The furnace, at pressures up to 200 megabars, provides a heated section of about15 m m . in length, within which the temperature difference is not more than 2' a t 1400'.

However, at 1000 megabars the temperature gradient is much larger and the hottestportion of t he furnace moves continually upwards with increasing pressure, until some-times at 1000megabars the best location for the crucible is a t the very top of the furnacetube, above the winding altogether. The furnace, whichat atmospheric pressure requires 10amperes at 22 volts to at tain a temperature of 1400requires 30 amperes a t 75 volts for the same temperature a t 1200megabars. Th e chargesThe heat losses are also large.


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1174 F . N A S T I N G S S M Y T H A N D I,IL\SON H. A D A M S Vol. 45used were not more than 5 to 7 m m . deep, and were, therefore, assured of a reasonablyuniform temperature distribution within the platinum crucible:It is necessary to circulate hot water through the bomb walls a t high pressures, soth at all of the gas may be above its critical temperature. If the walls are kept cool,cxtraordinary sudden temperature fluctuations as great as 50 " to 100' may be notedwithin the furnace. The reason for this is not clear, though it is not impossible thatliquid carbon dioxide may assume a spheroidal state and this cool liquid may penetratea t times the hot furnace. If the walls of the bomb are heated, however, th e temperatureof a crucible and charge may be controlled readily within a few tenths of a degree overperiods of an hour or more by using storage battcry current and rcgulating from time totime with a hand rheostat. Satisfactory smooth heating curves are also obtained withthis apparatus.Gas pressures up to 60 megabars may be obtained readily from a cylinder of com-mercial carbon dioxide. We can obtain higher pressures up to 1000 megabars by usinga hand pump with a small steel piston, the pump being cooled by ice in order t o keep thecarbon dioxide passing through it in the liquid state. The compressibility of liquidcarbon dioxide a t prcssures above 100 megabars does not appear to be great, so that largeqiiantitics of liquid do not have to be pumped into the bomb to givc the pressures whichwe have requircd.While in use, the bomb with the top and bottom pieces in place is set upright in a500-ton oil press, and sufficient pressurc is applied to hold the end pieces against theirseats and to prevent leakage around the copper gaskets. At 1000 megabars this re-quires about 200 tons. ' The crucible A has acover which extends down inside the crucible, which is entirely made of pure thermelplatinum and is joined to the platinum thermel lead. Inside the lower tip of the cover,well within the interior of the charge when it is in place, is fused the rhodium-alloy wire.Thus the tip of the cover itself becomes the thermel junction, an arrangement whichgives the temperature of the inside of the charge and a t the same time keeps the cruciblecovered. The depression in the cover is filled with ground quartz or alundum to preventexposure of the junction to convection currents. The cover is held in place by smallpieces of platinum foil bent as clamps around the edges of the flanges of the crucible andits cover. These flanges, shown in the figure, fit tightly together when the cover is inplace.In order to register very slight heat effects, a divided crucible (Fig. 2, B) is also used,with two junctions, one compartment of which holds the charge and t he other groundquartz . By means of a suitable potentiometer the difference in the temperature of thesejunctions and the actual temperature of the charge can be readpracticallysimultaneously.Thc double thermel junction tips arc made separately and carried on a bit of porcelaincapillary as shown, which also servcs as a support for the crucible. Connections withthe thermel leads are made by pinching the wires together through loops as shown in thefigure.

The gas pressures are read on gages of the Bourdon type, which have been carefullycalibrated during use against an absolute gage with weights. The various scales of thesegages reading in pounds and atmosphcres are convertcd into megabars and millimetersof mercury for the sakc of uniformity and convenicnce in plotting. Pressures between50 and 1000 pounds per square inch are read to a n accuracy of 1 pound, and abovethis point an accuracy of 1 megabar is obtained.

The types of crucibles used are shown in Fig. 2 a t A and B.

MaterialsThe calcium carbonate used is natural Iceland spar. This material is in the form oflarge, colorless crystals showing the correct index of refraction for calcite and found by


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M a y , 1023 SYSTEM, CALCIUM OXIDB-CARBON DIOXIDB 1175analysis in this Laboratory to be free from iron and manganese and to contain less than0,0570 of magnesium oxide.The carbon dioxide gas is from commercial cylinders and shows practically completeabsorption in potassium hydroxide solution. The gas is dried before use by passingthrough a steel tube containing fused calcium chloride. Now and then a cylinder ofcarbon dioxide contains oil which is troublesome if it enters a heated furnace. Anoily cylinder is, however, easily detected and discarded.

Exper imenta l Deta i l and Resul tsIn general, in determining points on the pressure-temperature curve,

a charge of ground calcium carbonate is heated in th e bom b a t a constantpressure until dissociation begins. By recording the heat ing curve, thepoin t of dissociation, or fusion, as t h e case m ay be, is eviden t by t h e slowingor stop ping of t h e temp eratur e rise. A t higher temperatures this method

2Fig. 3.-I,oglo p - / r a b a . Plot of experimental data throughout range covered by

The equation of the curve below the.eutectic s:gives accurate results without difficulty, and above l l O O o the charge isusually held in th e covered crucible of Fig. 2 , A, with a single junctionregistering the temperature. It is particularly necessary to have thecrucible covered throu gh a rang e of ab ou t 30' above th e eutect ic point ,for w ithin this range fusion is accompanied by so much dissociation t h a t thecharge usually blows ou t of th e crucible. Ev en a covered crucible does no tprevent th e loss of fusions ma de at or near t he eutect ic point.

A t lower temperatures and pressures, between 900O and 1100O there is agreat tendency for calcium carbonate t o superheat , and i t is only a t tem-peratures m uch ab ove th e true dissociation tem pera ture for a given pres-sure that the heat ing curve begins to show a defini te heat absorpt ion.Therefore, points determined by this method are always too high in tem-

Johnston and by Smyth and Adams.Loglo P --11355/T - 5.388 log T + 29.119.


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1176 P. HASTINGS SMYTIi A N D LEASON H . A D A M S VO l . 45perature. T h e points in brackets in Tab le I1 and shown in Fig. 3 aretyp ical of m an y such results.

But by using a divided crucible with a differential element, the pointof incipient dissociation is readily recognized before th e effect in th e heatin gcu rve is noticeable. Fi g. 4 shows such a heat ing curve with the accom-pany ing difference in tem pera ture of the tw o junctions plotted with it.10300m10200

101w

1woo99 W

9800

9700

Bo70605040302010010203040_ -

T h e tem pera ture -of dissociationjudged from the heat ing curvewould be read a t 10,225microvolts,whereas th e differential shows th a talready a t 10,140 microvolts . thecharge is dissociating. T h e differ-ential also brings ou t the interest-ing fact that slight dissociationdoes s ta r t prompt ly a t the equil ib-rium temperature. T h e calciumoxide formed, however, m ust coa t

XJ th e surface of th e calcium car-differential thermcl with CaC03 used in di- bona te grains, causing fu rth er lossvided crucible. Left-hand curve shows re- of carbon dioxide t o cease, SO t h a tcorded temperature of CaCOs in microvolts. heat absorption actually falls off

Fig. 4.--Typical plot of behavior of th:

....... . . .C . . again, al though the temperaturecon tinua lly rises. It is only a t sti llKignt-nana curve snows airrerence in texnper-ature between the two junctions.higher temperatures that the carbon dioxide pressure in the calciumcarbon ate crystals is able to crack this coat ing or t o burst the crystals open,when dissociation begins in earnest a nd th e heat effect is unmistakable.The Eutectic Point

As already noted, for abo ut 30" abov e the eutectic point i t is very difficultto get resul ts, owing to th e fact th a t th e charge always blows ou t of t h ecrucible, b u t when t h e pressure reaches 150 megabars and the temp era turea bou t 1300" quiet fusion is usually assured. T h e eutectic point i tselfhas been most difficult t o locate for this reason. I n such cases two kind sof superh eating are possible. Firs t , the calcium carbonate itself m ay(though rarely) be heated above the dissociat ion point without formingappreciable quan tities of calcium oxide and, second, th e calcium oxide andcalcium carbonate m ay remain unfused for some t ime even abov e the eu-tectic temperature. However, after repeated tr ial by approaching th eeutectic pressure from bo th directions, we believe t h a t th e tr u e pressure ofth e quadruple point is now known within about 600 mm., which corre-spon ds to a te m pera ture range of l i t t le over 2". T h e m etho d of locatingthis pressure consists in taking heating curves on successive charges ofcalcium carbonate, starting at a pressure of 28,000 mm., where no fusion


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M a y , 1923 SYSTEM,CALCIUM OXIDE-CARBON DIOXIDE 1177take s place, an d increasing th e pressure by a bou t 50 mm . a t each experi-m en t un til dissociation is imm ediately followed by fusion. T h e sam eprocess is followed by reducing t he pressure gradually from abo ut 32,000mm., until the charge a t th e end of th e experiment shows t h a t dissociation,

Temp.c.

842.3"852.9"854.5"868.9"904.3'906 5"937.0"937.0"

TABLE1E Q U I L I B R IU M PR E S S U R E S A N D TEMPERATURESOR TIlE Rl3ACTIONS

CaC03 (solid)* CaO (solid) 4-COZ gas) andCaCOa (solid)S CaO (liquid solution in CaC03) f COZ gas)Temp. 10s Pressure PressureOAbs. Mm. I-Ig Megabars

CaCOs (solid) S CaO (solid) -I- COZ (gas)1115.3 0.8966 343.0 . . .1125.9 0.8882 398. 6 . . .1127.5 0.8869 404.1 . . .1141.9 0.8757 510.9 . . .1177.3 0.8494 879.0 . . .1179.5 0.8478 875.0 . . .1210.0 0.8258 1350 * . .1210.0 0.8258 1340 . . .

[990.Bb 1263.6 0.7914 2053[1038.2* 1311.2 0.7627 36041049.3' 1322.3 0.7563 48941082.5c 1355.5 0.7377 67581157.7' 1430.7 0.6990 142021226.3 1499.3 0.6670 260931226.2 1499.2 0.6670 26093

2.734.806.529.9018.9434.834.8

1240.9 \ eutectic 1513.9 0.6605 28718 \ eutectic / 39.G1238.9' temp. i 1511.9 0.6614 30579 j pressure 40.7CaC03 (solid)e aO (liquid solution in CaC03)+ COI (gas)

1244.5 1517.5 0.6589 31528 42.01244.9 1517.5 0.6588 31528 42.01267.2 1540.2 0.6492 39690 52 .91270.1 1543.1 0.6480 39535 52 .71275.6 1548.6 0.6457 53505 71.41278.1 1551.1 0.6441 49875 66. 51296.0 1569.0 0.6373 107262 142.91296.9 1569.9 0.6369 106228 141.61304.8 1577.8 0.6338 108296 144.31306.4 1579.4 0.6332 157411 209.81314.0 1587.0 0.6301 193800 258.41324.2 1597.2 0.6260 449160 598.81325.0 1598.0 0.6257 426360 568.41334.7 1607.7 0.6220 798760 10651336.1 1609.1 0.6214 779000 10391338.9 1611.9 0.6203 779000 1039

10g10P ( m m . )2.53532.60052.60652.70832.94402.94243.13033.12713,312413.556813.68973.82984.15234.41664.41664.47304.48%4.49874.49874.59884.59714.72844.69805.03055.02635.02645.19715.28745.65245.62985.90245.89155.8915" Determined in glass apparatus with mercury manometer.* Results which show superheating by heating-curve method with single thermel.Differential thermel used.


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1178 I?. HASTINGS SMYTH AND LEASON H . ADAMS VOl. 45b u t no fusion, has taken place. T h e average values which come from theseresu lts ar e: eutec tic pressure, 30,000 mm . =t 300 mm.; eutectic tem-perature, 1240" * lo.It has been found possible to ex tend th e Johnston series of d a ta whichstops a t 894" and 716 mm. of mercury u p to 937" and 1350 mm., in th e low-pressure qua rtz-tub e appar atus which had been used previously in th e workon copper oxidela in this Labo ratory. T h e lowest determination made inthe pressure bomb is a t a tem pera ture 100" higher tha n this. B u t sinceth e latter d eterminations are consistent with t h e extrapolation of th e curve

6030"

50030

40000

30000

20000

10000

900 EGO 900 1000 1100 1200 1300Fig. 5.-Direct pressure temperature curve taken from the logarithmic plot of Fig. 3

together with experimental points of Johnston and of Smyth and Adams, u p to 53,500mm. and 1276'.determined by th e lower points, i t was no t thoug ht necessary to constructa special gage to use w ith th e high pressure app aratu s w hich would readw ith accuracy pressures between 1400 and 5000 mm. of mercury. Evenwith su ch a gage, the heating curve method w ould no t be satisfactory muchbelow 1050". Pressures and temperatures between 940" and 1050" canbest be read from t he curve which th e other points determine.T h e final experimental results toge ther with logarithm s of pressures an dreciprocals of abso lute tem pera tures ar e given in Tab le 11, and are plot tedin Figs. 3 and 5.

The direct pressure-temperature plot of Fig. 5 does not extend above60,000 mm . a nd 1280, b u t t he logari thmic plot is extended to the maximumpressure an d temp eratur e recorded. Th is final tem pera ture of 1339" a t apressure of 779,000 mm . (1040 m egab ars) m us t be very close to th e t ruemelting p oint of calcium carb ona te, since the loss in weight of th e charge


13/18

M a y , 1923 SYSTEM,CALCIUM OXIDZ-CARBON DIOXXDB 1179afte r fusion corresponded to the form ation of only 0.38% of calcium OX-idc. Exam ination of the fused m aterial un der the microscopee indicatesth e presence of n o t mo re th an this q ua nt ity of lime.T o get rid of the la st trac es of calcium oxide in th e melt wou ld dou btlessrequire a very much greater pressure, bu t we d o no t believe th a t th e fusiontemperature would be greatly raised. 1340 is thought to be a close ap-proximation to th e melting point of pure calcium carbo nate.The microscope also shows that the eutect ic mixture fusing at 1240contains abou t 50% of calcium oxide. Th is m ixture has no t been analyzeddirectly since, after fusion, sufficiently large quantities have not yet beenrecovered. T h e fused mixture has, however, a typical honeycombeutectic structure , withou t p redom inantly large crystals of either calciumcarbonate or calcium oxide, which is also an evidence that the chargesexamined, and from which the eutect ic temperature has been obtained,have been fused a t the proper eutectic pressure (30,000mm.) and th a t theliquid an d solid phases in eq uilibrium were of th e same comp osition.I n charges fused a t higher pressures, large crystals of calcium carbo nate ,which is th e primary phase, are readily seen unde r th e microscope.I n order to determine the form an d t he constants of the logari thmic equa-tion which best represents the curve passing through the experimentalpoints recorded for R eaction 1, we may start , as suggested in th e discus-sion above, with the C lapeyron equation

(1)A Hdp/dT =-AVAssuming th e ideal gas laws and t h a t AV = h e volum e of th e gas alone,we have A H = A H 0 + AC,T, if AC,, the difference in the heat capacity ofC a C 0 3 a n d C aO + COZ is assumed to remain constant throug hou t th etempera ture range s tudied, up to the eutec t ic point .W ith these assumptions we obtain on integrating,(2)n p = - AH PI^ T + constRT + Rwhich may be written in the general form, after converting to ordinarylogarithms,(3 )1log p = A T + B log T + C

T h e con$ants A , B , and C can be determined direct ly from the dataavailable by dividing the determ ined p oints, including those of Jo hns ton,into three groups, averaging the variables (log p and 1/T) in each groupto give three av erage points a t different par ts of th e curve, and solving forth e constants b y direct subs t i tut ion in the equation. This method utilizesthe experimental results only and involves no assumption concerning theactu al valu e of AH or of AC,.Microscopic work performed by H. E. Merwin, t o whom we are indebted for muchvaluable assistance.


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1180 F. I I A S T I N G S S M Y T R A N D LEASON H. A D A M S Vol. 45

Pressure Pressureh4m. Megabars

0.078 . . . . .0.41 . . . . .1.84 . . . , .8.90 . , , ,22.2 . . . . .63.2 . . . . .167 . . . . .372 . . . . .760 (1 atm.)l.013793 1.1

2.15772942 3.95196 6.98739 11.6

The eutect ic point i tself (1240' and 30,000 mm.), which we think isdeterm ined with considerable accuracy, is chosen as a definite poin t on t h ecurve. T he other two groups used are our own da ta between 842' and937 and the da ta of Johnston between 587 ' nd 711 .

lo g p = -11355/T - .388 og T + 29.119T he equat ion of the curve thus found isTh is curve is th e one plot ted up to t he eutect ic point in Fig. 3 and i t f i tsthe da ta wi thin the l imi ts of experimental error throughout the rangecovered by both Jo hnston 's a nd our own results.It is interesting t o note t h a t th e curve given by Johnston as determinedfrom his results alone with the aid of separately determined values for

AH and AC,, would give a eu tectic pressure of 24,770 mm., a nd th at th ebes t representative straigh t l ine through all th e points as determined by t h emethod of least squares gives a eutectic pressure of 32,360 mm . T h eexperimentally determined pressure being 30,000 mm., a straight l ineappears to fit th e da ta for the whole range bet ter than does the extrapola-tion of th e curv e given by Jo hnston.Table I11 contains th e equilibrium pressures a t even temp eratures from500' t o 1360' as read from th e logarithmic plot. Below th e eutectic, th epressures hav e been calculated from th e equation of the curve.

(4)

TABLB11PRESSUREST EVEN WPGRATURESALCULATEDROM T H E CURVE. T e m p . Temp . Pressure Pressurec. OAbs. Mm. Megabars

1150 1423 13750 18.31200 1473 21797 29.01240' 1513 30149 40.2Pressures above eutectic point read fromlogarithmic plot1250 1523 33490 44.61260 1533 39090 52.11270 1543 47210 62.91280 1553 59170 78.81290 1563 77630 103.41300 1573 108400 144.41310 1583 162200 216.11320 1593 288400 384.21330 1603 588900 784.41340 1613 851200 1133.8

Temp. Ternp.0 c. OAbs.500 773550 823600 873650 923700 973750 1023800 1073850 1123897 1170900 1173950 12231000 12731050 13231100 1373

a Eutectic.Equat ion 4ma y perhaps best b e regarded as an empirical equation chosend In pto represent the d a t a ; bu t we may note th a t in general A H = R T 2 1 d Tprovided only th a t th e gas follows th e ideal gas laws an d th a t th e volumes


15/18

May, 1923 SYSTEM, CALCIUM OXIDE-CARBON DIOX IDE 1181of the solids are negligible in comparison with t h a t of the gas. Hence, fromEquat ions 3 and 4 i t fo l lows that AH =-2.3026 RA- B T = 51990-10.71T. A t several temp eratures the values of AH as calculated from thisrelation are as follows: 48,800, 45,850, 42,640, 40,500, 38,360, and 36,210calories at 25", 300, 600, 800, 1000" and 1200", respectively. A t898.6", the temperature a t which p= 76 0 mm ., AH=39,440 cal. and a t theeutectic temperature, 1240O, AH = 35790 calories.T h e values of AH a t 25", 300", 800" and 1000" calculated from th ecurv e given b y Jo hnston ar e 42,870, 42,150, 38,740 an d 34,010 calories,respectively. T h e Johnston value a t 25" is in agreement with th e ex-perimental value of Tho mso n (42,900 cal.) since th e Johnston pressure-temperature curve is based on this value. T h e curve which we hav edeveloped, while i t gives t o AH a t 25 " a m uch higher value, is neverthelessin better agreement with th e actu al low temp eratu re results of Johnstonthan is his own curve.

Furtherm ore, from th e general relation, AC - dm9 i t follows th a tAC,=-RB= -l0.71, which is co nst an t on acc oun t of th e form of th eequation chosen t o represent p as a func tion of T. Th is value, however,is not t o be thou ght of a s more tha n a rough determ ination of the m eanvalue of AC, over th e rang e of tem pera tures a t which me asurem ents ha vebeen made. Th is is because we have differentiated Equa tion 4 twice inorder to ob tain AC,, an d, as is well know n, sm all errors in th e original d a tabecome magnified enormously when we pass to the second differentials.Nevertheless, we may note that at 900"-about th e middle of the to ta lexperim ental rang e of temperature - AC, calculated from th e equ ationwhich Johnston1 o took t o repre sent th e change of AC' with T is -11.8.Th ere is thu s a n agreement between th e mean values of AC, as determineddirectly from t he dissociation pressures a nd as calculated from Johnston'seq ua tion fo r AC,.T h e values calculated for AH, on the other hand, depend on the f irstderiv ative of p with respect to T and probably represent th e tru e values ofA H with fair accuracy, b ut only for temperatures within the experimentalrange. Extrapolation to temperatures outside this range may, and prob-ably does, lead to serious error. I n th e absence of directly determine dvalues of th e specific hea ts for each of th e substances involved and overth e whole desired range of tem pera ture, it is unsafe to e xtrapolate he ats ofreaction, dissociation pressures, equilibrium c ons tants or free energies.

The above calculation of AH involves the assumption tha t t he carbondioxide follows th e perfect gas law a nd t h a t th e volume of th e calciumcarbon ate a nd of th e calcium oxide m ay b e neglected.It is interesting to compare thes e values of AH wi th some of t h e values of

'- l

1ORef. 1, p. 943.


16/18

1182 F. HASTINGS S M Y T H A N D L E A S O N XI. A D A M S Vol. 45AH for Reaction 1 which may be calculated directly from the Clapeyronequa t ion dp /dT .=-H by determining th e value of d p /d T from t heexperimental curve (Equation 4), and the value of AV from the Keyesequ ation of sta te" for carbon dioxide. T hi s equ ation has been shown toapp ly to carbon dioxide with considerable accuracy within th e ranges ofavailable experimental data .By differentiation of Eq uatio n 4 after conversion to n atura l logarithm swe obtain,

T A V

( 4 42.3026 X 11355 5.388Q = P [T 1'2 - 7 1which expresses the change in equilibrium pressure for Reaction 1, inmillimeters of mercury per degree a t a ny given absolute temp erature.Values for dp /d T may be found by direct subst i tut ion in this equation ofcorrespon ding values of p and T taken from th e experimental logarithmiccurve (Equation 4).A t 1513' abs. and 30,000 m m . ( the eutect ic point) dp /d T =237.02 mm. =0.3157 megabars per degree.T he volumes a t th e eutectic point areCaCO8+CaO + COS36.89 cc . 18.15 cc. 3208.33 cc.and AV= 4-3190 cc.

AH whence1513 X 1390ubs tituting in t h e C lapeyron equation, 0.3157 =AH= 1 , 5 2 3 , 6 0 5 ~ ~ .egabars = 36,440 cal.Similarly, AH(897') = 39,420 cal.T h e use of th e Dieterici equ ation of s ta te would indica te a volume of th ecarbon dioxide nearer t o th e ideal volume than would th e Keyes equation.Indeed, t h e former equation yields a volume which departs from th e idealby an amount which nearly compensates for the difference in volumebetween calcium carbona te an d calcium oxide.From Equ ation 4 also we obtain th e equation for th e free energy change(-AF) for th e reaction CaC 03 -+- CaO + COZ(1 atm .), in term s of th eabsolute temp erature a t which th e change takes place.This equation is: -AF= 120.136 T -24.670 T log T -51991; or , if t h efinal gas pressure be 1 megabar: - F= 120 .149 T-24.670 T log T-51991.From these equations: a t 1240", -AF= 11,085 cal.The Non-existence of T w o Forms of CalciteI n none of the m any experiments carried o ut in t h e course of this inves-tigation were we able to detect the slightest evidence in the thermal be-havior of th e calcium carbonate th a t a transition from one form to an other

l1 Compare Keyes and Kenney, J . Am . SOC. efrig. Eng., 3, No. 4, 1-25 (1917).


17/18

M a y , 1023 SYSTEM, CALCIUM OXIDE-CARBON DIOXIDE 1183takes place a t or about 970" as found by Boeke,6 and recently by Eitel , I2using Boeke's appa ratus .

Th ere is threefold evidence th a t no transition take s place.Fi rst, large crystals of na tur al calcite m ay be hea ted a nd cooled re-peatedly through the supposed transiticni point under sufficient pressureto prevent dissociation, w ithout su bsequ ently betraying und er t he m icro-scope signs th a t they have been changed in any manner. If a molecularrearra nge m ent takes place a t 97O0, evidences of twinning sh ould be presentin calcite which has been heated above th a t point, which do no t exist in th enatu ral calcite before heating. T h e analogy or this result would be th ebehav ior of qu art z which has been heate d abo ve 575", th e changes in whichmay be used as a "geologic thermometer," as pointed out by Wrightand Larsen. 3 The se latt er changes are certainly, from a crystallographicstan dpo int, very slight, yet th e evidences of change which remain in aqua rtz crystal which has been heated are unmistakable. No such snalo-gous evidence is discernible in th e case of calc ite. La rge cry sta ls ofnatural calcite are not sha ttered af ter being repeatedly passed through t hesupposed transit ion tem perature, nor do t he crystal surfaces when etchedwith dilute acid after such heating show any change as is the case withquar tz.

evidence of tran sfo rm -ation. T he pressure-tem perature equilibrium curve shows no break untilth e eutectic point is reached. On the logarithmic curve which is almost astraig ht line, a sudden change in slope could b e readily de tected if the h ea tabsorption at the supposed transition is even a pa rt of t h a t suggested b yth e heating curve s of Boeke an d Eite l.Third, we have direct experimental evidence that no transition exists.I n one side of th e divided crucible (Fig. 2 , B) w as packed pure powderedquartz, and in the other powdered calcite. T he temperature of t he cru-

cible was read simultaneously with t he difference in tem pera ture betweenth e calcite an d qua rtz. All other conditions were identical with those ofother high pressure experiments. W ith this arrangem ent th e transitionfrom a: t o P quar tz a t 575" is readily discerned even with rapid heating a t apress ure of GO me gabars. Th is absorbs only ab ou t 3.5 calories per gram ofquartz; yet with 0.5 g. of quartz in the crucible, a rapid increase of t h edifference in temp eratu re between t he tw o junction s of th e double thermelcould be noted of 17 t o 20 microvolts, during th e transition process. Ahea t absorption of '/IO of this q ua nti ty would be readily recognized b y thismeans. Nevertheless, th e crucible could be heated through th e range 900"t o 1100" without any corresponding sign whatever of heat absorption bythe calcite.

Second, on thermodynamic grounds, there is

I * Compare Ref. 8.l 3 Wright and Larsen, Am . J . Scz., 27, 421 (1909).


18/18

1184 H.A. SPOEHR VOl. 45To us th e evidence seems conclusive th a t tw o forms of calcite do n otexist, bu t th a t th e heat effect at 970' in the appa ratus used by Boeke andb y E itel must b e due to some other mater ial present near t i le thermel junc-tion or t o some condition peculiar to th e furnace used.

Sbmmary1. An apparatus has been built with which the pressure-temperaturerelations in th e system, calcium oxide, carbon dioxide can bc studied u pt o 1390" and 1000 m egabars pressure.2. Equilibrium pressures have been determined which, with the pre-viously determined data of Johnston, define the system experimentallyfrom 587" to 1389', and from 1.0 mm . to 770,000 m m . pressure.3. An equation for the pressure-temperature curve which fits all datawithin limits of experimental error up t o the eutectic poiiit for the system,calcium carbonate-carbo n dioxide is given.4. T h e me lting point of calcium carb ona te containing only 0.38% ofcalcium oxide is given a s 1,789' a t 770,000 m m . pressure. 'l'his is pro bablyvery nea r t o th e melting point of pure calcium carbonatc.5 . T h e eutectic, experimentally determined, between calcium carbo nateand calcium oxide lies at 1240' * l o , nd 30,000 m m . * 300 mm. Th ecomposition as judged from microscopic examination is about 50(% ofcalcium carbonate: 50% of calcium oxide. CaO 4- COz, have beencalculated at various temperatures, and equations giving these two quan-tities in terms of th e tem perature are discussed.7. It has been shown by therm odynam ic and experimental evidence t h atonly one crystalline form of calcium carbona te exists within th e tem pera turerange investigated.

6. AH and - A F for the reaction, CaCOa

WASHINGTON,. C.[CONTRIBUTIONRO M TIIE COASTAL ABORATORY1) ' r m C A R N I W I ~ ~NSTITUTIONFT H E R E D U C T I O N OF CARBON DIOXIDE BY ULTRAVIOLETL I G H T

BY H. A . SPOEHR

WASHINGTON

Received February 12, 1023In the many applications of chemistry to biology there is probably noothe r purely h ypothetica l suggestion which has served as such a lure for

furth er speculation as th e Bacyer formaldehyde theory of photosynthesis.Attention is here confined to a discussion of some cxpcriments on the re-duc tion of carbonic acid to formaldch ydc in glass by m eans of u ltravio letlight. Although the conclusions draw n from these experiments can onlyL Baeyer, Bey., 3, 63-78 (1870).

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Cao Co2 System