EQUILIBRIA IN THE REDUCTION OF OXIDE8 BY CARBON. 205 XXI V .-Equilibria in the Reduction of Oxides by Carbon. By ROLAND EDGAR SLADE and GEOFFREY ISHERWOOD HIGSON. Equilibria of some reactiolns of the type: or have been investigated. I n either of the above systems there are three components, namely metal carbon and oxygen and four phases namely, metlal metal oxide carbon (or carbide) and carbon monoxide (gas). The number of degrees of freedom is thus 3 + 2 - 4 = 1. Therefore a t one temperature there is one pressure of carbon monoxide which determines the equilibrium of the syst'em The following experimental met'hod was adopted. A small quantity of the metal was heated in a vacuum to a certain texnpera-ture and carbon monoxide was then admitted until the pressure was greater than the equilibrium pressure.The reaction pro-ceeded in the direction from right t o left and carbon monoxide was absorbed until the equilibrium pressure was attained. Some carbon monoxide was then removed when the reaction proceeded from left to right until the equilibrium pressure was again attained. If the carbon formed in the first part of the experiment did not remain as a separate phase either as a carbon o r as a carbide but formed a solid solutlon with the metal there would be only t'wo solid phases present with the gas phase; the system would there-metal oxide + carbon carbon monoxide + metal metal oxide + metal carbide t carbon monoxide +metal K 206 SLADE AND HIGSQN EQUILIBRIA IN THE fore have two degrees of freedom and the pressure of carbon mon-oxide would depend on the relative amounts of metal and carbon present as well as on the temperature.When the equilibrium was attained from left to right there was less carbon monoxide present than when the equilibrium was attained from right to left, therefore the equilibrium pressures would have been different in these two cases if there were only two solid phases present. The equilibrium may be caIcuIated from the heat of reaction by making use of the Nernst heat theorem. where M is the weight in grams of any metal combining with 16 grams of oxygen and QC is the heat of reaction; in all cases Qt is positive so that increase in temperature will cause the form-ation of M+CO. That is to say pCo increases with the tempera-ture. This quantity of heat @ may be considered as the differ-ences of two quantities of heat Q1 and Q2 for if we write I n the reaxtion .. . . . . . M+CO=MO+C+Qt (1) (2) M+O=MO+& . . . . . . CO =C + 0 + Q2 (3) . . . . . . . then on addition M + CO = MO + C+ (Q1 + Q2). dissociation of carbon monoxide is - 29,000 calories therefore Q2 t#he heat of . . . . . . . &t= &I- 29,000 (4) Neglecting terms containing % the Nernst heat theorem requires that logpuo = ____- -‘O +1*76 log T + 2 6 . . . ( 5 ) 4.571 T where &, is the heat of reaction a t absolute zero. value given by Weigert in Abegg’s “ Handbuch.” between Q0 and Qt is given by the equation The thermodynamic constant used for carbon monoxide is the The relation . . . . . . . Qo=Qt+3*5T ( 6 ) Limits of the 1nuestigation.-The equilibrium mentioned above could only be determined a t temperatures a t which the equilibrium, is practically completely in the left-hand direction.Rhead and Wheeler (T. 1910 97 2187; 1911 99 1140) have investigated this equilibrium and from their results it is possible to calculate the partial pressure of carbon dioxide in equilibrium with carbon monoxide a t 760 mm. or 50 mm. These values are given in table I. 2co = CO,+ c REDUCTION OF OXIDES BY CARBON. 207 TABLE I. Pressure of carbon dioxide in mm. when prossure of carbon monoxide Temperature. is 760 mm. 850' 53.8 900 17.6 1000 4.6 1100 0.90 1200 0.46 Pressure of carbon dioxide in mm. when pressure of carbon monoxide is 50 mm. 0.23 0.076 0.0020 0.00082 0*00020 From these figures i t is seen that i f the equilibrium pressure is as low as 50 mm.there is no complication due to the presence of carbon dioxide at temperatures from 850° upwards. If however, the pressure is as great as 760 mm. the amount of carbon dioxide present is appreciable up t'o 1200O. If a carbide is present instead of free carbon the ratio of carbon dioxide to carbon monoxide will be greater for we have P2E- = K, Pco2Pc and pc (partial pressure of carbon vapour) will be lower over a carbide than over carbon. I n all our experiments the equilibrium pressures were sufficiently low and the temperature was sufficiently high for the pressure of carbon dioxide to be negligible. The metals with which we could determine the above equilibrium were only such as would fulfil the following conditions (1) The metals must not be volatile a t the temperature of the experiments.At these temperatures the vapour pressure of the metal should certainly not be more than 0.25 mm. o r it will distil rapidly on to parts of the platinum tube which are a t a somewhat lower temperature and probably attack the platinum. Platinum tubes were in fact destroyed by the volatility of boron and manganese. (2) The equilibrium pressure must not be greater than 50 mm. at 850° or the quantity of carbon monoxide in the gas phase will be appreciable. (3) The equilibrium pressure must be sufficiently great to be measurable. It must be a t least 1 mm. a t 1300O. Applying the Nernst heat theorem to the equilibrium we should only expect those elements of which the heat' of oxidation per gram-atom of oxygen lies between 75,000 and 114,000 calories to give an equilibrium pressure measurable in our apparatus.Of sub-stances with known heats of oxidation only silicon boron and manganese lie within this range. The only likely metals for which. K* 208 SLADE AND HIGCSON EQUILIBRIA IN 'THE the heats of oxidation were unknown and which were readily obtainable were vanadium tantalum and chromium. ,4 ppzratzcs.-The furnace which has been described by Slade (Proc. Roy. Soc. 1912 [A] 87 519) consists of a platinum tube 2 cm. in diameter heated by a current of 300 t o 400 amperes a t 2 to 4 volts. The furnace is placed in a vessel which can be ex-hausted to prevent the platinum tube from collapsing under the pressure of the atmosphere.A silver capillary tube is used to connect the furnace with the glass tube leading to the pressure gauge. The temperatures were dete'rmined by means of a platinum platinum (90 per cent.)-rhodium (10 per cent.) thermo-couple. The couple was calibrated up to the melting point, of copper 1083O and higher temperatures were determined by extrapolating by means of the formula log e = 1.22 log t - 2.65, where t is expressed in degrees centigrade and e in millivolts. The cold end of the couple was kept' a t Oo. It was found to be impossible to use a platinum boat for any of the substances investigated for although the temperatures were well below their melting poiints they were rapidly alloyed with platinum.Accordingly boats of unglazed Royal Berlin porcelain were employed. Pressures were read on a mercury vacuum manometer behind which was a glass millimetre scale illuminated by a lamp and a milk-glass screen. The readings were made with a telescope and were accurate to i-0.05 mm. The carbon monoxide was prepared by running pure formic acid into concentrated sulphuric acid a t 70-80°. The gas which was first passed through a long tube of soda-lime and then through a similar tube of phosphoric oxide was collected and stored over mercury in a vessel of 1 litre capacity. The gas was led from the reservoir to the furnace and pressure gauge by means of a tube in which were placed two taps separated by a capillary tube of such dimensions that the volume between the tlwo taps was 0.2 C.C.By filling this tube with carbon mon-oxide a t the ordinary temperature then closing one tap and open-ing the other 0.2 C.C. of carbon monoxide was allowed to flow into the exhausted furnace and the tubes connecting the furnace to the gauge and pump The total volume of this part of the apparaius was about 50 c.c. and when the furnace was heated t o about 1200° ibs effective volume was about 30 c.c. so that the introduc-tion of 0-2 C.C. under a pressure of one atmosphere caused a rise of pressure of o?ahosphere or about 5 mm. of mercury. 3 REDUCTION OF OXIDES BY CARBON. 209 I n most of the experiments 0.1 gram of the metal under investi-That this was sufficient may gation was introduced into the boat. be seen from ths following considerations.M + CO = RIO + C, If the reaction is where 11 is two equivalents of an element then two gram-equi-valents of the element would react wit-h 22,400 C.C. of carbon mon-oxide. I f the pressure) in the furnace was 60 mm. which was the maximum pressure used in several cases the volume of gas con-tained in the furnace was 2.4 C.C. when measured a t N.T.P. there-fore to absorb all t,his gas '.* x 2 = 2.2 x 10-4 gram-equivalent 23,400 of the element would be required. If the equivalent were as great as 100 only 0.02 gram would be required. The metal was usually broken into small pieces as the velocity of the action must be proportional t o the surface exposed. Experiments with Vanadium.-As vanadium is a very refractory substance and is difficultly reducible (that is the oxide has a high heat of formation) it was decided to attempt to measure the reduc-tion equilibrium.Some preliminary experiments were made on the action of vanadium on platinum and the melting point of vanadium. The vanadium was placed in very small pieces (about 0.5 mm. in diameter and less) on a platinum strip which was heakd in an atmosphere of hydrogen by an electric current.. . A t 1400° the vanadium adhered to the strip when the heating had been carried on for some three minutes. The temperature was determined by means of a Wanner pyrometer correction being made for black-body radiation of the platinum. In another experiment the strip was dusted with powdered vanadium and heated rapidly until it fused a t one point. Examination under the microscope showed that the vanadium had then fused into globules just round the portion of the strip which had fused.The melting point of this vanadium is tberefore just below the melting point of platinum, namely 1760O. The vanadium had been prepared in the electric furnacel and contaiqed 4.6 per cent. of carbon. The carbon prob-ably exists as the carbide VC and may be present in solid mlu-tion although the fact that so much carbon is present makes i t seem probable that the carbide exists as a separate phase. Experiments.-O.0636 Gram of the metal was placed in an un-glazed porcelain boat in the platinum tube furnace. The furnace was exhausted and left for sixteen hours when no rise of pressure was noticeable. The temperature was raised to 1000° and the occluded gas from the boat pumped off; 0.2 C.C.of carbon mon 210 SLADE AND HICISOX XQUTLIBBIA IN THE oxide was then admitted and this raised the pressure to 6 mm., at which it remained. Therefore the equilibrium pressure was greater than this or the velocity of reaction a t this temperature was very mall. The latter was found to be the case for when the pressure of carbon monoxide had been increased to 60 mm., there was still no reaction. The temperature was t'hen raised to 1340° and maintained a t this temperature for four and a-half hours During this time the pressure fell a t first rapidly and finally became steady a t 1 - 7 mm. The temperature was then reduced to 1145O where it was maintained for thirty minutes. The pressure fell rapidly to D-55 mm.where it remained constant. The temperature was then lowered to 900° when the pressure fell only to 0.2 mm. On the following day the furnace was heated to 1340O and the temperature kept constant. I n one hour the pressure rose to 1.2 mm. a t which it remained constant for three and a-half hours. The equilibrium pressure a t 1340O was therefore between 1.7 and 1-3 mm. The mean of these values is 1.45 mm. At lower temperatures the equilibrium was attained too slowly to be deter-mined. The reaction is probably The value vo-I-vc -f 2V+CO+Q. p c o = __- 1'45 atm. at 1340' 760 gives by the Nernst heat theorem the value Q0= 80,875 cals. Substituting this in equation ( 5 ) we find that p,,=1 atmosphere a t 1827O. This is the temperature a t which vanadium oxide would be reduced by the carbide under a pressure of one atmosphere.There is no direct evidence as t o the heat of formation of vanadium carbide that is of the reaction v+c -f vc, but usually the heats of formation of carbides are small (Warten-berg Zeitscla. anorg. Chem. 1907 52 299) that is to say not greater than 2000-3000 calories per gram-atom of carbon. If this heat of formatmion of the carbide is neglected an approximate value of the heat of oxidation of vanadium at 20° can be obtained : V.+O=VO+ 111,000 cals REDUCTION OF OXIDES RY CARBON. 211 Not much trust can be placed in this value however for the carbide may be in solid solution in the metal and not as a separate phase. Ezpem'ments with Tan8talum.-The tantalum used was a portion of a specimen obtained from the late Dr.Werner von Bolton and used by von Hevesy and Slade to determine the electrode potential of tantalum. It was in the form of a rolled sheet about 0.25 mm. thick. As only a small quantity of the metal was available 0.035 gram was used in each experiment. If the tantalum was oxidised to the oxide Ta,O this metal would absorb 3.5 C.C. of carbon monoxide. I n the first experiment with t'antalum the metal was in the form of one piece of sheet. At. 1000° 2 C.C. of carbon monoxide were admitted (p=60 mm.); the pressure fell and in two hours became constant a t 0.7 mm. The temperature was then raised to 1200O and 0.6 C.C. of carbon monoxide was admitt'ed so that the pressure was raised to 14 mm. As the pressure did not fall more carbon monoxide was admitted until the pressure was 40 mm.but there was still no action. The furnace was therefore exhausted but no appreciable rise in pressure took place in two hours. It therefore seemed probable that the constant pressure of 0.7 mm. obtained a t 1000° was not a true equilibrium pressure, but that the equilibrium pressure even atl 1200° was very low indeed. I n the next experiment the same quantity of metal was used, but i t was cut into as many strips as possible in order to increase the surface. After pumping outl all gases from the boat a t 1150°, 0.4 C.C. of carbon monoxide was admitted so as to raise the pressure to about 13 mm. In half an hour the pressure fell to 0.2 mm., and then became constant and remained so for half an hour.An attempt was now made to reach the equilibrium from the low pressure side. The furnace was exhausted and the temperature was raised to 1270O. In four hours the pressure rose slightly above 0.1 mm. (perhaps 0.12 mm.) and remained constant for about three hours. Carbon monoxide (about 0.1 c.c.) was then admitted to raise the pressure to 2.5 mm. and in one hour the pressure fell t o 0.1 mm. This value is therefore the equilibrium pressure a t 1270O. It was impossible to determine the equilibrium a t a higher temperature because a t this stage o€ the work the platinum tube had become weakened and slowly collapsed when kept exhausted for several hours a t 1270° although the external pressure on the tube was only 30-40 mm. of mercury. Experiments with Chromium .-The temperature of reduction of chromium sesquioxide was determined by Greenwood (T.1908 93 , 1438) who found that this oxide was reduced at 1195O under 212 SLADE AND HIGISON EQUILIBRIA IN T-nE prmsure of 2 mm. The boiling paint of chromium is 2200O (Green-wood) and from this value the vapo'ur pressure of liquid chromium can be calculated to be 0.07 mm. a t 1000°7 0.078 mm. at l l O O o , 0.7 mm. a t .1200° and 1-12 mm. a t 1300O. It was therefore not safe to heat chromium to a much higher temperature than 1200O in the platinum furnace. The chromium had been prepared by the Goldschmidt method, and therefore contained a trace of aluminium. As aluminium is e a d y and completely oxidised by carbon monoxide it is probable that it would only have a very slight influence on the equilibrium.0.45 Gram of metal in the form of a coarse powder was us'ed in the first experiment'. The furnace was heated to 936O and all adsorbed gases were pumped out. Carbon monoxide was then admitted until the pressure was 100 mm. I n nine and a-half hours the pressure fell to 22 mm. but did not appear to be, approaching a steady value. After remaining for eighty-five hoars, the furnace was heated to 1O1Oo and carbon inonoxide admitted until the preasure was 50 mm. I n six hours the pressure fell t o 0.75 mm. and appeared to be coast'ant. After eighteen hours the temperature was raised to 1292O and carbon monoxide admitted until the pressure waB 63 mm. In forty-five minutes the pressure fell to 6.2 mm. and remained const*ant. Carbon nionoxide was then pumped out until the pressure fell t o 5 mm.In fifteen minutes the prkssure rose to 6.2 mm. and remained constant. The temperature was then raised to 1339O and in twenty-five minutes the pressure had risen to 9.1 mm. and become constlant. The furnace was then cooled and next day was heated t o 1339O. I n twenty minutes the pre-ssure rose to 9.2 mm. The furnace was now cooled to 1292O and carbon monoxide was pumped off until the pressure was less than 1 mm. I n an hour t h s pressure became constant a t 4.4 mm. The furnace was cooled and the next day w m heated to 1292O; the pressure rose t o 4.4 mm. Carbon mon-oxide was then admitted until the pressure was 8.6 mm. I n forty-five minutes the pressure fell to 4.4 mm. The temperature was now raised to 1339O when the pressure rose to 9.2 mm.In the figure are given some of the time-pressure curves obtained. These show how accurately the results could be reproduced. A new sample of chromium (0.45 gram) was now introduced into the furnace and the temperature was raised to 129207 carbon mon-oxide being admitted until the pressure was 15 mm. In twenty minutes the pressure fell to 6.2 mm. and remained constant a t this value for one hour. The temperature was then raised to, 1339O and the pressure rose to 9.1 mm. but the platinum tube began to leak owing to its being attacked by the chromium which had distilled on to i t during this and former experimentn REDUCTION OF OXIDE8 BY OARBON. 213 Table I1 ehows the values for the equilibrium pressure obtained with chromium.All these equilibrium pressures were obtained twice from each side. The high value at 1292O is the value obtained when the furnace had not been raised t o a higher tempera-ture. After the temperature had been raised to 1339O and lowered again to 1 2 9 2 O the equilibrium pressure was 4.4 mm. and this value could be obt'ained again and again. Since chromium easily forms a carbide i t is probable that. the reaction taking place was 5Cr+CO Cr,C+-CrO+Q. 1 2 3 1 1 2 3 Time in hours. Calculating the heat of reaction per gram-atom of oxygen a t 1315O from the Nernst formula and the van't Hoff formula the values given in table I1 are obtained. TABLE 11. Pressure of carbon monoxide Qt calculated &t cdcul&tted, Temperature. in mm.Nernst. van't Hoff. 1292" 6-2 73,600 -1339 9.2 69,200 1292 4.4 69,200) 77,000 The value 77,000 calories is the heat of reaction calculated from the integrated form of the van't Hoff equation, I n this method of calculating Qt an error of 0.1 mm. in the deter-mination of the equilibrium a t 1293O would make a difference of a little more than 1000 calories in the value of Qt. The assump-$ion on which this formula is based however is only that the hea 214 EQTTlTJBRIA IN THE REDTTCTION OF OXIDES BY CARBON. of reaction does not change appreciably between the two tempera-tures. That two different values were obtained for the equilibrium at 1292O according to whether the furnace had been heated up to 1339O or not must be explained by supposing that the substances in equilibrium were different in the two cases.It is very improb-able that the first value is due to the presence of a trace of aluminium in the metal for the presence of aluminium would be expected to lower rather than to raise the equilibrium pressure, and in the two experiments in which the pressure was 6.2 mm., very different amounts of carbon monoxide had been absorbed by the same amount of metal. I n the first experiment 6-7 C.C. of carbon monoxide and in the second case only 0.6 c.c. were absorbed. The equilibrium in the gas phase is represented by = K . P c r r ~ e 2J)oxinr pll,etd The change in the system caused by raising the temperature to 1339O was to give a lower equilibrium pressure a t 1292O and this must be due to (1) increase in the partial pressure of chromium, (2) lowering of the partial pressure of the carbide or (3) lowering of the partial pressure of the oxide. Case (1) might be caused by the existence of a transition point of chromium between 1292O and 1339O. At first the metal is in the Otfom stable at lower temperatures; on heating the metal would change to the other or &form and on cooling to 12920 would not revert to the a-form but remain in the unstable &form, which would have a higher vapour pressure than the a-form. Case (2) might be caused by the formation of an unstable carbide in the first instance which on heating to 1339O changes into the stable form. On cooling now to the lower temperature the un-stable carbide is not formed in the presence of the more stable one. Case (3) might be caused by the chromium oxide combining with t.he silica in the boat to form a silicate but this reaction should not be different after the furnace had been raised to the higher temperature. The first explanation seems the more probable. This investigation was carried out in the Muspratt Laboratory of Physical Chemistry University of Liverpool. BRITISH PHOTOGRAPHIC RESEARCH ASSOCIATION LABORATORY. [Received Pebmary 14th 1919.