T H E CRITICAL COXSTANTS OF CARBOS DIOXIDEOXYGEN MIXTURES' B T HAROLD SIMMONS BOOTH A N D JAMES MIATRICE CARTER'
Purpose of the Investigation The medical profession have come to demand mixtures of carbon dioxide and oxygen varying from 5% carbon dioxide to 3 0 % carbon dioxide. The question has been raised as to whether or not such mixtures are liable to separate under the conditions of shipping and storage. If separation occurred, the gas coming out of the cylinder a t first would be rich in oxygen, which while not especially efficacious for de-anesthesia, would not be harmful. However the last gas coming from the cylinder would be principally carbon dioxide and might cause asphyxiation. There are no data available on the conditions under which separation of the gases might be expected to occur. On the scientific side, the question is also interesting. Critical phenomena, although well known for pure gases, and for gas mixtures a t ordinary and high temperatures, have been but little investigated a t low temperatures. A mixture of an easily condensible gas, such as carbon dioxide, with one of the permanent gases, such as oxygen, might be expected to show interesting behavior in this respect. I t was t o study the behavior of such mixtures, and to determine the limits of safety for medical use that the followinginvestigation wasundertaken. Historical Discussion The change from the liquid to the gaseous state and the reverse are wellknown phenomena, and take place a t definite pressures and temperatures. However, it was observed by Cagniard de la Tour,3 in 1 8 2 2 , that when a suitable quantity of liquid was placed in a glass tube, which was then sealed off and heated, that at a certain temperature the meniscus separating the gas and the liquid phases disappeared, leaving only one homogeneous phase. On cooling the tube an opaque, thick cloud first formed, followed by the reappearance of the meniscus separating the two phases. Further work on this subject was done by Andrews,4 whose classic researches on the continuity of the liquid and gaseous states of matter are well Contribution from the Morley Chemical Laboratory, Western Reserve University. Presented to the Graduate School of Western Reserve University, Cleveland, O., by James Maurice Carter in partial fulfillment of the requirements of the degree of Doctor of Philosophy, June, 1929. " Holder of one of the Ohio Chemical and hlfg. Company's Graduate Fellowships for pure science research in anesthetic gases. 3 Ann. Chim. Phys., (2) 21, 127, 178 (1822); 22, 4 1 0 (1822). 4 Phil. Trans., 159, j47 (1869); 166, 421 (1876); 178, j 7 (1888.)
2802
HAROLD SIMMONS BOOTH AND JAMES MAURICE CARTER
known. He confirmed the previous observations, and found that at temperatures above that of the disappearance of the meniscus gases were incapable of liquefaction, no matter how great the pressure. I n the place of closed tubes used before, Andrews employed tubes in which the gas was confined by mercury, so that’the pressure and volume of the gas could be altered and measured. I n working with carbon dioxide, Xndrews partially liquefied the gas by pressure alone at room t,emperature, and then gradually heated the mixture. ~ meniscus separating the liquid and the gas became faint, lost its At 3 1 the curvature, and a t last disappeared. The space was then occupied by a homogeneous fluid, which exhibited, when the pressure was suddenly diminished or the temperature slightly lowered, a peculiar appearance of flickering or moving striae throughout the entire volume. Xndrews termed the temperature and pressure a t which he observed these phenomena the critical point. The continuous transition from liquid to gas and vice versa was also described by Andrews. He placed carbon dioxide in a tube at a temperature below the critical, and completely liquefied it by pressure. On raising the temperature above the critical point, no transition could be seen, the tube remained filled with a homogeneous fluid. The pressure was then decreased to a value below the critical, and again the fluid behaved as if it were entirely homogeneous. The temperature was then decreased to the original value when the substance was obtained in the form of a gas, without having undergone any discontinuity of any sort. The process could also be reversed. The mist or opalescence which appears near the critical point has been studied by Travers and Usher’ particularly, as well as by many others. There are several theories t o account for i t ; the one generally accepted is that it is due to fortuitous aggregations of molecules to form momentary droplets. It has been stated by several observers that the critical point is not a definite property of a substance, but depends on the relat’ive amounts of the two phases present, the size of the container, and other factors. De Heen’ put forth a theory based on the change in the critical point with variations in the proportions of the phases present, and Traube3 postulated ‘gasogenic’ and ‘liquidogenic’ molecules as a result of similar observations. However, Travers and Usher, (loc. cit.), Sidney Young‘ and others, have shown that the differences observed are simply the effect of impurities in the samplesused. It was also proved by Young, (loc. cit.), that the temperature a t which the meniscus appears, and a t which the striae, opalescence, and so forth are ’Travers and Usher: Proc. Roy. SOC.,78A, 247 (1906); Einstein: Ann. Physik, 33, 1275-98 (1908); Onnes and Keesom: Proc. Acad. Amsterdam, 10, 611-23 (1909); Young, F. B.: Phil. Mag., 20, 793 (1910); Smoluchowski: Phil. Mag., 23, 165-73 (1912); Cardoso: J. Chim. phys., 13, 414-25 (1915); Audant: Compt. rend., 170, 1573-75 (1920); Ornstein and Zernicke: Verslag. Akad. Wetensch. Amsterdam, 23, 582 (1915); Zernicke: Proc. Akad. Wetensch., IS, 152e-27 (1916). Bull. Acad. roy. Belg., 24, 96 (1892); 40, 512 (1908). Z. anorg. Chem., 38,399 (1904). ‘Young: J. Chem. SOC.,71, 446 (1897).
CARBON DIOXIDE-OXYGEN MIXTCREB
2803
observed, is the same as the temperature a t which the gas and liquid phases have the same density. By means of a series of determinations on n-pentane, he showed that the densities of the gas and liquid phases actually do become equal, a fact which had been disputed by de Heen, (loc. cit.).
History of Critical Phenomena of Mixtures The first work done on the critical phenomena of gas mixtures was by KamsayL,who investigated the effect of small amounts of impurities on the critical constants of gases. The effects observed were small, as only minute amounts of impurities were present, but the difference between the critical constants of mixtures and of the pure substances was apparent. Further work on this subject was done by Hannay and HogarthJ2who investigated particularly the solubilities of solids, such as potassium iodide, in liquids above the critical point. Little further seems to have been done in this field. It would seem to have possibilities in the determination of the vapor pressures of substances of low volatility. Small amounts could be confined together with a gas such as oxygen and compressed until entirely evaporated. From the measurements of the pressure the vapor pressure could be calculated and extrapolated to atmospheric pressure. True mixtures of gases were first investigated by CailleteL3 He observed that on compressing a mixture of five volumes of carbon dioxide and one volume of air, a t a definite pressure liquid appeared, increased to a maximum as the pressure was increased, and with further increase of pressure, decreased in volume and finally disappeared. On reducing the pressure the liquid reappeared, passed through a maximum, and finally evaporated. Above 21’ the mixture could not be liquefied a t any pressure up to 3 9 0 atm. In the following year Cailletet4 observed a similar phenomenon in the case of a mixture of carbon dioxide and hydrogen, and van der WaalsJ5without knowing of the earlier experiments, published the results of an investigation of a mixture of carbon dioxide and air. For mixtures of carbon dioxide and hydrogen chloride he found a similar behavior. In a posthumous paper communicated to the Royal Society by Stokes in 1887, Andrew@ discusses the results found by him for mixtures of carbon dioxide with air and with nitrogen. Three volumes of carbon dioxide and four volumes of air could not be liquefied by him a t temperatures above zT., no matter how great the pressure. X mixture of six volumes of carbon dioxide with one volume of nitrogen commenced to liquefy a t 3-5’C. and 48.4 atms., and a t a pressure Proc. Roy. SOC.,30, 323 (1880); 31, 194 (1881). *Proc. Roy. SOC.,30, 178, 478 (1880). 3Compt. rend., 90, 210-11 (1880); 92, 901 (1881); J. Phys., (I) 9, 192 (1880); (3) 2, 389 (1883). Jamin: Compt. rend., 96, 1448 (1883). Published by Jamin on basis of an experiment by Cailletet. 5 “Die Continuitat des gasformigen und fliiasigen Zustandes,” 143 (1881). ‘Phil. Trans., 178, 45 (1887).
2804
HAROLD SIMMONS BOOTH AND JAMES MAURICE CARTER
of I O Z atm. was completely liquefied. At higher temperatures, however, the mixture showed the behaviour described by Cailletet and van der Waals. The analogous behaviour of another mixture is given in the table below. Mixture: 3.33 vol. COzand t"C.
6 3 2
vol. S 2 P1atm. 113 2
76 6
Io; 8
91.6
103
9 9 I3
I
P, atm. 68 7
2
I n all of the above experiments there was no provision for stirring the mixtures, and it is the concensus of opinion that equilibrium was therefore not reached. The conflicting data given for similar mixtures show this. Before any accurate experiments had been performed, Jamin (loc. cit.) assumed that the disappearance of the liquid with increasing pressures was only apparent, and that the fact was that the densities of the liquid and gas phases became identical. The two phases were assumed to diffuse into each other and become indistinguishable. One of the consequences of this theory was that the pressure a t which the liquid disappeared should be higher for gases of low density than for those of high density. This was confirmed by Cailletet, comparing mixtures of carbon dioxide with oxygen and with hydrogen. However, another consequence of the t'heory was that on increasing the pressure still more, the liquid should reappear in t,he upper portion of the tube. This phenomenon has never been observed for mixtures of any gases, except helium-hydrogen mixtures near absolute zero.' I n order t o test the theory, other workers experimented with different mixtures.2 I n the investigation of mixtures of carbon dioxide and sulphur dioxide, two liquid phases were observed, and another worker, in an investigation of a mixture of five volumes of carbon dioxide and one volume of air, reported no less than three liquid phases present a t once. Because of these contradictory results, caused by non-attainment of equilibrium,S the critical phenomena exhibited by gas mixtures remained a scientific curiosity. I n 1892, however, Kuenen,4 in an investigation on gaseous mixtures undertaken to furnish evidence for the correctness of the theory of van der \Taals mentioned above, hit upon the idea of placing a small iron rod in the tube containing the mixture, and of agitating it by means of an electro-magnet. In this way he could be sure that equilibrium conditions were maintained at all times, and obtain accurate and reproducible results, so that the study of gaseous mixtures was a t once placed on a firm experimental basis. Keesom. Comm. Leyd. Suppl., No. 186 (ryo;).
* Pictet: Compt. rend., 100, 329 (1885). 4
Wroblewski: K e d . Ann., 26, 134 (1885). Arch. d e r l . , 26, 394 (1892); 2. physik. Chem , 11, 38 (18yj).
2805
CARBOS DIOXIDE-OXYGES MIXTURES
B Liquid
Cas
Liquid
Cas
L
T
Cas + Liquid a
i"
DIAGRAMS I and I1 Two Kinds of Retrograde Condensation
I n the next few years Kuenen in Holland, and Duheml and his pupil Caubet in Bordeaux,-contributed much to the study of these mixtures, both experimentally and theoretically. The existence of three types of mixtures mas shown, and examples of each were studied. The first or normal types Duhem: (General Theory) J. Phys., (z), 1, 198 (1888); J. Phys. Chem., 1 273 (1897); 5, 91 (1901); Kuenen: (C&-N20) Phil. Mag., ( 5 ) , 40, 173 (1895); (COz-CHdl) Z. physik. Chem.. 24. 66796 ( 1 8 9 7 ) : (C.H2-N90) Chem. News. '71. 266 fr8o;): fC,H,-N,O\ - - ,Arrh ~ ~ ~ - - - -
n6erl.
z,'
(2) '1, &,'27&96'(i897j; Kuenen: (C,H,-CO,j P h k Mag., i 7 4 (i897); (HClMezO) Z.' physik. Chem., 37, 483 (1901); (HC1-Me20) Arch. n6erl.(z), 5, 306-11 (1901); Kuenen and Robson: (C0&2He) Phil, Mag., 48, 180-203 (1899); (COZ-CZH~) Z. hvsik. Chem., 28, 342 (1899); (C02-CzHs) Phd. Mag. (6), 3, 149-222 ; 4, 116-32 ( 1 9 0 2 ) ; & & e n and Clark: (Air) Trans. Roy. SOC.Canada, 11, 25-41 (1917); (Air) Proc. Acad. Sci. Amsterdam, 19, 108898 (1917);Kuenen, Verschoyle, and van Urk: (02-Nz) Proc. Acad. Sci. Amsterdam, 26, 49-64 (1923); Caubet: (COZ-SO,, COz-CH8Cl) Compt. rend., 131, 1200-02 (1900); (SO&HaC1), 131, 108-109 (1900); (COZ-CHJCI), 130, 167-69 (1900); (COZ-SOZ), 130, 828-29 (1900); All of mixtures above, Z. physlk. Chem., 40, 257-367(1902); (CotNzO), 49, 1 0 1 (1904).
2806
HAROLD SIMMONS BOOTH AND JAMES MAURICE CARTER
showed no peculiarities; both the critical temperatures and pressures lie between those of the components. The second type shows a point of maximum critical pressure, although the critical temperatures lie between those of the components. The third type is most irregular; there is either a maximum or a minimum critical temperature, and a maximum or minimum critical pressure. The mixtures belonging to this type are usually those in which the critical points of the components lie close together. The phenomenon of retrograde condensation, first accurately observed and named by Kuenen, may be of two kinds. The first kind has been described above: it consists in the appearance of a liquid phase with increase of pressure, its increase to a maximum, decrease, and final disappearance a t a higher pressure. The second kind is quite different. A consideration of the shape of the boiling and dew curves for mixtures shows that under certain conditions we could expect the momentary appearance of a gas phase arising from the liquid phase, passing through a maximum and disappearing, with rise in temperature. The accompanying Diagram I illustrates both kinds of retrograde condensation. Because of the shape of the experimentally found curves, retrograde condensation of the second kind has never been observed, even when conditions appear to favor it. The region in which it might be expected is so small that it is almost impossible to avoid changes in pressure or temperature which prevent its occurrence. There has been a quantity of work published on the subject of gas mixtures, although much of it is not of great value from an exact standpoint. The following table gives most of the work on gas mixtures that has been published up to the present time,’ in addition to the references already given.
.
Smits,2 in 1904,discussed the phenomena which occur if the triple point of one of the components of the mixture was at a higher temperature than the critical point of the other component. In such mixtures two cases arise: in the simpler case, in which the higher-boiling component is sufficiently soluble in the other, an entire series of critical points for a complete series of mixtures can he observed, as in the cases previously mentioned. At sufficiently low pressures, a solid phase separates and redissolves at higher pressures. Strauss: (H20-C2HjOH) 2. rusk. chim. obsc., 13, (21, 2 7 0 (1881); Ansdell: (HC1-COI) Proc. Roy. SOC.,34, 113 (1882); Pavlevski: (EtzO-CsHla) Ber., 15, 460 (1882); (EtBrCsH,,) 16, 2633 (1883); (EtOH-CeHB, 21, 2141 (1888); Olzewski: (Air) Compt. rend., 99, 184 (1884); (Air) Phil. Mag., 36, 328 21893); van t’Hoff: (HCI-PH3) Ber., 18, 2088 (1885); U‘roblewski: (Air) Compt. rend., 98, 982 (1884); (Air) W e d . .4nn., 25,402 (1885); 26, 134 (1886); Monatsheft, 6, 621 (1885); Compt. rend., 101,63j(188j);102,1o10(1886);Ramsav and Young: (Et20-EtOH) J. Chem. SOC.,51, 755 (1887);Blumcke: (several) Wied.Ann., 36, 911 (1889)’Galitzine: (Me20-Et20)41, 620 (1890); Schmidt: (Series) L. Ann., 266, 266 (1891); Virschaffelt: (C02-H2) Comm. Phys. Lab. Leiden, KO.47 (1899); Quint Gzn: (C2H6-HCI) 2. ph sik Chem., 39, 141 (1901); Hollman: (CH3CHO-(CH3CHO)n) 43, 148 (1903);Keesom: Comm. Phys. Lab.Leiden, No. 88 ( I 03), Young and Fortey: (Organic Liquids), J. Chem. SOC.,83, 4j (1903); Brinkmann: (802-MeC1) Diss. Amsterdam ( I 9 0 ; Erdrnann: (02-N2)Ch. Centr., 19, 1 1 2 7 (1904); Erdmann and Bedford: (02-Nz) Ber., 37, 1184 (1904); Centnerxwer and Zoppi: (Et20-MeOH) 2. physik. Chem., 54, 689 (1906); Sander: (C02Et20),78, 540 (1912); Holst and Hamburger: (Ar-N2) Proc. Akad. Wet. Amsterdam, 18, 872 (1916); Hartman: (Review), J. Phys. Chem., 5, 425 (1901). 2. physik. Chem., 51, 193 (1905).
(60b,-02)
CARBON DIOXIDE-OXYGEN MIXTURES
2807
If the higher boiling component is not sufficiently soluble in the other, a discontinuity results, that is, there are some mixtures which do not have a critical point. Smits’ later published data on ether and anthraquinone, a mixture which showed this discontinuity. Previous Work on Mixtures of Oxygen and Carbon Dioxide The only investigation carried out on mixtures of oxygen and carbon dioxide is that of Keesom (loc. cit.). He worked with two mixtures, containing ten and twenty per-cent of oxygen respectively. These mixtures showed the phenomena of retrograde condensation, and behaved as might be expected from the properties of the components. Keesom’s curves are reproduced in Diagram VIII, together with those obtained in this work, and fit in with them very well. Plan of the Investigation -1s has been stated, the only work that has been done on mixtures of oxygen and carbon dioxide is that of Keesom, and his results do not extend to mixtures containing more than twenty per cent oxygen. As the mixtures used in medicine contain from fifty to ninety-five percent oxygen, little could be predicted on the behavior of these mixtures from Keesom’s results. It was therefore decided to investigate the critical phenomena of a series of mixtures, containing from fifty to one hundred percent oxygen. Five mixtures were decided upon, with a difference of ten percent in the concentrations of oxygen in each. In order to carry out the investigation, the following apparatus was necessary: I. Apparatus for preparation, purification and mixing of the gases. Gases for use in critical constant work must be of the highest possible purity, as slight amounts of impurities exert large effects. Likewise the mixtures must be accurately made up, as for certain ranges of concentrations small changes in the composition exert large effects. The apparatus was designed t o prepare gases of the highest possible purity, and was found to function properly, as evidenced by tests made upon the gases. The mixing apparatus was designed to allow known concentrations t o be made up accurate to 0.055, . I successive S mixtures of the same concentration showed identical properties, it was shown that the apparatus functioned properly. 2 . Tubes in which the phenomena could be observed. I n order to have any amount of liquid to observe a t high pressure, it was necessary to have the total volume of the tubes large, about I O O cc., and also to have a very small volume where the critical phenomena could be readily observed. This was done by fusing a small bulb, of approximately 0.5 cc. capacity to a large glass tube of approximately roo cc. capacity by means of a very fine glass capillary tubing. Much difficulty was experienced in sealing the glass tubes into the steel dowels, by which they were held in the large steel cylinders. Using the 2. phyalk. Chem., 54, 498 (1906).
2808
HAROLD SIMMONS BOOTH AND JAMES MAURICE CARTER
cements described in the literature, the tubes invariably broke off in the cement or mercury leaked up through the cement between the glass and the steel. The same difficulty had been noted by Bradley and Rrowne,' and the leakage was remedied in much the same manner. It was found that a preliminary cement of ordinary hard grade de Khotinsky cement, (shown at AB, Fig. 2 ) , followed by a layer of Dennison's No. z grade red sealing wax (at B-C), would usually hold. If a layer of very soft' de Khotinsky cement, made by mixing the medium grade with pine oil until it would barely flow, were painted over the sealing wax (at' C), while this was warmed to 50-60", no leaks ever developed, even when the tubes stood a t high pressures for 3 long time. 3 . Means for measuring the pressure, temperature, and controlling the temperature of the part of the tube in which the phenomena were observed. The apparatus for these purposes is described under their heads below. It was planned to make a series of determinations on each of the five mixt'ures made up, and t o study other mixtures if the results seemed to call for it. As was expect,ed, the results obtained for the mixture with equal volumes of the two gases fitted in so well with the results of Keesom on mixtures high in carbon dioxide that the intermediate mixtures were not investigated. I n the investigation, the temperatures and pressures a t which each mixture underwent any change of phase, were t o be determined. This involved the determination of the maximum point of condensation; that is, the highest temperature at, which any liquid could be made to separate from the gas, together with the corresponding pressure. I t also involved the determination of the critical point, by which is meant in this paper, the highest temperature at which the gas can be complet,ely liquefied. I t is a t this point, and not a t the maximum point, that the mixture behaves like a pure gas, as the density and composition of the two phases are equal, the mixtures can be made t o change from the completely gaseous state to the completely liquid state with only very small changes in temperature and pressure, and the meniscus is very faint. A t the maximum point, the compositions of the two phases are very different, and only a minute amount of liquid can be separated from the mixture. The meniscus is always very distinct. For the mixtures containing more oxygen, the phase changes from gas to solid, and liquid to solid, and vice versa were also of interest. Owing to the difficulty of making observations, complete data could not be gathered on these changes, but a large number of observations were made. Finally, it was desired to determine, from the observations listed above, what t,he limits of safety might be for mixtures of carbon dioxide and oxygen, and to formulate precautions, if any were necessary, for the handling and use of such mixtures. J. Phys. Chem., 8, 37 (1904).
CARBOX DIOXIDE-OXYGEN MIXTURES
2809
FIG.I
Experimental Tubes These were similar to the ones used by Cardoso' in the determination of the critical constants of the permanent gases. Since mixtures were being investigated, and since diffusion had to be avoided as much as possible, the modification of Onnes and Hyndmann? was adopted. However, instead of the capillary steel tubes connecting the reservoir with the small bulb placed in the thermostat, glass capillary tubing was used. This had an inside diameter of less than 0.3 mm., so that diffusion was very slow. I n the calibration of these tubes the total volume, the volume of the small bulb, the volume of the projecting capillary, and the volume of the small connecting capillary were all determined by calibration with pure mercury. This made it possible to obtain some data on the compressibility of the mixtures a t various temperatures. Measurement of Pressure Two methods of pressure measurement were used. The first was by means of the closed type of manometer, which has been extensively described in the literature. These manometers were substantially the same as those described
* J. Chim. phys.,
13, 312-jo (1915). Onnes and Hyndrnann: Versl. Kon. Akad., 30, 668-674 (1901).
.
2810
HAROLD SIMMOSS BOOTH AXD JAMES JIAURICE CARTER
by Cardoso. After volume calibration with pure mercury, they were sealed on to the apparatus for preparation of the gases, and filled with oxygen under the same conditions as were the samples of the gas mixtures. As these closed manometers were necessarily of a rather limited range, and as the pressures used in the course of the experiments varied from less than 30 atm. to more than 130 atm., they mere rather inconvenient. For this reason an absolute manometer, (so-called deadweight manometer), manufactured by Shaeffer and Budenburg was later used. The weights used on this manometer were calibrated against, a standard kilogram, and the diameter of the piston measured with a calibrated micrometer caliper. The manometer was also compared with one of the closed manometers. It mas found to give accurate and reproducible pressures correct to 0.1atms. Measurement of Temperature The only thermometers suitable for temperature measurements in the entire range from +35'C. to -1z0'C. were the platinum resistance and thermocouple types. The resistance type was chosen for this work. The resistance coil was const,ructed of commercial platinum wire, 0.I 5 mm. in diameter. It was wound on a mica cross, I cm. in diameter and 8 cm. long. The method of winding was such as to eliminate strains in the wire, and was done as follows: the mica cross was cut from sheet mica, following the design given by Callendar' and others. The pieces were assembled, tied together with thread, and hard paraffin cast in a cylinder around them. This cylinder was then placed in a lathe, and double threads, twelve to the inch, cut in it, deep enough to notch the edges of the mica. The wire was doubled on itself to obtain an induction-free circuit, then wound in the grooves on the cylinder, and attached firmly to the mica by threading through holes in the upper and lower ends of the cross. The free ends were then silver-soldered to copper leads, and the paraffin melted away from the mica. This gave a circular winding, free from sharp bends. After a pair of dummy leads, consisting of one turn of platinum wire silver-soldered to copper leads of t,he same length and diameter as the true leads, had been attached, the coil was placed in a thin glass tube. The coil was then flashed to red heat by sending a momentary current through it, in order to insure good annealing of the wire, and to remove any strains which might have been introduced in handling. The case was pumped out, dried, and filled with dry air, to avoid errors due to moisture on the coil. The thermometer was then ready for calibration. The fixed points used in calibration were the ice-point, the steam-point, and the boiling point, of oxygen. For a check on these, the boiling-point of sulphur was also used. 'Callendar: Phil. Trans., 161 (1887); Griffiths: Proc. Roy. Soc., Dec. (1890) Phil.; Mag., ( 5 ) 48,519 (1890); Chappuis: Phil. Mag., ( 6 ) ,2,243 (19oz);Rothe: 2.Instrument., 23, 364 (1903); Chappuis and Harker: Trav. Mem. Bur. int., 12, 7 j (1900); Callendar and Grfiths: Phil. Trans., 182, 119 (1891).
CARBOS DIOXIDE-OXYGEN MIXTURES
2811
The temperature measured by this thermometer is indistinguishable from the thermodynamic temperature in the range used. The thermometer was also compared with a mercury thermometer calibrated by the Bureau of Standards a t a series of temperatures between oo and IOO', and found to agree within the limits of observation, (0.03'). It was also tested a t the subliming point of carbon dioxide, and found to agree within 0.1'. This point was not used in the calibration, because of the poor heat conductivity of carbon dioxide snow, and because, when liquids such as alcohol or ether were used to make a paste, the temperature invariably changed. Control of Te mp e m t u r e. The thermostat used in this work (see Figure 2 ) , consisted of two Dewar flasks, one within the other. The outer one was filled to a greater or less extent with liquid air, and the inner one with a mixture of halogenated hydrocarbons, which composed the cooling bath. The substances used for the bath, ethyl bromide, dichloro-ethylene, and trichloroethylene, are all non-inflammable, and suitable mixtures of them will not For the lower freeze or become viscous at temperatures as low as -120'. temperatures, a predominance of ethyl bromide must be used. For the higher temperatures, the inner Dewar flask used was one with a very good vacuum. The liquid in it cooled only very slowly with liquid air :wound it in the outer flask, and the application of a very little heat by means of the resistance coil H immersed in the liquid in it, was sufficient to maintain the temperature at any desired point in the region of -50'. For the lower temperatures, an inner Dewar flask having a vacuum of only about 0.01 mm. was used. This permitted rapid cooling to the temperature desired, and the temperature could be maintained by passing a small current through the heating coil, H. The bath was stirred by means of dry air. Slight amounts of moisture or of carbon dioxide cause the liquid in the thermostat to become turbid, so that observations are almost impossible. Therefore, the air was dried by passing it first through concentrated sulphuric acid, then over barium oxide,' to remove any carbon dioxide present, and then through a tube filled with phosphorus pentoxide. The dried air passed down through a glass tube containing the heating coil, H., mentioned above, and out through fine holes in the bottom of the tube. Placing a thermometer a t different positions in the bath showed no temperature difference. -1utomatic control of the temperature was attained by means of a vaporpressure manometer immersed in the bath. Ammonia was placed in the bulb of the manometer, and enclosed by means of mercury. A change in temperature altered the vapor-pressure of the ammonia, raising or lowering the mercury, which was connected to a relay in the usual manner. The relay opened or closed the circuit in the heating coil. Temperatures constant to 0.05' a t - 80" could be maintained for as long as an hour. By adding liquid air to the outer vessel from time to time it was possible to maintain the temperature constant within these limits for as long as desired. '"Barium Oxide as a Desiccant," Harold Simmons Booth and Lucille H. McIntyre: Ind. Eng. Chem., Analyt. Ed., 2, 1 2 (1930).
2812
HAROLD SIMMOKS BOOTH AND JAMES blAURICE CARTER
Preparation and Purification of Gases Ozygen. The source of the oxygen used was a tank of the electrolytically prepared gas. After passing through the tubes D I , Figure I , the first filled with potassium hydroxide and the second with phosphorus pentoxide, it was condensed in the distilling tube TI, by means of liquid air. It was then distilled six times, a t approximately atmospheric pressure, between the tubes T I and T2, the first and last fractions from each distillation being pumped off through the stopcocks 4 and j, and discarded. In each fractionation, after the last of the liquid had been removed from the distilling tube, it was heated to 100’ while attached to the pump, t o remove any water vapor. The purified gas was then stored in the bulb €31, which had been dried previously, washed out with the purified oxygen, and re-evacuated. From this bulb, the gas was drawn off as needed. Carbon Dioxide. Carbon dioxide was prepared by the action of potassium permanganate on oxalic acid in the presence of sulphuric acid. This method was adopted because Lidovl has reported the presence of oxane, C O S , in carbon dioxide prepared from carbonates. Both the potassium permanganate and the oxalic acid were three times recrystallized, the C.P. products of commerce being used as starting materials. The sulphuric acid used was the ordinary C.P. grade. For the preparation of the gas, a reaction vessel, partly filled with oxalic acid, was fused on to the inlet tube of the purifying apparatus beyond the tubes D4. A solution of potassium permanganate in sulphuric acid was then admitted through a fused-on dropping funnel, and let drop slowly on the oxalic acid. The gas generated was then led slowly through the tubes Dq, the first filled with moist potassium permanganate to oxidize any traces of carbon monoxide which might be present, and the second filled with phosphorus pentoxide. The gas was then condensed to a solid in the tube Tq by means of liquid 3ir. After the liquid air had been removed from around the tube, any permanent gases, such as S2,02,H?, CO, were removed by continuous pumping with the Hyvac pump. On further warming, the carbon dioxide melted t o a liquid, at approximately 5. j atmospheres pressure. I t was then fractionated between the tubes T4 and T3 a t approximately this pressure, to keep it in the liquid state. The first and last fractions in each distillation were pumped off through the stopcocks 11 and 13, and discarded. Each time, after the liquid had been removed from the tube, it was heated to 100’ while attached to the vacuum pump, to remove any water vapor. The purified gas was then stored in the bulb R z , which previously had been dried, washed out with the purified carbon dioxide, and re-evacuated. Preparation of the Samples. Oxygen was drawn from the storage bulb R I into the double baro-buret A2 The volume, temperature, and pressure were noted, an average of several readings a t different pressures being taken. J. Russ. Phys. Chem. S O C 43, , 6jo-653 (1912); 44,jz7-532 (1913); Orig. Comm. 8th Intern. Cong. App. Chem., 6, 1 8 j (1912);Reports Kharkov Tech. Inst. (1912). “The Baro-Buret--A new Accurate Gas Buret.” Harold Simmons Booth: Ind. Eng. Chem. Anal. Ed., 2, 182 (1930).
CARBON DIOXIDE-OXYGEN MIXTURES
2813
Then an amount of carbon dioxide sufficient to make up the desired mixture was drawn from the storage bulb B2, and the volume, temperature, and pressure noted as for the oxygen. I n this way, mixtures could be made up accurately to any desired composition. For the mixing, the two-way stopcocks of the burets were turned so as t o connect the two burets. The tube connecting them had been previously pumped out to as high a vacuum as
FIG.2
possible, (o.ooz mm.). The leveling bulbs L I and Lz attached to the burets were raised and lowered so as t o have the entire mixture first in one buret and then in the other; this was repeated 2 0 times, t o insure thorough mixing. The experimental tube, fused on at the point indicated-above stopcock I O , was dried by flaming, xhile it was connected t o the vacuum pump. Small amounts of oxygen were let in from time t o time, the tube flamed and again pumped out; this was repeated six to eight times. Finally the tube was rinsed out with a small quantity of the mixture to be placed in it, and again pumped out. After this, the stopcocks connecting the tube and the burets were opened and the mixture transferred to the bulb. In all casw, the tube was filled to within one or tw-o centimeters of atniospheric pressure. The stopcock on the lower stem of the experimental tube x i s then closed, and the tube with stopcock removed from the apparatus. After removal, the experimental tube was placed in a constant temperature room, and the stopcock broken off under mercury. The mercury level was then adjusted eo that the gas occupied a known volume. The difference in the mercury levels inside and outside the tube mas measured with a cathetometer, the temperature noted, and a reading of the barometer taken. The exact amount of gas in the tube could then be calculated.
2814
HAROLD SIMMONS BOOTH A X D JAMES MAURICE CdRTER
The experimental tube was then placed in the large steel container, Fig. 2 , care being taken to have the lower end under mercury at all times. Experimental Procedwe. The experimental tube, containing the mixture made up as previously described, was placed in the steel container, Fig. 2 , which was then connected to the compression pump and to the manometers by means of flexible steel tubing. The gas was then compressed to a pressure somewhat higher than that expected in the subsequent determinations, to make sure that there were no leaks, and to check on the strength of the glass tube. Volume and pressure readings were made during the compression, in order to have a room temperature isotherm for the mixture. The thermostat was then placed around the small bulb at the end of thr experimental tube, and the thermostat liquid, previously cooled to approximately the temperature desired, was rapidly filtered in to the inner Dewar flask. It was found that in cooling, the liquid always absorbed small amounts of water vapor and carbon dioxide, and unless it was filtered after cooling before being placed in the flask, was cloudy and obscured observations. T.4BLE
Phase equilibria for mixtures of Temp.
- 13.16"c'. '3.32 '3.96 14.62 19.44 '9.47 zj.11
28.38 29.04 29.68 29.68 30.20 30.20
30.32 33.04 33.44 33.38 33.50 33.60 35.22
35.71 35.87 38.09
37.63 37.58
Pressure
122.54 atm. 122.54
1 2 2 ,j4
103.64 1 3 1 .j 2 1 3 2 . I9 151.29 1j4.00 I 3 7 .so 140.21 '37,SO 137.50 138.86 138.86 139.54 140.21 140.21 35.63 35.63 140.66 140.91 140.81
141 . I 4
1 j
vols. O2 -
j
vols. C N ?
Observations
Gas only Gas and liquid Gas and liquid Gas and appreciable liquid Gas and liquid Gas only Gas only Gas only Gas and liquid Gas only Gas and liquid Gas and liquid Gas only Gas and liquid Gas and liquid Gas only Gas and liquid Gas and liquid Gas only Gas and liquid Liquid only, no gas in bulb Gas and liquid Gas and liquid, gas vanished on stirring Completely liquid Gas and liquid
CARBON DIOXIDE-OXYGEX MIXTCRES
281 j
When the bath had been brought to constant temperature, the pressure was gradually increased until the first drop of liquid appeared in the bulb. If the liquid remained after vigorous stirring, the pressure and temperature were noted, and also the total volume occupied by the gas. The pressure was then raised by small amounts until the liquid either filled the entire bulb or had completely re-evaporated. After each increase of pressure, the mixture was vigorously stirred by means of the magnetic stirrer, to insure equilibrium conditions. S e a r the critical point, small increases in pressure caused the meniscus to rise into the small capillary connecting the bulb with the rest of the tube, TA4BLE11 Phase equilibria for mixtures of 6 vols. O? - 4 vols. CO,
Temp.
- 20.23~C’ 20.26 20.66 20.89
Pressure
20.40
107.94atm. 107.94 108,62 108.62 108.62 108.62 108.62 89.j6 89.65
21.32
11; .26
21.29
I I i .94 118.57 119.91 89,56 118.5; 117.96
21 . I 2
21.03
21.06 20.56
21.22
21.2j ?I
.74
~1.6j 21.84 21.67
117,ji
22.49 22.26 24.97 24.97 30.99 30.99 30.92 30.95 31.12
119.91 119.91 128.81 129.11 134.66 137.28 137.28 136.90 137.05
37.28
141 .oj
37.25
141.35
37.25
141.20
Observations
Gas only Gas and liquid Gas only Gas and liquid Gas and liquid Gas only Gas and liquid Gas and liquid Gas only Gas and liquid Gas and liquid-less liquid Gas and trace liquid Gas only-possibly liquid Gas and liquid Gas only Gas and liquid Gas only Gas and liquid Gas only Gas and liquid Gas only Gas and liquid Gas only Gas only Gas and liquid Gas only Gas and liquid Gas only Gas only Gas and liquid Gas and liquid Gas only
2816
H.%ROLD SIMMOSY BOOTH A T D JAMES MAURICE CARTER
if the temperature was slightly below the critical value, and caused the meniscus to fall rapidly to the lower part of the tube and vanish if the temperature was slightly above the critical value. By observing the direction in which the meniscus moved on changing the pressure slightly, the critical point could be arrived at very closely. I t was not always possible to keep the mixture exactly at the critical point, as mixtures are much more sensitive to small changes in conditions than are pure gases. However, when the critical condition could be maintained, the mixture behaved like a pure gas. The meniscus became flat and indistinct, and slightly changing the pressure made the mixture opalescent. At temperatures near the maximum temperature of condensation, the liquefaction curve lies almost parallel to the pressure axis, making observations by the method just mentioned difficult and uncertain. An error of 0.0;' in temperature in this portion of the curve causes errors of as much as I O atm. in the determination of the pressure. For these portions of the curves a different procedure was followed. The pressure was set at some definite value, which could easily be maintained for as long as desired by means of the absolute manometer. The temperature was then lowered extremely slowly, until the first drop of liquid was observed. In this way, the curve was approached at very nearly right angles, so that errors were minimized.
TABLE 111 Phase equilibria for mixtures of j vole. Temp. -32,97OC. 33.14 33.64
Pressure 9 6 , 1 6 atm. 96.16
1'4.5'
0 2
- 3 vols. C 0 2 Ohservations
Gas only Gas and liquid Gas and liquid Gas only Gas and liquid Gas and liquid Gas and possibly liquid Gas only Gas only Gas and liquid Gas and liquid G a s only Gas and liquid (;:is only (;as and liquid Gas only Gas and liquid (;as only
