SOLUBILITY RELATIONS I N GAS-LIQCID SYSTEMS.* I. A XEW APPARATUS FOR DETERMINING GAS SOLUBILITIES BY J. LIVINGSTOX R. XORGAN AND H. RIVINGTON PYNE
I. Introduction While analytical methods for determining gas solubilities are in general the most accurat'e, there are many systems to which they are not applicable, so that the working out of a physical met,hod, as free as possible from the objections raised against such methods in the past, seemed desirable. Such methods depend on the measurement of the change in volume or pressure in the system when measured quant'ities of gas and liquid are mixed together while the other variables are kept constant. The volume changes may be follored with any desired degree of accuracy, and the only doubt as to the validity of t,he results obtained by these methods is in the possibility that true equilibrium betTveen the phases may not be obtained by the methods of mixing employed. The most usual method of mixing gas and liquid has been to shake the two phases together, as in the well-known OhstTvaldlapparatus and its many modifications. --In end-point is very rapidly obtained, but the possibility that this represents a condition of supersaturation rather than a true equilibrium, due to t,he violence with which the gas is forced into the liquid, has been pointed out by Cady, Elsey and Berger,? who made use of electromagnetic stirring in order to obviat,e this possibility. The use of gentle stirring to effect the mixture results in conditions antithetical t o those brought about by violent shaking. Solution is very slow, from 2 4 to 36 hours being required for a determination, and as the final portions of gas are taken up by the liquid Tvith extreme slowness the danger of undersaturation must be present. These considerat'ions led the present authors to use a third method of mixing the gas and liquid phases, that of bubbling the gas through the liquid. This method is almost invariably chosen when the object sought is a saturated solution for investigation by analytical or other means. The procedure is also used for freeing liquids of gaseous impurities-as in the preparation of standard baryta solutions-so that it seems reasonable to expect that if at any time supersaturation should occur a continuation of the process would remove the excess gas and restore a condition of true equilibrium. At the same time the gas must become saturated with the liquid vapor, as in the vapor pressure determinations based on this same process. It should be noted also that the time'required for a determination by bubbling through falls between the extremes of the other methods, being between one and two hours. * Contributions from the Chmnical Laboratories of Columbia University, No. 265 "OstxAd: "1.rhrbiich d e r ollg(,m. Chcmii,." ( z .4ufl.). Cady, Elsry and Berger: J. .Sin.Chem. doc., 44, 1 4 j 6 ( 1 9 2 2 ) .
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11. Apparatus Four different types of apparatus for determining gas solubilities by repeated bubbling of a sample of gas through a known volume of liquid and measuring the loss of gas volume a t constant temperature and pressure have been constructed and tried. The first was an adaptation of the apparatus of Hainsworth and Titus,' consistina essentially of an absorption vessel placed between two gas burettes, so that gas could be bubbled through the liquid while passing from one burette to the other. Stopcocks allowed the introduction of gas and liquid without removal of the apparatus from the thermostat, and the whole apparatus was rigidly mounted on a board in order to give it the necessary support. The use of two burettes with a large absorption vessel was found to be disadvantageous, and the mode was abandoned in favor of one using a single burette, from which the gas was forced through the liquid and to which it was returned, its passage in one direction only being controlled by means of a small glass valve. The design of this valve necessitated the construction of the apparatus in two pieces, a fault which was remedied in the third form tried by setting the valve in the bore of a large stopcock. This gave a convenient form of apparatus, and one capable of giving reproducible results, but the forcing of gas through the liquid and its withdrawal to a single burette effected constant changes in volume, and hence in pressure. While it was considered reasonable to assume the average pressure as the true pressure of the experiment, it was thought preferable t o construct an apparatus with FIG.I pumping mechanism separate from the burette, so that the pressure should remain constant and be independently measurable. X fourth form was therefore designed, and is described in detail below. The final apparatus is shown schematically in Fig. I. I n actuality the four small pumping bulbs lie close together on the side opposite to that shown while the two capillary tubes extending from near the top to the 1
Hainsworth and Titus: J. Am. Chem. SOC., 43, I (1921).
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stopcock F lie together on the near side. The apparatus is therefore more compact than would appear, and can be conveniently supported by means of forks and burette clamps on a portable triangular brass frame, A gas burette of convenient shape is attached to the tube A by means of rubber tubing, or may be sealed on if necessary. A rubber tube attached at B leads through the lower wall of the thermostat, as do tubes C and D and the tube for filling and controlling the burette. All of these tubes end in levelling bulbs, which are supported on ring stands outside of the bath. The operation of the apparatus is as follows: through the tubes A, B, C and D the apparatus is filled with mercury as far as the upper three-way stopcock E. Gas is then allowed to flow freely through E until all the air in the connecting tubes has been expelled. I t is then possible to fill the entire apparatus with gas by the withdrawal of mercury. In practice, however, the 300 cc. absorption vessel marked “Liquid” is allowed to remain filled with mercury by suitable manipulation of the stopcocks. Stopcocks F and G are then placed in position to give communication between tubes B and H through the absorption vessel, and connection is made between the mercury in the apparatus and the supply of gas-free liquid through the stopcock I. Rithdrawal of mercury thereupon causes the absorption vessel to be filled with liquid between the stopcocks F and G. The apparat’us is now in the condition indicated in the figure, with the exception of the stopcock J, which must be turned over to invalidate the action of the two small valves contained in its double bore and allow free communication between the gas in the burette and that in other parts of the gas space. Liquid and gas are separated by the stopcocks F and G, and stopcock E is closed to shut off the gas supply. The init,ial reading is taken on the burette, and all is ready for a determination. Stopcocks F and G are turned t o establish communication between the liquid and gas phases, and stopcock J is turned t o the position shown, in which gas may pass upward through it, but not downward. The two small levelling bulbs h: and L are set in motion. They are carried a t opposite ends of a walking-beam operated by an electric motor through a reducing gear making approximately one revolution per minute. As they rise and fall the mercury in the four small pumping bulbs must rise and fall, the quantity of mercury added t o one pair of bulbs being equal to that withdrawn from t,he other pair. Gas is thereby forced upward on one side of the apparatus, impinging on one of the valves contained in the stopcock J. Being unable to pass downward through the valve, it passes down the capillary tube to stopcock F, bubbles up through the liquid in the absorption vessel and the small overflow bulb 31,and passes through the other valve in stopcock J to the pumping bulbs on the other side of the apparatus. Reversal of the motion of the levelling bulbs causes a similar flow of gas in the opposite direction, but always upward through the absorption vessel. Absorption of the gas by the liquid is followed on the burette in the usual
SOLUBILITY RELATIONS IN GAS-LIQUID SYSTEMS
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manner. It may be pointed out that, when desirable, increase of liquid volume may be measured at the end of an experiment, the absorption vessel and capillary side tubes of this apparatus forming a practical dilatometer.
111. Experimental A determination of the solubility of carbon dioxide in water a t 2 jowas made with the apparatus described above. For this purpose a burette of the Bleier' type was used, consisting of a straight tube and a bulbed tube joined together a t the top and making possible the measurement of all volumes from o to 350 cc. The use of such a dual burette of course doubles the error of reading. Carbon dioxide was formed in a generator by the action of sulphuric acid on a solution of sodium bicarbonate, and was passed through a series of wash bottles containing distilled water before being admitted to the apparatus, the last wash bottle being suspended in the thermostat. Distilled water was used, and was freed from dissolved gases by boiling for one hour under the reflux in the usual manner.
IV. Results While a number of determinations of the solubility of carbon dioxide in water were made a t different pressures, only those will be included in which measurement of volume and saturation were effected at the same pressure, later work having indicated that any other procedure gave results which were inconclusive for purposes of comparison. The results are expressed in terms of the Ostwald Solubility Coefficient, I , which represents the volume of gas absorbed a t the temperature and pressure of the experiment divided by the volume of the liquid, in this case 2 9 0 . 5 2 cc. The average value obtained is 0.8265, which should be compared with the value of 0.8267 obtained with the single-acting apparatus described above, and with the following figures* taken from the literature: Observer
I
Geffeken Just Bohr
0.82jj
0.82j6 0.826
Obszrver
Kunerth Findlay and Creighton Findlay and Shen
I
0.818 0.817 o 825
Findlay and Creighton report their gas o.j8%, Findlay and Shen theirs impure. V. Discussion The chief point of interest in these results seems to be the vindication of the shaking method of saturation, by which practically all of the results taken from the literature for comparison were obtained. The fact that our o.oj%
'Bleier: Ber 30, 2753 (1897). Geffeken: Z. physik. Chern., 49, 268 (1904); Just: 37,342 (1901); Bohr: Ann. Physik, ( 3 ) , 6 8 , 5 0 3 (1899);Kunerth:Phys,Rev., (2) 19, 512 (1922);Findlayand Creighton: J. Chern. Soc., 97,536, (1910); Findlay and Shen: 101, 1459 (1912).
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results are if anything even higher than the older ones mould indicate that the supersaturation feared by some investigators does not develop. As the shaking method is the most rapid of all, it is probably the most advantageous for use where solubilities alone are to be determined.
VI. Summary X new apparatus for determining gas solubilities by repeated bubbling of the gas through a known volume of liquid a t constant temperature and pressure has been described. The solubility of carbon dioxide in water has been determined a t and the result compared with those of other investigators.
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