SUPERSATURATION O F GASES I N LIQUIDS BY FRANK B. KENRICK, K. L. WISMER AND K . S. WYATT
The work described in this paper was begun some years ago and part of it has already been reported in abstract1. Although it is not yet completed the recent appearance of an article under the same title2 makes it inadvisable further to delay publication of the details. I n most of the previous work on supersaturation the phenomenon has probably been complicated by one or more of three factors: (I) crystalline form of the separating substance, ( 2 ) effect of the walls of the containing vessel, (3) dust particles suspended in the liquid. We have avoided the first difficulty by using gases. The effect of the vessel walls has been eliminated t o a certain extent in the present work, and it is hoped later to be able t o employ dust free liquids prepared by some of the methods already worked out in this laboratory. The work may be taken up under three heads: (A) Solutions made by shaking water and the gas under pressure, (B) Supersaturated solutions prepared by chemical formation of dissolved gas, (C) Ultra-microscopic investigation of capillary tubes of solution under pressure.
A. Solutions made by shaking Water and the Gas Refore giving the details of the experiments it may be well to quote the conclusions already reached in the work by Wismer3. “Solutions of oxygen and of carbon dioxide were investigated at atmospheric pressure a t concentrations corresponding to pressures up to about 50 atmospheres in the case of oxygen and 35 in the case of carbon dioxide. The results obtained up to the present seem to justify the following conclusions: I . A long heating of tube and solution a t high temperature was found t o favour supersaturation. 2 . The time interval between the reduction of pressure and appearance of a bubble varies between wide limits even under apparently identical conditions. 3 , Suspended particles (e.g. colloidal platinum) introduced into the liquid rapidly lose their effectiveness in starting the bubbles. 4. It is almost certain that in all cases the bubbles were initiated at the surface of the glass, although the location on the surface was by no means constant except in tubes in which there were obviously imperfections in the glass. 5 . Although carbon dioxide is nearly thirty times as soluble as oxygen, the average time interval before formation of bubbles is about the same for 1K. I,. Wismer: Trans. Roy. SOC.Canada (3) 16, 217 (1922). 2 J. Metschl: J. Phys. Chem. 28, 417 (1924). 3 LOC.cit,. This has not yet been reviewed in Chemical Abstracts.,
SUPERSATURATION O F GASES IN LIQUIDS
I309
the two gases a t the same temperature when the supersaturation corresponds to the same equilibrium pressure. 6. On the assumption that the bubble originates from a spherical particle acting as a nucleus which the bubble just encloses! the diameter of such a particle was calculated to be at most 5 X IO-~CM*”. Method and Apparatus. The difficulty encountered with the ether in preventing the growth of large bubbles (see preceding article) is almost wholly overcome in experiments with a supersaturated solution of gas in water. I n this case the formation of a bubble of gas in the liquid depletes the immediate neighborhood of dissolved gas, and the bubble must grow by the diffusion of gas from a more concentrated portion of the liquid. Hence the bubble grows comparatively slowly and there is no violent reaction to disturb the rest of the liquid.
n
FIG.I
Oxygen and carbon dioxide were chosen for study, as examples of slightly soluble and very soluble gases. The oxygen used was the commercial gas from a cylinder; the carbon dioxide was made in a Kipp apparatus from marble and hydrochloric acid. The form of apparatus used is shown in Fig. I. The tube, A , 60 mmX6 mm inside diamet,er, contains the supersaturated liquid. D is a bulb large enough to hold the contents of A. E and BC are capillary tubes used for filling the apparatus. The small manometer M is used to indicate atmospheric pressure. The whole is made of well-annealed soft glase of a suitable thickness to stand over 50 atmospheres pressure. The apparatus is first filled with water. The open tip, C, is then brought up under a test tube of oxygen inverted in a dish of water. By unscrewing the screw of the pressure machine the liquid in A is drawn back into D, and is followed by oxygen. A little water is then drawn in to fill the tube BC, and the tip C sealed off. By applying a pressure of from 30 to 40 atmospheres the gas is compressed to a small bubble. A motor-driven mechanical a.rrangement slowly raises and lowers the apparatus through about 30 cm. by means of a string attached to a wheel on the ceiling. The apparatus is attached t o
-
*This is wrong. This was the number for superheated ether put in here through error. The value should have been 23 X IO-’.
13I O
FRANK B. KENRICK, K . L. WISMER AND K. S. WYATT
the pressure machine and hydraulic pressure gauge by a horizontal flexible glass capillary about a meter long, and the gas bubble is thus made to run from end to end in tube A . Even with this arrangement an hour or more is necessary to effect solution. When the gas is all dissolved the pressure is reduced to one atmosphere, the tip C broken off and a fresh supply of gas drawn in. The operation may be repeated many times until the desired concentration is reached. The equilibrium pressure of the gas ir, solution was found by dissolving all but a minute bubble, and then adjusting the pressure until the bubble was seen to grow neither smaller nor larger on measurement every three minutes with a microscope provided with an etched micrometer scale. On complete solution the pressure was, of course, increased by a slight but almost negligible amount. Results. (I) An amount of oxygen equal to approximately half the volume of A was drawn into the tube and dissolved under 3 0 or 40 atmospheres pressure, giving a solution whose equilibrium preseure was about 16 atmospheres. With this concentration of gas bubbles did not appear on reducing the pressure to one atmosphere, a t room temperature, even after an hour, and it was found necessary to heat the tube to the neighborhood of 60 or 70' to obtain bubbles within a reasonable time. (2) Experiments were performed which had another advantage over those with superheated ether, namely, that after the formation of one bubble time could be allowed for the formation of one or two more bubbles. The following record is typical of a number of experiments. Different portions along the length of the tube may be represented by a, b, c , d, e, J", g , h. The equilibrium pressure of the oxygen was 16 atmospheres. At 61' and I atmosphere a bubble appeared in 4 seconds a t a ; one second later another bubble appeared at e; I O seconds later no more bubbles had appeared and pressure was applied to redissolve the gas. At 58' and I atmosphere a bubble appeared in 75 sec. at g; I O seconds later another a t a ; 20 seconds later another a t d. (3) The effect of temperature on the time interval for which a liquid can remain a t one atmosphere is shown very strikingly by one tube whose time intervals in the neighborhood of 80' varied from I O seconds to 4 minutes. This tube a t room temperature gave rise to a bubble only aftei more than 76 hours, although a t this temperature the absorption coefficient of oxygen is scarcely twice as great as that at 80'. (4) One tube was filled with a solution of colloidal platinum which had been prepared by Bredig's method some months before, and from which all the larger particles had settled out. Oxygen equal to about half the volume of the tube was dissolved in it giving an equilibrium pressure of about 16 atmospheres. On the first reduction of pressure to one atmosphere a t room temperature g bubbles appeared almost simultaneously after two seconds. These were dissolved, and a second time g bubbles appeared after three seconds. On a third trial 3 bubbles appeared in three seconds, and no more appeared in the next 40 seconds, A fourth reduction of pressure gave no
SUPERSATURATION O F GASES IN LIQUIDS
1311
bubbles in 1 1 minutes, and pressure was again applied. Thus the effectiveness of the colloidal platinum in starting bubbles is rapidly lost. (5) One tube, in which a concentration of oxygen corresponding to an equilibrium pressure of 30 atmoepheres a t zoo had been built up, held for 75 seconds a t 72’ a t atmospheric pressure, without formation of bubbles. (No doubt much higher concentrations could have been obtained for shorter time intervals.) From Winkler’s solubility values1 for oxygen a t different temperatures, and assuming Henry’s law the equilibrium pressure a t 7 2’ was calculated t o be 51.5 atmospheres. The radius r of a bubble which could exist in equilibrium with the liquid under those conditions would be 23 X IO-’ cm. If the maximum supersaturation of oxygen attainable be calculated on the basis of r= 7.3 X 10-7 (the value obtained for water from superheating experiments; see preceding article) a concentration is found corresponding to a pressure of 2 0 2 atmospheres a t 20’ and 168 atmospheres at 72’. (6) A few experiments were made with carbon dioxide in water. Since the absorption coefficient of this gas at room temperature is 0.878~a great many fillings were necessary to build up a high equilibrium pressure. After 14 fillings of the tube the pressure was found to be g atmospheres; aft’er 24 fillings the pressure was 14 atmospheres. At room temperature and a t 40’ a bubble did not form in I O minutes. It was necessary to raise the temperature to from 55’ to 70’ before bubbles would form in ten minute intervals. The capriciousness of the behaviour is very similar to that of the oxygen solutions. I n one test it was possible to keep the liquid a t atmospheric pressure at 79’ for 15 seconds. The equilibrium pressure was again determined a t the conclusion of the experiments and found to be 1 2 atmospheres, the loss being due to slow diffusion through the capillary connecting A and D. Using an extrapolated value for the absorption coefficient a t 79’ and assuming Henry’s law, which is not strictly true for carbon dioxide, an equilibrium pressure of over 3 5 atmospheres was calculated.
B. Supersaturated Solutions prepared by Chemical Formation of the Gas Method and Apparatus. Highly concentrated solutions of nitrogen were prepared by warming, under pressure, Folutions in which dissolved nitrogen is produced by chemical reaction. Fig. z gives 8 diagrammatic representation of the apparatus. A is a screw pressure machine from which the IOO atmosphere hydraulic gauge used in part A has been removed. This is connected by a small copper tube to the needle side of a needle valve. The seating side is connected by similar tubing to a sealing wax joint and thence by glass capillary tubing to the thick-walled glass bulb B of about gcc. capacity, in which is the meniscus between the oil of the press and the water ueed in the rest of the system. The air-filled manometer whose lower half was made from 0.9 mm and upper half from 0.3 mm tubing was of such dimensions 1
Ber. 24, 3602 (1891). Seidell: “Solubilities of Inorganic and Organic Compounds.”
. 1312
FRANK B. KENRICK, K. L. WISMER AND K. S. WYATT
that at 2 0 0 atmospheres the mercury stood about 20 mm from the top. This was joined by a loop E of about two metres of capillary glass tubing (about 2 mm outside and 0.7 mm inside) to a T-piece made of very small tubing, the lower end of which led to the solution vessel F , 8 mm outside diameter and IO cm long with about 1.5 mm wall thickness. This vessel was painted black with the exception of four narrow vertical slits where the paint was scraped off as shown in the sectional view, Fig. 2. The tubes C were used for filling the system and for blowing into when making sealed joints or repairs. The apparatus was always tested to 2 0 0 atmospheres before beginning an experiment. It may be worth noting that most of the breakages consisted in the bursting of F and in the splitting of T-pieces. It was originally in-
Fro. 2
tended to heat F to 70°, keeping the lower end cooled to prevent bubbles forming at, the bottom, but we never succeeded in getking a tube to stand the pressure under this condition, although Pyrex, lead glass, and Jena glass were tried in addition to soft soda glass which was found to be the best. All tubes were made as Pmall as possible. The only T-pieces that could be relied upon were made very carefully from tubing about 3 mm outside diameter and I mm bore. The advantage of thick glass walls was apparently more than offset by the difficulty of avoiding strains in the glass-blowing. The method of procedure was in general as follows. Dissolved nitrogen was generated in a warmed aqueous solution in the vessel F while the pressure was maintained at a value sufficient to prevent bubbles forming, until the reaction was complete. The time necessary for this was found by blank experiments in test tubes with fine rubber delivery tubes, the gas from which was measured by collecting over water. I n these blank experiments the solution, temperature, and duration were the same as in the actual experiments, but in the blanks the tubes were shaken vigorously every few minutes t o
SUPERSATURATION OF GASES IN LIQUIDS
I3 I3
cause evolution of the gas. As the reactions used might safely be assumed to be non-reversible, the fact that the gas was evolved in one case and not in the other could not affect the quantity, and would be very unlikely to affect the rate of formation of the nitrogen. When the reaction was complete the tube A was allowed to cool to the room temperature. There is now in the tube a solution which, when the pressure is reduced to I atmosphere will be very highly supersaturated. A beam of light from a n arc lamp was then passed through the tube from right t o left and observation was made through the front slit by means of a lens. With this method of illumination bubbles rising from the sides of the vessel, although visible by light reflected inside the tube could be easily distinguished from bubbles rising through the middle of the tube which shone out like sparks. Bubbles rising from the bottom could be distinguished from bubbles formed in the body of the liquid by the fact that the former could not come into view near the top of the tube till a couple of seconds after the pressure was reduced. There are not many reactions suitable for producing the gas. The reaction must be slow enough to give time for thorough mixing, for filling the tube through a fine capillary, and for sealing up the tube; and fast enough to be complete in a reasonable time, for the needle valve was never so tight that the apparatus could be left overnight a t 150 atmospheres without a fall of 2 0 or 30 atmospheres. Also it was not possible to heat the tube to above 70' without greatly increasing the risk of it bursting, and as it generally broke even*at room temperature there was not much t o come and go on. Many trials were made with ammonium chloride and sodium nitrite mixtures containing various acids. Acetic acid worked fairly well, but so much of i t had to be added that the results obtained were inconclusive on account of the probably great increase it causes in the solubility of nitrogen. Finally the reaction between hydrazine sulphate, ferric alum and sulphuric acid was found to be exactly what was needed. Results. Only two successful experiments have been made as yet. A description of one will euffice t o show the nature of the result. 3cc of a solution containing about 6 percent hydrazine sulphate were mixed with 6cc of a solution containing, per IOOCC, about 8 g. cone. sulphuric acid and 40 g. hydrated ferric alum. This was run quickly into the vessel A . Then a few drops of water were added to rinse out the top of the capillary and make a clean seal possible. The tip was then closed and the pressure raised to 150 atmospheres. A beaker of water surrounding A was next slowly raised to 60' and kept a t that temperature for 30 minutes and finally allowed to cool to room temperature. Finally after adjusting the lantern and the necessary screens etc., the pressure was suddenly lowered to I atmosphere. Bubbles rose from the sides and bottom immediately but none from the body of the liquid. It is not possible to determine the equilibrium pressure from the quantities of mat'erial used, since the effect of the various dissolved substances on the solubility of nitrogen is not known, but the following facts give information.
I3 I4
FRANK B . KENRICK, K. L. WISMER AND K. S. WYATT
Two or three times during the cooling of the tube the pressure inadvertently fell to about IOO atmospheres and a stream of very minute bubbles was seen slowly rising from a point near the bottom of the tube. This stream could be stopped by screwing the pressure up to I 2 5 atmospheres. This, of course, does not show that the equilibrium pressure is between I O O and 125 atmospheres, but it showe that it must be a t least IOO atmospheres. It is clear therefore that a liquid with a concentration of nitrogen corresponding to over I 00 atmospheres can be brought to atmospheric pressure for an appreciable time without bubbles forming in the body of the liquid. As stated in Part A one might expect supersaturation corresponding to zoo atmospheres to be about the limit that could be reached in glass vessels with walls free from imperfections. Unfortunately this pressure is so near the breaking point of the apparatus that we have not yet succeeded in determining whether bubbles form in the body of the liquid with this degree of supersaturation. To settle this our apparatus will have to be very considerably modified.
C. Ultra-Microscopic Investigation of Capillary Tubes In these experiments which are as yet quite incomplete, mixtures of hydrazine sulphate and ferric alum similar to those used in Part B are drawn into a very fine flattened capillary tube sealed horizontally to the pressure system just described. The mixture however in these experiments was coloured red by addition of a little ammonium sulphocyanate solution in order to make it easier to be sure it was in the tube. The tip of the capillary was then sealed off, the pressure raised and a dish of water kept a t 60" brought up round the capillary for 30 minutes. After that the tube was cooled and dried and the microscope slid into position. The cover-glass, below which was the cedar oil in which the capillary was immersed, was supported on two little rods slightly thicker than the capillary, so that the microscope could be slid backwards and forwards to examine different parts of the tube, and could be slid completely away if it was necessary to heat the tube again or refill it. When the pressure is lowered the growth of the bubbles can be observed and it is hoped by this method to locate their position in reference t o the motes visible on the surface of the glass. Although no breakages have occurred in the flattened capillary repeated accidents to other parts of the apparatus have prevented the completion of more than two experiments. I n the first in which the concentration of nitrogen corresponded t o about 7 0 atmospheres, no gas bubbles appeared a t all even after more than five minutes, which confirms and extends the result recorded in Part A. In the second, in which a stronger mixture was used bubbles had already formed when the tube was examined, though there were still long stretches of capillary with no bubbles. This result, however, is not free from objection for if the bubbles were formed before reduction of pressure this would cause considerable stirring of the liquid and the intermediate parts of the liquid might have lost nitrogen.
SUPERSATURATION OF GASES IN LIQUIDS
131.5
Summary I. Experiments have been made with water supersaturated with oxygen, carbon dioxide and nitrogen. 2. Liquids have been saturated with gases a t concentrations corresponding to pressures of over IOO atmospheres and the pressure has been reduced to one atmosphere without bubbles forming in the body of the liquid. Liquids containing gaees at Concentrations corresponding to nearly I 00 atmospheres have been reduced to I atmosphere without bubbles forming even on the walls. 3. As in the case of superheating of liquids there is, with rise in temperature, a very rapid shortening of the time interval between the lowering of pressure and formation of bubbles, in spite of the fact that the absorption coefficient of the gas decreases but little with rise of temperature. 4. Colloidal platinum in solutions supersaturated with oxygen is favourable to the formation of bubbles, but it rapidly loses its effectiveness. 5 . A long heating of the tube containing the solution a t a high pressure was found to favour supersaturation. 6. Although carbon dioxide is nearly 30 times as soluble as oxygen the average time interval before bubble formation is about the same for the two gases, at the dame temperature, when the supersaturation corresponds to the same equilibrium pressure. Chemical Deportment, liniuernity of Toronto, June leth, 1924.
