T H E REACTION OF SODIUM WlTH DRY OXYGEN B. L. HERRINGTON Department of Dairy Industry, Cornell University, Ithaca, N e w York Received November 3, 1.533
Bonnsdorff (1) stated that sodium and potassium were not oxidized by dry oxygen, and later Holt and Sims (4) published a paper on the oxidation of the alkali metals in which they announced: “Potassium and sodium (and probably lithium) are not attacked by dry oxygen and may be distilled in it without undergoing oxidation.” The truth of this statement has apparently never been questioned. Work done in this laboratory, however, has indicated that the statement is only partially correct. In the absence of moisture, sodium is coated with a film of oxide which protects it from further oxidation. In the presence of moisture, the film is unstable and, because of this fact, sodium is slowly but completely oxidized in ordinary air. The experiments upon which this belief is based will be described in detail, and it will be shown that the earlier workers either overlooked this surface film or else observed it, but failed to realize its significance. THE EXPERIMENTS
All of the observations reported here were made upon liquid sodiummercury amalgams. The mercury was purified by washing carefully with acid mercuric nitrate solution and then distilling under reduced pressure in a current of air. The amalgams were prepared by electrolysis of specially purified sodium chloride solutions. A platinum anode was used. It was first observed that such amalgams could be preserved in vacuo indefinitely, but when they were stored under dry hydrogen, obtained from a cylinder in the laboratory, they behaved in an abnormal manner. If the flask holding such an amalgam was rocked, the amalgam would wet the walls and would not drain back completely. A thin mirror-like film wa8 left adhering to the glass. The experiments reported in this paper were performed in order to explain the formation of these mirror films.
Experiment I A bulb was blown on the end of a glass tube and then partially filled with sodium amalgam (0.5 per cent). The bulb was then connected to a Cenco Hyvac pump and evacuated. After several minutes, the bulb and the tube were heated until the mercury was boiling freely. The tube was 675
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then sealed off, leaving the amalgam in an evacuated bulb with a tube several inches long. A similar bulb was prepared containing pure mercury. The bulb containing the amalgam was corroded on the inside by the sodium oxide fused into the glass, but when the amalgam and the pure mercury were run into the clean tubes attached to their respective bulbs, it was not possible to distinguish between the two. Such tubes containing amalgam and mercury have been kept for three years without any visible changes.
Experament 2 When one of the bulbs containing amalgam was sealed off and cooled, a minute crack appeared in the glass. This bulb was watched to determine the effect of the slow admission of air. Before the appearance of the crack, the amalgam could not be distinguished from mercury of the highest purity. Afterwards the amalgam became coated with a thin film which was made visible only by the wrinkles which appeared when the surface was agitated. At the same time, the amalgam acquired the property of wetting the glass to form mirrors. At first the surfaces exhibited a brilliant metallic luster, but this slowly became tarnished and dull.
Experiment 3 A bulb containing amalgam was evacuated and heated as described previously, but before sealing it off, a small amount of air was admitted. I n this bulb, the surface films appeared a t once. There was no tarnishing; the surfaces remained bright. Experiment 4 Previously experiments had shown that some material present in the air caused the formation of the mirror films, and that an excess of air caused the films to tarnish. It was suspected that moisture was responsible for these effects. In order to eliminate the effect of moisture, the apparatus shown in figure 1 was constructed. Bulb C was partially filled with phosphorus pentoxide. Its capacity was approximately 35 cc. Amalgam was placed in bulb A and the apparatus was then sealed at D. The entire system was then evacuated through E and F. About 1 cc. of mercury was distilled over into B, and then all of the apparatus to the left of stopcock E was heated as uniformly as possible. Mercury distilled over into C sweeping out the last traces of air from the amalgam bulbs. The stopcockR were then closed and the apparatus was allowed to cool. The bulb C was then allowed to fill with air by opening F momentarily. The apparatus was then shaken to coat the walls of the drying chamber with the pentoxide and thereby hasten the drying process.
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After 44 hours, the amalgam was transferred to bulb B. At that time, the amalgam resembled pure mercury. The dried air was then admitted to the amalgam chamber. No change could be observed until the bulbs were tilted, Then the amalgam was found to be coated with a rigid surface film, and a mirror film was left on the glass. It should be noted in particular that neither of these phenomena could have been detected if solid sodium had been exposed to dry air. The brilliance of the surface was not impaired.
Expeyiment 5 Experiment 4 was repeated using hydrogen from a cylinder instead of air. The gas was admitted to the amalgam after drying for 4 days. A film was formed over the surface as in the previous experiments. However, it did not appear instantly. Several seconds were required for its formation.
E
FIG.
1.
APP.4RATCS U S E D T O
ELIMINATE T H E EFFECT OF
F
hfOISTCRE
Experiment 6 The hydrogen used in the previous experiment had been obtained from an experimental plant and was suspected of containing traces of oxygen, though this has never been proved directly. Experiment 5 was repeated in duplicate using hydrogen which had been passed through freshly prepared alkaline pyrogallol solution. After intervals of 6 and 4 days, the gas was admitted to the amalgam chamber. No change could be detected. The purified hydrogen was without effect.
Experiment 7 Experiments 5 and 6 had shown that there was a small amount of the substance responsible for film formation in the hydrogen used, 'and that this could be removed by alkaline pyrogallol. In this experiment, carbon dioxide dried for 14 days was found to have no action upon the sodium amalgams.
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Experiment 8 Nitrogen dried for 14 days over phosphorus pentoxide had no action upon a sample of sodium amalgam.
Experiment 9 Oxygen was dried for 14 days over phosphorus pentoxide, using the apparatus shown in figure 1. Before admitting the gas to the bulbs containing the amalgam, a globule of amalgam was transferred to the tube leading to the stopcock. When the oxygen was admitted, this globule was shot down the tube by the inrushing gas, leaving the tube coated with a mirror-like film, proving that the formation of the film was practically instantaneous. No change could be observed in the appearance of the amalgam in the bulbs until the apparatus was tilted. The surface crumpled like tinfoil.
FIG.2. APPARATUS USEDTO DETERMINE TBE EFFECT OF PUREWATERVAPOR UPON A SODIUM
AMALGAM
Experiment 10 The apparatus shown in figure 2 was constructed in order to determine the effect of pure water vapor upon a sodium amalgam. An alkaline solution of stannous chloride was placed in bulb D and a few pieces of tin were added. It was believed that this would fix any oxygen which might remain in that bulb, and that no volatile substance except water would be introduced into the system. This bulb was then heated until the solution boiled vigorously, and then the inlet was sealed off. Bulb E was partly filled with solid sodium amalgam to remove oxygen, and liquid sodium amalgam was placed in bulb A. This compartment was boiled out as usual before closing stopcocks 1 and 2. A small amount of moisture was then admitted to the stlid amalgam by opening stopcock 3. This was done to insure the absorption of oxygen from the bulbs. After 11 days, the liquid amalgam was transferred to the clean bulb B, and stopcock 1 was opened. No change could be observed. Stopcock 4
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was then closed and stopcock 3 was opened momentarily. Small lenses of water appeared almost instantly upon the surface of the amalgam, and a gas (hydrogen?) was liberated slowly until the water disappeared entirely, leaving small white flakes floating upon the surface. When more water vapor was admitted, the same cycle was repeated. The characteristic mirror formation was never in evidence, nor could any film be detected upon the surface.
Experiment 11 The apparatus shown in figure 1 was modified by providing it with two sets of amalgam bulbs, and by enlarging the drying chamber to 250-cc. capacity. Oxygen was dried in this for 49 days. The gas chamber was then connected momentarily to each of the amalgam chambers in turn. In both cases the amalgams remained bright, but the presence of a rigid film was disclosed when the apparatus was shaken. The oxygen reservoir was then connected permanently with the first amalgam chamber. After four months, there was no visible difference in the appearance of the two amalgams in contact with different amounts of oxygen. Experiment 19 It is known that metallic sodium in air phosphoresces wibh a greenish light. Linneman ( 5 ) attributed this to oxidation; Reboul (7) attributed it to the reaction of sodium oxide with water vapor. More recently, Woodrow and Bowie (9) and Bowie ( 2 ) have concluded that the reaction is between metal and water vapor, and not a direct oxidation. It was believed that if the film formation was due to a surface oxidation, and that if this oxidation gave rise to luminescence, it might be possible to detect a flash of light when the oxygen was first admitted. For this experiment, oxygen was dried for 10 days in the apparatus shown in figure 1. The oxygen was then admitted to the amalgam in a completely darkened room. There were two observers. One saw nothing, the other observed a faint flash of light in the tube leading from the stopcock to the amalgam. A small globule of amalgam had previously been transferred to this tube. This experiment was repeated, drying the oxygen for 51 days. Two observers noticed the flash, a third did not. This experiment was again repeated, drying the oxygen for 13 months. When the oxygen was admitted, one observer uttered an exclamation of surprise when he saw the light. He had no way of knowing the instant a t which the oxygen was to be admitted. I n these experiments, no light was ever observed upon the surface of the amalgam in bulk. It was only seen in the tube where a small amount of amalgam was probably suspended in the rush of oxygen, and where the oxidation continued longer than it did on a stationary surface. It might be
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added that it is not possible to estimate the actual intensity of the light because the duration of the flash is exceedingly short, and because the greater part of the walls of the tube was instantly covered with an opaque mirror film.
Experiment i3 Previous experiments had indicated that sodium amalgams react to a limited extent with dry oxygen, but no indication had been obtained as to the actual amount of oxygen involved. I n order to learn more regarding the quantities involved, the apparatus shown in figure 3 was constructed. After placing phosphorus pentoxide in bulb B, the system was evacuated. It was then filled with oxygen, a streamof oxygen being forcedout through
n
FIG.3. APPARATUSU S E D
TO
DETERMINE THE
AMOUNT O F O X Y G E N IXVOLVED
the manometer for about five minutes. After closing stopcock 2, the oxygen was allowed to dry for 167 days. The graduated tube T was then filled with amalgam. A small amount of gas was formed and this was removed by evacuating for 2 hours. Four hours later, 1.0 cc. of the amalgam was allowed to flow into the oxygen chamber (capacity 200 cc. +). The amalgam was instantly covered by a rigid film. The absorption of oxygen caused less than 1 mm. change in pressure. After 24 hours, no change could be detected. An additional 2.0 cc. of amalgam was then added. After 2 days, the loss of oxygen was still too small to detect (less than 1mm. of mercury). The apparatus was then rocked gently for 2 hours to break up the film and expose fresh surfaces to the oxygen. During that time the
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pressure fell 25.0 mm., and the surface of the amalgam became coated with a heavy scum. DISCUSSION
As a result of the experinients which have been described, it is believed that sodium is attacked by dry oxygen, and that an impervious film is formed which prevents further oxidation. From the nature of the problem, it is always possible to argue that the gases used were not dry enough for the results to be conclusive. It is necessary, therefore, to review the original evidence for the non-reaction of dry oxygen and sodium. Bonnsdorff (1) states: “In einer vollkommen trocknen und von Kohlensaure freien Atmosphare oxydiert sich kein Metall. Auch Kalium und Natrium bleiben in derselben ohne Oxydation.” However, he admits that there is a slight oxidation for he goes on to say, “Das Kalium lauft zwar gewohnlich in lrurzer Zeit unbedeutend an, aller Wahrscheinlichkeit nach eine Folge davon, dass die Versuch nicht mit solcher Genauigkeit gemacht werden kann, dass die 1;CTirkung des Wassers vollig ausschlossen bleibe, aber das Metall erhalt sich als dann unverandert.” In his experiments, the air used was dried over sulfuric acid, and it doubtless contained much more moisture than the oxygen used in our experiments. On a later page he stages that the bright metal exposed to dry air “sich unverandert mit metallischen Glanz erhielten.” Our experiments proved that the presence of metallic luster was not a safe indication of the absence of a surface film. The paper of Holt and Sjms (4)is frequently quoted on the subject of the oxidation of sodium by dry oxygen. I n their conclusion they state, “Potassium and sodium (and probably lithium) are not attacked by dry oxygen and may be distilled in it without undergoing oxidation.” Howerer, a study of their paper reveals that they did not actually distill these metals. They merely fused them in an atmosphere of oxygen, and heated until fumes appeared. This is analogous to the experiments of Deville (3), who reported that aluminum was absolutely resistant to oxygen even when fused. Furthermore, Holt and Sims make the enlightening statement: “In a few experiments made by passing a stream of dried oxygen direct into molten sodium at a temperature of 235”C., no oxidation took place except a t the surface, . . . .1, In both of these papers it is admitted that some surface oxidation took place. In both cases the authors chose to ignore this as “unbedeutend,” and assumed that since the reaction did not go to completion, it would not have occurred a t all if water vapor had been entirely eliminated. According to their own writings, they did not actually reach this ideal condition. Hence it still remains to be proved that oxygen can be made so dry that it will not react with sodium.
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It is not possible to prove that the oxygen used in these experiments was dry, but the evidence should be considered. All gases were dried over phosphorus pentoxide, which leaves no measurable amount of water vapor in the gas ( G ) , and the drying periods were greatly prolonged. Other investigators have used calcium chloride or sulfuric acid, which are of doubtful value. In fact, Russel (8) has reported that the amount o€ water left in a gas by sulfuric acid is the optimum concentration for the rapid oxidation of phosphorus. There is the possibility that all of the water was not removed from the glass surfaces by t’lis treatment. Even if that were the case, the partial pressure exerted by the adsorbed water must have been infinitely small and can scarcely account for the extreme speed of the initial reaction,-an action which ceased immediately, but which began again the instant that a fresh surface was exposed. Regarding the reaction responsible for the chemiluminescence of sodium, it is evident that no light could be detected by the eye unless the reaction continued for an appreciable inteival of time. Therefore the evidence in the literature favoring the reaction with water is not valid, since it does not eliminate the possibility that water is not directly responsible for the luminescence, but merely makes it possible for the oxidation to proceed slowly by preventing the formation of protective films. CONCLUSIONS
1. It remains to be proved that oxygen can be made so dry that it will not react with sodium. 2. When sodium amalgams were exposed to oxygen dried over phosphorous pentoxide for more than a year, the amalgams were instantly covered with a protecting film which prevented further action. 3. Sodium will react with dry oxygen a t room temperature with the emission of light, but the reaction ceases instantly unless water vapor is present to prevent the formation of protective films. REFERENCES (1) VON BONNSDORFF, P. A.: Ann. Physik Chem. 41,293-314 (1837). (2) BOWIE,R. M.: J. Phys. Chem. 36, 2964 (1931). (3) DEVILLE,STE.-CLAIRE:Ann. chim. phys. [3] 43, 12 (1855). (4) HOLT, W., AND SIMS,W. E.: J. Chem. SOC.66,432-44 (1894). ( 5 ) LINNEMANN: J. prakt. Chem. 76, 128 (1858). (6) MORLEY, E. W.: J. Am. Chem. SOC.26, 1171-3 (1904). (7) REBOUL:Compt. rend. 168, 1195-6 (1919). (8) RUSSEL,E. J.: J. Chem. SOC.83, 1283 (1903). J. W., A N D BOWIE,R. M.: Phys. Rev. 36, 1423 (1930). (9) WOODROW,
