378

The Luminous Reduction of Dioxide.

By H. J. E meleus, Imperial College of Science and Technology, London, and H. L. R il ey , Armstrong College, Newcastle.

(Communicated by J. C. Philip, F.R.S.—Received November 22, 1932).

[Plate 3.]

Introduction.

In the course of some recent investigations* on the oxidizing action of selenium dioxide on organic compounds it was found that, if the temperature was raised above the point required for the specific oxidizing reaction, the reduction of the dioxide was accompanied by a characteristic moonlight-like flame. This was first observed in the reaction of ethylene with selenium dioxide at a temperature of circa 400° C, but subsequently a number of other substances were found to behave similarly, the following having been studied in some detail: ethylene, acetylene, methyl-, ethyl-, and propyl-alcohols, acetaldehyde, acetone, benzene, toluene, ether, carbon disulphide, and ammonia.

When the oxidation takes place a t lower temperatures, i.e., at temperatures which are not sufficiently high for the luminous reaction, intermediate products are formed from all of the carbon compounds. Acetaldehyde, for example, is oxidized quantitatively to glyoxal, the dioxide being reduced to elementary selenium, whilst ethylene also yields glyoxal. The present investigation was undertaken in the hope that the nature of the light emitted when the reactions are carried out at higher temperatures might possibly explain the highly selective nature of the oxidations at the lower temperatures.

Experimental.

The conditions used for producing the flames in selenium dioxide vapour were as follows. Powdered resublimed oxide (15-20 gm.) was heaped near the closed end of a clear silica tube, 2 • 0 cm. in diameter, 20 cm. long, with a side inlet tube at 0*5 cm. from the closed end ; this served as a window in photographing the spectra. The open end of the tube was attached to a con-

* Riley, Morley and Friend, * J. Chem. Soc.,’ p. 1875 (1932); Riley and Friend, p. 2342 (1932); Imperial Chemical Industries and Riley, ‘ B .P .,’ 354, 798 (1931) and 376, 306 (1932); I. G. Farb. A.G., 5 B .P .,’ 347, 743 (1931); Schwenk and Borgwardt,‘ Ber. deuts. chem. Ges.,’ vol. 65, p. 1601 (1932).

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The Luminous Reduction of Selenium Dioxide . 379

denser for removing liquid products of reaction and reactants, any uncon­densed gases being led into an efficient draught cupboard. A fairly rapid stream of gas from a cylinder, or, with liquids, a stream of vapour from a distilling flask containing the gently boiling liquid, was passed over the oxide to displace air from the apparatus. The tube was then heated rapidly, either electrically or with a Bunsen burner. Reduction started at below 100° C. in every case, and at 350°-400° C. a blue flame suddenly appeared. With the aliphatic compounds, carbon disulphide, and ammonia the flames were fairly persistent, occasional heating being sufficient to maintain them continuously. With the aromatic compounds—particularly benzene—strong and continuous heating was necessary. One charge of selenium dioxide gave a flame of 10-2-5 minutes duration, after which a new tube had to be set up.

The rate of passage of gas or vapour could be varied within wide limits, provided the oxide was sufficiently heated. The only effect of altering this factor was to change the size of the flame, which took the form of a tongue of light 5-20 cm. long, stretching down the tube from the heated oxide. The flame spectra were photographed end-on through the closed end of the silica tube, which was placed directly in front of the spectrograph slit. I t was necessary to withdraw the tube and heat its end with a free flame every few minutes during an exposure, in order to remove a film of condensed material from the window.

Ethylene, from a cylinder of the compressed gas, was used in the first series of experiments. The approximate ignition temperature, measured with a thermocouple in contact with the heap of selenium dioxide, was 350°-400° C. and the reading then rose rapidly to circa 600° C. The flame was definitely in the gas phase, and not at the surface of the oxide. The vapour pressure of selenium dioxide at 320° C. = 849 mm. (Landolt-Bornstein Tables), so that a plentiful supply of vapour would be available. The light intensity was sufficient to render small print visible at 1 metre from the tube. When the temperature was allowed to fall until the flame was just extinguished there was a very short temperature interval in which a faint white chemiluminescence was visible.

Acetylene, prepared by the action of water on calcium carbide, behaved in the same way as ethylene, as also did methyl-, ethyl-, and propyl-alcohols, acetaldehyde, acetone, ether, and carbon disulphide. These latter substances were all obtained from the ordinary commercial samples, but highly purified ethyl alcohol gave results identical with those from “ rectified spirit.” During the reaction with acetylene the end of the tube rapidly became covered with a

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380 H. J. Emeleiis and H. L. Riley.

deposit of carbon, for which reason the flame spectrum could not be photo­graphed. Hydrogen sulphide was also found to give a flame with selenium dioxide, but the deposition of sulphur prevented photographs being taken, Benzene and toluene inflamed in selenium dioxide vapour at temperatures above 450° C., benzene being more difficult to ignite than toluene, whilst both substances burned with a smoky flame.

The reaction between selenium dioxide and ammonia started at room temperature if the oxide were at all damp, or at about 40° C. if the sample were dry. The white oxide rapidly blackened, owing to the separation of selenium, and minute flashes of pale blue light were visible in a dark room. On warming the tube to 80°-100° C. a steady bluish chemiluminescence could be observed. As the temperature was raised the luminescence increased in volume and became brighter, a rose pink tinge appearing at circa 350° C. The normal flame, resembling those obtained with the organic substances, made its appear­ance at 400°~430° C. On cooling, these phenomena could be observed in the reverse order.

The Chemical Reactions.I t is already known that when selenium dioxide reacts with ketones, alde­

hydes, ethylene, and acetylene belowr 230° C. the principal products are glyoxals (Riley and co-workers, loc. cit.).The reaction with alcohols is more complex, and is the subject of a separate paper, in course of publication. When the reaction temperature is allowed to rise so that the flame appears, the yield of the particular glyoxal falls off, and the amount of carbon monoxide, carbon dioxide, and water formed increases. These three substances are the principal products from the flames of all of the organic compounds examined. At 400° C. intermediate products would probably be unstable, and the oxides of carbon and water might come from their decomposition, or, alternatively, from a direct oxidation process, differing in mechanism from the non-luminous oxidation. The lack of control of the flame conditions would make it difficult to carry out complete analyses of the reaction products, which would serve to decide this point.

In the experiments with carbon disulphide large amounts of sulphur dioxide and carbon dioxide were detected among the products. The reaction with ammonia gave almost exclusively nitrogen and water, a little nitric acid also being detected. Indications of the formation of a detonating substance (possibly a selenium-nitrogen compound) were also obtained in the course of the reaction, very slight explosive cracks occurring from time to time in the cooler parts of the apparatus.

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According to Klages* tlie reduction of selenium dioxide witli hydrogen is accompanied by luminous phenomena. The authors failed to verify this observation with hydrogen from a cylinder. The oxide could, in fact, be sublimed quite freely in a stream of hydrogen with only slight reduction. I t is possible tha t the hydrogen used by Klages was contaminated with hydro­carbons. I t is also of interest tha t sulphur dioxide had only a slight reaction with selenium dioxide under the above conditions, no luminous phenomena being observed.

The Flame Spectra.

Photographs of the flame spectra were taken on (a) a glass grating spectro­graph, with a linear dispersion of 35 A. per millimetre ; (6) a Hilger small quartz spectrograph; (c) a quartz spectrograph with lenses working at an aperture of f. 4-5. The bright flames needed exposures of 5-30 minutes on the grating instrument. Since band heads in the flame spectra were always partly obscured by a continuous radiation, and were difficult to see when using a comparator, measurements were made with an accurate scale on enlarge­ments of the negatives printed on contrast paper, and with a copper arc reference spectrum. An additional check on the identity of band systems was obtained by superposing plates taken on the same instrument.

The hot flames supported by selenium dioxide gave bands and continuous radiation in the visible spectrum, and no light in the ultra-violet. There were 12 broad bands between 5620-4600 A., of which six between 5620-5100 A. were very diffuse and had a breadth of 50-70 A. The remainder were some­what sharper and were definitely degraded towards the red, some of the bands showing evidence of fine structure and having double heads, of which, however, only the more intense were measured. The continuous radiation extended over the whole of the visible spectrum, and had a maximum intensity at 4400- 4600 A. Between 4500-4200 A. there were further bands, almost obscured by the continuous background, but apparently belonging to the same series as above. Between 4100-3800 A. was a series of closely spaced and very diffuse bands, considerably less intense than the above, and extending to 3600 A. when photographed with a quartz spectrograph.

Plates showing all of these characteristics were obtained from the flames of selenium dioxide with methyl-, ethyl-, and propyl-alcohols, acetone, ether, ethylene, and toluene. Only the stronger bands between 4700-4400 A. were observed in the case of the flames with acetaldehyde, benzene, ammonia,

The Luminous Reduction of Selenium Dioxide. 381

* ‘ Chem. Z.,’ vol. 45, p. 450 (1898).

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382 H. J. Emeleus and H. L. Riley.

and carbon disulphide, though there is little doubt that longer exposures would show the remainder. All of the plates taken with the grating instrument were compared by direct superposition with that from the selenium dioxide- methyl alcohol flame, and the spectra were found to be identical in all cases for bands above 4600 A. Where bands at shorter wave-lengths were sufficiently well defined for comparison to be made there was again a complete corre­spondence between the different negatives. Reproductions of the flame spectra of methyl and propyl alcohols are shown in Plate 3. (a) is the spectrum ofthe methyl alcohol-selenium dioxide flame, photographed on the grating instrument with an exposure of 30 minutes, and with a copper arc comparison spectrum. I t shows the broad diffuse bands particularly well. (6) is the spectrum of the propyl alcohol-selenium dioxide flame, photographed on the same instrument with an exposure of 15 minutes. The continuous back­ground is less intense, and a number of the sharper bands are visible. Both plates show the sodium lines, partly resolved. They were always present in the flame spectra, and probably represent impurity in the selenium dioxide. These two reproductions are typical of all of the spectra so far described.

In Table I is given average wave-length measurements for the violet edges of the bands. The error of measurement is probably less than i 5 A.

In the second column of the table the positions of the band heads in the spectrum of the selenium flame in oxygen are recorded. The bracketed values indicate bands found to correspond with those measured in the selenium dioxide flames, but not measured directly. To produce this flame the selenium was heated strongly in oxygen in a quartz tube, when it ignited readily. The flame was distinctly more blue in appearance than when combustion occurred in the vapour of the dioxide. The spectrum was photographed on the grating instrument and on the small quartz spectrograph ; in the latter case no ultra­violet spectrum was observed. The visible spectrum consisted of broad diffuse bands between 5700-4600 A., identical with those observed in the selenium dioxide flames. Below 4300 A. there was a well-developed series of closely spaced bands, degraded to the red. The positions of their heads are recorded in the second column of Table I. They were sharper than the corre­sponding bands in the selenium dioxide flames, and relatively more intense, whilst the two sets of wave-length measurements show very definite differences, so that the two groups of bands are not definitely identical. The closely spaced bands in the selenium-oxygen flame agree well with a series of absorption bands observed with selenium dioxide at 350° C. by Evans and Antonoff.*

* ‘ Astrophys. J .,’ vol. 34, p. 277 (1911).

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The Luminous Reduction of Selenium Dioxide.

Table I.

383

Flames burning in Se02.

Flame of Se in 0 2.

Absorption of Se02.

Se in Geissler tube and flame (Salet).

A. A. A. A.5620 ------ - — 56505530 — —■ 55505410 — —

5370 (5370) — 53705245 (5245) — 52705140 (5140) — 51605025 (5025) — 50504930 (4930) — 49504850 (4850) — 48404750 (4750) — 47504685 (4685) —• 46704600 (4600) —- 4610

4435 _ 4437 _4405 — . 4395 44004375 — 4365 —

4340 — . 4330 —

4310 4295 4290 —

4270 4260 4256 —

4240 4225 4220 — .

4200 4190 4185 —

4165 4160 4157 —

4130 4125 4117 — .

4100 4095 4083 —

4060 4060 4055 -—

4030 4030 4020 —

4000 4000 3990 —

— 3965 3960

The measurements of these authors are given in column 3, Table I, and a comparison of the two sets of values shows fairly definitely that the band system in the selenium-oxygen flame must be the emission spectrum of selenium oxide. In Plate 3 (c) is a photograph of the selenium-oxygen flame spectrum, taken on the grating spectrograph with an exposure of 30 minutes. A strong continuous background masked the broad diffuse bands at longer wave-length, but the bands attributed to Se02 (or SeO) are visible.

Special tests were made with the large aperture spectrograph to verify that no ultra-violet bands were given by the flames burning in selenium dioxide vapour, and, in particular, that the water bands, commonly observed in the spectra of burning hydrogen compounds, were not excited. The large aperture spectrograph recorded them in the spectrum of a Bunsen flame placed at 10 cm. from the slit with an exposure of 1 minute. With a 90-minute exposure of the selenium dioxide-methyl alcohol flame no trace of the (OH) bands was observed, nor did similar tests with the flames of selenium dioxide and propyl alcohol, acetone, ammonia, or carbon disulphide give any indication of ultra-violet

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bands or continuum (below 3600 A.). Unless, therefore, the transparency of the quartz had been impaired, it was clear that any ultra-violet emission from these flames must have been of very low intensity compared with that in the visible spectrum. During the exposures the end of the quartz tube was heated every few minutes to remove any film of condensed material, and the transparency of the quartz in the ultra-violet was specially tested. Evans and Antonoff ( loc. tit.) found that absorption by selenium dioxide vapour was continuous below 4000 A., but with the method used in producing the flames absorption by the vapour of the dioxide is unlikely to have masked short wave-length emission bands, as the reacting vapour or gas always approached the selenium dioxide from the direction of the window, and combustion took place in a definite flame front.

In order to determine if the intensity of the water bands in a hydrogen flame burning at a silica jet was influenced by the admixture of selenium vapour, the image of a flame 1 • 5 cm. high was focussed on the slit of the small quartz spectrograph, and an exposure of 30 minutes was made. Next an equal exposure was made with sufficient selenium volatilized into the hydrogen stream to colour the whole flame an intense blue. In this case, although bands in the visible due to selenium were observed, the intensity of the water bands was the same as in the simple hydrogen flame.

The normal flame of ammonia burning in the vapour of selenium dioxide, as already described, gave the same spectrum as the flames of the organic compounds and carbon disulphide. The reaction with ammonia was, however, unique in giving a well-marked phosphorescent flame. The spectrum of this was photographed on the large aperture spectrograph with rapid plates, the same type of tube being used as for the hot flames, but the temperature being controlled to about 300° C. With an exposure of 5J hours seven band heads between 5200-4600 A. common to the spectrum of the hot selenium dioxide- ammonia flame, were recorded. Very much longer exposures would be needed to verify all of the detail, but it is very probable that the low temperature flame has the same spectrum as that at higher temperatures. A reproduction of this photograph on the quartz spectrograph is shown in (d), Plate 3. Owing to the low dispersion of the instrument in the visible the bands can scarcely be distinguished.

Discussion.

The emission spectra of selenium in the Geissler tube and flame has been studied by Salet* using a low dispersion (this being necessary, if the results

* ‘ Ann. ehim. phys.,’ vol. 28, p. 5 (1873).

384 H. J. Emeleus and H. L. Riley.

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Emeleus and Riley. Proc. Roy. Soc., A , vol. 140, 3.

(Faring p . 384 . )

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The Luminous Reduction of Selenium Dioxide. 385

are to be of use for comparison witb those given here), it was found to consist of a series of bands between 5650-4610 A. These values are included for comparison in the last column of Table I. They agree approximately with the diffuse emission bands in the spectra of the flames in selenium dioxide vapour, and the latter are therefore probably to be attributed to the selenium molecule. The same bands occur in the flame of selenium burning in oxygen. I t is of interest that a similar series of bands in the visible spectrum, attributed to S2, was observed in the flame spectrum of sulphur or carbon disulphide in oxygen by Fowler and Vaidya.*

In the selenium flames there is also some evidence that the Se2 molecule is responsible for emission, for Rosenf has recorded a series of broad diffuse bands at the red end of the selenium fluorescence spectrum, his measurements agreeing approximately with the values for the selenium dioxide flames. Patemo and MazzucchelliJ observed a continuous emission with diffuse bands on heating selenium in a quartz tube to 1400° C., and Rosen ( cit.) has identified these bands with the diffuse system in the fluorescence spectrum. At 800°-1000° C., when the thermal emission first becomes visible, the dissocia­tion of selenium into Se2 molecules is almost complete. Probably, therefore, the emission of these broad diffuse bands is to be attributed to the simple molecule. The origin of the faint group of closely spaced bands in the region 4400-4000 A. cannot be regarded as se ttled ; there is partial agreement with the absorption spectrum of Se02, however, and there are indications of the bands in all of the flames burning in selenium dioxide, so that they are probably associated with selenium.

These results prove definitely that the chief emission from the flames of very diverse substances burning in selenium dioxide vapour is characteristic of selenium, and perhaps also of its oxide, but not of the substance undergoing oxidation. This is surprising, because all carbon-hydrogen and carbon- hydrogen-oxygen compounds when burnt in oxygen give very characteristic flame spectra. The chief emission bands are due to (CH), (C2), and (OH), or in the case of the phosphorescent combustion of some of the carbon compounds, a series of bands of unknown origin is observed in the blue.§ With the possible exception of a band at 4310 A., which might be due to (CH), there is no indication of the above band systems in the flame spectra which we have examined

* ‘ Proc. Roy. Soc.,’ A, vol. 132, p. 310 (1931).t ‘ Z. Physik,’ vol. 43, p. 95 (1927).% Gf. Kayser, “ Handbuch d. Speotroscopie,” vol. 6, p. 461 (1912).§ Emel6us, ‘ J. Chem. Soc.,’ p. 1733 (1929).

2 OVOL. CXL.— A.

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386 The Luminous Reduction of Selenium Dioxide.

Bands in the visible might be blotted out by strong selenium emission and continuum, but there should have been no difficulty in observing the ultra­violet OH bands. The same is true of the flame of ammonia in oxygen, which normally gives systems due to NH and OH,* and of the normal carbon di­sulphide flame, which gives bands attributed to S2, SO, S02, and CS, and has a considerable emission in the ultra-violet.f

The reason why all of these characteristics of the normal flames are missing when the oxygen needed for combustion is supplied by selenium dioxide is not clear. The experiment described, in which selenium was burnt with hydrogen, shows that it does not interfere with the emission of the (OH) bands under such conditions.

In the present state of the problem there is too little evidence for the possible reaction mechanism to be discussed in detail. The spectroscopic results do, however, show that selenium dioxide does not simply furnish oxygen for the combustion by dissociation. If this were so, then the emission bands of selenium might be superposed on the normal flame spectra, but the latter would not be suppressed. The alternative is that the Se02 molecule forms a definite intermediate compound with the substance undergoing oxidation— this might be the counterpart of the peroxides postulated in combustion in oxygen. From such a complex selenium must be eliminated, leaving inter­mediate oxidation products which may be isolated at lower temperatures, but which undergo a non-luminous thermal decomposition if the temperature is sufficiently high. The radiation from the selenium molecule is evidently a process which occurs readily, for selenium is capable of giving a thermal emission, and the energy of the oxidation reaction would be sufficient for its excitation at the higher temperatures at which the transition from the dark to the luminous reaction is observed.

The authors are indebted to Dr. L. C. Martin for the loan of the grating spectrograph used.

Summary.

(1) Selenium dioxide vapour will react with methyl-, ethyl-, and propyl- alcohols, acetone, acetaldehyde, ether, ethylene, acetylene, benzene, toluene, ammonia, hydrogen sulphide, and carbon disulphide at circa 400° C., giving a bright blue flame.

(2) The principal oxidation products are oxides of carbon, carbon, and

* Fowler and Badami, * Proc. Roy. Soc.,’ A, vol. 133, p. 325 (1931). f Fowler and Vaidya, loc. cit.

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Investigations on the Spectrum of Selenium. 387

water, from the carbon compounds; nitrogen and water from ammonia ; and sulphur dioxide, sulphur, and oxides of carbon from carbon disulphide.

(3) All of the flames have the same spectrum, consisting of bands and con­tinuous radiation between 5700-3600 A .; the band systems being attributed to the Se2 molecule, and possibly also to the oxide of selenium. The same bands occur in the flame of selenium in oxygen, which also shows emission bands attributed to Se02. None of the flames has an ultra-violet spectrum.

(4) These flame spectra are totally different from those of the corresponding flames in oxygen.

(5) A phosphorescent flame has been obtained by heating selenium dioxide in an ammonia stream at 100°-300° C. Its spectrum has been photographed, and is the same as that of the hot flame of ammonia burning in selenium dioxide.

Investigations on the Spectrum of Selenium.—Part II, Se III.

By J. S. B adami, Ph.D., Physikalisch-Technische Reichsanstalt, Berlin, and K. R. R ao, D.Sc., Science College, Andhra University, Waltair, India.

(Communicated by A. Fowler, F.R.S.—Received November 25, 1932.)

Introductory.

The present paper is a continuation of the work on the spectra of selenium, carried out by the authors partly in Professor Fowler’s Laboratory at the Imperial College and partly in Professor Siegbahn’s Laboratory a t Uppsala. A detailed account of the experimental part of the investigation together with the analysis of Se IV and Se Y has been given in a previous communication.* As already reported in ‘ Nature ’f the experimental data there described have led to the identification of the system of energy levels characteristic of the spectrum of doubly-ionized selenium. I t is the purpose of the present paper to give an account of the structure of this spectrum so far as it is revealed by a study of the spectrograms of selenium under different conditions of excita­tion.

* * Proc. Roy. Soe.,’ A, vol. 131, p. 154 (1931). t ‘ Nature,’ vol. 128, p. 496 (1931).

2 C 2

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