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Slow Oxidation of Phosphorus





The Glow of Phosphorus

The similarity in appearance between the glow observed on the surface of phosphorus when exposed to air and that observed in the case of substances like commercial calcium and barium sulphides after exposure to light is of course only superficial. While the name " phosphorus " was soon restricted to the element, the term " phosphorescence" in its scientific usage now refers to photoluminescence, while the glow observed on smouldering phosphorus, or fish or wood in certain stages of decay, is called " chemiluminescence." The slow combustion of phosphorus is seen as a pale blue " cold flame " which continually spreads away from the surface and is associated with a peculiar smell. The volatile products are poisonous and consist of phosphorous oxide, and, in the presence of water, phosphorous acid. It was soon discovered that the presence of air was necessary—" the fire and flame of phosphorus have their pabulum out of the air " — but low air pressures, such as remained in the best air pump vacua of the older experimenters, were still sufficient to maintain the glow, whereas the combustion of charcoal was completely extinguished. It follows that in the well-known lecture experiment the oxygen can be removed almost completely from the air by smouldering white phosphorus in the presence of water. The glow, however, is extinguished in pure oxygen at atmospheric pressure. Ozone is produced in the combustion and its presence may be demonstrated by the usual tests. Since this striking phenomenon early challenged the attention of chemists it has been the subject of much investigation. Many of the qualitative effects were early demonstrated by Boyle. The increase in luminosity when the air is rarefied was described by van Marum and Davy. A bibliography of the older literature is given in papers by Cent- nerszwer and Downey.


Effect of Pressure upon the Oxidation of Phosphorus

At each temperature there is a certain critical pressure below which the glow appears. Also at each pressure (or partial pressure) of oxygen there is a certain critical temperature above which the glow appears.

Thus the pressures had to be reduced below their critical values at the particular temperatures, or the temperatures had to be raised to their critical values at the particular pressures, in order to initiate the slow combustion. Some values of temperature and pressure are:—

t° C1.43.05.08.911.514.219.2
p (mm.)355387428519580650760


As already stated, in ordinary moist oxygen or air the glow begins at very low pressures, increases to a maximum with pressure and then decreases, vanishing at an oxygen pressure of 600 to 700 mm.

The limits of pressure within which the glow occurs have also been found by means of a photoelectric counter of the Geiger-Muller type. A quartz tube was coated inside with a thin film of platinum which acted as a photoelectric emitter. Along the axis of the tube ran an insulated tungsten wire which was connected to the earthed positive terminal of a 1200-1500-volt battery. The tube was filled with air at a pressure of 4.7 cm. mercury. The upper threshold of the radiation to which the counter was sensitive was at 2800 Å. Radiations of lower wavelength are emitted by phosphorus glow. Under the influence of these a photoelectron is emitted which ionises the air by collision, causing a drop of potential of the wire, which acts on the grid of an amplifying valve and is recorded as an impulse on a standard recorder of the telephone type. A residual effect of 10-15 impulses per minute due to cosmic ray was constantly present. This number was greatly increased when the wire was surrounded by oxygen and phosphorus vapour at glow pressures, and began to be marked between pressures which gave a yellow and those which gave a bluish glow. The lower limit was found to be 0.3 mm. of mercury and the upper 595 mm. when the oxygen was pure, or 400 mm. when the oxygen was mixed with nitrogen. It was considered that these results were best interpreted by the " chain reaction " theory, as proposed by Semenoff. The oxide is built up by a series of stages, some of which result in the production of an active molecule, thus:—



In these expressions only some of the active products (marked ') have been included. Radiation is associated with the spontaneous deactivation of an active product according to (2) below, if it does not happen to activate a molecule of oxygen according to (1):—

P4O10' + O2P4O10 + O + O (1)

P4O10' → P4O10 + hν (2)

h and ν have their usual meaning of Planck's constant and the frequency of the radiation.

Velocity of the Reaction

Quantitative determinations of the velocity of oxidation have been based on the diminution in pressure which follows on absorption of oxygen. This became appreciable, with appearance of glow, at about 700 mm. oxygen pressure.

The rate of oxidation, calculated from the rate of decrease of pressure in a constant volume, at first increases rapidly as the pressure falls, reaches a maximum at about 300 mm., and then falls slowly to 100 mm. Below 100 mm. the rate diminishes rapidly with further decrease of pressure.

The velocity constant K of a unimolecular reaction, i.e.



(p and pt are the partial pressures of the oxygen at the beginning and after the lapse of t minutes) is given in the following table:—

Rate of absorption of oxygen in moist air by phosphorus

Time in Minutes.Total Pressure in mm. of Mercury.Partial Pressure of Oxygen.KK1
0773.1157.8......
25750.6135.30.0026742.0
50729.7114.00.0028243.1
75714.399.00.0027140.1
100697.482.10.0028442.3
130682.266.90.0028642.1


The constant K increases on the whole with time (although somewhat irregularly), showing that the reaction takes place somewhat faster at a lower pressure than would be expected assuming that it is directly proportional to the partial pressure of the oxygen. Since the reaction takes place between the phosphorus vapour and the oxygen, the reason for the increase in the constant was sought in the increased rate of evaporation of the element at the lower pressures. A correction was introduced for the vapour pressure px of the phosphorus in the form—

(1)

in which P is the total pressure. The integration of this equation gave the corrected constants K1 in the above table. The results given above refer to air or oxygen in its usual state, i.e. slightly moist, and apply also when the air has been partly dried, as by bubbling through concentrated sulphuric acid, which still leaves enough water vapour for the continuance of slow oxidation. If, however, the air or oxygen is carefully dried, the velocities at all pressures are much lower, and are between 0 and 70 mm. proportional to the square roots of the oxygen pressures. The maximum velocity of oxidation is also attained at a lower pressure (about 100 mm. of oxygen) and the upper limit of chemiluminescence is found at lower pressures, i.e. at about 200 mm. of oxygen. If the oxygen is quite dry, no combustion takes place, no glow is seen and no ozone is formed—the production of ozone and hydrogen peroxide is due to the presence of moisture.

The results in the table may be summarised in the statement that from low pressures up to a certain limiting pressure the velocities are proportional to the oxygen pressure and are also a function of the rate of supply of vapour from the surface of the phosphorus. In fact, if in the equation plotted against p, the graph is rectilinear from the lowest value of oxygen pressure up to 520 mm., then rapidly decreases to zero at 700 mm.

Low Pressures

The velocity of oxidation of phosphorus vapour at low pressures has been further investigated by Semenoff, who found that an inert gas increases the reaction velocity and lowers the lower critical oxidation pressure. The subject was taken up at this point by Melville.

According to Semenoff "if Pp4, Po4 and px are the pressures of phosphorus, oxygen and inert gas respectively, then, at the lower explosion limit

(1)

provided that the explosions are confined to a vessel of a given size. Now (1) indicates that the inert gas effect should be independent of the nature of the gas. Subsequent work has shown that (1) does not describe exactly the effect of gases on the lower critical oxidation limit of phosphorus. From (1) it is seen that if 1/Po2 is plotted against 1+px/(Pp4 + Po2), a straight line is obtained. The slope A of this line is, however, dependent on the nature of x".

The critical pressure or glow pressure is affected by the presence of inhibitors.

The explanation why such a complicated reaction as the slow oxidation of phosphorus appears to be unimolecular is one which has often been brought forward in similar cases. The slowest part of the process is a diffusion, either of oxygen molecules to the reaction surface, or of phosphorus molecules from the surface, or of the inhibiting molecules of phosphorus oxide from the surface. According to the theory of diffusion the rates will follow the unimolecular law. Since the vapour pressure of phosphorus is low at ordinary temperatures, the concentration of oxygen molecules is many times greater than that of phosphorus molecules, even at the lower oxygen pressures.

Phosphorus vapour when apparently inactive in an atmosphere of oxygen may really be combining with the oxygen at isolated centres, but the combination does not spread, on account of the inhibitory effect of the enormous excess of oxygen molecules. The combination has obvious similarities to the combustion of hydrocarbons, etc., in air or oxygen, which, as is well known, is limited and finally inhibited by an excess either of the combustible substance or of oxygen. An excess of oxygen probably hinders the diffusion of phosphorus across the solid-gas interface on which a layer of vapour is continually being formed and removed. If the rate of formation of oxide (especially in a dry atmosphere) exceeds that of evaporation of the phosphorus, a protective coating of oxide will also inhibit further action.

The products of oxidation have been investigated. When the oxidation was carried out in moist air or oxygen the partial pressures of which were varied between 100 and 1200 mm. the product was a nearly constant mixture of the tetroxide and pentoxide. The empirical composition PO2.10 is nearly the same as that of the corresponding product derived from P4O6. It is considered that the phosphorus trioxide is formed as an intermediate stage which does not accumulate in the system.

Effect of Temperature

The pressures corresponding to the maxima of chemiluminescence diminish with fall of temperature; at 0° C. the maximum oxygen pressure is 320 mm., while by extrapolation this pressure would become zero, and the effect vanish, at -13.8° C. The pressures at which the glow first appears also decrease with fall and increase with rise of temperature. At 27° C. the glow appeared in oxygen at atmospheric pressure. At higher temperatures the velocity, as is usual, increases greatly, until at about 60° C. (or at lower temperature if pressure is reduced) the combustion changes its character, and the phosphorus ignites.

Production of Ozone

In the first definite description of ozone by Schonbein in 1840 the discoverer noted that this gas is produced by the slow oxidation of phosphorus.

It was observed later that half an atom of oxygen is activated for every atom of phosphorus which is oxidised. This may be explained on the supposition that the reaction takes place in stages, thus:—

2P + O2 = P2O + O
O + O2 = O3
P2O + O2= P2O3

The amount of ozone formed is proportional to the intensity of the glow. and is 0.5 to 5.0×10-6 gram per c.c. of gas which passes over the phosphorus. The radiation from the glow has been proved capable of producing ozone.

The effect of ozone is to raise the upper critical explosion limit for phosphorus and oxygen and to lower the lower limit. The effect is much greater than that for an ordinary neutral gas. The presence of 5 per cent, of ozone in phosphorus vapour and oxygen diminishes the lower critical pressure by about 20 per cent., whereas the normal effect for an inert gas at this concentration would be 1 to 2 per cent.

Inhibition of the Glow

The following substances diminish or destroy the glow at ordinary temperatures, or allow it to appear only at higher temperatures—chlorine, iodine, nitrous and nitric oxides, hydrogen sulphide, sulphur dioxide, turpentine, alcohol, benzene, chloroform, aniline, ethylene, acetylene and other unsaturated hydrocarbons, and lead tetraethyl. The glow is not diminished by nitrogen, sulphur, acetic acid, hydrogen chloride or ammonia in small quantities. Some of the inhibitors will combine with ozone, but it has been stated that their relative activity in inhibiting the glow is not the same as that shown in destroying ozone.

The diminution in the rate of propagation of the glow was shown by measuring the velocity of an air current which was just sufficient to carry the glow to a point marked in a tube. This method was also used in examining the effect of varying the partial pressures of oxygen in nitrogen. In the presence of 0.16 per cent, of ethylene this velocity was 140 cm. per second, and when the percentage was increased to 0.43 the velocity required fell to 1 cm. per second. The temperature at which the glow is first seen increases with increasing concentration of ethylene, etc., and can be raised to well over 60° C., the normal ignition point of the phosphorus. When air containing 8 and 26 per cent, of ethylene was heated for long periods in contact with phosphorus a certain amount of non-luminous oxidation did occur. It has been shown further that the partial pressure of the ethylene which will just inhibit the glow is proportional to the partial pressure of the phosphorus over the range 60° to 97° C.

Phosphorus trioxide itself inhibits the glow of phosphorus, being about three times as powerful in this respect as ethylene. The ratio of the partial pressure of trioxide to that of ethylene which just stops the glow is about 145.

The inhibitory effect may also be expressed as a function of the "glow pressure" at which the glow can just be detected. An empirical equation was used by Tausz and Gorlacher—

px(x+a) = K

in which px represents the partial pressure of oxygen at which the glow appears, x is the percentage by volume of the inhibitor or anticatalyst and a and K are empirical constants for each substance. This equation holds for certain percentage admixtures in the case of each inhibitor—thus for 0.1 to 0.9 per cent, sulphur dioxide; for 2 to 10 per cent, benzene; for 0.01 to 0.1 per cent, isoprene; for 0.046 to 0.047 per cent, iron carbonyl. The corresponding values of the constants are as follows:—

Sulphur DioxideBenzeneIsopreneIron Carbonyl
a23210.0250.0033
K1352010805201.7
1/K0.04740.04930.050.59


The maximum pressure at which the glow will persist is lowered by these substances, and their activity in this respect is proportional to 1/K. It is noteworthy that lead tetraethyl and iron carbonyl, which are known as "anti-knock" additions to petrol, are both very active inhibitors.

The results of these experiments lend support to the theory that oxidation probably takes place only in the gaseous phase and is catalysed by the active oxygen produced in the reaction.

Nature of the Chemiluminescence

Many observations lead to the conclusion that the glow is produced by the combination of gases only. The glowing zone may be removed from the surface of the phosphorus by a current of air, leaving a dark space in the immediate neighbourhood of the phosphorus.

The glow is exhibited by ordinary phosphorus trioxide, but is then really due to small quantities of dissolved phosphorus. The oxide when purified as described on p. 126 gave only a momentary glow at the commencement of the oxidation (by oxygen), which afterwards proceeded without emission of light. The glow of phosphorus, which actually is inhibited by the trioxide, is restored continuously as this is hydrated by small amounts of water vapour. The inhibitory effect of P4O6 is also removed by ozone.

Ionisation by the Glow

Air which has been drawn over smouldering phosphorus is rendered electrically conducting, so that it allows an electroscope to discharge itself whether it is positively or negatively charged; therefore gaseous ions of both signs are produced. These may be charged atoms, O+ and O-, one of which may combine with the phosphorus and the other form ozone. The conductivity is not due to the ozone itself, since after removal of this, conductivity is retained.

The fact that below 100 mm. the rate of oxidation is proportional to the square root of the partial pressure of oxygen suggests that the active atoms of oxygen are produced before the combination. The difficulty in this supposition is that there is no apparent source of the high energy of activation required, which could, however, be drawn from a coupled reaction, or from the formation and decomposition of a peroxide.

It has been shown by numerous investigators that the connection between ionisation, oxidation and chemiluminescence is a close one. If air or oxygen is mixed with the vapours of turpentine, etc., the conductivity of the mixture after passing over the phosphorus is only slight, corresponding to the suppression of the glow.

The mobilities of the positive and negative ions were found to be equal at first, but the mobilities and sizes change with time. Maxima and minima were found at different temperatures, including a maximum mobility just below the ignition temperature, at 40° C.

Moisture is favourable, perhaps even essential, to the ionisation, which leads to the opinion that the conducting ions may be those of an acid.

The gases in flames are ionised as a rule, and it has been stated that the vivid combustion of phosphorus also gives rise to ionisation.

One cause of the ionisation is indicated by the observation that the light from glowing phosphorus when transmitted through a window of fluorite, is capable of ionising air on the other side. This proves that the light contains ultraviolet radiations. This light also produces ozone. Under otherwise similar conditions ozone is produced from oxygen in greater amount by light transmitted through a window of fluorite than by that transmitted through a window of quartz. Now it is known that quartz (2 mm. thick) is weakly transparent to light of wavelength lower than 200 mμ and opaque to that of wavelength below 160 mμ; while fluorite is opaque to light of wavelength below 125 mμ. This, taken in conjunction with the observation that light of wavelength 120 to 180 mμ is a strong ozonising agent, while that of wavelength 230 to 290, especially 257, decomposes ozone, explains the effect mentioned above.

The Emission Spectrum

A spectroscopic examination of the light emitted by glowing phosphorus reveals a number of narrow bands in the ultraviolet at

λ = 3270 2600 2530 2470 2390 Å.

The same bands are shown when phosphorus burns under reduced pressure with a flame which has a temperature of 125° C. and in the glow of phosphorous oxide. The broader bands which have been reported as present in the light from glowing phosphorus have been shown to be due to incipient combustion; they are the same as those shown by phosphorus burning in air enriched with oxygen at a flame temperature of 800° C. and by phosphine burning in air at reduced pressure with a flame temperature of 160° to 230° C., and also in oxygen at atmospheric pressure. The bands have, in reality, a complex structure. Such spectra are associated with molecular rather than atomic vibrations. There must therefore be some excited system common in all these kinds of combustion. In this connection it was noted by Emeleus and Purcell that the band at about λ = 3270 had already been observed in the light emitted by phosphorus pentoxide when volatilised in an oxyhydrogen flame.
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