Sulphur Trioxide, SO₃

The first mention of sulphur trioxide is by Basil Valentine at the end of the fifteenth century. The method of formation by heating ferrous sulphate was first described in 1675 by Lemery, whilst the preparation by heating fuming sulphuric acid was discovered a century later by Bernhardt. Scheele and Guyton de Morveau first recognised the compound as sulphuric anhydride.

1. Sulphur trioxide is formed to a small extent together with sulphur dioxide when sulphur or compounds of sulphur are burned in oxygen or air. In an oxygen tomb, in the presence of a compound which on combustion yields water vapour and oxides of nitrogen, for example ammonium nitrate, the combustion goes completely to sulphur trioxide.
2. 1. Sulphur dioxide, when exposed to strong illumination, undergoes partial and reversible conversion into sulphur and sulphur trioxide.
   2. Sulphur dioxide is easily converted into the trioxide by the action of oxygen and gentle heat. Above 450° C. there is a tendency towards the formation of an equilibrium mixture of sulphur dioxide and trioxide with oxygen, but below this temperature the amount of dioxide in the equilibrium mixture is almost inappreciable. It is inadvisable, therefore, to allow the reaction to occur at too high a temperature, but, at the same time, the temperature must not be too low, otherwise the rate of change will be very slow.
      
      The change has been submitted to careful physico-chemical examination and is found to accord with the usual method of expression,
      
      2SO₂ + O₂ ⇔ 2SO₃,
      
      in agreement with which are the facts that the reaction is termolecular and that increase in pressure greatly favours the formation of trioxide.
      
      The presence of traces of moisture exerts a very considerable favourable effect on the rate of combination of the gases; reaction after drying with phosphorus pentoxide is relatively sluggish.
      
      Many solid substances are remarkably active in accelerating the change; especial mention may be made of finely divided platinum in this respect. In the presence of platinum black at a temperature of 432° C., 96.8 per cent, of the dioxide may be converted into the trioxide. Sugar charcoal and the oxides of iron, copper, vanadium and arsenic are amongst other substances which possess catalytic power over the reaction, and although they are weaker catalysts than platinum, the final proportions of dioxide and trioxide in the equilibrium mixture are but little affected by the nature of the catalyst used. The present commercial process for the manufacture of sulphur trioxide is based on the use of such catalysts.
   3. When a mixture of sulphur dioxide and oxygen is subjected to the silent electric discharge, more or less complete conversion into sulphur trioxide occurs, the amount of conversion depending upon the composition and pressure of the gaseous mixture. If oxygen is activated alone by the discharge it readily unites with sulphur dioxide after removal, but sulphur dioxide is not itself activated by the discharge.
3. 1. Fuming sulphuric acid, when carefully distilled into a well-cooled receiver, gives a crystalline deposit of sulphur trioxide as the first fraction; ordinary sulphuric acid remains in the retort:
      
      H₂S₂O₇ = H₂SO₄ + SO₃.
   2. The pyrosulphates resemble their parent acid (see (3) (a)) in yielding sulphur trioxide when heated; a similar result is obtained on heating with sulphuric acid:
      
      Na₂S₂O₇ = Na₂SO₄ + SO₃,
      Na₂S₂O₇ + H₂SO₄ = 2NaHSO₄ + SO₃.
4. 1. The elements of water can be removed from sulphuric acid by treating with an excess of phosphorus pentoxide, when the resulting sulphur trioxide may be removed by distillation:
      
      H₂SO₄ + P₂O₅ = 2HPO₃ + SO₃.
   2. Many metal sulphates derived from the more feebly basic metals when strongly heated in an anhydrous condition are converted into oxide, with loss of sulphur trioxide. Ferric sulphate is one of the best known examples. On account of the high temperature necessary there is a tendency, however, for the sulphur trioxide to undergo partial decomposition into dioxide and oxygen, and a similar tendency is observable with the pyrosulphates (see (3) (b)). With both classes of salts reduction of pressure allows evolution of the trioxide to occur at a lower temperature and so renders the result more satisfactory.
5. Especially interesting from the evidence which it supplies as to the possibility of a bimolecular structure for sulphur trioxide is the formation of this substance when sulphuryl chloride and silver sulphate are heated together. This reaction would be expected to follow the course
   
   
   
   and, as will be seen later, sulphur trioxide can exist in a bimolecular condition.

The trioxide, produced by any of the preceding methods, can be purified by repeated distillation, followed by treatment with phosphorus pentoxide at 90° to 100° C., being finally separated from the pentoxide by distillation or decantation.

It has long been recognised that sulphur trioxide is capable of existence in more than one form, but the exact nature of the polymorphs has been difficult to define owing to gross irregularities observable in the physical properties of different specimens and even of individual specimens after lapse of time.

Two solid forms, distinguished as α- and β-modifieations, were described by Marignac and others and are now generally recognised. The α- or " ice" form is that corresponding with ordinary molten sulphur trioxide; it consists of colourless prismatic crystals which melt at 16.8° C. to a fairly mobile liquid, less viscous than sulphuric acid. The pure liquid is colourless, but the presence of organic matter causes a brown coloration. The density of the liquid has been found to be as follows:

Temperature, °C.	Density.
11	1.944
16	1.940
20	1.9255
25	1.9040
30	1.8819
40	1.8335
50	1.7812
60.4	1.718
80.3	1.617
100.0	1.529

The liquid has a coefficient of expansion of 0.002005 for the temperature range 15° to 20° C., a refractive index, n^20°_D = 1.40965, and boils at 44.8° C. under 760 mm. pressure. When kept at a temperature below 25° C. for a considerable period, it undergoes gradual conversion into the more stable β-form, which slowly separates; for this reason the earlier boiling-point data in the literature show considerable variation. Experiments with solutions of sulphuric acid, sulphonal and trional in the liquid indicate a value of 13.5 for the ebullioscopic constant.

β-Sulphur trioxide, the more stable solid form, usually consists of long fibrous needles, and is designated the " asbestos " form. When heated above 50° C. it vaporises without melting, but in a sealed tube the crystals melt over the range 50° to 80° C.

As would be expected from its increasing dissociation at higher temperatures, sulphur trioxide is formed from sulphur dioxide and oxygen with evolution of heat, the amount being 32.1 Calories per gram- molecule formed, whilst the heat of formation of the liquid trioxide from its elements is 103.2 Calories per gram-molecule. The latent heat of vaporisation per gram-molecule of liquid sulphur trioxide is 10.3 Calories.

In accordance with its nature as the more stable form, β-sulphur trioxide is found to give lower vapour pressures than the α-modification, whilst its heat of formation with respect to its elements, namely 111.6 Calories, is greater.

The existence of two polymorphs was questioned by Weber, who maintained that the β-form was a hydrate of sulphur trioxide, and this also was the conclusion arrived at by Berthoud, after determinations of the melting-points of the two forms, and of the vapour pressures of liquid sulphur trioxide. The latter investigator, however, described it as a hydrate unique in its remarkably small water content-estimated at less than 1 molecule per million molecules of trioxide.

Again, Le Blanc and Ruhle, in order to interpret the results of a large number of observations of the vapour pressures of solid and liquid sulphur trioxide, using specimens prepared by different methods, and also from melting-point determinations, found it necessary to postulate four different modifications, namely "A" (m.pt. 95° to 100° C.), "B" (m.pt. 31° C.), "C" (m.pt. 16.8° C.), "D" (m.pt. lower than 16.8° C.). "C" was obviously the α-form.

Oddo expressed the view that the α-form-liquid sulphur trioxide could be represented by the simple formula SO₃, whilst the more stable β-form-ordinary fibrous or "asbestos" sulphur trioxide was to be regarded as a dimeride (SO₃)₂. This view followed from a series of determinations of freezing-point depressions and vapour density measurements. In the former experiments irregular values were obtained when liquid sulphur trioxide was used as solute, owing to the formation of solid solutions. The results, however, using phosphorus oxychloride as solvent, agreed with α-molecular weight corresponding to the simple formula. With sulphuryl chloride and ethyl chloro-acetate as solvents the values for the fibrous form corresponded to a theoretical molecular weight of 160. Vapour density determinations for liquid and fibrous sulphur trioxide gave mean values of 82.68 and 83.77 for the respective molecular weights. Thus a small degree of association in the vapour state is indicated, though this can scarcely be seen in the following data obtained by earlier investigators:

Pressure, mm. Hg.	Temperature, ° C	Density (Hydrogen = 1).
22.1	22.8	40.0
40.5	22.7	39.2
56.7	22.1	40.9
760	46	39.4-39.7

The behaviour of the asbestos-like β-form is more complex, however, than can be accounted for by regarding it as a simple dimeride, and several chemists have expressed the belief that it contains two distinct constituents, one unimolecular and the other bimolecular; the former is described as a fusible crystalline solid, melting at about 30° C., and the latter as an infusible substance.

What appears to be a probable solution to the problem has been put forward by Smits and Schoenmaker after an investigation of the changes in vapour pressure and the corresponding changes in the melting temperature of the intensively dried and purified substance. Two " asbestos " forms were separated by distillation under reduced pressure, a high-melting form, m.pt. 62.2° C., and a low-melting form, m.pt. 32.5° C. The third form corresponded to α-sulphur trioxide, or the " ice " form, of m.pt. 16.8° C. It was found that with each form the vapour pressure diminishes continually. Thus, after the " ice " form had stood for fifty-six hours at 18° C. the vapour pressure at 0° C. diminished by 71.4 mm. of mercury, and on repeated distillation it fell continuously to 22 mm., the initial value having been 207.8 mm. During this fall, the initial melting-point first rose and then fell. Similarly with the high-melting "asbestos " form, the fall of vapour pressure after distillation was very marked. At 50° C. a decrease from 591 to 37 mm. of mercury was noted, but on keeping the substance at this temperature the value slowly increased, after nineteen hours being 214-5 mm., and recovering its original value after 159 days, further increasing to 620-2 mm. after 255 days. It was found that the sample both before and after these changes gave identical X-ray photographs. The effect of the X-rays was to give a rapid increase in the value of the vapour pressure. The low-melting "asbestos" form, which was obtained as clusters of fine needles, did not lend itself to intensive drying, since it readily passed to the high-melting form; but samples which were not intensively dried showed behaviour similar to the foregoing. The high-melting form, which is the stable form of sulphur trioxide, was obtained by cooling the "ice" form in liquid air or distilling repeatedly at 18° to -80° C. Above 18° C. the "ice" form is a clear liquid.

In order to explain the foregoing behaviour, Smits and Schoenmaker assume that sulphur trioxide consists of a mixture of two different kinds of molecules which not only change one into the other, but combine to give a dissociable compound, the reactions leading to the attainment of an inner equilibrium which may be represented as



The various modifications of sulphur trioxide will therefore act as mixtures of these three constituents. In an ordinary preparation the condition of equilibrium is more or less rapidly attained, but in an intensively dried material the velocity of such inner transformation is considerably retarded. On realising this condition, as already shown, both the solid and liquid states behave as mixtures. Exposure to X-rays accelerates the attainment of the inner equilibrium.

The critical data for sulphur trioxide have been found to be as follows: Critical pressure, 83.8 atmospheres; critical temperature, 218.3° C.; critical density, 0.633.

Both the α- and β-forms of sulphur trioxide exhibit as a rule the same chemical behaviour, although the β-variety is less active.

At the ordinary temperature sulphur trioxide forms dense fumes in moist air, caused by the formation of sulphuric acid. The anhydrous substance possesses no acidic properties, but combines violently with water, producing pyrosulphuric and then sulphuric acid, the heat evolution with a large excess of water amounting to 40.3 Calories per gram-molecule of SO₃. The heats of dilution (H) for one gram-molecule of SO₃ with n molecules of H₂O are as follows:

n	1	2	3	5	1600
H Cals.	21.3	28.04	31.31	34.14	40.34

Its affinity for water causes sulphur trioxide to carbonise many organic substances.

From examination of the vapour density it is known that the trioxide undergoes no appreciable dissociation below 430° C., but at higher temperatures the percentage of dissociated molecules increases, until at near 1000° C. the vapour consists entirely of sulphur dioxide and oxygen. The catalysts which accelerate the formation of sulphur trioxide also facilitate its dissociation:

2SO₃ ⇔ 2SO₂ + O₂.

The trioxide possesses marked oxidising properties, in the exercise of which it is generally reduced to the dioxide. Yellow phosphorus soon inflames in the vapour at the ordinary temperature, some of the trioxide being reduced even to sulphur. Phosphine is oxidised by solid sulphur trioxide with formation of phosphorus. Phosphorus trichloride is converted into the oxychloride. Hydrogen bromide and iodide yield bromine and iodine, respectively, whereas hydrogen chloride, as already described, gives rise to chlorosulphonic acid. At a red heat the metals iron, magnesium and zinc are converted into a mixture of the corresponding metallic sulphide and oxide, whilst mercury reacts at a much lower temperature, with the formation of sulphate and sulphur dioxide. Many metal sulphides, for example those of the alkalis, and of lead and antimony, are oxidised to sulphates, with simultaneous production of sulphur dioxide or sulphur. Sulphur trioxide frequently also exerts oxidising power towards organic substances, as for instance towards carbon disulphide, which at higher temperatures gives carbon oxysulphide (see below), but the action with organic compounds is more commonly one of sulphonation.

Reduction of sulphur trioxide by hydrazine gives rise to sulphur sesquioxide, possibly by primary formation of sulphur, which combines with unaltered trioxide.

Under the influence of the silent electric discharge sulphur trioxide combines with oxygen to form the so-called heptoxide or persulphuric anhydride, S₂O₇. At the ordinary temperature it forms an unstable blue sesquioxide, S₂O₃, with sulphur, whilst with selenium and tellurium it forms the analogous compounds SeSO₃ and TeSO₃. Iodine combines with a termolecular proportion of sulphur trioxide, giving a product I(SO₃)₃, from which the trioxide can be removed in several stages.

With many acid anhydrides sulphur trioxide condenses to form "mixed anhydrides"; thus, nitric anhydride gives N₂O₅.4SO₃, possibly of the constitution NO₂.O.SO₂.O.SO₂.O.SO₂.O.SO₂.O.NO₂; nitrogen trioxide dissolved in carbon tetrachloride yields 2N₂O₃.5SO₃; at 120° C. boric anhydride gives B₂O₃.SO₃, whilst at 230° C. the product is B₂O₃.2SO₃; chromic anhydride at 70° C. yields CrO₃.SO₃; phosphoric oxide, arsenious oxide and selenium dioxide give the compounds P₄O₁₀.6SO₃, As₄O₆.2SO₃ and SeO₂.SO₃, respectively. In the cold, carbon disulphide combines with sulphur trioxide forming a crystalline additive compound, in which the former constituent, as a thio-anhydride, possibly functions in a similar manner to the ordinary anhydrides in the compounds just described; when warmed, this additive compound decomposes with formation of carbon oxysulphide, as already mentioned. Sulphur dioxide is absorbed by sulphur trioxide, giving a liquid containing up to approximately 70 per cent, of the former and probably representing an unstable compound.

Sulphur trioxide combines with sulphuric acid, producing pyrosulphuric acid, whilst the sulphates yield pyrosulphates. Perdisulphates also absorb the trioxide, giving the so-called "per-pyrosulphates "; thus potassium perdisulphate combines with a bimolecular proportion of the trioxide to form potassium per-pyrosulphate, K₂S₄O₁₄, probably K.S₂O₆.O₂.S₂O₆.K.

It has already been recorded that hydrogen chloride and sulphur trioxide react to produce chlorosulphonic acid. The alkali chlorides, on the other hand, form chloropyrosulphonates, e.g. ClSO₂.0.SO₂.ONa, which are crystalline solids, decomposed by water; the alkali fluorides, however, yield the corresponding fluorosulphonates, such as F.SO₃Na. Sodium nitrite produces a "nitrosotrisulphonate," NO₂(SO₃)₃Na. Alkali chlorides may be converted to sulphates with evolution of chlorine by the action at 300° to 600° C. of a mixture of sulphur trioxide and air. No fusion occurs nor does the evolved gas contain any sulphur dioxide.

Towards many chlorinating agents the α- and β-varieties behave differently. Thus with phosphorus pentachloride the α-form yields phosphorus oxychloride with some pyrosulphuryl chloride, S₂O₅Cl₂, and evolution of sulphur dioxide and chlorine; the β-form yields the two former products, but neither sulphur dioxide nor chlorine is evolved. Sulphur monochloride with the a-form yields the dichloride and sulphur dioxide; with the β-form no gas is evolved. With carbon tetrachloride, pyrosulphuryl and carbonyl chlorides are formed, the reaction being much more rapid with the α- than with the β-variety. The following has been suggested as the course taken by the latter reaction, which is shown by the velocity constant to be unimolecular:

2SO₃ + CCl₄ → [SO₂Cl.O]₂CCl₂ → [SO₂Cl]₂O + COCl₂.

As has been stated earlier, sulphur trioxide in the gaseous condition is unimolecular and is therefore correctly represented by the molecular formula SO₃.

At one time, influenced largely by the fact that in sulphur tetrachloride sulphur attained its highest state of valency towards any univalent element, many chemists preferred to regard sulphur as never exceeding a quadrivalent condition, and therefore expressed the molecular structure of sulphur trioxide by the formula . This argument was destroyed by the discovery of sulphur hexafluoride, and to-day the molecular constitution for the unimolecular condition is fairly generally accepted.

For the bimolecular substance, which appears to be the essential constituent of the "asbestos" form or the β-trioxide, the molecular constitution is probable, the possibility of such a structure being indicated by the method of formation (5), and supported by the differences in chemical behaviour, especially towards chlorinating agents as already indicated. Moreover, that the change from the α- to the β-form involves polymerisation is suggested by the fact that the α-form can be stabilised for many months by the presence of certain negative catalysts, such as small amounts of sulphur, tellurium, carbon tetrachloride, or phosphorus oxychloride; this is analogous to the preservative action of such catalysts on an aldehyde. A specimen of asbestos-like sulphur trioxide, after being kept for twelve years, has been found by cryoscopic methods to have a molecular weight corresponding approximately with (SO₃)₅.