Sulphur trioxide

Sulphur Trioxide. The He I photoelectron spectrum of SO3 has been measured and compared with the spectra of SO2 and BF3. The results indicate a substantial stabilization of the sulphur lone-pairs in SO2 and the possible involvement of central atom /-orbitals in SO3. An anharmonic force-field for SO3, based on the valence force model, has been investigated. Gas-phase Raman and i.r. spectra together with a band-contour calculation have been used to establish beyond doubt the assignment of at 497.5 and at 530.2 cm for SO3. 

A study of the kinetics of reactions between oxygen and nitrogen atoms with SO3 has shown that the reaction of oxygen atoms with SO3 yields SO2 and O2 as stable products. A white metastable solid condensing at — 10°C was observed as a probable intermediate. The reactions of SO3 with the sodium borates Na2B407 and NaBgOg have been studied by chonical, X-ray diffraction, and thermal analyses. Reaction, which took place between 190 and 200 °C, gave three products  

The reactions of SO3 with sodium and barium nitrates are now thought to give the compounds KNO2S3O10 and Ba(N02)2(S30i3)2, respectively. Previously the reaction was thought to be  

Sulphates.—The structures of sulphates have been discussed in terms of two models (a) the SO4 group is considered to be a large pseudosphere and 

Pevergne, P. Legrand, and J. Henbel, Bull. Soc. chim. France, 1972, 4106. 

Sulphur trioxide is produced by the catalytic oxidation of sulphur dioxide. Exposure to sulphur trioxide may occur due to leaks in pipes and process equipment. On escape to the atmosphere, sulphur trioxide reacts with water to form sub-micron particles of sulphuric acid smoke which may be sufficiently dense to obscure the source of the leak. 

There exist three solid forms of SO3 called A, B and C. Only the A-form is stable, while B and C are not stable. The melting point of the A-form is 62°C, at which temperature the vapour pressure of liquid SO3 is 2.5 atmospheres. However, it appears that the A-form never crystallises from the liquid phase, that only forms if B crystals are present initially. These B-form crystals will in time (which might be very long) transform into A, therefore it is essential to prevent the formation of B crystals. 

Liquid SO3 is relatively stable between 32°C and 44.5°C at 1 atm. Below 32 C the B-form starts crystallising and the dwger then exists that B crystals might transform into the A-form, which cannot be melted without increasing the pressure to 2.5 atm, as noted above. 

The stabilisers which are used in liquid SO3 only serve to prevent the crystallisation of the B-form from liquid SO3 below 32°C. If the temperature drops further to 16.5 C, the C-form will crystallise, though there does not appear to be much danger of the C-form transforming directly into A. Stabilisers in liquid SO3 are patented inhibitors, such as boron compounds, methane sulfonyl chloride and phosphorous oxychloride. 

Sulphur trioxide is normally kept in the liquid state at a temperature of about 35 C and ambient pressure. The vapour pressure at 35°C is about 410 mm Hg. The commercial sulphan has a melting point of 16.8and boiling point of 45°C. 

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The sulphonic acids are usually prepared by the action of sulphuric acid upon a compound. The concentration of the acid and the temperature of reaction are varied according to the reactivity of the compound. Often oleum is used or even chiorosulphonic acid. Alternatively sulphur trioxide complexed to pyridine or dioxan can be used with reactive substrates. Aminosulphonic acids such as sulphanilic and naphthionic acids are most conveniently prepared by heating the sulphate of the amine at ISO C. 

Sulphur trioxide, SO3, m.p. 17 C, b.p. 49 C. Formed SO2 plus O2 over a catalyst (contact process - see sulphuric acid). The solid exists... 

The reaction with oxygen converts phosphorus trichloride to phosphorus trichloride oxide (oxychloride), POCI3 the trichloride is able to remove oxygen from some molecules, for example sulphur trioxide... 

At high temperatures oxygen reacts with the nitrogen in the air forming small amounts of nitrogen oxide (p. 210). Sulphur burns with a blue flame when heated in air to form sulphur dioxide SO2, and a little sulphur trioxide SO3. Selenium and tellurium also burn with a blue flame when heated in air, but form only their dioxides, Se02 and Te02. 

Sulphur trioxide was first prepared by heating iron(III) sulphate Fe2(S04)3 FcjOj + 3SO3... 

Sulphur trioxide can be collected as a white solid in a receiver surrounded by a freezing mixture of ice and salt. 

In the vapour state, sulphur trioxide has the formula SO3. The molecule is planar with all the S—O bonds short and of equal length. The structure can be represented simply as... 

Solid sulphur trioxide reacts explosively with liquid water ... 

Sulphur trioxide unites exothermically with basic oxides to give sulphates, for example... 

Sulphur trioxide is used on an industrial scale for sulphonating organic compounds. 

Although the left to right reaction is exothermic, hence giving a better equilibrium yield of sulphur trioxide at low temperatures, the reaction is carried out industrially at about 670-720 K. Furthermore, a better yield would be obtained at high pressure, but extra cost of plant does not apparently justify this. Thus the conditions are based on economic rather than theoretical grounds (cf Haber process). 

The conversion of sulphur trioxide to sulphuric acid arises as a separate reaction only in the Contact process. 

Sulphur trioxide is not very soluble in water but dissolves readily in concentrated sulphuric acid. 

The sulphur trioxide from the Contact chamber is passed into... 

Pure sulphuric acid is a colourless, viscous and rather heavy liquid (density 1.84 g cm ). On heating, it decomposes near its boiling point, forming sulphur trioxide and a constant boiling (603 K) mixture of water and sulphuric acid containing 98% of the latter. This is concentrated sulphuric acid, which is usually used. Further heating gives complete dissociation into water and sulphur trioxide. 

When sulphur trioxide is dissolved in concentrated sulphuric acid the pure 100% acid is first formed then a further molecule of the trioxide adds on ... 

The formation of other polysulphuric acids H2S30io up to H2 0(S0j) , by the addition of more sulphur trioxide, have been reported. 

Hence the strength of the acid goes up as sulphur trioxide is dissolved in it. The acidity of pure and fuming sulphuric acids is not so apparent as in ordinary aqueous acids because it is masked by the oxidising and other properties moreover, the conductivity... 

When solid sodium hydrogensulphate is heated, sodium pyro-sulphate is formed further heating gives sodium sulphate and sulphur trioxide ... 

The sulphates of the alkali and alkaline earth metals and man-ganese(II) are stable to heat those of heavier metals decompose on heating, evolving sulphur trioxide and leaving the oxide or the metal ... 

This structure is perhaps best visualised by regarding it as built up from a sulphur trioxide molecule and an oxide ion (this happens in practice). 

Nitrations are usually carried out at comparatively low temperatures at higher temperatures there may be loss of material because of the oxidising action of the nitric acid. For substances which do not nitrate readily with a mixture of concentrated nitric and sulphuric acids ( mixed acid ), the intensity of the reaction may be increased inler alia by the use of fuming sulphuric acid (containing up to 60 per cent, of sulphur trioxide) or by fuming nitric acid. Thus nitrobenzene is converted by a mixture of fuming nitric acid and concentrated sulphuric acid into about 90 per cent, of wi-dinitrobenzene and small amounts of the o- and p-isomers the latter are eliminated in the process of recrystallisation ... 

Sulphur trioxide (including the sulphur trioxide content in oleum) 15 t... 

Hot oleum (>50°C), strong alkalis, fluoride solutions, sulphur trioxide Strong alkalis, especially >54°C, distilled water >82°C, hydrofluoric acid, acid fluorides, hot concentrated phosphoric acid, lithium compounds >1 77°C, severe shock or impact applications Strong oxidizers, very strong solvents... 

For example, sulphur dioxide is highly water soluble and tends to be absorbed in the airways above the larynx. Responses at various concentrations are summarized in Table 5.3. However, in the presence of particulate catalysts and sunlight, conversion to sulphur trioxide occurs and the in itant response is much more severe. 

Sulphur oxides G Sulphur dioxide Sulphur trioxide Coal distillation Combustion of coal and heavy fuel oil Detergents (sulphonation of alkyl benzenes) Electricity generation... 

Griffiths, R. (ed.) (1995) Sulphur Trioxide, Oleum and Sulphuric Acid Mist, Institution of Chemical Engineers, Rugby. Hamngton, J.M. and Gardiner, K. (1995) Occupational Hygiene, 2nd edn, Blackwell Science, Oxford. 

Sulphur trioxide Approximately 1 % of the sulphur dioxide conlem... 

Sulphur dioxide in the air originates from the combustion of fuel and influences rusting in a number of ways. For example, Russian workers consider that it acts as a cathodic depolariser , which is far more effective than dissolved oxygen in stimulating the corrosion rate. However, it is the series of anodic reactions culminating in the formation of ferrous sulphate that are generally considered to be of particular importance. Sulphur dioxide in the air is oxidised to sulphur trioxide, which reacts with moisture to form sulphuric acid, and this in turn reacts with the steel to form ferrous sulphate. Examination of rust Aims formed in industrial atmospheres have shown that 5% or more of the rust is present in the form of iron sulphates and FeS04 4H2 0 has been identified in shallow pits . 

Lead will resist chlorine up to about 100°C , is used for dry bromine at lower temperatures and is fairly resistant to fluorine . Hydrofluoric acid does not passivate lead, so lead should not be used in this environment. Lead is very resistant to sulphur dioxide and fairly resistant to sulphur trioxide, wet or dry, over a wide temperature range . ... 

Tantalum is severely attacked at ambient temperatures and up to about 100°C in aqueous atmospheric environments in the presence of fluorine and hydrofluoric acids. Flourine, hydrofluoric acid and fluoride salt solutions represent typical aggressive environments in which tantalum corrodes at ambient temperatures. Under exposure to these environments the protective TajOj oxide film is attacked and the metal is transformed from a passive to an active state. The corrosion mechanism of tantalum in these environments is mainly based on dissolution reactions to give fluoro complexes. The composition depends markedly on the conditions. The existence of oxidizing agents such as sulphur trioxide or peroxides in aqueous fluoride environments enhance the corrosion rate of tantalum owing to rapid formation of oxofluoro complexes. 

It is completely inert to 98% sulphuric acid to at least 160°C and to even higher temperatures at lower concentrations. Practically, it may be used to 200°C in all concentrations and to 225-250°C at concentrations between 80% and 90%. Fuming sulphuric acid containing sulphur trioxide attacks tantalum at room temperature as do hydrofluoric and fluorosilicic acids. 

Tantalum has excellent resistance to virtually all salts including chlorides (especially cupric and ferric chloride), sulphates, nitrates and salts of organic acids, provided (a) they do not contain fluorides, fluorine and free sulphur trioxide, or (b) hydrolyse to produce strong alkalis. 

Sulphur Trioxide Tantalum is attacked by sulphur trioxide at ambient conditions at rates higher than 1 mm/y. 

Sulphur Trioxide (SO2 -I- O2) Linear reaction rates are observed due to phase boundary control by adsorption of the reactant, SO3. Maximum rates of reaction occur at a SO2/O2 ratio of 2 1 where the SO3 partial pressure is also at a maximum. With increasing 02 S02 ratio the kinetics change from linear to parabolic and ultimately, of course, approach the behaviour of the Ni/NiO system. At constant gas composition and pressure, the reaction also reaches a maximum with increasing temperature due to the decreasing SO3 partial pressure with increasing temperature, so that NiS04 formation is no longer possible and the reaction rate falls. 

The majority of the applications of anodic protection involve the manufacture, storage and transport of sulphuric acid, more of which is produced world-wide than any other chemical. Oleum is 100% sulphuric acid containing additional dissolved sulphur trioxide. The corrosion rate of steel in 77-100% sulphuric acid is 500-1 000 my" at 24°C and up to 5 000 my at 100°C which indicates the necessity for additional protection. 

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