Of sulphur dioxide

On the industrial scale it is produced in large quantities for the manufacture of sulphuric acid and the production methods are dealt with later. It was once estimated that more than 4 000 000 tons of sulphur dioxide a year entered the atmosphere of Britain from the burning of coal and oil. 

The molecule of sulphur dioxide has a bent structure. Both S—O distances are equal and short and since sulphur can expand its outer quantum level beyond eight, double bonds between the atoms are likely i.e. 

When a saturated solution of sulphur dioxide is titrated against approximately 2 M sodium hydroxide solution the following pH curve is obtained Figure 10.4) ... 

Barium sulphite is soluble in dilute hydrochloric acid unlike barium sulphate which is insoluble. Hence this reaction, and the evolution of sulphur dioxide on addition of an acid, distinguishes a sulphite from a sulphate. 

The combination of sulphur dioxide and oxygen to form the trioxide is slow and does not proceed to completion ... 

It is extensively used industrially as a catalyst, notably in the oxidation of sulphur dioxide to the trioxide in sulphuric acid manufacture. It is an essentially acidic oxide, dissolving in alkalis to give vanadates however, addition of acid converts the anionic vanadate species to cationic species, by processes which are very complex, but which overall amount to the following ... 

Thus, filler paper which has been dipped into a solution of potassium dichromate turns green in the presence of sulphur dioxide. This reaction provides the usual test for sulphur dioxide. 

Hydrobromic acid. Method 1 (from bromine and sulphur dioxide). A mixture of 600 g. (or 188-6 ml.) of bromine, 250 ml. of water and 760 g. of crushed ice is placed in a 1 6 litre round-bottomed flask and a rapid stream of sulphur dioxide (from a siphon of the liquefied gas) is passed into the flask, care being taken that the outlet of the gas-delivery tube is below the surface of the bromine layer. The rate of flow of the gas is adjusted so that it is completely absorbed. It is advisable to cool the flask in ice and also to shake the contents from time to time. The reduction is complete when the mixture assumes a uniform yellowish-brown or yellow colour, which is unaffected by further introduction of sulphur dioxide excess of the latter gas should be avoided as it will be... 

Sulphuric acid method. Place 20 g. of commercial cycZohexanol and 0-6 ml. of concentrated sulphuric acid in a 150 or 200 ml. round-bottomed or bolt head flask, add 2-3 chips of porous porcelain, and mix well. Fit the flask with a fractionating column, a Liebig condenser, adapter and filter flask receiver as in Section 111,10 (1). Heat the flask in an air bath (Fig. II, 5, 3) at such a rate that the temperature at the top of the column does not rise above 90° alternatively, an oil bath, heated to a temperature of 130-140°, may be used. Stop the distillation when only a small residue remains and the odour of sulphur dioxide is apparent. Transfer the distillate to a small separatory funnel. 

The above simple process cannot be applied to the preparation of the homo-logues a higher temperature is requir (di-n-amyl ether, for example, boils at 169°) and, under these conditions, alkene formation predominates, leading ultimately to carbonisation and the production of sulphur dioxide. If, however, the water is largely removed by means of a special device (see Fig. Ill, 57,1) as soon as it is formed, good 300 of ethers may be obtained from primary alcohols, for example ... 

Preparation of SchlfT s reagent. Method 1. Dissolve 0- 2 g. of pure p.rosaniline hydrochloride in 20 ml. of a cold, freshly-prepared, saturated aqueous solution of sulphur dioxide allow the solution to stand for a few hours until it becomes colourless or pale yellow. Dilute the solution to 200 ml. and keep it in a tightly, stoppered bottle. If the bottle is not adequately stoppered, the reagent will gradually lose sulphur dioxide and the colour wUl return. The solution keeps well if not unnecessarily exposed to light and air. 

Dissolve the solid in 700 ml. of water in a 1500 ml. round-bottomed flask, and add a solution of 88 ml. of concentrated sulphuric acid in about 200 ml. of water until the liquid has a distinct odour of sulphur dioxide sufficient heat will be liberated in the neutralisation to cause the solution to boil. Immediately steam distil the liquid (Fig. II, 40, 1 it is better to use the apparatus shown in Fig. II, 41, 3) until a sample of the distillate gives only a slight precipitate with bromine water. About 700 ml. of distillate should be collected. Saturate the steam distillate with salt, extract the dl with ether, dry the extract with a little anhydrous magnesium or calcium sulphate, distil oflF the ether (compare Fig. II, 13, 4, but with a 50 ml. Claisen flask replacing the distilling flask) and distil the residue under diminished pressure. Collect the p-cresol at 95-96°/15 mm. the colourless liquid solidifies to a white crystalline solid, m.p. 31°. The yield is 24 g. 

CAUTION. The preparation of o-nitrobenzoyl chloride, o-nitrophenacetyl chloride and all o nitroacid chlorides should not be attempted by the above methods a violent explosion may occur upon distilling the product or when the last traces of thionyl chloride are removed in vacuo at 100°. Perhaps the safest method is to treat the pure acid in benzene solution with 1 1 mols of thionyl chloride and to reflux until evolution of sulphur dioxide and hydrogen chloride has ceased the solution of the acid chloride in benzene may then bo employed for most reactions. 

Bisulphite compounds of aldehydes and ketones. These substances are decomposed by dilute acids into the corresponding aldehydes or ketones with the liberation of sulphur dioxide. The aldehyde or ketone may be isolated by steam distillation or by extraction with ether. Owing to the highly reactive character of aldehydes, the bisulphite addition compounds are best decomposed with saturated sodium bicarbonate solution so um carbonate solution is generally employed for the bisulphite compounds of ketones. 

FIG. 17-17 Quench tank for overflow or cyclone solids discharge. [Gilfillan et al., "The FluoSolids Reactor as a Source of Sulphur Dioxide, J. Chem. Metall. Min. See. S. Afr. (May 1954).]... 

When polyethylene is chlorinated in the presence of sulphur dioxide, sulphonyl chloride as well as chlorine groups may be incorporated into the polymer Figure 10.10). This reaction is used to produce a useful elastomer (Hypalon, see Chapter 11). 

Table 5.3 Typical effects of sulphur dioxide concentrations in air...

Various methods can be used to determine the concentration of sulphur dioxide in the vicinity of steel test specimens. They all provide suitable information on the relationship between sulphur dioxide and corrosion. However, the actual amount of sulphur dioxide in contact with the steel surface is more important than the concentration, as shown in the work reported by Walton et al. ... 

The significance of the amount of sulphur dioxide rather than the concentration has been demonstrated by other workers who have studied the effects of atmospheric flow rate. An increase in steel corrosion with increase in atmospheric flow rate at a constant volume concentration of sulphur... 

Preserves and fruit juices Much use is made of sulphur dioxide to preserve fruit pulps and juices, and for these applications molybdenum-bearing steels should be specified. 

In laboratory tests Vernon showed that the relative humidity and the presence of sulphur dioxide have a profound effect on the rate of corrosion of copper, as of many other metals. When the relative humidity was less than 63%, there was little attack even in the presence of much sulphur dioxide, but when the relative humidity was raised to 75%, corrosion became severe and increased with the concentration of sulphur dioxide present. 

Ambler has attempted to find a relationship between the corrosion of zinc and iron and atmospheric salinity in the UK. This followed previous tests in Nigeria, when it was concluded that the governing factor in the corrosion of steel and zinc was airborne salt and that there was a relationship between corrosion and the distance from the sea. In the UK, however, no such relationship was found to exist, and the governing factor in the corrosion of zinc in the atmosphere is confirmed to be the amount of sulphur dioxide pollution. 

In most of its uses, e.g. the external surfaces of tinplate cans, tinned steel has only to resist condensed moisture. In the absence of pollution of the atmosphere by unusually large amounts of sulphur dioxide or chlorides, or of several days of continuous wetting, tinned steel remains unrusted even the thin porous coatings on the common grades of tinplate remain bright and unmarked over the periods involved in the commercial handling and domestic storage of cans, and the domestic use of kitchenware. When... 

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