Sulphur dioxide, reactions

The gas is passed through caustic soda solution to remove any sulphur dioxide or carbon dioxide produced in side reactions. Carbon monoxide is also obtained when an ethanedioate (oxalate) is heated with concentrated sulphuric acid ... 

A method of estimating small amounts of water in organic liquids (and also in some inorganic salts) is that of Karl Fischer. The substance is titrated with a mixture of iodine, sulphur dioxide and pyridine dissolved in methyl alcohol. The essential reaction is ... 

Although sulphur dioxide, as a gas, is a reducing agent in the sense that it unites with oxygen, free or combined (for example in dioxides or peroxides) most of its reducing reactions in aqueous solution are better regarded as reactions of sulphurous acid (in acid solution), or the sulphite ion (in alkaline solution). 

This reaction is a useful test for a sulphite or for moist sulphur dioxide, which turns dichromate paper (filter paper soaked in potassium dichromate) from yellow to green. 

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. 

In the laboratory it is commonly prepared by the reaction between sulphur dioxide and oxygen at high temperature in the presence of a platinum catalyst ... 

Concentrated sulphuric acid is an oxidising agent, particularly when hot, but the oxidising power of sulphuric acid decreases rapidly with dilution. The hot concentrated acid will oxidise non-metals, for example carbon, sulphur and phosphorous to give, respectively, carbon dioxide, sulphur dioxide and phosphoric(V) acid. It also oxidises many metals to give their sulphates cast iron, however, is not affected. The mechanisms of these reactions are complex and the acid gives a number of reduction products. 

In both reactions above, the oxide dichloride is refluxed with the acid or the hydrated chloride the sulphur dioxide and hydrogen chloride pass off and any unused sulphur oxide dichloride is distilled off in vacuo. 

The presence of chloric(I) acid makes the properties of chlorine water different from those of gaseous chlorine, just as aqueous sulphur dioxide is very different from the gas. Chloric(I) acid is a strong oxidising agent, and in acid solution will even oxidise sulphur to sulphuric acid however, the concentration of free chloric(I) acid in chlorine water is often low and oxidation reactions are not always complete. Nevertheless when chlorine bleaches moist litmus, it is the chloric(I) acid which is formed that produces the bleaching. The reaction of chlorine gas with aqueous bromide or iodide ions which causes displacement of bromine or iodine (see below) may also involve the reaction... 

Bromine has many oxidising reactions ( = + 1.07 V) and like chlorine it will oxidise sulphur dioxide in aqueous solution to sulphuric acid, and hydrogen sulphide to sulphur. 

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. 

The sulphuric acid and ethyl hydrogen sulphate required in reactions 1 and 3 respectively are regenerated in reactions 2 and 4, but the water formed is retted in the acid mixture and ultimately results in such a dilution that the caiversion into ether is no longer efficient. Furthermore, some ethylene is always formed this partly polymerises to give materials capable of reacting with sulphuric acid and reducing it to sulphur dioxide. In industrial practice, sulphuric acid is sufficient for the production of about 200 parts of ether. 

An alternative procedure for the above test is as follows. Mix 2-3 ml. of 2 per cent, aqueous paraperiodic acid solution with 1 drop of dilute sulphuric acid (ca. 2 5N) and add 20-30 mg. of the compound. Shake the mixture for 5 minutes, and then pass sulphur dioxide through the solution until it acquires a pale yellow colour (to remove the excess of periodic acid and also iodic acid formed in the reaction). Add 1-2 ml. of Schiff s reagent (Section 111,70) the production of a violet colour constitutes a positive test. 

Aromatic aldehydes react with the dimedone reagent (Section 111,70,2). All aromatic aldehydes (i) reduce ammoniacal silver nitrate solution and (ii) restore the colour of SchifiF s reagent many react with sodium bisulphite solution. They do not, in general, reduce Fehling s solution or Benedict s solution. Unlike aliphatic aldehydes, they usually undergo the Cannizzaro reaction (see Section IV,123) under the influence of sodium hydroxide solution. For full experimental details of the above tests, see under Ali-phalic Aldehydes, Section 111,70. They are easily oxidised by dilute alkaline permanganate solution at the ordinary temperature after removal of the manganese dioxide by sulphur dioxide or by sodium bisulphite, the acid can be obtained by acidification of the solution. 

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. 

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). 

Neutralization of strong mineral acids from metal finishing trades (sulphide and hypochlorite contamination common) Fierce reaction Possibility of mixing with water or organic materials Chlorine Nitrogen dioxide Sulphur dioxide Hydrogen sulphide... 

Thus for non-ferrous metals, SO is consumed in the corrosion reactions whereas in the rusting of iron and steel it is believed that ferrous sulphate is hydrolysed to form oxides and that the sulphuric acid is regenerated. Sulphur dioxide thus acts as a catalyst such that one SOj" ion can catalyse the dissolution of more than 100 atoms of iron before it is removed by leaching, spalling of rust or the formation of basic sulphate. These reactions can be summarised as follows ... 

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 . 

Sulphur Dioxide An excellent account of the reaction of nickel with SO2 and SO3 (SO2 -I- O2) has been given by Kofstad and is summarised below. 

In sulphur dioxide linear kinetics are generally observed due to control by phase boundary reactions, i.e. adsorption of SOj. RahmeF suggested that this is one of the conditions which favours simultaneous nucleation of sulphide and oxide at the gas/scale interface. The main reaction products are NiO, NijSj, Ni-S,j, and NiS04, depending on the temperature and gas pressure for example, according to the following reaction ... 

The Karl Fischer procedure has now been simplified and the accuracy improved by modification to a coulometric method (Chapter 14). In this procedure the sample under test is added to a pyridine-methanol solution containing sulphur dioxide and a soluble iodide. Upon electrolysis, iodine is liberated at the anode and reactions (a) and (b) then follow the end point is detected by a pair of electrodes which function as a biamperometric detection system and indicate the presence of free iodine. Since one mole of iodine reacts with one mole of water it follows that 1 mg of water is equivalent to 10.71 coulombs. 

Another group of Japanese workers91 found that the sulphoxonium salt, 7, was reducible to sulphoxides with either alkyllithiums or lithium dialkylcuprates, the exact reaction pathway being complicated by halide ions originating from the preparation of the metal alkyls. However, good yields of methyl phenyl sulphoxide were obtained by reduction of 7 with sulphur dioxide or a thiol in pyridine (equation 37). 

The photoextrusion of sulphur dioxide to form cyclophanes or other novel aromatic molecules has been reviewed and studied by Givens208-210, while the photodecomposition of aromatic sulphones to form products of radical coupling reactions has recently also received attention211. 

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