Sulphur dioxide bleaching

It acts as a bleach, when it is damp or in solution. Some coloured things lose colour when they lose oxygen—that is, when they are reduced. Sulphur dioxide bleaches them by reducing them. 

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... 

Alternatively cellulose is produced from wood via wood pulp. A number of processes are used in which the overall effect is the removal of the bulk of the non-cellulosic matter. The most widely used are the sulphite process, which uses a solution of calcium bisulphite and sulphur dioxide, the soda process using sodium hydroxide and the sulphate process using a solution of sodium hydroxide and sodium sulphide. (The term sulphate process is used since sodium sulphate is the source of the sulphide.) For chemical purposes the sulphite process is most commonly used. As normally prepared these pulps contain about 88-90% alpha-cellulose but this may be increased by alkaline purification and bleaching. 

For bleaching with alkaline peroxide, the 304 group is satisfactory but with moist sulphur dioxide or hypochlorites the 316 group should be used. In the case of hypochlorites, contact should be short or confined to alkaline sodium hypochlorite. 

Sulphur dioxide is used as a preservative for beer, wine and meats in the production of sulphites and hydrosulphites in solvent extraction of lubricating oils as a general bleaching agent for oils and foods in sulphite pulp manufacture in the cellulose and paper industries and for disinfection and fumigation. 

Bleaching. It is often necessary in the manufacture of lacquer nitrocellulose to remove all traces of coloured substances by bleaching. One method consists in oxidizing with potassium permanganate in the proportion of 1 kg KMn04 to 100 kg of lacquer nitrocellulose in the presence of a little sulphuric acid. After the reaction is over the nitrocellulose is rinsed with water, and the brown colour of manganese dioxide is removed by the action of sulphur dioxide or sodium sulphite. 

There are several methods of manufacturing is on a commercial scale, for instance the calcination of oxalic add blended with potassium chloride, treatment of sodium chlorite with chlorine, or of sodium chlorate with sulphur dioxide in the presence of concentrated sulphuric add. Chlorine dioxide is utilized as a strong bleaching agent for cellulose, paper, flour, oils and such like, as well as a solvent for removing lignin from wood pulp. 

To obtain maximum colour stability a careful balance must be achieved between sulphur dioxide and ascorbic acid contents to avoid bleaching the colours. 

Where other bleaching agents, such as hydrogen peroxide, permanganate and the like, have been used instead of sulphur dioxide, the test is inapplicable. 

The salt has also been found as an efflorescence on bricks which have been continuously exposed to sulphur dioxide in bleaching chambers.4 When the efflorescent mass is broken up it exhibits a silken or fibrous mass of white crystals not unlike asbestos in texture, whence the popular names of hair salt and feather alum. 

Bleaching of wool with sulphur dioxide (stoving process)... 

This was carried out extensively in this country until 1940 when, owing to the scarcity of sulphur, it fell out of use and it is doubtful whether it has been revived to any great extent. It did, however, give an adequate bleach for lower quality goods, was extremely cheap, and only required simple equipment. The process was known as stoving because the wool was exposed to sulphur dioxide in what was called a stove. 

Tussur and other wild silks require special treatment. Tussur silk, in particular, is so difficult to bleach that it is commonly used in its natural fawn colour. Bleaching in a 0-6 per cent w/w solution at 70°, C (158°F) for 6 hours will give a fairly good white without excessive loss of tensile strength. The colour is improved by after-treatment in a liquor containing 2 g per litre of sodium hydrosulphite at 100°C. Silk can be bleached by stoving with sulphur dioxide, but the method has now virtually fallen into disuse. Treatment with sulphur dioxide after a peroxide bleach is sometimes used for exceptionally good whites. 

Natural flowers, more especially roses, dahlias and violets, are soon bleached when thrown into a jar full of sulphur dioxide gas. The colouring matter is not, however, destroyed as when chlorine is used, and may... 

As well as flowers, silk, straw, wool, etc., may be bleached by the action of sulphur dioxide. 

The bleaching brought about by sulphur dioxide differs chemically from that produced by chlorine, or, more strictly speaking, by hypochlorous acid. The chlorine bleaching consists of the oxidation of the colored substances and this action usually so far destroys the substance that no reversal of the action takes place that is the bleaching is permanent. 

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