A solution of sulphur(IV) oxide in water rapidly bleaches both artificial and natural dyes. The bleaching property of sulphur(IV) oxide is as a result of its strong reducing power. Sulphur(IV) oxide dissolves in water to form trioxosulphate(IV) acid which subsequently donates electrons to the dye and then becomes oxidized to tetraoxosulphate(VI) acid. Dye is reduced in this process to a colorless compound.
The bleaching acting of this compound is similar to that of chlorine in that water is a prerequisite for bleaching to occur. However, the bleaching action of sulphur(IV) oxide differs slightly from that of chlorine because sulphur(IV)oxide bleaches by reducing the dye while chlorine bleaches by oxidizing the dye. Again, bleaching by sulphur(IV) oxide is not usually long lasting as compared to that of chlorine because the bleached dye may become re-oxidized by atmospheric oxygen to form the initial colored compound. This well explains why a white straw hat turns yellow after some time.
Uses of Sulphur(IV) Oxide
1. One of the most important use of sulphur(IV) oxide is its use in making tetraoxosulphate(VI) acid.
2. Liquid sulphur(IV) oxide is sometimes used as a refrigerant because of its property of easy liquefaction and its high latent heat of vaporization.
3. This compound is used as a germicide and as a fumigant used often in the destruction of termites and other insects.
4. Sulphur(IV) oxide is used as a preservative in some liquid foods such as orange juice. It destroys bacteria, protozoa and fungi and prevents the oxidation of the juice, by reacting with the oxygen. It is also used in the preservation of grains and fruits.
It is also used as a bleaching agent for straw, silk, sponges and wool. It is also used to bleach other fabrics that would otherwise be damaged by chlorine, which is a more commonly used bleaching agent. This compound is also used in the manufacture of Calcium hydrogentrioxosulphate(IV), which is a bleaching agent used in bleaching artificial silk and wood pulp.
The oxidizing power of this oxide can be confirmed by its action on hydrogen sulphide, which is a strong reducing agent. In the presence of moisture, hydrogen sulphide is reduced to sulphur by sulphur(IV) oxide. Sulphur(IV) oxide also oxidizes carbon at 1000*C while being reduced to sulphur. This is the process used in the recovery of sulphur from some industrial fuel gases.