Sulphur Derivatives

The most important sulphur compound is sulphur dioxide (S02)
This is extensively made for the manufacture of sulphuric acid by roasting iron pyrites. Liquid sulphur dioxide is now an article of trade, but the gas is usually made where it is to be used, by burning sulphur with a propel' supply of air. It is occasionally made by decomposing sulphuric acid by means of copper or charcoal:

2 H2S04 +Cu = CuS04+2 H20 + S02;
2 H2S04+C =2 H20 + CO2 +2 S02.

purposes. Processes for the recovery of sulphur dioxide from furnace gases, by passing them through towers filled with coke, over which water trickles, have been proposed, but are not very practicable.
Sulphur dioxide is used in making sulphuric acid; as a bleaching agent for wool, hair, straw, and other tissues; as a disinfectant and germicide; for use in ice machines; for making the acid sulphite liquor used in manufacturing wood pulp; for the preparation of sodium bisulphite; and to a small extent in the leather and glucose industries.
Substances such as wool and straw, when bleached by exposure to sulphur dioxide gas, slowly regain their original color on exposure to the light. The coloring matter is not destroyed, but probably unites with the sulphur dioxide to form a colorless compound, which slowly decomposes.
Sodium bisulphite (NaHS03) is formed by saturating sodium carbonate solution with sulphur dioxide:-

Na2COa+H20 +2 S02 = 2 NaHS03 + CO2,

It forms a strong smelling solution used as an "antichlor" to remove excess of chlorine from the fibres of bleached cotton or linen goods. Its reaction is probably as follows:

   Ca (ClO)2+2 NaHS03 = 2 NaCl +CaS04 +H2S04; or,
                                = Na2S04 + CaS04 + 2 HCl;
   2 Cl +NaHS03 +H20 = NaCl +H2S04 + HCl; or,
                              =NaHS04 + 2 HCl.

It also finds some use in other industries, such as chrome tannage, brewing, glucose, and starch making. The solution of bisulphite decomposes on evaporation, giving off part of the sulphur dioxide, and forming neutral sulphite of sodium.
Calcium bisulphite [CaH2(S03)2] is made by passing sulphur dioxide into milk of lime. It is probably a solution of neutral sulphite in an excess of aqueous sulphurous acid. It is used in much the same way as the sodium salt.
Hyposulphurous acid (H2S02) and hyposulphite of sodium are of some importance as powerful bleaching and reducing agents. The acid is formed in solution by the action of iron or zinc on aqueous sulphurous acid: -

   H2S03 +Zn = ZnO +H2S02.

The zinc oxide then combines with another molecule of sulphurous acid, to form zinc sulphite and water.
Sodium hyposulphite (NaI-IS02) is made when zinc is dissolved in sodium bisulphite:

3 NaHS03 + Zn = Na2Zn(S03)2+H20 +NaHS02.

The zinc-sodium sulphite crystallizes, leaving the sodium hyposulphite in solution. The latter salt is very unstable, rapidly absorbing oxygen from the air, and should be made immediately before needed.
It is much used for reducing indigo in the so-called "hydrosulphite vat." It is to be noted here that the salt sold in commerce under the name of "hyposulphite of sodium" is properly the thiosulphate (Xa~8"03' 5 H20). This is made by digesting sulphur with a solution of neutral sodium sulphite or sodium hydroxide:-

    Na2S03 + S = Na2S203;
    6NaOH +4 S = Na2S203+2Na2S + 3 H20.

Sodium thiosulphate is also obtained from the waste sulphide liquors of the Leblanc soda process. It crystallizes with five molecules of crystal water, Na2S203 . 5 H20, and is largely used in chrome tanning, in paper bleaching, and in photography.