Sodium dithionite manufacture

Traditionally, these dyes are appHed from a dyebath containing sodium sulfide. However, development in dyeing techniques and manufacture has led to the use of sodium sulfhydrate, sodium polysulfide, sodium dithionite, thiourea dioxide, and glucose as reducing agents. In the reduced state, the dyes have affinity for cellulose (qv) and are subsequendy exhausted on the substrate with common salt or sodium sulfate and fixed by oxidation. 

A higher quality a-modification with enhanced tinctorial strength and transparency is prepared from the leuco form of bromoisoviolanthrone. This intermediate in turn is manufactured by vatting crude bromoisoviolanthrone with sodium dithionite/aqueous sodium hydroxide. The product is separated and oxidized in an aqueous alkaline medium in the presence of surfactants. Application of shearing forces, preferably by means of a sand or pearl mill, and maintaining a temperature of 50°C produces improved pigment quality [26]. 

Sodium dithionite is also known as sodium hydrosulphite. It is a stable white powder with strong reducing properties. The first stage in its manufacture is to pass sulphur dioxide through an aqueous suspension of zinc dust when the following reaction takes place ... 

Before the availability of sodium dithionite (1871), the reduction of indigo was brought about by bacteria with reducing properties (fermentation vat). The vat of indigo has a brown-yellow colour. The cloth is dipped in the vat and then exposed to air to allow reoxidation to indigo which is precipitated and finely distributed onto the fibre. A consequence of this process is the low rubbing fastness of the dyes. It causes the faded appearance of indigo-dyed jeans and makes possible the manufacture of faded jeans . 

In the main product Ri=CH3 and R2=—CH2CH2OH although small quantities of the products where Ri=R2=CH3 and Ri=R2=CH2CH20H are present, as will be seen from the synthetic method used in manufacture. Quinizarine is reduced with sodium dithionite to give leuco quinizarin which is probably an equilibrium mixture (Figure 2.69). 

Sodium hydroxymethanesulfinate Dithionites are the starting materials in the manufacture of sodium hydroxymethanesulfonate by the reaction of dithionites with formaldehyde in the presence of sodium hydroxide. In a variant of this process, zine dithionite, as starting material, reacts with formaldehyde in aqueous solution forming zinc salts of hydroxymethanesulfinic acid and hydroxymethanesulfonic acid. The latter is reduced to zinc hydroxymethanesulfinate with added zinc at 90 °C. 

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