Sodium hydrosulfite, also known as sodium dithionite or hydros,
has a formula of Na2S2O4. It is a white
crystalline powder and exists in both the anhydrous and dehydrated forms.
Sodium hydrosulfite has a powerful reducing capacity and allows vat dyes to be
reduced readily at room temperatures. It can also be used as antioxidants in
hair treatment processes.
Sodium hydrosulfite is stable when dry, but will slowly be
oxidized by air. Even with the absence of air, it will be deteriorated and
hence it cannot be kept for long and has a comparatively shorter shelf life.
When sodium hydrosulfite is wet and in the absence of oxygen, it is unstable
and will be decomposed to sodium thiosulfate and sodium bisulfite.
Sodium hydrosulfite is a versatile chemical and is used mainly
in textile dyeing, manufacturing of pulp and paper, commercial bleaching agent,
bleaching of foodstuffs and as a reducing agent in hair treatment.
Sodium Formate Process
Sodium hydrosulfite can also be manufactured using sodium
formate, a by-product in the manufacture of pentacrythritol mainly used in the
paint industry. Using sodium formate in an alkaline alcoholic solution, it
reacts with sulfur dioxide and yield sodium hydrosulfite. With increased
concern over metal pollution, manufacturer shifts away from zinc process to the
environmental friendly amalgam and sodium formate processes.
Using sodium amalgam, formed in the manufacturing of chlorine
using mercury cells at a controlled pH in the range of 5 to 7), it reduces
sodium sulfite to sodium hydrosulfite. This method has an advantage and is one
of the methods widely used today, as the end product does not contain any trace
of zinc, iron or mercury.
Sodium hydrosulfite is used in the bleaching of wood pulp. By
reaction with formaldehyde, it produces rongalite that
bleaches the wood pulp. This decreases the colour of the pulp so that it
becomes whiter. This method is of high efficiency and is also known as
reductive bleaching since reduction of the colour-causing compound causes it to
lose its colour-causing property.
Being a reducing agent, sodium hydrosulfite is used an as
antioxidant in hair treatment process since it scavenges for oxygen and have
the ability to split disulfide bonds in hair. It is also used in the
formulation of body and hand products, deodorants, hair dyes and colours. In addition,
it aids in water treatment and the purification of gas. Moreover, Sodium
hydrosulfite is used in the food industry and serves as a food
additive. It is used in industrial dyeing processes. It is a water-soluble
salt and can be used as a reducing agent, potently reducing water-insoluble
dyes into water-soluble dyes. Meanwhile, it is also used in bleach, together
with formaldehyde. In addition, it removes residual oxides and excess dye,
improving the overall quality of the product. Sodium hydrosulfite is also
used as commercial bleach for treatment of leather, kaolin clay, fur and chrome