Sodium dithionite (also known as sodium hydrosulfite) is one of the most important reductive bleaching agents. It is a white crystalline powder with a weak sulfurous odor. It can be obtained by reacting sodium bisulfite with zinc
- 2 NaHSO3 + Zn → Na2S2O4 + Zn(OH)2
It is used as such in some industrial dyeing processes to eliminate excess dye, residual oxide, and unintended pigments and for bleaching wood pulp.
Reaction of sodium dithionite with formaldehyde produces Rongalite,
- Na2S2O4 + 2 CH2O + H2O → NaHOCH2SO3 + NaHOCH2SO2
which is used in bleaching wood pulp, cotton, wool, leather and clay.
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“Single acts of tyranny may be ascribed to the accidental opinion of a day; but a series of oppressions ... too plainly prove a deliberate, systematical plan of reducing us to slavery.”
—Thomas Jefferson (17431826)