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Sodium dithionite (Na2S2O4) is a water-soluble alkali metal salt. It is a reducing agent in dyeing, bleaching and other chemical processes that use water as a carrier or as a reaction medium. It eliminates excess dye, oxide, and unintended pigments to improve overall colour quality in textiles.
It is commonly used in the pulp and paper industry, primarily for bleaching purposes, but it also has an important role in the zinc dust process. Besides its reducing and sulfonating properties, dithionite can be used for other purposes including water treatment, aquarium water conditioning, gas purification and cleaning.
The boiling point of sodium dithionite is relatively low, which makes it a good choice for industrial applications that require a strong reducing agent. It is soluble in aqueous solutions, a feature that makes it a useful alternative to sodium formate for a number of reducing tasks.
However, it is unstable and may undergo decomposition in acidic conditions or at high temperatures under aerobic conditions (Wayman and Lem 1970; Kilroy 1980). The rate of decomposition increases with increasing concentration of the dithionite ion and with increasing acidity, temperature and other sulphur-containing species.
The thermal and alkali stability of dithionite was investigated as a function of temperature, heating time, and concentration of the solution at the pHs 9 to 12.5. Generally speaking, the thermal stability decreased with increasing concentration of the solution at weak and moderate alkalinity (pH 9 to 11) and with increasing temperature at high alkalinity (pH 14). At moderate alkalinity, the dithionite solutions were rather stable.