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28.31 Dithionites and sulphoxylates.
Dithionites (hydrosulphites) are the salts of dithionous ("hydrosulphurous") acid (H2S2O4) which has not been isolated in the free state. They are obtained by reducing (with zinc powder) solutions of hydrogen sulphites saturated with sulphur dioxide. They are reducing agents employed in the chemical, textile or sugar industries, mainly for bleaching.
The most important is sodium dithionite (Na2S2O4), anhydrous white powder, soluble in water, or hydrated (with 2 H2O) in colourless crystals. It is used in organic synthesis, in the dyeing industry and for paper making. It deteriorates rather rapidly, even when crystallised. For certain uses (e.g., as a discharge in the textile industry), sodium dithionite must therefore be stabilised with formaldehyde, sometimes with the addition of zinc oxide or glycerol. It may also be stabilised with acetone.
Dithionites of potassium, calcium, magnesium and zinc, which may be stabilised by similar processes, are products similar to sodium dithionite, with similar properties and uses.
The heading includes all these stabilised dithionites and also formaldehyde sulphoxylate which is a similar product.
Sulphites and thiosulphates are excluded (heading 28.32).
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