What is "dithionous acid"

n. (context chemistry English) the unstable acid, H₂S₂O₄, known only as its derivatives, the dithionites

Dithionous acid is a sulfur oxoacid with the chemical formula HSO. It is unstable in pure form, but its salts, known as dithionites, are stable.

It was initially assumed that the C symmetric structure HOS(=O)-S(=O)OH is the most stable among molecules with the formula using ab initio calculations. The reason for this is the existence of intermolecular hydrogen bonds. It is now known that dithionous acid spontaneously decomposes to SO and S(OH).

Sodium dithionite is a white powder used as a reductant and a bleaching agent. It is also used to reduce the nitro group to an amino group in organic reactions.