Sulfoxylic acid

Sulfoxylic acid (H2SO2) (also known as hyposulfurous acid or sulfur dihydroxide) is an unstable <a href="/facts/Oxoacid/p0bMdwSx">oxoacid</a> of <a href="/facts/Sulfur/IK0amTfM">sulfur</a> in an intermediate <a href="/facts/Oxidation_state/W53W6s6V">oxidation state</a> between <a href="/facts/Hydrogen_sulfide/UbCqjMMk">hydrogen sulfide</a> and <a href="/facts/Dithionous_acid/M51ouvK9">dithionous acid</a>. It consists of two <a href="/facts/Hydroxy_group/1Hs2QAEv">hydroxy groups</a> attached to a sulfur atom. Sulfoxylic acid contains sulfur in an oxidation state of +2. <a href="/facts/Sulfur_monoxide/CI8PbSJX">Sulfur monoxide</a> (SO) can be considered as a theoretical <a href="/facts/Inorganic_anhydride/2h1jor4I">anhydride</a> for sulfoxylic acid, but it is not actually known to react with water.
The complementary base is the sulfoxylate anion SO2−2 which is much more stable. In between these states is the HSO−2 ion, also somewhat stable. 
Sulfoxylate ions can be made by decomposing <a href="/facts/Thiourea_dioxide/uwoALBuC">thiourea dioxide</a> in an alkaline solution. To do this, thiourea dioxide first forms an <a href="/facts/Amidine/dhB1LGfz">amidine</a>-<a href="/facts/Sulfinic_acid/Ur6P5qNX">sulfinic acid</a> tautomer, H2NC(=NH)SO2H, which then breaks apart. Sulfoxylate reacts with <a href="/facts/Formaldehyde/Ck8RLfqk">formaldehyde</a> to yield a hydroxymethanesulfinate called <a href="/facts/Rongalite/F4eh3d2V">rongalite</a>:

HSO−2 + H2CO → HOCH2SO−2,
which is an important chemical for dyeing.

Sulfoxylic acid open-in-new

Sulfoxylic acid