thiosulfuric acid

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun An acid, H₂S₂O₃, formed by replacement of an oxygen atom by a sulfur atom in sulfuric acid, found only in solution or as its salts and esters.

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• noun inorganic chemistry An unstable acid, H2S2O3, formally derived from sulfuric acid by the substitution of a sulfur atom for an oxygen one; it is known only in solution or as its salts and esters, the thiosulfates.