Acid test (gold) mark is tested by applying aqua fortis (nitric acid), which dissolves the mark of any item that is not gold. If the mark remains, it is tested by applying aqua regia (nitric acid and hydrochloric acid). If the mark is removed, then this test dissolves the gold, proving the item to be genuine gold. More accurate testing of the item for its fineness or purity can be done through the use of differing strengths of aqua regia and comparative testing of gold items of known fineness. The figurative meaning of the expression, where it is applied to tests of character,
Thiosulfurous acid They are immiscible in water, but dissolve in benzene or carbon tetrachloride. These species are less rigid than the thiosulfite esters. Thiosulfurous acid Thiosulfurous acid (HS−S(=O)−OH) is a low oxidation state (+1) sulfur acid. It is the equivalent acid for disulfur monoxide. Thiosulfurous acid is formed by reacting hydrogen sulfide and sulfur dioxide at room temperature. The thiosulfurous acid molecule is unstable when condensed, reacting with itself. It decomposes to polysulfane oxide or polythionic acids in water which is termed Wakenroder's liquid. Other possible isomers are dihydroxydisulfane or hypodithionous acid HOSSOH, a linear chain, and thiothionyl hydroxide (S=S(OH)) a tautomer