Denigés' reagent An acidic solution of mercury sulfate is known as Denigés' reagent. It was commonly used throughout the 20th century as a qualitative analysis reagent. If Denigés' reagent is added to a solution containing compounds that have tertiary alcohols, a yellow or red precipitate will form.[10] Hydration reactions Mercury compounds such as mercury sulfate and mercury(II) acetate are commonly used as catalysts in oxymercuration-demercuration, a type of electrophilic addition reaction which results in hydration of an unsaturated compound. The hydration of an alkene results in an alcohol that follows the regioselectivity predicted by Markovnikov's rule. For an alkyne, the result is an enol, which tautomerizes to give a ketone.[11] An example is the conversion of 2, 5-dimethylhexyne-2, 5-diol to 2, 2, 5, 5-tetramethyltetrahydrofuran using aqueous mercury sulfate without the addition of