Volhard
Volhard refers primarily to two distinct concepts in chemistry, both associated with the German chemist Jacob Volhard (1834-1910):
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Volhard Titration: This is a titrimetric method used to determine the amount of halide ions (such as chloride, bromide, and iodide) or silver ions in a solution. It is a type of back titration, meaning an excess of a standard solution is added, and then the excess is titrated with another standard solution. In the classic Volhard method, a known excess of silver nitrate solution is added to the sample containing halide ions, precipitating the halide as a silver halide. The remaining, unreacted silver ions are then titrated with a standard solution of thiocyanate ions, typically potassium thiocyanate (KSCN). Ferric ions (Fe3+) serve as an indicator; at the endpoint, the thiocyanate reacts with the ferric ions to form a colored complex (typically red), indicating that all of the silver ions have reacted. The Volhard titration is particularly useful when the precipitate formed (e.g., silver chloride) is less soluble than silver thiocyanate, which prevents the precipitate from redissolving during the back titration. It can also be used for indirect determination of other ions that can be precipitated with silver.
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Volhard-Erdmann Cyclization: This is a chemical reaction involving the intramolecular cyclization of a substituted aryl or alkyl amine with cyanogen bromide to form an aminotriazole. This method is employed in the synthesis of various heterocyclic compounds.