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Tollens reagent A reagent used in testing for aldehydes, named after German chemist B. C. G. Tollens (1841–1918). It is made by adding sodium hydroxide to silver nitrate to give silver(I) oxide, which is dissolved in aqueous ammonia (giving the complex ion [Ag(NH3)2]+). The sample is warmed with the reagent in a test tube. Aldehydes reduce the complex Ag+ ion to metallic silver, forming a bright silver mirror on the inside of the tube (hence the name silver-mirror test). Ketones give a negative result.