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Fehling's reagent[′fāl·iŋz rē‚ā·jənt]
(also Fehling solution, Fehling’s solution), a mixture of equal volumes of a 7-percent solution of copper sulfate and a 34.6-percent solution of potassium sodium tartrate in a 10-percent solution of sodium hydroxide. Fehling’s reagent is used for the qualitative and quantitative determination of aliphatic aldehydes and, especially, monosaccharides. The action of the reagent is based on the reduction of a divalent copper ion to a monovalent ion as a result of the oxidation of the aldehyde (that is, carbonyl) groups of the compounds under study. An orange-yellow precipitate of CuOH is formed at low temperatures; upon heating, a red precipitate of Cu2O is formed. The reagent was proposed in 1850 by the German chemist H. Fehling.