Barfoed's reagent

Barfoed's solution contains cupric ions in an acidic medium. The milder condition allows oxidation of monosaccharides but does not oxidize disaccharides. If the time of heating is carefully controlled, disaccharides do not react while reducing monosaccharides give the positive result (red Cu2O precipitate). Ketoses do not isomerize with this reagent. Carbohydrates are dehydrated in the presence of nonoxidizing acids to form furfural and hydroxymethylfurfural.