The reagents consist of
resorcinol and concentrated
hydrochloric acid. Generally, 6M HCl is used to run this test.
Ketoses are dehydrated faster and give stronger colors.
Aldoses react very slowly and give faint colors. The
acid hydrolysis of
polysaccharide and
oligosaccharide ketoses yields simpler sugars such as fructose. In the presence of strong acid, fructose dehydrates to give
hydroxymethylfurfural (HMF). The formyl group of the HMF condenses with two equivalents of
resorcinol to produce a deep cherry red analogue of
fluorescein. Aldoses may react slightly to produce a faint pink color. When the test solution is added to a solution containing ketoses, a red color is formed rapidly indicating a positive test. When added to a solution containing aldoses, a slower forming light pink is observed instead.
Fructose and
sucrose are two common sugars which give a positive test. Sucrose gives a positive test as it is a disaccharide consisting of fructose and glucose. ==Further reading==