Shows positive test for:
Pentoses

Bial's test is a chemical test for the presence of pentoses. The components include orcinol, hydrochloric acid, and ferric chloride. In the presence of a pentose, the pentose will be dehydrated to form furfural. The solution will turn bluish and a precipitate may form. The solution shows two absorption bands, one in the red between B and C and the other near the D line . It is named after Manfred Bial, German Physician

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Reactions:
The test reagent dehydrates pentoses to form furfural. Furfural further reacts with orcinol and the iron ion present in the test reagent to produce a bluish product (reaction not shown).

Composition

Bial's reagent consists of 0.4g orcin, 200 ml of concentrated hydrochloric acid and 0.5 ml of a 10% solution of ferric chloride . Bial's test is used to distinguish pentoses from hexoses; this distinction is based on the color that develops in the presence of orcinol and iron (III) chloride. Furfural from pentoses gives a blue or green color. The furfural from hexoses may give a muddy-brown or gray solution, but this is easily distinguishable from the green color of pentoses.


How to perform the test:
Two ml of a sample solution is placed in a test tube. Two ml of Bial's reagent (a solution of orcinol, HCl and ferric chloride) is added. The solution is then heated gently in a Bunsen Burner or hot water bath. If the color is not obvious, more water can be added to the tube.

A positive test is indicated by:
The formation of a bluish product. All other colors indicate a negative result for pentoses. Note that hexoses generally react to form green, red, or brown products.