Bial's test is a chemical test for the presence of pentoses. It is named after Manfred Bial, German Physician. The components include orcinol, hydrochloric acid, and ferric chloride. A pentose, if present, will be dehydrated to form furfural which then reacts with the orcinol to generate a colored substance. The solution will turn bluish and a precipitate may form. The solution shows two absorption bands, one in the red between Fraunhofer lines B and C and the other near the D line. An estimate of the relevant wavelengths can be made by referring to the Fraunhofer lines article.
Bial's reagent consists of 0.4g orcinol, 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 related hydroxymethylfurfural from hexoses may give a muddy-brown or gray solution, but this is easily distinguishable from the green color of pentoses.
The test may be performed as a quantitative colorimetric test using a spectrophotometer. Fernell and King have published a procedure for simultaneous determination of pentoses and hexoses from measurements at two wavelengths  Various versions of this test are widely used for a quick chemical determination of RNA; in this context it is usually called the orcinol test.
- Baldwin, E. and Bell, D.J., Cole's Practical Physiological Chemistry, published by Heffer, Cambridge, 1955, page 189
- Baldwin and Bell, page 189
- W. R. Fernell and H. K. King, The simultaneous determination of pentose and hexose in mixtures of sugars. Analyst, 1953,78, 80-83
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