Bial's test

A pentose, if present, will be dehydrated to form furfural which then reacts with orcinol to generate a colored substance.

[1] An estimate of the relevant wavelengths can be made by referring to the Fraunhofer lines article.

[2] 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.

The related hydroxymethylfurfural from hexoses may give a muddy-brown, yellow or gray solution, but this is easily distinguishable from the green color of pentoses.

Fernell and King published a procedure for simultaneous determination of pentoses and hexoses from measurements at two wavelengths.