# Bial's test

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**Bial's test** is a chemical test for the presence of [pentoses](/source/Pentose) originally developed for the diagnosis of [pentosuria](/source/Pentosuria). It is named after [Manfred Bial](/source/Manfred_Bial), a German physician. The components include [orcinol](/source/Orcinol), [hydrochloric acid](/source/Hydrochloric_acid), and [ferric chloride](/source/Ferric_chloride). A pentose, if present, will be dehydrated to form [furfural](/source/Furfural) which then reacts with orcinol to generate a colored substance. The solution will turn bluish and a precipitate may form. The solution shows two [absorption bands](/source/Absorption_bands), one in the red between Fraunhofer lines B and C and the other near the D line.[1] An estimate of the relevant [wavelengths](/source/Wavelength) can be made by referring to the [Fraunhofer lines](/source/Fraunhofer_lines) article.

## Composition

Bial's reagent consists of 0.4 g [orcinol](/source/Orcinol), 200 ml of concentrated hydrochloric acid and 0.5 ml of a 10% solution of ferric chloride.[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. Furfural from pentoses gives a blue or green color. The related [hydroxymethylfurfural](/source/Hydroxymethylfurfural) from hexoses may give a muddy-brown, yellow or gray solution, but this is easily distinguishable from the green color of pentoses.

## Quantitative version

The test may be performed as a quantitative colorimetric test using a [spectrophotometer](/source/Spectrophotometer). Fernell and King published a procedure for simultaneous determination of pentoses and hexoses from measurements at two wavelengths.[3] Various versions of this test are widely used for a quick chemical determination of [RNA](/source/RNA); in this context it is usually called the orcinol test.[4]

## See also

- [Dische test](/source/Dische_test)

## References

1. Baldwin, E. and Bell, D.J., *Cole's Practical Physiological Chemistry*, published by Heffer, Cambridge, 1955, page 189

1. Baldwin and Bell, page 189

1. Fernell, W. R. & King, H. K. (1953). "The simultaneous determination of pentose and hexose in mixtures of sugars". *[Analyst](/source/Analyst_(journal))*. **78** (78): 80–83. [Bibcode:1953Ana....78...80F](https://ui.adsabs.harvard.edu/abs/1953Ana....78...80F). [doi:10.1039/an9537800080](https://doi.org/10.1039/an9537800080)

1. Hanson, R. S. & Phillips, J. A. (1981). "Chemical composition". *Manual of methods for General Bacteriology*. Phillip Gerhardt (ed.). [American Society for Microbiology](/source/American_Society_for_Microbiology). p. 349.

## External links

- [original description of Bial's test in 1903](https://zenodo.org/record/1577291/files/article.pdf)

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