Bial's test is a chemical test for the presence of

pentose In chemistry, a pentose is a monosaccharide (simple sugar) with five carbon atoms. The chemical formula of many pentoses is , and their molecular weight is 150.13 g/mol.Manfred Bial, a German physician. The components include

orcinol Orcinol is an organic compound with the formula CH3C6H3(OH)2. It occurs in many species of lichens including ''Roccella tinctoria'' and ''Lecanora''. Orcinol has been detected in the "toxic glue" of the ant species '' Camponotus saundersi''. It i ...

hydrochloric acid Hydrochloric acid, also known as muriatic acid, is an aqueous solution of hydrogen chloride. It is a colorless solution with a distinctive pungent smell. It is classified as a strong acid. It is a component of the gastric acid in the dig ...

, and

ferric chloride Iron(III) chloride is the inorganic compound with the formula . Also called ferric chloride, it is a common compound of iron in the +3 oxidation state. The anhydrous compound is a crystalline solid with a melting point of 307.6 °C. The colo ...

. A pentose, if present, will be dehydrated to form

furfural Furfural is an organic compound with the formula C4H3OCHO. It is a colorless liquid, although commercial samples are often brown. It has an aldehyde group attached to the 2-position of furan. It is a product of the dehydration of sugars, as occur ...

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 A spectral line is a dark or bright line in an otherwise uniform and continuous spectrum, resulting from emission or absorption of light in a narrow frequency range, compared with the nearby frequencies. Spectral lines are often used to ident ...

, one in the red between Fraunhofer lines B and C and the other near the D line. An estimate of the relevant

wavelength In physics, the wavelength is the spatial period of a periodic wave—the distance over which the wave's shape repeats. It is the distance between consecutive corresponding points of the same phase on the wave, such as two adjacent crests, tr ...

s can be made by referring to the

Fraunhofer lines In physics and optics, the Fraunhofer lines are a set of spectral absorption lines named after the German physicist Joseph von Fraunhofer (1787–1826). The lines were originally observed as dark features ( absorption lines) in the optical spectr ...

article.

Composition

Bial's reagent consists of 0.4 g

, 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 Hydroxymethylfurfural (HMF), also 5-(hydroxymethyl)furfural, is an organic compound formed by the dehydration of reducing sugars. It is a white low-melting solid (although commercial samples are often yellow) which is highly soluble in both water ...

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. Fernell and King 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 Ribonucleic acid (RNA) is a polymeric molecule essential in various biological roles in coding, decoding, regulation and expression of genes. RNA and deoxyribonucleic acid ( DNA) are nucleic acids. Along with lipids, proteins, and carbohydra ...

; in this context it is usually called the orcinol test.

References

Chemical tests {{analytical-chemistry-stub