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.[pentosuria
Pentosuria, a component of Garrod’s tetrad, is a condition where the sugar L-xylulose, a pentose, presents in the urine in unusually high concentrations. It was characterized as an inborn error of carbohydrate metabolism in 1908. It is associa ...]
. It is named after
Manfred Bial, a German physician. The components include
orcinol,
hydrochloric acid
Hydrochloric acid, also known as muriatic acid or spirits of salt, is an aqueous solution of hydrogen chloride (HCl). It is a colorless solution with a distinctive pungency, pungent smell. It is classified as a acid strength, strong acid. It is ...
, and
ferric chloride
Iron(III) chloride describes the inorganic compounds with the formula (H2O)x. Also called ferric chloride, these compounds are some of the most important and commonplace compounds of iron. They are available both in anhydrous and in hydrated f ...
. 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 occu ...
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, 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 and mathematics, wavelength or spatial period of a wave or periodic function is the distance over which the wave's shape repeats.
In other words, it is the distance between consecutive corresponding points of the same ''phase (waves ...
s can be made by referring to the
Fraunhofer lines
The Fraunhofer lines are a set of spectral absorption lines. They are dark absorption lines, seen in the optical spectrum of the Sun, and are formed when atoms in the solar atmosphere absorb light being emitted by the solar photosphere. The l ...
article.
Composition
Bial's reagent consists of 0.4 g
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
Hydroxymethylfurfural (HMF), also known as 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 b ...
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 that is essential for most biological functions, either by performing the function itself (non-coding RNA) or by forming a template for the production of proteins (messenger RNA). RNA and deoxyrib ...
; in this context it is usually called the orcinol test.
See also
*
Dische test
References
External links
original description of Bial's test in 1903
Carbohydrate methods
Chemical tests
{{analytical-chemistry-stub