Bial's reagent
Bial's reagent is a reagent used to test for pentoses, named after German physician Manfred Bial (1870–1908).[1] It is a solution of 3,5-dihydroxytoluene (orcinol) in concentrated hydrochloric acid with a catalytic amount of iron(III) chloride.
Reagent
The reagent is prepared by dissolving 2 g of 3,5-dihydroxytoluene (orcinol) in concentrated hydrochloric acid with 1.5 cm3 of 370 mmol dm−3 iron(III) chloride solution,[note 1] and diluting to 1 litre with concentrated hydrochloric acid.[2][note 2] The final solution is 16 mmol dm−3 in 3,5-dihydroxytoluene and 550 µmol dm−3 in iron(III). The reagent should be stored in dark glass bottles away from direct light.
Test
A few drops of the solution to be tested are added to about 1 cm3 of the reagent in a test tube. The mixture is heated to 100 °C and kept at that temperature for three to five minutes, then allowed to cool. The appearance of a green-blue colouration indicates the presence of pentoses (including nucleosides), while hexoses can give a greenish-yellow colouration.
Notes and references
Notes
- ↑ The iron(III) chloride solution is prepared by dissolving 100 g of iron(III) chloride hexahydrate and 20 cm3 of concentrated hydrochloric acid in water and diluting to one litre.
- ↑ An alternative preparation dissoves the 3,5-dihydroxytoluene in 500 cm3 of concentrated hydrochloric acid with the iron(III) chloride solution, then diluted to one litre with water: the final acid concentration is approximately 6 mol dm−3. This less acidic reagent is said to be more stable to storage than the reagent made up entirely with concentrated hydrochloric acid, and to give fewer fumes of hydrogen chloride during the test: the reaction is, however, slightly slower and the colour slightly less pronounced.
References
- ↑ Attwood, Teresa K.; Cammack, Richard Oxford dictionary of biochemistry and molecular biology, 2nd ed.; Oxford University Press, 2006; p 72. ISBN 0198529171, <http://books.google.es/books?id=XpUjsqD7lFUC>.
- ↑ CRC Handbook of Chemistry and Physics, 62nd ed.; Weast, Robert C., Ed.; CRC Press: Boca Raton, FL, 1981; p D-111. ISBN 0-8493-0462-8.
External links
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