What Is Bial’s Test?
Bial’s test named after a German physician Manfred Bial, is a chemical test for detection of presence of pentoses. Bial’s test reagent contains concentrated Hydrochloric acid (HCl) as a dehydrating acid, orcinol and traces of ferric chloride as condensation reagent. A pentose if present in a given sample, will be dehydrated to form furfural which then reacts with the orcinol to generate a colored substance. The solution will turn bluish-green and a precipitate may form.
Objective
- To detect the presence of pentoses and pentosans (derivatives of pentoses).
- To distinguish between reducing monosaccharides and disaccharides.
Bial’s Test Principle
Bial’s test is used to differentiate between pentose monosaccharide and hexose monosaccharide. Bial’s test reagent contains concentrated Hydrochloric acid (HCl) as a dehydrating acid, orcinol and traces of ferric chloride as condensation reagent. The test reagent dehydrates pentoses to form furfural and dehydrates hexoses to form 5-hydroxylmethyfurfural. The furfural further reacts with orcinol and the iron ion present in the ferric chloride to produce a bluish or green product, while 5-hydroxylmrthyfurfural yield muddy-brown to grey condensation product. The intensity of the precipitation is directly proportional to the concentration of the pentoses in the sample whereas the intensity of the color that develops depends on the concentration of Hydrochloric acid (HCl).
Experiment
Reagent And Material Required
Reagent
- 5% ribose solution (pentose).
- 5% Glucose solution (hexose).
- 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).
Material Required
- Test tube
- Test tube stand
- Water bath
- Pipette
Bial’s Test Procedure
- In clean dry test tube add 1 ml of 5% ribose solution (pentose).
- In the second test tube add 1 ml of 5% glucose solution (hexose).
- In each test tube add 2.5 ml of Bial’s reagent and mix thoroughly.
- Keep both tubes in boiling water for one minute and allow the tubes to cool down to room temperature.
- Observe the appearance of blue-green color for ribose and brown color for glucose.
Bial’s Test Result Interpretation
- Positive Test: The presence of a bluish-green color indicates the presence of pentoses.
- Negative Test: Absence of bluish-green color indicates absence of pentoses. Given that in our experiment we have used glucose solution, a brown color is observed indicating presence of hexoses.
Uses Of Bial’s Test
- Used to distinguish between monosaccharides and disaccharides.
- It is also used to distinguish pentoses and hexoses.
Limitations
- On prolonged heating, glucoronates might also give a blue-green colored precipitate which might result in false-positive results.
- The color produced might be different with different sugars, and the concentration might not be proportional to the intensity at higher levels.