Bial's Test for Pentoses 2.0

[@hiramzen]

good day professor, why is the boiling necessary for the reduction of sugars to take place, what causes the color change in the tubes, and what are the reactions called in this test? i am currently researching for our chem project and there are some things we havent understood properly due to our own professor's poor teaching so i apologise for the lack of knowledge

[@ProfessorDrewCollop]

I am sure there must be ample resources to explain the reaction if you were to Google it. In short, the heat usually needed to open up the carbon ring in the sugars. In Bial's test, the sugar is first dehydrated (loses water molecules) to create a new compound. Pentoses create furfural, while hexoses create hydroxyfurfural. These products will react with Bial's reagent, turns it from a yellow solution to a coloured solution. Furfural will turn blue-green. Hydroxyfurfural will turn brown. The chemicals in Bial's reagent include orcinol, ferric chloride and hydrochloric acid.

[@lalalayeyeyeye3576]

i've found an article that uses this test to do a colorimetric assay. I did it once and 5 minutes of heating, the color still gets darker and darker. is there any exact time for the reaction to stop? I'm afraid i stopped halfway through and the assay won't be accurate

[@ProfessorDrewCollop]

Thanks for the question Lalala. Heating for too long can result in the production of monosaccharides from the disaccharides and polysaccharides. This is caused by the breaking of the Glycosidic bond and may result in an unexpected positive result for Bial's test. I cannot tell you the exact optimal time for heating, as there are many variables involved. I would recommend attempting a practice run with some positive controls to determine what you optimal time should be. That is what I did. In the video I included my results from heating the sample at both 3 min and 1 min. I hope this help you achieve collection of high quality data.