Qualitative Analysis of Sugars

This exercise introduces students to the use of three classic qualitative tests for reducing sugars, ketoses, and pentoses. First with known sugars, and then with acid hydrolysates of naturally-occurring polysaccharide sources, they examine the color changes that are characteristic of these tests and use them to identify the types of monosaccharides found in each hydrolysate.

Known sugars are provided as 1% solutions in water: fructose, glucose, lactose, ribose, sorbitol, and sucrose.

Hydrolysates are prepared by adding 0.25 g of a polysaccharide source (sawdust, dandelion root, cornmeal, and gum arabic) with 5 mL of 1 M HCl in a screw-capped plastic tube. The tubes are heated in an oven at 80 degrees Celsius for two days, then refrigerated until the next lab period.  Students neutralize the extracts with 1 mL of 20% NaOH, then filter using a fast paper (Whatman 4 or Fisher P8). Gum arabic and corn meal extracts can be diluted with 8 mL of water, but the sawdust and dandelion root extracts probably shouldn't. Class extracts are combined in four flasks for common use.

Benedict's reagent, Seliwanoff's reagent, and Bial's reagent identify reducing sugars, ketoses, and pentoses, respectively. To 1 mL of each reagent, 4 drops (Benedict's and Seliwanoff's) or 2 drops (Bial's) of known or unknown sugar are added. The resulting solutions are heated in a boiling water bath for 2-5 minutes and results are recorded.  From the results for known sugars, students are asked to construct a table describing both positive and negative results for each test.  They are then to use those descriptions to identify the types of sugars present in each extract.

 

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Revised 10/24/06