Seliwanoff’s test for aldo and keto sugar

Seliwanoff’s test is used to differentiate between keto sugar and aldo sugar.

PRINCIPLE:

Carbohydrates are converted to furfural derivatives by dilute HCl. Only furfural derivatives of keto-hexoses condense with Resorcinol to form a cherry red complex. This test is positive for sugar (Keto) when it gives cherry red colour complex.

REAGENTS:

1. Seliwanoff’s Reagent: i. Resorcinol, and ii. Concentrated HCl. 2. Original solution (0.8.) containing a carbohydrate

PROCEDURE:

• Take 3ml of seliwanoff’s reagent in a test tube.
• Add 5ml of original solution.
• Heat the test tube just to boil.
• Avoid prolonged heating otherwise; it gives a false test.

INDICATION:

The production of a Red colour indicates a positive reaction.

RESULT:

• No change in colour = Aldo sugar
• If cherry red colour appears = Keto sugar is present.

NOTE:

Avoid prolonged heating. Prolong boiling with aldoses is bound to give a false positive result, as the aldoses also start giving a similar reaction.

Precautions:

• Wash the apparatus before and after the experiment.
• Avoid prolonged heating as it will then gives a false result.