Question

If a sample of 10% solution of an unknown carboyhydrate yielded positive results for Seliwanoff's test, which carbohydrate is the unknown? If more than one carbohydrate is possibly the unknown, which other test(s) can be used to help identify the unknown?

Answer 1

Seliwanoff’s test is a chemical test which differentiates aldose and ketose sugars.

Seliwanoff s reagent is 0.5% resorcinol in concentrated HCl diluted 1:1 with water. A 3ml of abve reagent is added t the 10% solution.

Ketose sugar like fructose will bring about cherry red colour, while the aldose sugars like glucose give a negative result with no red shading and upon boiling aldose sugars will give a pink colour.

While the sucrose (a blend of fructose and glucose) will produce a cherry red shading due to presence of fructose.

The other tests to identify the unknown a) Benedict’s solution (Anhydrous sodium carbonate-100 gm
Sodium citrate – 173 gm and Copper(II) sulfate pentahydrate -17.3 gm r 1 lt solution) is a deep-blue alkaline solution used to test for the presence of the aldehyde functional group, – CHO (reducing sugars).

The Benedict's solution is added to the test sample solution and upon boiling is changed into green which indicates 0.1 to 0.5 percent sugar in solution.

If it changes color to yellow which indicates 0.5 to 1 percent sugar is present.
If it changes to orange which indicates1 to 1.5 percent sugar is present.
If color changes to red which indicates 1.5 to 2.0 percent sugar is present.
And if color changes to brick red which indicates more than 2 percent sugar is present in solution.

Formation of a reddish precipitate within three minutes indicate glucose (reducing sugar) and negative test indicate reducing sugars are absent. Example: Sucrose.

Other test Bial's test : To 2 mL 0f Bial’s reagent (1.5 g of orcinol in 100 mL of conc. HCl and 20-30 drops of 10% Ferric Chloride solution ) add 4-5 drops of test solution.

Upon boiling formation of bluish green solution and precipitate indicates the presence of a pentose sugar whereas hexoses generally react to form green, red, or brown products