Basis of biochemical tests; Biuret test, Benedict test, Iodine/potassium iodide test, Emulsion test

Biuret test
A test for peptide bonds to identify proteins.

In the presence of dil.copper sulphate in alkaline solution, nitrogen atoms in the peptide chain form a purple complex with copper(ll) ions. Biuret is a compound derived from urea which also contains the –CONH- group and  gives a positive result.

Benedict test

A test for sugar

Reducing sugars- Benedict’s solution contains copper sulphate. Reducing sugars reduce soluble blue copper sulphate. Here the containing copper(ll) ions will change to insoluble red- brown copper oxide containing copper(l). This is seen as the precipitate.

Non-reducing sugars- The disaccharide will hydrolysed to its monosaccharide elements by boiling with dil. HCl. Such as Sucrose will be hydrolysed to glucose and fructose which are both reducing sugars. That will give the reducing sugar results with the Benedict’s test.

Iodine/potassium iodide test

A test for starch.

What is given at last is a poly-iodide complex formed with starch.

Emulsion test

A test for Lipids

Lipids are iinsoluble with water. Adding water to a solution of the lipid in alcohol results in an emulsion of lipid droplets or micelles in the water. These reflect light and give a white, opalescent appearance.