Question

How can urea be detected by the Biuret test?

Answer 1

Biuret test is a test performed to detect the presence of peptide bonds in the compounds. In the presence of peptide bonds, a copper (II) ion forms colored coordination complexes in the alkaline solution. In the case of urea, the amino acids are being analyzed and thus, the protein structure and the peptide bonds associated with it are determined.

Complete step by step answer:
In the Biuret test, an aqueous sample is treated with an equal volume of 1% strong base (sodium or potassium hydroxide) followed by a few drops of aqueous copper(II) sulfate. If the solution turns purple, protein is present in the sample. Peptides with the presence of at least 3 amino acids are necessary for a significant, measurable color shift with these reagents. These tests can be measured using Beer-Lambert’s law as there is a colored solution and the maximum wavelength will be around $540nm$ .
Note:
The Biuret reagent is made up of sodium hydroxide ($NaOH$ ) and hydrated copper(II) sulfate ($CuS{O_4}.5{H_2}O$ ), together with potassium sodium tartrate, the latter of which is added for the chelation and thus, to stabilize the cupric ions in the coordinated complex formed. The reaction of the cupric ions with the nitrogen atoms involved in peptide bonds leads to the movement of the peptide hydrogen atoms under the alkaline conditions due to the presence of strong sodium hydroxide. A tri- or tetra-dentate chelated complex with the peptide nitrogen produces the characteristic color. This is found with dipeptides. The reagent is commonly used in the biuret protein test, which is a colorimetric test used to determine the concentration of protein by UV/VIS spectroscopy at a wavelength of $540\;nm$ .