Biuret test for protein
Biuret reagent reacts with peptide bonds, turning from blue to purple. Unlike the Bradford test it will give equally good results with any protein, but it is unable to detect the low concentrations that can be achieved with the Bradford test.
A qualitative test can be performed simply by adding equal quantities of 1% sodium (or potassium) hydroxide and a 1% solution of copper sulphate to the sample. If the solution turns purple it means that protein is present.
The results described here use quantitative Biuret solution which will give a good estimate of protein concentrations in the range 0.1-10 mg/cm3.
Similar results were obtained using either powdered egg white or gelatine as the protein standard