Egg albumen (egg white) is a good source of protein. Some eggs contain up to 10.0% w/v (100gdm-³) protein. A biuret assay can be used to determine the concentration of protein in egg albumen. • Biuret reagent is a mixture of copper(II) sulfate solution and sodium hydroxide solution. • In a biuret assay, biuret reagent is added to a solution. • If protein is present, copper(II) ions (Cu2+) in the biuret reagent bind with the nitrogen atoms that form peptide bonds in the protein. • This binding causes the reagent to change from a clear, pale blue to a purple colour. • Different concentrations of protein result in different intensities of purple colour. A colorimeter can be used to measure differences in the intensity of the purple colour in samples tested. Fig. 1.1 shows a colorimeter that measures the absorbance of light when it passes through different coloured solutions. [Figure 1.1]