To test for proteins a BCA assay can be used. This test is performed with a stock BCA solution which contains Bicinchoninic acid, salts of sodium and Cupric sulfate pentahydrate in a solution with a high alkalinity.
During this test, when the BCA solution is added to proteins the peptide bonds in the proteins reduces the Cu 2+ ions to Cu 1+ ions. This reduction is dependent on the amount of protein in the solution. Following this, two molecules of bicinchoninic acid chelate with each of the Cu1+ ions, and form a purple-colored product which strongly absorbs light at a wavelength of 562 nm.
Looking at the absorption spectra after the chelation and comparing it with solutions with a known protein concentration allows us to determine the protein concentration in the solution being tested.