Biuret test for proteins. The Biuret reagent contains copper(II) sulfate (CuSO4) and sodium hydroxide (NaOH), with potassium sodium tartrate (KNaC4H4O6) added to stabilize copper ions. Here equal amounts of blue-colored Biuret reagent was added to distilled water (left beaker, negative control) and to a solution of albumin (egg white, right beaker). The solution in the left beaker did not change color from blue, indicating no proteins. The solution in the right beaker turned violet (deep purple), indicating presence of proteins.