Editorial use only . 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 blue-coloured Biuret reagent was added to a solution of albumin (egg white) in a beaker. The solution turned violet (deep purple), indicating presence of proteins.