The biuret reagent contains CuSO₄. The Cu²⁺ ion is a Lewis acid.
The biuret reaction is a test for the presence of peptide bonds.
The Cu²⁺ ion forms violet-colored coordination complexes with peptides in an alkaline solution.
The biuret reagent is made of NaOH and CuSO₄·5H₂O.
Potassium sodium tartrate is added to stabilize the Cu²⁺ ions and prevent their precipitation as Cu(OH)₂.
The Cu²⁺ ions are Lewis acids. They react with the lone pairs on the peptide N atoms and form a purple tetradentate chelate.