Bicinchoninic acid () or BCA is a weak acid composed of two carboxylated quinoline rings. It is an organic compound with the formula (C<sub>9</sub>H<sub>5</sub>NCO<sub>2</sub>H)<sub>2</sub>. The molecule consists of a pair of quinoline rings, each bearing a carboxylic acid group. Its sodium salt forms a purple complex with cuprous ions.
Bicinchoninic acid is most commonly employed in the bicinchoninic acid (BCA) assay, which is used to determine the total concentration of protein in a solution. Bicinchoninic acid is used to detect the presence of cuprous ions, due to its purple coloration via a biuret reaction. In this assay, two molecules of bicinchoninic acid chelate a single Cu<sup>+</sup> ion, forming a purple water-soluble complex that strongly absorbs light at 562 nm.