Alcian blue is a water-soluble copper phthalocyanine and is a cationic stain that binds to the polyanions of acidic mucopolysaccharides through saline bridges, allowing selective staining of acidic and non-neutral mucins. The specificity of the reaction varies depending on the pH (0.5 to 2.5); a low pH is used to selectively identify acid sulphurised mucins, while a higher pH is used to detect carboxylated mucins in connective tissue cartilage and also weakly sulphurised mucins, hyaluronic acid, sialomucins.