Tolonium chloride

Tolonium chloride (INN, also known as toluidine blue O) is a compound used in histology.

Test for lignin
Toluidine blue solution is used in testing for lignin, a complex organic molecule that bonds to cellulose fibres and strengthens and hardens the cell walls in plants. A positive toluidine blue test causes the solution to turn from blue to blue-green. A similar test can be performed with phloroglucinol-HCl solution, which turns red.

Toluidine Blue is often used to identify Mast cells. It is also used to stain proteoglycans and glycosaminoglycans in tissues such as cartilage.