Ponceau S belongs to a group of azo dyes that can stain proteins in red/pink color. Chemically it is a tetrasodium salt of diazo dye. It is available commercially in the form of crystalline dark brown color power (as seen by the naked eye at room temperature). It can also be purchased as a ready-to-use solution for staining proteins on nitrocellulose and PVDF membranes.
It is often used to evaluate the transfer efficiency of western blotting. It reversibly stains the protein on the nitrocellulose and PVDF membranes. It can detect as little as 250 ng protein/band on the nitrocellulose membrane. It can also be used to detect protein on cellulose acetate membranes. However, Ponsue S staining is not suitable for nylon membranes due to difficulties in destaining the negatively charged nylon membrane from positively charged Ponceau S stain.