The crude material is treated with hot EtOH to remove soluble impurities, then dissolve in 20% aqueous MeOH and chromatographed on a cellulose powder column with propanol/EtOH/water (5:5:4) as eluent. The upper of three zones was eluted to give the pure dye which is precipitated as the monosodium salt trihydrate by adding conc HCl to the concentrated eluate [Ito & Ueno Analyst 95 583 1970]. It can be converted to the trisodium salt wih the calculated amount of alkali. Indigocarmine (2[1,3-dihydro-3-oxo-5-sulfo-2H -indol-2-ylidene]-2,3-dihydro-3-oxo-1H -
