Dissolve 10g of the dye in 100mL of hot water. Sodium dithionite (0.4g) is then added, followed by active carbon (1.5g) and is filtered hot. To the colourless or slightly yellow filtrate a solution of saturated NaCl is added, and the mixture is cooled. The needles are filtered off, washed with cold water, dried at room temperature, and stored in a dark bottle (light sensitive). [Anderson & Steedly J Am Chem Soc 76 5144 1954, Erdey Chem Analyst 48 106 1959, Beilstein 16 H 602, 16 I 371, 16 II 307, 16 III 575. ]
