To a solution of 27.0 g of 8-amino-1,3,6-naphthalenetrisulfonic acid trisodium salt in 100 ml of water and 60 ml of N sodium hydroxide is added 22.5 g of p-nitrobenzoyl chloride and 30 ml of diethyl ether. The reaction mixture is shaken in a separatory funnel for about 15 minutes, and another 60 ml portion of base is added. After shaking for 15 minutes the procedure is repeated with two additional 60 ml portions of base. The reaction mixture is acidified with 7.5 ml of concentrated hydrochloric acid and diluted with 250 ml of water, then extracted 9 times with 300 ml portions of ether. The aqueous phase is neutralized with 5N sodium hydroxide and allowed to stand overnight at room temperature, then is filtered through diatomaceous earth. The filtrate is concentrated to a low volume in vacuo at 55°-60° C. The crystalline solid formed is collected by filtration, washed and slurried with absolute ethyl alcohol to yield 28.6 g of 8-(p-nitrobenzamido)-1,3,6-naphthalenetrisulfonic acid trisodium salt.
