Hydrochloric Acid → m-(2-Carboxy-4-amino-phenylazo)benzenephosphonic acid

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Into water (500 ml), m-aminobenzenephosphonic acid (87.0 g) was suspended with stirring. To the mixture, 35% hydrochloric acid (101.3 g) was added, and the mixture was cooled to 0° C. While the solution was maintained at a temperature below 5° C., 36% sodium nitrite (105.4 g) was added dropwise to the solution. The mixture was stirred for 50 minutes to diazotize m-aminobenzenephosphonic acid. Thiourea was then added to the resulting aqueous solution, and the unreacted nitrite was removed to give a diazotized compound containing solution. On the other hand, 3-aminobenzoic acid (58.6 g) was dispersed into methanol (800 ml) and heated to 55° C. to form a solution containing a coupling compound, which was then cooled to 0° C. The diazotized compound containing solution prepared above was added dropwise while the mixture was maintained at a temperature below 5° C. After stirring the mixture at 3°-5° C. for 18 hours, the resulting precipitates were collected by filtration, washed with water to give a monoazo compound, m-(2-carboxy-4-amino-phenylazo)benzenephosphonic acid in a yield of about 125.3 g.

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