Phenylhydrazine → Sodium 2-phenylhydrazonobenzenesulfonate

104 g of phenylhydrazine was dissolved in 800 ml of methanol, and a solution obtained by dissolving 56 g of sodium o-formylbenzenesulfonate in 200 ml of methanol was dropwise added thereto at room temperature. The resulting mixture was heated for 2 hours under reflux with stirring. After cooling, the crystal formed was separated by filtration and then washed with methanol to obtain 69 g of sodium 2-phenylhydrazonobenzenesulfonate (Intermediate A). 50 g of Intermediate A thus obtained was dissolved in 250 ml of water and then hydrogenated in an autoclave by adding palladium-carbon catalyst thereto. After the catalyst was removed by filtration, 250 ml of isopropanol was added to the hydrogenated product whereupon a colorless product precipitated out. This product was separated out by filtration and washed with isopropanol and then dried to obtain 43 g of sodium 2-phenylhydrazinobenzenesulfonate (Intermediate B). Next, 40 g of Intermediate B, 19 g of diethyl malonate, 25 g of sodium methylate (28% methanol solution) and 100 ml of n-butanol were blended and heated for 10 hours under reflux. N-butanol was concentrated and 200 ml of water was added to the residue. Next, the aqueous layer was made acidic with concentrated hydrochloric acid, whereby a crystal precipitated out. This was separated out by filtration and washed with a small amount of methanol to give 39 g of Intermediate C (1-phenyl-2-(2-sulfobenzyl)- 3,5-pyrazolidinedione sodium salt).