Acrylic Acid → Ammonium acrylate acrylic acid

To a 300 ml separable flask, were added 171.13 g of a 40% by weight aqueous ammonium acrylate solution from Production Example 1, 18.53 g of acrylic acid, and 35.34 g of water to obtain a 38% by weight aqueous ammonium acrylate/acrylic acid=75/25 solution, which was then mixed with 7.8 g of activated carbon and stirred for one hour while shielding the light. The resulting solution was filtered to remove the activated carbon, followed by adding and dissolving 0.0968 g of trimethylolpropane triacrylate as a crosslinking agent. To a 300 ml separable flask, was transferred 90 g of the solution, which was then purged with nitrogen at 30° C. for 30 minutes under stirring. To the resulting solution, was added 0.018 g of sodium hydrogen sulfite and then 0.0276 g ammonium persulfate. Polymerization was performed for one hour to obtain a polymer, which was then crushed to about 5 mm and dried at 150° C. for one hour in an inert oven with a nitrogen atmosphere. The dried polymer was milled by a homogenizer and then classified into particles having a particle size of 106 to 850 μm. The classified polymer in an amount of 100 parts was mixed with 2 parts of propylene glycol, 3 parts of water, and 30 parts of isopropyl alcohol, followed by heat treatment at 150° C. for 60 minutes. The resin obtained as described above was named as a comparative water-absorbing resin (1).