Polyacrylic acid (salt)-based water absorbent resin and method for producing same

A method for producing a polyacrylic acid (salt)-based water absorbent resin, sequentially comprising the steps of:

• storing or producing acrylic acid;
• mixing acrylic acid containing a polymerization inhibitor with water, a crosslinking agent, and a basic composition, and/or neutralizing them, to prepare an aqueous monomer solution;
• polymerizing the aqueous monomer solution by use of a polymerization initiator and/or irradiation with active energy rays;
• drying a water-containing gel-like crosslinked polymer thus obtained;
• and
• optionally carrying out surface crosslinking,
• wherein the acrylic acid containing the polymerization inhibitor has a water content of less than 80 ppm (by mass, the same applies hereinafter), and the basic composition has an iron content of 0.0007 to 7 ppm, and optionally the aqueous monomer solution has a formic acid content of 1 to 700 ppm, relative to the monomer, and/or the following conditions (2) or (3) are satisfied:
• (2) 0.01 to 5 parts by mass of a cationic polymer is mixed in the polymer, relative to 100 parts by mass of the polymer, after the drying step;
• and
• (3) 0.0001 to 5 parts by mass of formic acid is allowed to be present and/or mixed in the monomer at the time of polymerization and/or the polymer after the polymerization, relative to 100 parts by mass of the monomer at the time of polymerization and/or the polymer after the polymerization, and wherein the monomer at the time of polymerization has a formic acid content of 0 to 700 ppm, relative to the monomer.