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(57) [Abstract] [Purpose] To provide a method for quantifying sodium ions that is accurate, convenient, stable, and has a wide measurement concentration range of sodium ions. [Structure] In the method for quantifying sodium ion in a sample using β-galactosidase, 1,2-cyclohexanediamine-N, N, N ′, N′-tetraacetic acid, ethylenediaminetetraacetic acid, triethylenetetraminehexaacetic acid, Diethylenetriamine-N, N, N ', N ", N" -pentaacetic acid, 1,3-diaminopropan-2-ol-N, N, N ', N'-tetraacetic acid, ethylenediamine-N, N' Conducting the β-galactosidase reaction in the presence of at least one chelating agent selected from the group consisting of dipropionic acid dihydrochloride, ethylenediamine tetrakis (methylene sulfonic acid), iminodiacetic acid, hydroxyiminodiacetic acid and nitrilotriacetic acid. A method characterized by. |