The present invention provides a production method of a medium composition, including previously removing divalent metal cations contained in a polymer compound having an anionic functional group, preparing a dry powder of the polymer compound, and mixing the dry powder and the medium, and the like. The present invention also provides a purification method of a polymer compound having an anionic functional group, which includes mixing a suspension of the polymer compound having an anionic functional group in water and a cation exchanger that exchanges a divalent metal cation with a monovalent metal cation to give a polymer compound having an anionic functional group, which shows a reduced amount of contaminating divalent metal cations.
