Cation-π interactions in aqueous media are known to play critical roles in various biological activities. However, quantitative experimental information, such as the binding ratio of metal ions to aromatic groups, is hardly available due to the lack of a suitable test system and method. Herein, we proposed a hydrogel Donnan potential method to determine the binding ratio of metal ions to aromatic groups on polymer networks in aqueous media. In this method, we adopted recently developed poly(cation-π) hydrogels with a rich adjacent sequence of the cationic group and the aromatic group on the polymer network. A microelectrode technique (MET) is used to measure the Donnan potential of the poly(cation-π) hydrogels. From the Donnan potential, the binding ratios of various metal ions to aromatic groups are quantitatively determined for the first time.