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A counterion (pronounced as two words, i.e. "counter" "ion", and sometimes written as two words) is the ion that accompanies an ionic species in order to maintain electric neutrality. In table salt (NaCl), the sodium cation is the counterion for the chlorine anion and vice versa. In a charged transition metal complex, a (i.e. non-coordinated) ion accompanying the complex is termed the counterion. In the older literature, the term ''gegenion'' (from ''gegen'' which means counter in German) is sometimes used. Counterions are generally vaguely defined in biological systems. Depending on their charge, proteins are associated with a variety of smaller anions and cations. In plant cells, the anion malate is often accumulated in the vacuole to decrease water potential and drive cell expansion. To maintain neutrality, K+ ions are often accumulated as the counterion. Ion permeation through hydrophobic cell walls is mediated by ion transport channels. Nucleic acids are anionic, the corresponding cations are often protonated polyamines. ==Interfacial chemistry== Counterions are the mobile ions in ion exchange polymers and colloids.〔http://goldbook.iupac.org/C01371.html〕 Ion exchange resins are polymers with a net negative or positive charge. Typically, cation exchange resins are supplied with sodium as the counterion. Because the resin is insoluble and imobile, the countercations are relatively easily exchanged, e.g., by Ca2+ in the case of water softening. Similarly, anion exchange resins are typically provided in the form of chloride, which is a highly mobile couteranion. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「counterion」の詳細全文を読む スポンサード リンク
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