What is anion?

A negatively charged ion. Anions are the opposite of CATIONS, which carry positive charges. Important anions are formed when weak acids ionize. Anions, together with their cation counterparts, occur in blood and are called electrolytes. They are required to maintain the appropriate effective concentration of ions and PROTEINS in the
blood. Key anions in blood are chloride (Cl–), phosphate (H2PO4–), and bicarbonate (HCO3
–).
Chloride (Cl–) is the predominant anion in body fluids. Neither chloride nor phosphate can be made by the body; they are essential nutrients to be supplied by the diet. Phosphate and bicarbonate ions help buffer blood at nearly a constant pH. These anions are examples of “conjugate bases,” formed when weak acids ionize.