isopoly anion

Isopoly anions are formed when amphoteric metals in specific oxidation states condense to create anions containing multiple molecules of an acid anhydride. If only one type of acid anhydride is present, they are called isopoly acids, and their salts are isopoly salts.1

For example, the yellow chromate ion (CrO₄²⁻) can condense to form the orange isopoly dichromate ion (Cr₂O₇²⁻). This process is reversible and depends on the pH of the solution. In acidic conditions, the dichromate ion predominates, while in basic conditions, the simple chromate ion is more common.1

These compounds are distinct from heteropoly acids, which form when acid anhydrides condense with other acids, such as phosphoric or silicic acids.1