Anion

An ion with a negative charge is called an anion (pronounced an-eye-on). Conversely, an ion with a positive charge is named a cation (pronounced cat-eye-on). Salts contain one or more cations and one or more anions to form a neutral compound. In solution, the cations and anion become solvated and disassociate from each other. An anion can be a single negatively charged atom, such as the halide ions F^-, Cl^-, Br^- and I^-, or be comprised of a group of atoms, often containing the electronegative element oxygen. Some anions, such as the cyanide anion, are poisonous. Anions can evolve gasses, act as reducing agents or as oxidizing agents, and these differences are used to test for them in solution. There are 13 common anions which are described below.

Common anions and properties useful for their identification

The presence or absence of each of the thirteen common anions can be detected through a series of five tests, some with several steps, when performed in a particular order. The properties of the anions that are used in such test are indicated in the following table.

Anion name	Formula	Notable properties used to detect their presence
Carbonate	CO32-	Evolves carbon dioxide (CO2) gas when sulfuric acid is added
Sulfite	SO32-	Evolves sulfur dioxide (SO2) gas when sulfuric acid is added; Reduces Fe(III) to Fe(II)
Sulfide	S2-	Evolves hydrogen sulfide (H2S) gas when sulfuric acid is added; Reduces Fe(III) to Fe(II)
Nitrite	NO2-	Evolves nitrogen dioxide (NO2) gas when sulfuric acid is added; Reduces Fe(III) to Fe(II); Oxidizes Mn(II) to Mn(III);
Nitrate	NO32-	Oxidizes Mn(II) to Mn(III);
Oxalate	C2O42-	Forms an insoluble calcium salt in acetic acid solution
Chromate	CrO42-	Oxidizes Mn(II) to Mn(III); Forms an insoluble BaCrO4 salt in acetic acid solution containing calcium;
Sulfate	SO42-	Forms an insoluble BaSO4 salt in acetic acid solution containing calcium;
Phosphate	PO43-	Forms insoluble Ba3(PO4)2 in solution when excess ammonia is added.
Chloride	Cl-	Forms insoluble salt with silver, AgCl, white in color, in nitric acid solutions
Bromide	Br-	Forms insoluble salt with silver, AgBr, very pale yellow in color, in nitric acid solutions
Iodide	I-	Reduces Fe(III) to Fe(II); Forms insoluble salt with silver, AgI, somewhat intense yellow color, in nitric acid solutions
Thiocyanate	SCN-	Forms insoluble salt with silver, AgSCN, white in color, in nitric acid solutions