Chemical Nomenclature and Analytical Methods for Anions

Anions: Definition and Characteristics

An anion is an ion (an atom or molecule) that possesses a negative electrical charge, meaning it has an excess of electrons. Anions are typically described with a negative oxidation state. They are broadly classified into two main types: monatomic and polyatomic.

Monatomic Anions

Monatomic anions are typically formed when nonmetals gain electrons to complete their valence shell.

Traditional Nomenclature (Monatomic)

These are named using the word ion followed by the name of the nonmetal, ending in the suffix -ide. (Note: Amide and Cyanide are often included here for simplicity, though technically polyatomic.)

• Cl^-: Chloride ion
• H^-: Hydride ion
• S^2-: Sulfide ion
• NH₂^-: Amide ion
• CN^-: Cyanide ion

Systematic Nomenclature (Monatomic)

For simple monatomic anions, the systematic nomenclature often aligns with the traditional naming convention:

• Cl^-: Chloride ion
• H^-: Hydride ion
• S^2-: Sulfide ion

Polyatomic Anions

Polyatomic anions (or oxyanions) are complex ions that can be considered as originating from a molecule (often an acid) that has lost one or more protons (H⁺ ions).

Traditional Nomenclature (Polyatomic)

These are named using the word ion followed by the name of the central nonmetal. The suffix depends on the oxidation state of the central atom:

• Suffix -ite: Used if the central atom acts with a lower valence (oxidation state).
• Suffix -ate: Used if the central atom acts with a greater valence (oxidation state).

Example:

• SO₄^2-: Sulfate ion

Systematic Nomenclature (Polyatomic)

Systematic names are derived from the corresponding acid name, but the word ion is prefixed, and the term "hydrogen" is removed (if applicable). The oxidation state of the central atom is indicated by Roman numerals.

Examples:

• SO₄^2-: Ion tetraoxosulfate(VI)
• NO₂^-: Ion dioxonitrate(III)
• ClO₄^-: Ion tetraoxochlorate(VII)

Acid Anions (Hydrogen Anions)

Acid anions are derived from a polyprotic acid (an acid capable of donating more than one proton) that has lost only some, but not all, of its acidic protons.

Traditional Nomenclature (Acid Anions)

These are named like the corresponding polyatomic ion, but prefixed with the word hydrogen. Multiplicative prefixes (e.g., dihydrogen) are used when more than one hydrogen atom remains.

Systematic Nomenclature (Acid Anions)

Systematic naming involves prefixing the name of the corresponding ion with hydrogen, using multiplicative prefixes (e.g., di-, tri-) as necessary. This classification scheme is often used for better understanding, although the overall classification of anions is not always rigid.

Analytical Classification of Anions

Anions are often classified analytically based on their reactivity towards specific reagents, such as dilute acids or precipitation agents. This classification is not always rigid.

Class A: Anions Reacting with Dilute Acids

These anions typically react with dilute hydrochloric acid or sulfuric acid, often resulting in the evolution of gases.

Examples include:

• Carbonate, Bicarbonate, Sulfite, Thiosulfate, Sulfide, Nitrite, Hypochlorite, Cyanide, and Cyanate.
• Also included are anions that do not evolve gas but are often grouped here: Fluoride, Chloride, Bromide, Iodide, Nitrate, Chlorate, Perchlorate, Bromate, Iodate, Borate*, Ferrocyanide, Ferricyanide, Thiocyanate, Formate, Acetate, Oxalate, Tartrate, and Citrate.

Class B: Anions Identified by Precipitation or Redox Reactions

These anions are typically identified through precipitation reactions or oxidation-reduction reactions in solution.

Precipitation Reactions:

• Sulfate, Persulfate**, Phosphate, Phosphite, Hypophosphite, Arsenate, Arsenite, Silicate, Fluorosilicate, Salicylate, Benzoate, and Succinate.

Oxidation and Reduction Reactions in Solution:

• Manganate, Permanganate, Chromate, and Dichromate.

*Note: Borate behavior can vary. **Note: Persulfate is sometimes classified differently.

Analytical Procedure for Common Anions

The most common anions encountered in a laboratory setting cannot be separated as clearly as cations. Often, anions are identified directly. In other cases, they are separated by precipitation in large groups along with specific cations, and then identified within those precipitates.

Analyzing anions is generally considered more challenging than analyzing cations in the laboratory.

The typical analytical procedure for anions involves first removing all existing cations, usually by precipitation using reagents like NaOH or CO₃^2-. Following this removal, three preliminary tests are usually performed.