Inorganic anhydride

An inorganic anhydride is a chemical compound that is related to another by the loss of the elements of water, H₂O. It shares this definition with acid anhydrides but inorganic anhydrides do not contain any organic moiety. For example, carbon dioxide is the anhydride of carbonic acid:

H₂CO₃ – H₂O = CO₂

Although the term is somewhat archaic, it survives in current use in the name of the enzyme carbonic anhydrase, which catalyzes this reaction. Other inorganic acids have anhydrides which are oxides. Examples include

• silicon dioxide is the anhydride of silicic acid: Si(OH)₄ – 2H₂O = SiO₂
• phosphorus pentoxide is the anhydride of phosphoric acid: 2H₃PO₄ – 3H₂O = P₂O₅.
• vanadium pentoxide is the anhydride of vanadic acid: 2H₃VO₄ – 3H₂O = V₂O₅.
• sulfur trioxide is the anhydride of sulfuric acid: H₂SO₄ = SO₃ + H₂O. This reaction is important in the manufacture of the acid.
• chromium trioxide is the anydride of chromic acid: H₂CrO₄ – H₂O = CrO₃
• iron(II) oxide is the anhydride of the aqueous ferrous ion: Fe(H₂O)₆]²⁺ – 6H₂O = FeO + 2H⁺ + 5H₂O

A significant property of these anhydrides is that they are acidic oxides. This follows from the definition, since loss of water from an acid is not an acid-base reaction. Although it is difficult to classify these oxides as acids, the property is manifest in reactions with bases. For example, carbon dioxide reacts with alkali.

CO₂ + OH⁻ ⇌ HCO₃⁻ + OH⁻ ⇌ CO₃²⁻ + H₂O

For this reason, alkali is kept in stoppered vessels to inhibit reaction with atmospheric carbon dioxide. In geochemistry complex silicates are often written as though they are the products of an acid-base reaction. For example the chemical formula of the mineral olivine can be written either as (Mg,Fe)₂SiO₄ or as (MgO,FeO)₂SiO₂. This mineral is said to be ultramafic, meaning that it has a very high nominal content of the bases magnesium oxide and iron oxide and hence, a low content of the acid silicon dioxide.