Lead(II,IV) oxide, also called red lead or minium, is the inorganic compound with the formula Pb3O4. A bright red or orange solid, it is used as pigment, in the manufacture of batteries, and rustproof primer paints. It is an example of a mixed valence compound, being composed of both Pb(II) and Pb(IV) in the ratio of two to one. Structure Lead(II,IV) oxide has a tetragonal crystal structure at room temperature, which then transforms to an orthorhombic (Pearson symbol oP28, Space group Pbam, No. 55) form at temperature . This phase transition only changes the symmetry of the crystal and slightly modifies the interatomic distances and angles. Preparation Lead(II,IV) oxide is prepared by calcination of lead(II) oxide (PbO; also called litharge) in air at about 450–480 °C: 6 PbO + O2 -> 2 Pb3O4 The resulting material is contaminated with PbO. If a pure compound is desired, PbO can be removed by a potassium hydroxide solution: PbO + KOH + H2O -> K[Pb(OH)3] Another method of preparation relies on annealing of lead(II) carbonate (cerussite) in air: 6PbCO3 + O2 -> 2Pb3O4 + 6CO2 Yet another method is oxidative annealing of white lead: 3Pb2CO3(OH)2 + O2 -> 2Pb3O4 + 3CO2 + 3H2O In solution, lead(II,IV) oxide can be prepared by reaction of potassium plumbate with lead(II) acetate, yielding yellow insoluble lead(II,IV) oxide monohydrate Pb3O4.H2O, which can be turned into the anhydrous form by gentle heating: K2PbO3 + 2Pb(OCOCH3)2 + H2O -> Pb3O4 + 2KOCOCH3 + 2 CH3COOH Natural minium is uncommon, forming only in extreme oxidizing conditions of lead ore bodies. The best known natural specimens come from Broken Hill, New South Wales, Australia, where they formed as the result of a mine fire. Reactions Red lead is virtually insoluble in water and in ethanol. However, it is soluble in hydrochloric acid present in the stomach, and is therefore toxic when ingested. It also dissolves in glacial acetic acid and a diluted mixture of nitric acid and hydrogen peroxide. When heated to 500 °C, it decomposes to lead(II) oxide and oxygen. At 580 °C, the reaction is complete. 2Pb3O4 -> 6PbO + O2 Nitric acid dissolves the lead(II) oxide component, leaving behind the insoluble lead(IV) oxide: Pb3O4 + 4HNO3 -> PbO2 + 2Pb(NO3)2 + 2H2O With iron oxides and with elemental iron, lead(II,IV) oxide forms insoluble iron(II) and iron(III) plumbates, which is the basis of the anticorrosive properties of lead-based paints applied