Lead(II,IV) oxide

Lead(II,IV) oxide, also called red lead, lead tetroxide, 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 in 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 2:1.

Structure

Lead(II,IV) oxide is lead(II) orthoplumbate(IV), [Pb2+]2[PbO4−4]. It 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 170 K (−103 °C). 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 (842–896 °F):

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:

6 PbCO3 + O2 → 2 Pb3O4 + 6 CO2

Yet another method is oxidative annealing of white lead:

3 Pb2CO3(OH)2 + O2 → 2 Pb3O4 + 3 CO2 + 3 H2O

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 + 2 Pb(OCOCH3)2 + H2O → Pb3O4 + 2 KOCOCH3 + 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.