Page 202 - Handbook of Battery Materials

6.2 Lead/Oxygen Compounds 171

covers any piece of lead exposed to air with a dull gray layer of lead oxide (cf. the
milling process in Section 6.4.2.1).
Two modifications are known:
1) red PbO, the tetragonal modification (litharge);
2) yellow PbO, the rhombic modification (massicot).
The crystallographic structure is described in Ref. [5], p. 11.
Red PbO is stable at low temperatures, whereas yellow PbO is the high-
◦
temperature modification. The temperature of conversion is 488 C:
◦
Red PbO → 488 C → Yellow PbO (6.2)
At low temperatures the conversion is a slow reaction, and yellow PbO also exists
at room temperatures as a metastable modification.

6.2.2
Minium (Pb 3 O 4 )

Minium, also called red lead, is formed when lead oxide is exposed to air at about
◦
500 C according to Equation 6.3 Roasting ovens are used for the technical process.
(6.3)
3PbO + 1/2O 2 → Pb 3 O 4
Minium contains Pb 2+ and Pb 4+ ions, but it is not simply a mixture of PbO and
PbO 2 . According to its composition and structure it can be regarded as a lead(II)
salt of the lead(IV) acid (Pb 3 O 4 = 2PbO·PbO 2 )(cf.[5],Figure 2.4,p.17).Inthe
technical process, the stoichiometric composition Pb 3 O 4 is usually not attained
and the (fictive) percentage of PbO 2 is often used to specify the grade of oxidation.
Stoichiometric Pb 3 O 4 contains 34% PbO 2 (239.2 g of PbO 2 per 685 g Pb 3 O 4 ,cf.
Table 6.2), and the technical products contain PbO 2 in the range between 25 and
30%.
When minium comes in to contact with sulfuric acid it is converted into lead
sulfate and lead dioxide according to Equation 6.4.
Pb 3 O 4 + 2H 2 SO 4 → 2PbSO 4 + PbO 2 + 2H 2 O (6.4)

6.2.3
Lead Dioxide (PbO 2 )
Lead dioxide exists in two modifications:

1) a rhombic modifications called α-PbO 2 ;
2) a tetragonal modification called β-PbO 2 .
Besides the crystalline material, a certain portion of amorphous lead dioxide is
always observed. In the working electrode such amorphous material is apparently