Lead(II) oxide

Preparation

PbO may be prepared by heating lead metal in air at approx. 600 °C. At this temperature it is also the end product of oxidation of other lead oxides in air:

PbO₂ –(293 °C)→ Pb₁₂O₁₉ –(351 °C)→ Pb₁₂O₁₇ –(375 °C)→ Pb₃O₄ –(605 °C)→ PbO

Thermal decomposition of lead(II) nitrate or lead carbonate also results in the PbO formation:

2 Pb(NO₃)₂ → 2 PbO + 4 NO₂ + O₂PbCO₃ → PbO + CO₂

PbO is produced on a large scale as an intermediate product in refining raw lead ores into metallic lead. The usual lead ore is galena (lead(II) sulfide). At high temperature (1000 °C) the sulfide is converted to the oxide:

2 PbS + 3 O₂ → 2PbO + 2SO₂

Metallic lead is obtained by reducing the PbO with carbon monoxide at around 1200 °C:.

PbO + CO → Pb + CO₂

Structure

As determined by X-ray crystallography, both polymorphs, tetragonal and orthorhombic feature a pyramidal four-coordinate Pb center. In the tetragonal form the four Pb-O bonds have the same length, but in the orthorhombic two are shorter and two longer. The pyramidal nature indicates the presence of a stereo-chemically active lone pair of electrons. When PbO occurs in tetragonal lattice structure it is called litharge; and when the PbO has orthorhombic lattice structure it is called massicot. The PbO can be changed from massicot to litharge or vice versa by controlled heating and cooling. The tetragonal form is usually red or orange color, while the orthorhombic is usually yellow or orange, but the color is not a very reliable indicator of the structure. The tetragonal and orthorhombic forms of PbO occur naturally as rare minerals.

Reactions

The red and yellow forms of this material are related by a small change in enthalpy: PbO(red) → PbO(yellow) ΔH = 1.6 kJ/mol

PbO is amphoteric, which means that it reacts with both acids and with bases. With acids, it forms salts of Pb²⁺ via the intermediacy of oxo clusters such as [Pb₆O(OH)₆]⁴⁺. With strong base, PbO dissolves to form plumbite(II) salts: PbO + H₂O + OH⁻ → [Pb(OH)₃]⁻

Applications

The kind of lead in lead glass is normally PbO, and PbO is used extensively in making glass. Depending on the glass, the benefit of using PbO in glass can be one or more of (1) increasing the refractive index of the glass, (2) decreasing the viscosity of the glass, (3) increasing the electrical resistivity of the glass, and (4) increasing the ability of the glass to absorb X-rays. Adding PbO to industrial ceramics (as well as glass) makes the materials more magnetically and electrically inert (raises the Curie temperature) and is often used for this purpose. Historically PbO was also used extensively in ceramic glazes for household ceramics, and it is still used, but not extensively any more. Other less dominating applications include the vulcanization of rubber and the production of certain pigments and paints. PbO is used in cathode ray tube glass to block X-ray emission, but mainly in the neck and funnel because it can cause discoloration when used in the faceplate. Strontium oxide is preferred for the faceplate.

The consumption of lead, and hence the processing of PbO, correlates with the number of automobiles because it remains the key component of automotive lead-acid batteries.

Niche or declining uses

A mixture of PbO with glycerine sets to a hard, waterproof cement that has been used to join the flat glass sides and bottoms of aquariums, and was also once used to seal glass panels in window frames. It is a component of lead paints.

In powdered tetragonal litharge form, it can be mixed with linseed oil and then boiled to create a weather-resistant sizing used in gilding. The litharge would give the sizing a dark red color that made the gold leaf appear warm and lustrous, while the linseed oil would impart adhesion and a flat durable binding surface.

PbO is used in certain condensation reactions in organic synthesis.

Health issues

Lead oxide may be fatal if swallowed or inhaled. It causes irritation to skin, eyes, and respiratory tract. It affects gum tissue, central nervous system, kidneys, blood, and reproductive system. It can bioaccumulate in plants and in mammals.