Lead(IV) oxide, commonly known as lead dioxide, is an inorganic compound with the chemical formula . It is an oxide where lead is in an oxidation state of +4. It is a dark-brown or black solid which is insoluble in water and slightly soluble in concentrated nitric and sulfuric acids. It exists in two crystalline forms. It has several important applications in electrochemistry, in particular as the positive plate of lead acid batteries and as a relatively inexpensive anode in electrolysis reactions.
Properties
Physical
thumb|left|120px|Crystal structure of α-
thumb|left|120px|Crystal structure of β-
Lead dioxide has two major polymorphs, alpha and beta, which occur naturally as rare minerals scrutinyite and plattnerite, respectively. Whereas the beta form had been identified in 1845, α- was first identified in 1946 and found as a naturally occurring mineral in 1988.
The alpha form has orthorhombic symmetry, space group Pbcn (No. 60), Pearson symbol oP12, lattice constants a = 0.497 nm, b = 0.596 nm, c = 0.544 nm, Z = 4 (four formula units per unit cell). and related to the rutile structure and can be envisaged as containing columns of octahedra sharing opposite edges and joined to other chains by corners. This contrasts with the alpha form where the octahedra are linked by adjacent edges to give zigzag chains.
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Electrochemical
Although the formula of lead dioxide is nominally given as , the actual oxygen to lead ratio varies between 1.90 and 1.98 depending on the preparation method. Deficiency of oxygen (or excess of lead) results in the characteristic metallic conductivity of lead dioxide, with a resistivity as low as 10<sup>−4</sup> Ω·cm and which is exploited in various electrochemical applications. Like metals, lead dioxide has a characteristic electrode potential, and in electrolytes it can be polarized both anodically and cathodically. Lead dioxide electrodes have a dual action, that is both the lead and oxygen ions take part in the electrochemical reactions.
Production
Chemical processes
Lead dioxide is produced commercially by several methods, which include oxidation of red lead () in alkaline slurry in a chlorine atmosphere, The reaction of with nitric acid also affords the dioxide:
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reacts with sodium hydroxide to form the hexahydroxoplumbate(IV) ion , soluble in water.
Electrolysis
An alternative synthesis method is electrochemical: lead dioxide forms on pure lead, in dilute sulfuric acid, when polarized anodically at electrode potential about +1.5 V at room temperature. This procedure is used for large-scale industrial production of anodes. Lead and copper electrodes are immersed in sulfuric acid flowing at a rate of 5–10 L/min. The electrodeposition is carried out galvanostatically, by applying a current of about 100 A/m<sup>2</sup> for about 30 minutes.
The drawback of this method for the production of lead dioxide anodes is its softness, especially compared to the hard and brittle which has a Mohs hardness of 5.5. This mismatch in mechanical properties results in peeling of the coating which is preferred for bulk production. Therefore, an alternative method is to use harder substrates, such as titanium, niobium, tantalum or graphite and deposit onto them from lead(II) nitrate in static or flowing nitric acid. The substrate is usually sand-blasted before the deposition to remove surface oxide and contamination and to increase the surface roughness and adhesion of the coating.
Applications
Lead dioxide is used in the production of matches,
Lead dioxide is used as an anode material in electrochemistry. β- is more attractive for this purpose than the α form because it has relatively low resistivity, good corrosion resistance even in low-pH medium, and a high overvoltage for the evolution of oxygen in sulfuric- and nitric-acid-based electrolytes. Lead dioxide can also withstand chlorine evolution in hydrochloric acid. Lead dioxide anodes are inexpensive and were once used instead of conventional platinum and graphite electrodes for regenerating potassium dichromate. They were also applied as oxygen anodes for electroplating copper and zinc in sulfate baths. In organic synthesis, lead dioxide anodes were applied for the production of glyoxylic acid from oxalic acid in a sulfuric acid electrolyte.
is not combustible, but it enhances flammability of other substances and the intensity of the fire. In case of a fire it gives off irritating and toxic fumes.
Lead dioxide is poisonous to aquatic life, but because of its insolubility it usually settles out of water.