Lead(IV) oxide is the

inorganic compound In chemistry, an inorganic compound is typically a chemical compound that lacks carbon–hydrogen bonds, that is, a compound that is not an organic compound. The study of inorganic compounds is a subfield of chemistry known as '' inorganic chemi ...

with the formula PbO₂. It is an oxide where

lead Lead is a chemical element with the Symbol (chemistry), symbol Pb (from the Latin ) and atomic number 82. It is a heavy metals, heavy metal that is density, denser than most common materials. Lead is Mohs scale of mineral hardness#Intermediate ...

is in an

oxidation state In chemistry, the oxidation state, or oxidation number, is the hypothetical charge of an atom if all of its bonds to different atoms were fully ionic. It describes the degree of oxidation (loss of electrons) of an atom in a chemical compound. ...

of +4. It is a dark-brown solid which is insoluble in water. It exists in two crystalline forms. It has several important applications in

electrochemistry Electrochemistry is the branch of physical chemistry concerned with the relationship between electrical potential difference, as a measurable and quantitative phenomenon, and identifiable chemical change, with the potential difference as an outc ...

, in particular as the positive plate of lead acid batteries.

Properties

Physical

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-PbO₂ was first identified in 1946 and found as a naturally occurring mineral 1988. The alpha form has orthorhombic symmetry,

space group In mathematics, physics and chemistry, a space group is the symmetry group of an object in space, usually in three dimensions. The elements of a space group (its symmetry operations) are the rigid transformations of an object that leave it ...

Pbcn (No. 60), Pearson symbol ''oP''12, lattice constants ''a'' = 0.497 nm, ''b'' = 0.596 nm, ''c'' = 0.544 nm, ''Z'' = 4 (four formula units per unit cell). The lead atoms are six-coordinate. The symmetry of the beta form is tetragonal,

P4₂/mnm (No. 136), Pearson symbol ''tP''6, lattice constants ''a'' = 0.491 nm, ''c'' = 0.3385 nm, ''Z'' = 2 and related to the

rutile Rutile is an oxide mineral composed of titanium dioxide (TiO2), the most common natural form of TiO2. Rarer polymorphs of TiO2 are known, including anatase, akaogiite, and brookite. Rutile has one of the highest refractive indices at visible wa ...

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.

Chemical

Lead dioxide decomposes upon heating in air as follows: :24 PbO₂ → 2 Pb₁₂O₁₉ + 5 O₂ :Pb₁₂O₁₉ → Pb₁₂O₁₇ + O₂ :2 Pb₁₂O₁₇ → 8 Pb₃O₄ + O₂ :2 Pb₃O₄ → 6

PbO Lead(II) oxide, also called lead monoxide, is the inorganic compound with the molecular formula Pb O. PbO occurs in two polymorphs: litharge having a tetragonal crystal structure, and massicot having an orthorhombic crystal structure. Modern ap ...

+ O₂ The stoichiometry of the end product can be controlled by changing the temperature – for example, in the above reaction, the first step occurs at 290 °C, second at 350 °C, third at 375 °C and fourth at 600 °C. In addition, Pb₂O₃ can be obtained by decomposing PbO₂ at 580–620 °C under an oxygen pressure of . Therefore, thermal decomposition of lead dioxide is a common way of producing various lead oxides. Lead dioxide is an amphoteric compound with prevalent acidic properties. It dissolves in strong bases to form the hydroxy plumbate ion, b(OH)₆sup>2−: :PbO₂ + 2 NaOH + 2 H₂O → Na₂ b(OH)₆ It also reacts with basic oxides in the melt, yielding orthoplumbates M₄ bO₄ Because of the instability of its Pb⁴⁺ cation, lead dioxide reacts with hot acids, converting to the more stable Pb²⁺ state and liberating oxygen: :2 PbO₂ + 2 H₂SO₄ → 2 PbSO₄ + 2 H₂O + O₂ :2 PbO₂ + 4 HNO₃ → 2 Pb(NO₃)₂ + 2 H₂O + O₂ :PbO₂ + 4 HCl → PbCl₂ + 2 H₂O + Cl₂ However these reactions are slow. Lead dioxide is well known for being a good

oxidizing agent An oxidizing agent (also known as an oxidant, oxidizer, electron recipient, or electron acceptor) is a substance in a redox chemical reaction that gains or " accepts"/"receives" an electron from a (called the , , or ). In other words, an oxid ...

, with an example reactions listed below: :2 MnSO₄ + 5 PbO₂ + 6 HNO₃ → 2 HMnO₄ + 2 PbSO₄ + 3 Pb(NO₃)₂ + 2 H₂O :2 Cr(OH)₃ + 10 KOH + 3 PbO₂ → 2 K₂CrO₄ + 3 K₂ PbO₂ + 8 H₂O

Electrochemical

Although the formula of lead dioxide is nominally given as PbO₂, 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 Electrical resistivity (also called specific electrical resistance or volume resistivity) is a fundamental property of a material that measures how strongly it resists electric current. A low resistivity indicates a material that readily allows ...

as low as 10⁻⁴ Ω·cm and which is exploited in various electrochemical applications. Like metals, lead dioxide has a characteristic

electrode potential In electrochemistry, electrode potential is the electromotive force of a galvanic cell built from a standard reference electrode and another electrode to be characterized. By convention, the reference electrode is the standard hydrogen electrod ...

, 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 (Pb₃O₄) in alkaline slurry in a chlorine atmosphere, reaction of lead(II) acetate with "chloride of lime" ( calcium hypochlorite), The reaction of Pb₃O₄ with

nitric acid Nitric acid is the inorganic compound with the formula . It is a highly corrosive mineral acid. The compound is colorless, but older samples tend to be yellow cast due to decomposition into oxides of nitrogen. Most commercially available ni ...

also affords the dioxide: :Pb₃O₄ + 4 HNO₃ → PbO₂ + 2 Pb(NO₃)₂ + 2 H₂O PbO₂ reacts with sodium hydroxide to form the hexahydroxoplumbate(IV) ion b(OH)₆sup>2−, 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 PbO₂ anodes. Lead and

copper Copper is a chemical element with the symbol Cu (from la, cuprum) and atomic number 29. It is a soft, malleable, and ductile metal with very high thermal and electrical conductivity. A freshly exposed surface of pure copper has a pinkish ...

electrodes are immersed in sulfuric acid flowing at a rate of 5–10 L/min. The electrodeposition is carried out

galvanostat A galvanostat (also known as amperostat) is a control and measuring device capable of keeping the current through an electrolytic cell in coulometric titrations constant, disregarding changes in the load itself. Its main feature is its ''nearly ...

ically, by applying a current of about 100 A/m² 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 PbO₂ which has a Mohs hardness of 5.5. This mismatch in mechanical properties results in peeling of the coating which is preferred for bulk PbO₂ production. Therefore, an alternative method is to use harder substrates, such as

titanium Titanium is a chemical element with the Symbol (chemistry), symbol Ti and atomic number 22. Found in nature only as an oxide, it can be reduced to produce a lustrous transition metal with a silver color, low density, and high strength, resista ...

, niobium, tantalum or graphite and deposit PbO₂ 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,

pyrotechnic Pyrotechnics is the science and craft of creating such things as fireworks, safety matches, oxygen candles, explosive bolts and other fasteners, parts of automotive airbags, as well as gas-pressure blasting in mining, quarrying, and demolition ...

s, dyes and the curing of

sulfide Sulfide (British English also sulphide) is an inorganic anion of sulfur with the chemical formula S2− or a compound containing one or more S2− ions. Solutions of sulfide salts are corrosive. ''Sulfide'' also refers to chemical compounds l ...

polymer A polymer (; Greek ''poly-'', "many" + '' -mer'', "part") is a substance or material consisting of very large molecules called macromolecules, composed of many repeating subunits. Due to their broad spectrum of properties, both synthetic and ...

s. It is also used in the construction of high-voltage lightning arresters. Lead dioxide is used as an

anode An anode is an electrode of a polarized electrical device through which conventional current enters the device. This contrasts with a cathode, an electrode of the device through which conventional current leaves the device. A common mnemoni ...

material in electrochemistry. β-PbO₂ is more attractive for this purpose than the α form because it has relatively low

, good

corrosion Corrosion is a natural process that converts a refined metal into a more chemically stable oxide. It is the gradual deterioration of materials (usually a metal) by chemical or electrochemical reaction with their environment. Corrosion engi ...

resistance even in low- pH medium, and a high

overvoltage When the voltage in a circuit or part of it is raised above its upper design limit, this is known as overvoltage. The conditions may be hazardous. Depending on its duration, the overvoltage event can be transient—a voltage spike—or perm ...

for the evolution of oxygen in sulfuric- and nitric-acid-based electrolytes. Lead dioxide can also withstand

chlorine Chlorine is a chemical element with the symbol Cl and atomic number 17. The second-lightest of the halogens, it appears between fluorine and bromine in the periodic table and its properties are mostly intermediate between them. Chlorine is ...

evolution in

hydrochloric acid Hydrochloric acid, also known as muriatic acid, is an aqueous solution of hydrogen chloride. It is a colorless solution with a distinctive pungent smell. It is classified as a strong acid. It is a component of the gastric acid in the dig ...

. Lead dioxide anodes are inexpensive and were once used instead of conventional

platinum Platinum is a chemical element with the symbol Pt and atomic number 78. It is a dense, malleable, ductile, highly unreactive, precious, silverish-white transition metal. Its name originates from Spanish , a diminutive of "silver". Pla ...

and graphite electrodes for regenerating potassium dichromate. They were also applied as oxygen anodes for

electroplating Electroplating, also known as electrochemical deposition or electrodeposition, is a process for producing a metal coating on a solid substrate through the redox, reduction of cations of that metal by means of a direct current, direct electric cur ...

and

zinc Zinc is a chemical element with the symbol Zn and atomic number 30. Zinc is a slightly brittle metal at room temperature and has a shiny-greyish appearance when oxidation is removed. It is the first element in group 12 (IIB) of the periodic t ...

in sulfate baths. In organic synthesis, lead dioxide anodes were applied for the production of glyoxylic acid from

oxalic acid Oxalic acid is an organic acid with the systematic name ethanedioic acid and formula . It is the simplest dicarboxylic acid. It is a white crystalline solid that forms a colorless solution in water. Its name comes from the fact that early inve ...

in a sulfuric acid electrolyte.

Lead acid battery

The most important use of lead dioxide is as the cathode of lead acid batteries. Its utility arises from the anomalous metallic conductivity of PbO₂. The lead acid battery stores and releases energy by shifting the equilibrium (a comproportionation) between metallic lead, lead dioxide, and lead(II) salts in sulfuric acid. :Pb + PbO₂ + 2 + 2 H⁺ → 2 PbSO₄ + 2 H₂O ''E''° = +2.05 V

Safety

Lead compounds are poisons. Lead dioxide is a strong oxidizer, so any contact of skin, eyes with either lead dioxide or its vapours may cause severe injury in the form of burns which can even lead to death. PbO₂ 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 and other lead compound pose a huge enviornmental hazard when they are not disposed of properly. it is especially poisonous to aquatic life.https://www.ltschem.com/msds/PbO2.pdf

References

External links

National Pollutant Inventory: Lead and Lead Compounds Fact Sheet
{{Oxides Oxides Lead(IV) compounds Acidic oxides Pyrotechnic oxidizers Oxidizing agents