Difference between revisions of "Lead(IV) oxide"
|Line 174:||Line 174:|
Lead dioxide is best kept in closed bottles.
Lead dioxide is best kept in closed bottles.
Revision as of 19:18, 18 May 2019
| IUPAC name
| Other names
|Molar mass||239.1988 g/mol|
|Appearance||Dark brown or brown-black|
|Density|| 9.773 g/cm3 (α-PbO2)|
9.55 g/cm3 (β-PbO2)
|Melting point||290 °C (554 °F; 563 K) (decomposes)|
|Solubility|| Reacts with mineral acids|
Soluble in acetic acid, alkalis
Insoluble in alcohols, ketones, hydrocarbons
|Vapor pressure||~0 mmHg|
Std enthalpy of
|Safety data sheet||Sigma-Aldrich|
| Lead(II) oxide|
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Lead(IV) oxide also called plumbic oxide, anhydrous plumbic acid (sometimes wrongly called lead peroxide), though more commonly called lead dioxide, is a chemical compound with the formula PbO2. It is an oxide where lead is in an oxidation state of +4 and the bond type is predominantly covalent.
Lead dioxide decomposes upon heating in air as follows:
- PbO2 → Pb12O19 → Pb12O17 → Pb3O4 → PbO
- PbO2 + 2 NaOH + 2 H2O → Na2[Pb(OH)6]
It also reacts with basic oxides in the melt, yielding orthoplumbates M4[PbO4].
Because of the instability of its Pb4+ cation, lead dioxide reacts with hot acids, converting to the more stable Pb2+ state and liberating oxygen, though the reaction is slow:
- 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
- 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 K2PbO2 + 8 H2O
Lead dioxide is an odorless dark-brown crystalline powder, practically insoluble in water. It exists in two crystalline forms:
- The alpha phase (α-PbO2): 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). The lead atoms are six-coordinate.
- The beta phase (β-PbO2): has tetragonal symmetry, space group P42/mnm (No. 136), Pearson symbol tP6, lattice constants a = 0.491 nm, c = 0.3385 nm, Z = 2 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.
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 as low as 10−4 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.
Lead(IV) oxide is sold by many chemical suppliers.
It's encountered in nature as the minerals plattnerite (β-PbO2) and scrutinyite (α-PbO2).
- Pb3O4 + 4 HNO3 → PbO2 + 2 Pb(NO3)2 + 2 H2O
- Pb(CH3COO)2 + Ca(ClO)2 → PbO2 + Ca(CH3COO)2 + CaCl2
- PbCl2 + 2 NaClO → PbO2 + 2 NaCl + Cl2
Lead dioxide forms on pure lead metal, 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 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/m2 for about 30 minutes. The drawback of the lead electrode 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. Therefore, an alternative method is to use harder substrates, such as titanium, 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.
The electrochemical method has been widely used by amateur chemists to make cheap PbO2 electrodes, useful for the production of chlorates and perchlorates.
- Make PbO2 electrodes (β-PbO2 is more desired for this purpose than the α-PbO2 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-acid- and nitric-acid-based electrolytes)
- Oxidizing agent
- Make chromates
- Mineral collecting
Lead dioxide is extremely toxic and ingestion may be fatal.
Lead dioxide is best kept in closed bottles. Any clean plastic or glass bottle is sufficient.
Should be taken to waste disposal companies. Scrap metal facilities that collect lead may accept it.
- White, W. B.; Dachille, F.; Roy, R.; Journal of the American Ceramic Society; vol. 44; (1961); p. 170 - 174
- Povarennikh, O. S.; Dopovidi Akad. Nauk Ukr.RSR; (1963); p. 805 - 808; C. A.; vol. 59; (1963); p. 13411,
Relevant Sciencemadness threads
- More on PbO2 electrodes
- Graphite Substrate Lead Dioxide anode
- Lead Dioxide Electrodes
- Which lead dioxide anode should I make?
- Lead dioxide battery plate as chlorate cell anode
- Lead Dioxide on Magnetite
- PbO/PbO2 from Electrolysis?
- Lead dioxide anode-possibility of an unusual synthesis method
- PbO2 synthesis (not an anode)
- Lead dioxide - vinegar reaction?
- Making PbO2 plates for chlorate production - from Pb lead-acid battery plates