Oxidation of lead

Curve A in Fig. 1 corresponds to the oxidation of lead to its divalent ion, described by the reaction 

At small pH values, the curve A is below the Hj/H curve, which means that lead is not stable in an aqueous solution under these conditions, but is converted into Pb ions, and simultaneously water is decomposed with formation of hydrogen 

Curve B describes the corresponding relation for the oxidation of divalent lead ions Pb - Pb , which implies the formation of lead dioxide since Pb does not exist alone  

PbO and Pbj04 are oxides of lead that are important primary products. Under certain circumstances, PbO is also observed in lead-acid batteries (cf. Sec. 4.4.5.1). Fig. 1 shows that these oxides are only stable in a neutral or alkaline environment. Their equilibrium potentials are represented by curves C-F and their standard values are compiled in Table 3. 

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All Group IV elements form both a monoxide, MO, and a dioxide, MO2. The stability of the monoxide increases with atomic weight of the Group IV elements from silicon to lead, and lead(II) oxide, PbO, is the most stable oxide of lead. The monoxide becomes more basic as the atomic mass of the Group IV elements increases, but no oxide in this Group is truly basic and even lead(II) oxide is amphoteric. Carbon monoxide has unusual properties and emphasises the different properties of the group head element and its compounds. 

Process. The QSL process (14) is a continuous single-step process having great flexibiUty in regard to the composition of the raw materials. In this process the highly exothermic complete oxidation, ie, the roasting reaction, can be avoided to some extent in favor of a weakly exothermic partial oxidation directly producing metallic lead. However, the yield of lead as metal is incomplete due to partial oxidation of lead to lead oxide. 

If the temperature of a molten lead—calcium (tin)—aluminum ahoy is not kept sufficiently high, finely divided aluminum particles may precipitate and float to the top of the melt. These may become mixed with oxides of lead in the dross. The finely divided aluminum particles can react violently with the oxides in the dross if ignited. Ignition can occur if attempts are made to melt or bum the dross away from areas of buildup with a torch. The oxides in the dross can supply oxygen for the combustion of aluminum once ignited. 

Tetravalent lead is obtained when the metal is subjected to strong oxidizing action, such as in the electrolytic oxidation of lead anodes to lead dioxide, Pb02 when bivalent lead compounds are subjected to powerful oxidizing conditions, as in the calcination of lead monoxide to lead tetroxide, Pb O or by wet oxidation of bivalent lead ions to lead dioxide by chlorine water. The inorganic compounds of tetravalent lead are relatively unstable eg, in the presence of water they hydrolyze to give lead dioxide. 

Lead borate moaohydrate [14720-53-7] (lead metaborate), Pb(B02)2 H20, mol wt 310.82, d = 5.6g/cm (anhydrous) is a white crystalline powder. The metaborate loses water of crystallization at 160°C and melts at 500°C. It is iasoluble ia water and alkaHes, but readily soluble ia nitric and hot acetic acid. Lead metaborate may be produced by a fusion of boric acid with lead carbonate or litharge. It also may be formed as a precipitate when a concentrated solution of lead nitrate is mixed with an excess of borax. The oxides of lead and boron are miscible and form clear lead-borate glasses in the range of 21 to 73 mol % PbO. 

Lead zirconate [12060-01 -4] PbZrO, mol wt 346.41, has two colorless crystal stmctures a cubic perovskite form above 230°C (Curie point) and a pseudotetragonal or orthorhombic form below 230°C. It is insoluble in water and aqueous alkaUes, but soluble in strong mineral acids. Lead zirconate is usually prepared by heating together the oxides of lead and zirconium in the proper proportion. It readily forms soHd solutions with other compounds with the ABO stmcture, such as barium zirconate or lead titanate. Mixed lead titanate-zirconates have particularly high piezoelectric properties. They are used in high power acoustic-radiating transducers, hydrophones, and specialty instmments (146). 

Much confusion exists concerning the number, composition, and structure of the oxides of lead. PbO exists as a red tetragonal form (litharge) stable at room temperature and a yellow orthorhombic form (massicot) stable above 488°C. Litharge (mp 897°, d 9.355 gcm ) is not only the most important oxide of Pb, it is also the most widely used inorganic compound of Pb (see Panel on p. 386) it is made by reacting molten Pb with air or O2 above 600° and has the SnO structure (p. 383, Pb-O 230 pm). Massicot (d 9.642 gcm ) has... 

It is prepared artificially, for use as a synthetic perfume, by several methods, for example, by heating benzyl chloride with oxide of lead to 100", or by heating benzyl chloride with potassium acetate and saponifying the benzyl acetate so formed, with caustic potash. 

Ruetschi, P. and Angstadt, R. T., Anodic Oxidation of Lead at Constant Potential , J. Elec-trochem. Soc., Ill, 1323 (1964)... 

Minium (Pb,04) represents a more highly oxidized form of lead oxide that enhances the electrochemical oxidation of lead oxide to lead dioxide. 

The production of tubular positive plates is in principle similar to that of pasted plates. A number of manufacturers use the same gray oxide as the basic filling substance. Sometimes the share or red lead or minium (Pb304) is increased above 25 or even to 100wt.%. The latter is more economic when the manufacturer runs his own minium plant then the expense of the chemical oxidation of lead oxide (PbO) to minium (Pb304) may be compensated by reduced formation cost. Furthermore, curing is not required, because of the high oxidation state, and the battery starts with full capacity when formed. 

After discontinuing the stirring, the potential is scanned positively, causing the amalgamated lead to be oxidized back into the solution (i.e., stripped out of the electrode). This oxidation of lead gives a current peak, ip, the magnitude of which is determined by the concentration of Pb in the mercury electrode which is in turn proportional to the amoimt of Pb " in the sample. 

The losses of lead as litharge (PbO) can be minimized if the impurity elements could be oxidized at an oxygen potential lower than that which causes the oxidation of lead. This can be achieved if the activities of the oxides of the impurity elements in the slag are decreased, for example, by the addition of an oxide which reacts very much more strongly with the oxides of the impurities than it does with lead oxide. Sodium hydroxide is a useful reagent for this purpose and sodium nitrate can be used as the source of oxygen. The reaction involved in this process, known as the Harris process, can be formally written as... 

Bamberger, M. et al., Z. Ges. Schiess-u. Sprengstoffiv., 1927, 22, 125-128 Mixtures of trioxane with 60% hydrogen peroxide solution are detonable by heat or shock, or spontaneously after contact with metallic lead. The latter may be owing to the heat of oxidation of lead. 

We shall not discuss here oxygen interaction with nickel since, although it is one of the most extensively studied systems (59), there is still considerable ambiguity and confusion regarding possible models. Two systems that, however, have lent themselves to investigation by XPS are the oxidation of lead (60, 60a) and aluminum surfaces (10). In both cases substantial shifts were observed in the metal core levels, which as emphasized previously is more the exception than the rule. 

The re-oxidation of lead was expected to occur rapidly at 600 K the surface of the lead oxide would then remain unchanged in the presence of oxygen. The authors concluded that, as a consequence, general hydrocarbon combustion in which formaldehyde is a degenerate branching intermediate is inhibited in the presence of PbO by the rapid removal of formaldehyde. 

In October 1772 Lavoisier performed some experiments using a large burning lens owned by the Academy of Sciences. He found that when litharge (an oxide of lead) was heated with charcoal, large quantities of air were released. Of course it wasn t air at all—it was carbon dioxide. At the time, Lavoisier was unaware that carbon dioxide—or fixed air as it was then called—has properties very different from ordinary air. He was also unaware that Priestley had experimented with numerous different airs and had shown that atmospheric air has more than one component. Lavoisier also performed experiments that showed that sulfur and phosphorus also gain weight when they are burned. Marie Anne wrote up the results... 

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