Lead carbonate, basic

The crystals are very fragile and are handled only in dispersions. They settle very fast because of their density of 6.14 g/cmv The use of basic lead carbonate is also hmited by its low chemical stability, the toxicity of its by-products, and toxicological concerns in its application areas. 

The first commercially successfull synthetic nacreous pigments were hexagonal platelet crystals of lead salts Thiosulfate, hydrogen phosphate, hydrogen arsenate and most important nowadays, basic carbonate. Basic lead carbonate [1319-46-6] Pb(OH2) 2PbC03, Mr 775.7, is precipitated from aqueous lead acetate or lead propionate solutions with carbon dioxide  

Under appropriate reaction conditions, regular hexagonal platelets (ca. 50 nm thick and 20 pm in diameter) can be obtained. The high refractive index (n — 2.0), high aspect ratio ( 200), and the extremely even surface of basic lead carbonate make it an optical match to natural pearl essence. 

The platelet thickness can be adjusted to produce interference colors by modifying reaction conditions. When aligned with its plane orthogonal to the incident light, the platelet crystal behaves as a thin, solid, optical film (see Fig. 72) with two phase-shifted reflections from the upper and lower crystal planes (the phase boundaries). 

The crystals are mechanically sensitive and their high density (6.4 g/cm3) results in fast sedimentation. In view of their agglomeration tendency and occupational health (toxicity) risks, they are not produced in powder form, but are flushed from the aqueous phase into suitable organic solvents or resins and handled as stabilized dispersions. 

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By far the most common lead salt used for PVC stabilization is tribasic lead sulfate. It can be found either alone or combined with another lead salt in almost every lead-stabilized PVC formulation. Many of the combinations are actually coprecipitated hybrid products, ie, basic lead sulfophthalates. Dibasic lead stearate and lead stearate are generally used as costabilizers combined with other primary lead salts, particularly in rigid PVC formulations where they contribute lubrication properties dibasic lead stearate provides internal lubrication and lead stearate is a good external lubricant. Basic lead carbonate is slowly being replaced by tribasic lead sulfate in most appHcations due the relatively low heat stabiHty of the carbonate salt which releases CO2 at about 180°C during PVC processing. 

Lead Chloride. Lead dichloride, PbCl2, forms white, orthorhombic needles some physical properties are given in Table 1. Lead chloride is slightly soluble in dilute hydrochloric acid and ammonia and insoluble in alcohol. It is prepared by the reaction of lead monoxide or basic lead carbonate with hydrochloric acid, or by treating a solution of lead acetate with hydrochloric acid and allowing the precipitate to settle. It easily forms basic chlorides, such as PbCl Pb(OH)2 [15887-88 ] which is known as Pattinson s lead white, an artist s pigment. 

Lead Carbonate. Lead carbonate [598-63-0] PbCO, mol wt 267.22, d = 6.6g/cm, forms colorless orthorhombic crystals it decomposes at about 315°C. It is nearly insoluble in cold water (0.00011 g/100 mL at 20°C), but is transformed in hot water to the basic carbonate, 2PbC03 Pb(OH)2. Lead carbonate is soluble in acids and alkalies, but insoluble in alcohol and ammonia. It is prepared by passing CO2 iuto a cold dilute solution of lead acetate, or by shaking a suspension of a lead salt less soluble than the carbonate with ammonium carbonate at a low temperature to avoid formation of basic lead carbonate. 

Basic Lead Carbonate. Basic lead carbonate [1319-46-6] (white lead), 2PbC03 Pb(OH)2, mol wt 775.67, d = 6.14g/cm, forms white... 

Although white lead was the oldest white hiding pigment ia paints, it has been totally replaced by titanium dioxide, which has better covering power and is nontoxic (see Pigments). Nevertheless, basic lead carbonate has many other uses, including as a catalyst for the preparation of polyesters from... 

Lead Whites. Basic lead carbonate, sulfate, siHcosulfate, and dibasic lead phosphite are commonly referred to as lead whites. Usage is limited because of environmental restrictions placed on the use of lead-containing compounds. 

Lead chromates are prepared by precipitation techniques from soluble salts ia aqueous media. The raw material Hst iacludes a number of different lead compounds, eg, Htharge, lead nitrate, basic lead acetate, basic lead carbonate, as well as acids, alkahes, sodium bichromate, and sodium chromate. The typical reaction can be represented by the foUowiag equation ... 

Vulcanization was first reported in 1839 with the discovery that heating natural mbber with sulfur and basic lead carbonate produced an improvement in physical properties (2). In 1906, aniline was the first organic compound found to have the abiUty to accelerate the reaction of sulfur with natural mbber (3). Various derivatives of aniline were soon developed which were less toxic and possessed increased acceleration activity. 

Carbon dioxide is used as a chemical reagent in the manufacture of sodium saHcylate, basic lead carbonate or white lead, and sodium, potassium, and ammonium carbonates and bicarbonates. 

Gaseous CO2 is extensively used to carbonate soft drinks and this use alone accounts for 20% of production. Other quasi-chemical applications are its use as a gas purge, as an inert protective gas for welding, and for the neutralization of caustic and alkaline waste waters. Small amounts are also used in the manufacture of sodium salicylate, basic lead carbonate ( white lead ), and various carbonates such as M2CO3 and M HC03 (M = Na, K, NH4, etc.). One of the most important uses of CO2 is to manufacture urea via ammonium carbamate ... 

The reaction of lead with concrete differs from that of aluminium and of zinc in that it is not normally rapid during the early wet stage. It is, however, progressive in damp conditions, and this is said to be due to the fact that the concrete prevents the formation of a protective basic lead carbonate film on the surface of the lead. The packing of lead cables in plaster of Paris is reported to be of doubtful value in preventing corrosion from surrounding concrete. 

Basic pigments Typical pigments in this class are basic lead carbonate, basic lead sulphate, red lead and zinc oxide. 

It has been found that red lead, litharge and certain grades of metallic lead powder render water alkaline and inhibitive this observation has been confirmed by Pryor . The effect is probably due to a lead compound, e.g. lead hydroxide, in solution. Since, however, atmospheric carbon dioxide converts these lead compounds into insoluble basic lead carbonate, thereby removing the inhibitive materials from solution, these pigments may have only limited inhibitive properties in the absence of soap formation. 

Freshly cast lead has a bright, silvery appearance. On exposure to the atmosphere, however, lead in the surface layer combines with atmospheric oxygen and carbon dioxide to form a dark, stable gray coating of mixed lead oxide and basic lead carbonate. This layer usually protects the metal from further oxidation and corrosion (see Fig. 38). Protected by a weathered surface layer, solid lead is stable to further corrosion. Lead is also very ductile and soft, being the softest metal known in antiquity. It is mainly because of these properties that lead was widely used for building, to make pipes and roofs, and in naval construction, for example. Solid lead flows, albeit very... 

Lead carbonate-lead hydroxide (Basic lead carbonate)... 

Bischof An obsolete process for making white lead (basic lead carbonate), invented by G. Bischof around 1900. Lead monoxide was reduced by the carbon monoxide in water-gas to form black lead suboxide, oxidized in damp air to lead hydroxide, slurried in dilute acetic acid, and carbonated with carbon dioxide. Piloted in Willsden, London, and commercialized by L. Mond in the Brimsdown White Lead Company. A sample of Bischof s white lead was used by Holman Hunt in his painting Light of the World, now in St. Paul s Cathedral, London. 

Carter Also known as H.T.S. Carter. A proces for making basic lead carbonate pigment (white lead). Lead monoxide, in a slowly revolving drum, is moistened and sprayed with acetic acid. Carbon dioxide is then introduced. Carbonation is subsequently completed in a separate vessel. See also Dutch, Thompson-Stewart. 

Dutch An obsolete process for making basic lead carbonate pigment (white lead) by exposing metallic lead to vinegar and carbon dioxide. Reportedly first described by Theophrastos around 300 BC. Also known as the Stack process because the metal ingots were arranged in vertical stacks. See also Thompson-Stewart. 

Thenard A process for making white lead pigment (basic lead carbonate) by boiling litharge (lead monoxide) with lead acetate solution and passing carbon dioxide gas into the suspension. 

Thompson-Stewart A process for making basic lead carbonate ( white lead, 2PbC03Pb(0H)2), by reacting lead monoxide ( litharge ) with acetic acid and then with carbon dioxide. Basic lead acetate is an intermediate. See also Dutch, Carter. 

Sandmeyer s valuable isatin synthesis involves the removal of sulphur from diphenylthiourea (I) (p. 169) with basic lead carbonate. Hydrogen cyanide is combined with the reactive diphenylcarbodiimide (II) so obtained, and the nitrile (III) produced is converted by means of hydrogen sulphide into tbe thioamide (IV). Concentrated sulphuric acid brings about ring closure and the product is the a-anil of isatin (V). Then, by hydrolysis with dilute sulphuric acid, aniline is removed ... 

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