Lead chromate

Chromates and dichromates are used in industry as oxidising agents, for example in the coal tar industry, in the leather industry (chrome taiming), and in the dye industry as mordants. Some chromates are used as pigments, for example those of zinc and lead. Chromates and dichromates are poisonous. 

Addition of lead(II) nitrate in ethanoic acid solution gives a yellow precipitate of lead chromate. PbCr04. 

Other compounds are of industrial value lead chromate is chrome yellow, a valued pigment. Chromium compounds are used in the textile industry as mordants, and by the aircraft and other industries for anodizing aluminum. 

Other salts include lead arsenates and lead arsenites (see Insect control technology), lead chromates and lead sihcochromates (see Pigments), lead cyanide (see Cyanides), lead 2-ethyIhexanoate (see Driers and metallic soaps), and lead fluoroborate (see Fluorine compounds, inorganic). 

Owing to environmental regulations, the consumption of lead-based pigments, lead chromate [7738-97-6], in particular, has been decreasing steadily at an average aimual rate of about 5% since the early 1980s. In 1993 production was estimated to be around 20,000 t (see Lead compounds). 

Bismuth V n d te. The use of lead chromate pigments has been slowly phased out of many appHcations. Thus a search for more environmentally acceptable relatively inexpensive yeHow pigments having exceHent coloristic properties has been ongoing. 

The basic compositions of the most common commercially available lead chromate pigments are given ia Table 2 (44). 

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 ... 

Insoluble lead chromate can be chemically treated where appHcable, then filtered, washed, dried, and ground. 

The principal producers of lead chromate pigments ia North America are Cookson, DCC (owned by Kikuchi), Engelhard, and Wayne Pigment. In Europe, lead chromates are produced by Ciba ia Holland, CapeUe ia Erance, and Heubach GmGH ia Germany. 

Sihcon carbide is comparatively stable. The only violent reaction occurs when SiC is heated with a mixture of potassium dichromate and lead chromate. Chemical reactions do, however, take place between sihcon carbide and a variety of compounds at relatively high temperatures. Sodium sihcate attacks SiC above 1300°C, and SiC reacts with calcium and magnesium oxides above 1000°C and with copper oxide at 800°C to form the metal sihcide. Sihcon carbide decomposes in fused alkahes such as potassium chromate or sodium chromate and in fused borax or cryohte, and reacts with carbon dioxide, hydrogen, ak, and steam. Sihcon carbide, resistant to chlorine below 700°C, reacts to form carbon and sihcon tetrachloride at high temperature. SiC dissociates in molten kon and the sihcon reacts with oxides present in the melt, a reaction of use in the metallurgy of kon and steel (qv). The dense, self-bonded type of SiC has good resistance to aluminum up to about 800°C, to bismuth and zinc at 600°C, and to tin up to 400°C a new sihcon nitride-bonded type exhibits improved resistance to cryohte. 

The first chromium compound was discovered in the Ural mountains of Russia, during the latter half of the eighteenth century. Crocoite [14654-05-8] a natural lead chromate, found immediate and popular use as a pigment because of its beautihil, permanent orange-red color. However, this mineral was very rare, and just before the end of the same century, chromite was identified as a chrome bearing mineral and became the primary source of chromium [7440 7-3] and its compounds (1) (see Chromiumand chromium alloys). 

Basic lead silicochromate [11113-70-5] (National Lead Co. designation Pigment M-50) is a composite in which basic lead chromate, ie, chrome orange, is precipitated onto a lead siUcate—siUca base. It does not have an appreciable chromate solubiUty and depends on lead oxide for its effectiveness. 

Lead Chromates and Molybdates. The lead chromates appear in several shades of yellow. The primrose and lemon are solid solutions of lead sulfate in the chromate and have the stable monoclinic stmcture. The medium shade contains no sulfate. Chrome orange is a compound with lead oxide (PbCrO TbO). Molybdate orange is a combination of lead chromate and sulfate with molybdate (PbMoO ). These pigments have the advantages of... 

The most common toxic metals in industrial use are cadmium, chromium, lead, silver, and mercury less commonly used are arsenic, selenium (both metalloids), and barium. Cadmium, a metal commonly used in alloys and myriads of other industrial uses, is fairly mobile in the environment and is responsible for many maladies including renal failure and a degenerative bone disease called "ITA ITA" disease. Chromium, most often found in plating wastes, is also environmentally mobile and is most toxic in the Cr valence state. Lead has been historically used as a component of an antiknock compound in gasoline and, along with chromium (as lead chromate), in paint and pigments. 

One other case Involving metal compounds should be noted. Some metal compounds may contain more than one listed metal. For example, lead chromate Is both a lead compound and a chromium compound. In such cases, if applicable... 

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