Lead chromate, common

Chromium metal is commercially produced in the United States by the reduction of chromite ore with carbon, aluminum, or silicon, and subsequent purification. Sodium chromate and dichromate are produced by roasting chromite ore with soda ash. Most other chromium compounds are produced from sodium chromate and dichromate (Hartford 1979 Westbrook 1979). For example, basic chromic sulfate (Cr(0H)S04), commonly used in tanning, is commercially produced by the reduction of sodium dichromate with organic compounds (e.g., molasses) in the presence of sulfuric acid or by the reduction of dichromate with sulfur dioxide. Lead chromate, commonly used as a pigment, is produced by the reaction of sodium chromate with lead nitrate or by reaction of lead monoxide with chromic acid solution (IARC 1990). 

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

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. 

Before lead in paint was discontinued, lead chromate was a common pigment in yellow paint. A 1.0-L solution is prepared by mixing 0.50 mg of lead nitrate with 0.020 mg of potassium chromate. Will a precipitate form What should [Pb2+] be to just start precipitation ... 

Other oxidizers, including barium chromate (BaCrO,), lead chromate (PbCrO 4), sodium nitrate (NaNO 3), lead dioxide (PbO 2), and barium peroxide (BaO 2) will also be encountered in subsequent chapters. Bear in mind that reactivity and ease of ignition are often related to the melting point of the oxidizer, and the volatility of the reaction products determines the amount of gas that will be formed from a given oxidizer /fuel combination. Table 3.2 contains the physical and chemical properties of the common oxidizers, and Table 5.8 lists the melting and boiling points of some of the common reaction products. 

Mixed chrome greens usually contain considerable quantities (up to 80-90%) of inert substances in the commoner types the content of lead chromate varies from 3 to 10% and that of Prussian blue from 5 to 20%. 

The rate of aging is strongly influenced by other solutes in solution and thus can be increased or decreased by the presence of excess lattice ions in solution. Barium sulfate ages more slowly in barium ion solution than in sulfate and more slowly in sulfate than in water. The aging of silver chloride is impeded by silver ion, but speeded by chloride ion a similar effect exists for silver bromide. For lead chromate no particular lattice ion effect was noticed. Apparently the rate of aging does not parallel solubility, which is decreased by the common ion effect. Kolthoff and others postulated that the solubility in the adsorbed water layer may be different from that in the bulk of the solution. For example, in the case of silver chloride in the presence of adsorbed chloride ion, the solubility may be increased owing to the formation (Section 7-7) of AgCl2 in the immediate vicinity of the surface. It appears likely that the adsorbed lattice ion also has a pronounced effect on the rate of recrystallization, which is not necessarily parallel with solubility even in the adsorbed water layer. 

The common ion effect and Le Chatelier s principle A saturated solution of lead chromate (PbCr04) is shown in Figure 18-17a. Note the solid yellow PbCr04 in the bottom of the test tube. The solution and solid are in equilibrium according to this equation. 

The banning of lead chromate pigments has resulted in an enormous demand for yellow azo pigments as replacements. Pigment Yellow 74 (XXX) has become important as such a replacement in the past 10 years. The structure is written in the hydrazone form. There has been evidence over recent years that this is the preferred tautomeric form of these types of pigments in the solid state (39-41) versus the more commonly shown azo form (XXXl). 

E. The addition of lead nitrate (PbfNOs)) to a saturated solution of lead chromate (PbCr04) will produce the common ion effect. 

The main use for lead today is in solder, bullets (it is one of the heaviest stable elements), and car (lead-acid) batteries. Because it s so malleable and soft, yet not prone to rusting (oxidation), it was thought to be excellent for piping until its toxicity was understood. Lead can still be found in some paints because it creates certain colors such as chrome yellow (lead chromate), white (lead sulfate), and red (lead tetroxide). Lead was also widely used as an additive to automotive fuel in the form of tetra-ethyllead, but it is not as common anymore. 

Chromate-containing minerals are unusual, but an example of a fairly common chromate substance is crocoite, which is a form of lead chromate (PbCrO ) and is a brilfiant orange color. 

HISTORY. Chromium was discovered by the French chemist Vauquel in 1797, while he was studying the properties of crocoite, an ore which is rich in lead chromate, its common name of chrome was derived from the Creek word chroma, which means color, because the element is present in many different colored compounds. These compounds have long been used as pigments in dyeing, and in the tanning of leather. In the early 1900s, chromium became... 

Synonym, variant or common name See lead chromate(VI) oxide. 

According to Haug (1951), monoclinic lead chromate shows tile greater lightfastness of the chrome yellows, which tend to darken with age (for example, Haug Watson and Clay), and as stated by Kuhn and Curran, the modem hghtfast pigments are commonly based on the monoclinic form, to which photochemical stabilisers are added. 

Lead pigments are commonly used in paints, although less toxic pigments are presently used preferentially. Red lead (minimum) is used extensively in the painting of structural steel, and lead chromate is used as a yellow pigment (see Table 1). At... 

The purity of the zinc is unimportant, within wide limits, in determining its life, which is roughly proportional to thickness under any given set of exposure conditions. In the more heavily polluted industrial areas the best results are obtained if zinc is protected by painting, and nowadays there are many suitable primers and painting schemes which can be used to give an extremely useful and long service life under atmospheric corrosion conditions. Primers in common use are calcium plumbate, metallic lead, zinc phosphate and etch primers based on polyvinyl butyral. The latter have proved particularly useful in marine environments, especially under zinc chromate primers . 

Zinc chromate and zinc tetroxychromate have also been used successfully in anticorrosive paints. Both pigments function by releasing chromate ions which passivate the steel surface. In common with lead pigments, those... 

The least expensive (and most common) objectives are the achromatic objectives that are designed to limit the effects of chromatic and spherical aberration. Achromatic objectives are corrected to bring two wavelengths of light (typically red and blue) into focus in the same plane. The limited correction of achromatic objectives leads to problems with color microscopy and photomicrography. 

As you learned in Chapter 8, this phenomenon is called the common ion effect. The observed result is predicted hy Le Chatelier s principle. Adding a common ion to a solution increases the concentration of that ion in solution. As a result, equilibrium shifts away from the ion. In this example, adding chromate ions causes the equilibrium to shift to the left, and lead(II) chromate precipitates. 

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