Coprecipitation impurities

A coprecipitated impurity in which the interfering ion occupies a lattice site in the precipitate. 

Precipitate particles grow in size because of the electrostatic attraction between charged ions on the surface of the precipitate and oppositely charged ions in solution. Ions common to the precipitate are chemically adsorbed, extending the crystal lattice. Other ions may be physically adsorbed and, unless displaced, are incorporated into the crystal lattice as a coprecipitated impurity. Physically adsorbed ions are less strongly attracted to the surface and can be displaced by chemically adsorbed ions. 

Occlusions, which are a second type of coprecipitated impurity, occur when physically adsorbed interfering ions become trapped within the growing precipitate. Occlusions form in two ways. The most common mechanism occurs when physically adsorbed ions are surrounded by additional precipitate before they can be desorbed or displaced (see Figure 8.4a). In this case the precipitate s mass is always greater than expected. Occlusions also form when rapid precipitation traps a pocket of solution within the growing precipitate (Figure 8.4b). Since the trapped solution contains dissolved solids, the precipitate s mass normally increases. The mass of the precipitate may be less than expected, however, if the occluded material consists primarily of the analyte in a lower-molecular-weight form from that of the precipitate. 

A coprecipitated impurity trapped within a precipitate as it forms. 

A coprecipitated impurity that adsorbs to the surface of a precipitate. 

A special type of aging occurs in calcium oxalate, which at room temperature is precipitated as a mixture of dihydrate and trihydrate. Upon digestion at higher temperatures these products become metastable with respect to the monohydrate. As a result of the drastic recrystallization, coprecipitated impurities are largely removed by digestion. 

Coprecipitated impurities may cause either negative or positive errors in an analysis. II the contaminant is not a compound of the ion being determined, a positive error will always result. Thus, a positive error is observed whenever colloidal silver... 

Coprecipitated impurities, especially those on the surface, can be removed by washing the precipitate after filtering. The precipitate will be wet with the mother liquor, which is also removed by washing. Many precipitates cannot be washed with pure water, because peptization occurs. This is the reverse of coagulation, as previously mentioned. 

The sulphide leaching plant (SUP) receives feed from all of the fiont-end zinc plants roasters, ZPL, and OLP. This plant treats calcine, ZPL slurry, and OLP electrolyte using a weak acid and neutral leaching process to produce impure SLP electrolyte and residue. The residue consists mainly of zinc foiites, paragoethite, jarosites, lead sulphate, as well as coprecipitated impurities. The residue slurry is fed to the lead smelter. 

Inclusions, occlusions, and surface adsorbates are called coprecipitates because they represent soluble species that are brought into solid form along with the desired precipitate. Another source of impurities occurs when other species in solution precipitate under the conditions of the analysis. Solution conditions necessary to minimize the solubility of a desired precipitate may lead to the formation of an additional precipitate that interferes in the analysis. For example, the precipitation of nickel dimethylgloxime requires a plT that is slightly basic. Under these conditions, however, any Fe + that might be present precipitates as Fe(01T)3. Finally, since most precipitants are not selective toward a single analyte, there is always a risk that the precipitant will react, sequentially, with more than one species. 

In order to make an efficient Y202 Eu ", it is necessary to start with weU-purifted yttrium and europium oxides or a weU-purifted coprecipitated oxide. Very small amounts of impurity ions, particularly other rare-earth ions, decrease the efficiency of this phosphor. Ce " is one of the most troublesome ions because it competes for the uv absorption and should be present at no more than about one part per million. Once purified, if not already coprecipitated, the oxides are dissolved in hydrochloric or nitric acid and then precipitated with oxaflc acid. This precipitate is then calcined, and fired at around 800°C to decompose the oxalate and form the oxide. EinaHy the oxide is fired usually in air at temperatures of 1500—1550°C in order to produce a good crystal stmcture and an efficient phosphor. This phosphor does not need to be further processed but may be milled for particle size control and/or screened to remove agglomerates which later show up as dark specks in the coating. 

Steps are normally taken to prevent the simultaneous precipitation of materials other than the desired analyte species. Incorporation of impurities into the precipitate may however occur by coprecipitation or post-precipitation. The former arises during the formation of the precipitate, and the latter after it has been formed. The various modes of coprecipitation are summarized in Table 5.16. 

This method requires the addition of a mixed M(II)/M(III) salt solution to an alkaline solution containing the desired interlayer anion. Preparations under conditions of high supersaturation generally give rise to less crystalline materials, because of the high number of crystallization nuclei. Because this method leads to a continuous change in solution pH, the formation of impurity M(0H)2 and/or M(OH)3 phases, and consequently an LDH product with an undesired M(II)/M(III) ratio, often results. Thermal treatment performed following coprecipitation may help increase the crystallinity of amorphous or badly crystallized materials. 

The impurities can become incorporated in the solid phase in three modes i) interstitially, i.e. between regular lattice positions, ii) by coprecipitation as a separate insoluble phase or iii) by isomorphous substitution of... 

Adsorbed, occluded, and included impurities are said to be coprecipitated. That is, the impurity is precipitated along with the desired product, even though the solubility of the impurity has not been exceeded. Coprecipitation tends to be worst in colloidal precipitates... 

Re(I) product [Re (DMPE)3] + along with small amounts of the red-purple Re(II) analog [Ren(DMPE)3]2+ and the red Re(III) impurity [Re111 (DMPE)2(SPh)2]+.6 This purple solution is concentrated by solvent evaporation to provide a semisolid which is redissolved in dichloromethane (or methanol) (1 mL). Diethyl ether is added dropwise until no further white precipitate appears impurities may coprecipitate if the diethyl ether is added too rapidly. The purple solution can then be removed and the white solid collected by filtration to afford [Re(DMPE)3]CF3SC>3 in about 50% yield. 

As discussed earlier in Section 3.17, the excessive application of arsenic-bearing pesticides and phosphate fertilizers on agricultural lands, golf courses, and lawns may locally contaminate surface waters and ground-waters (Welch et al., 2000), (Lewis et al., 2002), 590. Phosphates desorb arsenic from mineral surfaces and readily interfere with the sorption and coprecipitation of arsenic onto iron (oxy)(hydr)oxides (Campos, 2002). Commercial phosphate fertilizers also frequently contain >13 mg kg-1 of arsenic impurities (Campos, 2002), which may further contribute to groundwater contamination. 

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