Coprecipitation

In the coprecipitation method, trace metals are concentrated by adsorbing onto the surface of a precipitate. As the precipitate, hydroxides of Fe(III), Mn(IV), Al(IH), Bi(II), and Zr(IV) and sulfides of Co(II), Pb(II) and Fe(II) are frequently used. In AAS the use of hydroxide precipitates is more popular than that of sulfides. Coprecipitation using the hydroxide precipitates is generally unspecific, and many trace metals may be concentrated simultaneously. However, some limitations are often found in the coprecipitation technique [10], 

Contamination of the analytes from the carriers (the precipitates) should be first examined, and the blank test carried out carefully. Great care should also be taken in terms of the recoveries of the analytes, because the procedures in the coprecipitation are sometimes time-consuming and irre-producible. Some efficiencies of recovery for Zr(IV) coprecipitation along with the determined values of trace elements in seawater are summarized in Table 7, where inductively-coupled plasma (ICP) emission spectrometry was applied for the simultaneous multielement analysis [45]. In this experiment, 10 mg of Zr(IV) was added to 11 of seawater, the precipitation made 

DETERMINATION OF TRACE ELEMENTS IN SEAWATER AND RECOVERIES USING THE ZIRCONIUM-COPRECIPITATION TECHNIQUE 

Element Concentration (ng ml 1) Recovery (%)a Element Concentration (ngml-1) Recovery (%)b 

The coprecipitation method combined with colloid flotation using stearylamine, sodium oleate, etc. has been used to preconcentrate analytes in seawater [49, 50], 

Preparation by coprecipitation forms two active phases. This is based on two different precursors and a precipitating agent. As examples, we consider the preparation of copper chromite and Co-Cu spinel, using different coprecipitation methods. 

The solutions Na2Cr207 and Cu(N03)2 are mixed in stoichiometric amounts and precipitated in the presence of a precipitant agent NH4(OH). The reactions are as follows [10]  

Preparing mixed oxides having different ratios of Cu/Co by coprecipitation, starting from the solutions Co(N03)4, Cu(N03)2, we obtain oxides with spinel-type stmcrnre CU tCOy04 [10]. 

The temperature is constant at 70 °C, and pH varies between 8 and 9 during the precipitation process. Nitrate concentrations are equal to 0.5 M and precipitant equal to 1 M. The following reaction occurs  

The precipitate should be washed repeatedly to remove the ions, dried at 110 °C/ 15 h, and calcined at 350 °C/6 h under flowing air at 30 ml/min. The calcined precipitate showed different mixed oxides, as presented in Table 7.1. 

This method is closely related to precipitation, but makes use of the fact that when / ertain substances are precipitated other substances are coprecipitated. In radiochemistry in particular, coprecipitation with isotopic or non-isotopic carriers is still commonly practised today. In classical water analysis coprecipitation is often used to concentrate trace elements. In this way, for example, arsenic can be more or less completely coprecipitated and isolated with iron (III) hydroxide. With manganese in a manganese (III) or manganese (IV) compound, thallium can be coprecipitated and concentrated. Sections 3.2 to 3.5 and Section 3.7 deal with cases where precipitation and coprecipitation are still in general use in water 

By coprecipitating the catalytically active component and the support to give a mixture that is subsequently dried, calcined (heated in air), and reduced to yield a porous material with a high surface area. This procedure is followed when materials are cheap and obtaining the optimum catalytic activity per unit volume of catalyst is the main consideration. 

Some characterisation of catalysts prepared in this way has been reported.59,60 It is not however clear whether all the gold in solution is 

This method of ferrite formation is based on the formation of aqueous solutions of chlorides, nitrates or sulphates of Fe, and of divalent Ni, Co, Mg, Ba, Sr, etc., in the concentrations required for the ferrite composition, and their simultaneous precipitation in the form of hydroxides by NaOH. The precipitate is then filtered, washed and dried. The ferrite particles are obtained by calcination at 180-300 °C in air. Particles with a narrow size distribution in the range 50-500 nm may be obtained, with high purity. The final ferrite body is obtained by sintering at temperatures considerably lower than in the case of the ceramic method. 

A common source of inhomogeneity in the ferrite product is associated with changes of concentration and pH during precipitation. The various metal hydroxides can have different solubility dependences on pH, and the cation ratios in the precipitate can be different from those in the solution. To avoid this problem, the solution of metal ions is usually added to the alkaline solution slowly and dropwise (McColm Clark, 1988). If instead the alkaline solution is added to the solution of metal ions, the pH of the suspension must be brought to a value above 10 to avoid differences in precipitate composition (Takada Kiyama, 1971). 

Ferrites can also be synthesised by the coprecipitation of oxalates. The hydrated iron oxalate, FeC204 2H2O and the oxalates of Mn, Co, Ni or Zn can be coprecipitated from solution by solvent evaporation (Wickham, 1967). The Fe/Me stoichiometry can be accurately controlled to within 1%. 

Iron hydroxide, Fe(0H)3-nH20, decomposes with temperature to y-Fc203, which is more reactive than a-Fc203. As a result, strontium hexa-ferrite, SrFei20i9, can be obtained from coprecipitates of Fe(0H)3-nH20 and strontium laureate, Sr[CH3(CH2)ioCOO]2, at temperatures as low as 550 °C (Qian Evans, 1981). By contrast, mechanical mixtures of a-Fe203 and SrO show an appreciable reaction rate only at T 720 °C. 

Some ferrites that have been obtained by coprecipitation are shown in Table 3.2. Besides the preparation of powders for the ferrite industry, raw materials prepared by coprecipitation are used for the manufacture of pigments and magnetic toners and for the removal of heavy metal ions from waste water (Takada, 1982). 

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More water soluble than di-methylglyoxime less subject to coprecipitation with metal chelate. 

The stoichiometry must be exact. Coprecipitation by solid-solution formation, foreign ion entrapment, and adsorption are possible sources of error. 

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. 

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. 

A coprecipitated impurity trapped within a precipitate as it forms. 

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

Trace metals in sea water are preconcentrated either by coprecipitating with Ee(OH)3 and recovering by dissolving the precipitate or by ion exchange. The concentrations of several trace metals are determined by standard additions using graphite furnace atomic absorption spectrometry. 

However, the quantity of Pa produced in this manner is much less than the amount (more than 100 g) that has been isolated from the natural source. The methods for the recovery of protactinium include coprecipitation, solvent extraction, ion exchange, and volatility procedures. AH of these, however, are rendered difficult by the extreme tendency of protactinium(V) to form polymeric coUoidal particles composed of ionic species. These caimot be removed from aqueous media by solvent extraction losses may occur by adsorption to containers and protactinium may be adsorbed by any precipitate present. 

Powder Preparation. The goal in powder preparation is to achieve a ceramic powder which yields a product satisfying specified performance standards. Examples of the most important powder preparation methods for electronic ceramics include mixing/calcination, coprecipitation from solvents, hydrothermal processing, and metal organic decomposition. The trend in powder synthesis is toward powders having particle sizes less than 1 p.m and Httie or no hard agglomerates for enhanced reactivity and uniformity. Examples of the four basic methods are presented in Table 2 for the preparation of BaTiO powder. Reviews of these synthesis techniques can be found in the Hterature (2,5). 

K. Osseo-Asare, F. J. Arriagada, and J. H. Adair, "Solubility Relationships in the Coprecipitation Synthesis of Barium Titanate Heterogeneous Equihbria in the Ba—Ti—C2O4—H2O System," in G. L. Messing, E. R. Fuller, Jr., and Hans Hausin, eds.. Ceramic Powder Science Vol. 2,1987, pp. 47-53. 

Homogeneous distribution can be attained by controlled coprecipitation of hydroxides which are then decomposed by calciaation yielding powders of fine particle si2es. Active siaterable powders are produced commercially, usually by hydrolysis of a mixture of ZrOCl2 and YCl to precipitate the mixed... 

The magnesia and alumina suspension is prepared by treatment of an aqueous solution, containing aluminum and magnesium salt in the desired proportion, with sodium hydroxide. The coprecipitated aluminum and magnesium hydroxides are collected by filtration, washed free of soluble salts, and stabilized by the addition of a suitable hexatol. 

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. 

Another important class of titanates that can be produced by hydrothermal synthesis processes are those in the lead zirconate—lead titanate (PZT) family. These piezoelectric materials are widely used in manufacture of ultrasonic transducers, sensors, and minia ture actuators. The electrical properties of these materials are derived from the formation of a homogeneous soHd solution of the oxide end members. The process consists of preparing a coprecipitated titanium—zirconium hydroxide gel. The gel reacts with lead oxide in water to form crystalline PZT particles having an average size of about 1 ]lni (Eig. 3b). A process has been developed at BatteUe (Columbus, Ohio) to the pilot-scale level (5-kg/h). 

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. 

The lanthanum phosphate phosphor is usually prepared by starting with a highly purified coprecipitated oxide of lanthanum, cerium, and terbium blended with a slight excess of the stoichiometric amount of diammonium acid phosphate. Unlike the case of the aluminate phosphor, firing is carried out in an only slightly reducing or a neutral atmosphere of nitrogen at a temperature 1000° C. Also this phosphor is typically made with the addition of a flux,... 

The GdAlgB O QiCe ", Tb " is synthesized by a soHd-state firing of the rare-earth coprecipitated oxide plus boric acid and MgCO at 900° C in a slightly reducing atmosphere. As in the case of the lanthanum phosphate phosphor, a flux is usually used. The synthesis of this phosphor is further comphcated, however, by the fact that it is a ternary system and secondary phases such as gadolinium borate form and must then react to give the final phosphor. 

The toxic nature of mercury and its compounds has caused concern over environmental pollution, and governmental agencies have imposed severe restrictions on release of mercury compounds to waterways and the air (see Mercury). Methods of precipitation and agglomeration of mercurial wastes from process water have been developed. These methods generally depend on the formation of relatively insoluble compounds such as mercury sulfides, oxides, and thiocarbamates. MetaUic mercury is invariably formed as a by-product. The use of coprecipitants, which adsorb mercury on their surfaces facihtating removal, is frequent. 

Uranium. The uranium product from the PUREX process is in the form of uranyl nitrate which must be converted to some other chemical depending on anticipated use. One route to MO fuel is to mix uranium and plutonium nitrates and perform a coprecipitation step. The precipitate is... 

Raw juice is heated, treated sequentially with lime (CaO) and carbon dioxide, and filtered. This accomplishes three objectives (/) microbial activity is terminated (2) the thin juice produced is clear and only lightly colored and (J) the juice is chemically stabilized so that subsequent processing steps of evaporation and crystalliza tion do not result in uncontrolled hydrolysis of sucrose, scaling of heating surfaces, or coprecipitation of material other than sucrose. 

Coprecipitation is a technology by which many metals such as arsenic will adsorb on alum or iron docs and be effectively removed over a near-neutral pH range. The disadvantage of coprecipitation is the generation of large quantities of sludge. 

Purification actually starts with the precipitation of the hydrous oxides of iron, alumina, siUca, and tin which carry along arsenic, antimony, and, to some extent, germanium. Lead and silver sulfates coprecipitate but lead is reintroduced into the electrolyte by anode corrosion, as is aluminum from the cathodes and copper by bus-bar corrosion. 

The method of preparation of a support material has a tremendous effect on its properties (11). For example, zeoHtes, which are highly stmctured aluminosihcates, are known to be extremely sensitive to the conditions employed both during and after crystallization (12). Also, when siUca—titania is precipitated by a coprecipitation method using ammonia, in which localized hydroxide ion gradients are estabUshed by the precipitation process itself, the product is much more acidic than when it is precipitated using urea, which suppHes hydroxide ion slowly and uniformly during precipitation (13). 

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