Additive coprecipitation

Precipitation via Extended Chemicai Reaction. This can, perhaps, be considered as a subset of the previously described coprecipitation methodology however, in this case, a number of solution or surface processes occur prior to the formation of the final catalyst. In addition, coprecipitation involves the reaction between an acid and a base, but, in many cases, a catalyst precursor can be formed in the absence of a base. This is particularly the case for the formation of phosphates (eg, AIPO4, BPO4, and VPO catalysts). Typically, a metal salt is reacted with a phosphorus compound, typically phosphoric acid, under controlled conditions. The choice of the metal salt can influence the catalyst precursor structure. For example, for AIPO4, a catalyst for the formation of isoprene from the dehydration of 2-methylbutanal, the reaction of aluminium chloride with phosphoric acid gives a mixture of cristobalite and tridymite phases of AIPO4, whereas... 

Several additional coprecipitates were prepared with Nl/Al, Cu/Al and Cu/Cr in which sequential precipitation was conducted for atomic ratios of 0.1-0.2 with oxalic present prior to addition of the Ni or Cu to the A1 or Cr hydroxide. The results for these catalysts are summarized in Tables 3... 

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. 

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. 

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. 

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 method of stripping voltammetry (SV) is one of the most perspective methods in concentration range of thallium(I) determination of 10 -10 M. Achievement of high sensitivity of thallium(I) determination needs carrying out its additional concentration and sepai ation from other metals which ai e close by electrochemical properties. For these purposes it is offered to use a method of coprecipitation with collector. The combination of SV and a method of coprecipitation on a collector have shown that minimum detectable concentration can be decreased by 2-3 orders of magnitude. 

An additional 3-5 g. (6-10%) of crude product can be obtained by concentrating the filtrate to about 800 ml., cooling the concentrate to 0°, and filtering the solid that separates. This solid must be washed well with cold water to remove coprecipitated potassium chloride. 

Ml Thickener It is a coarse, granular material ranging in color from light tan to brown. Chemically, it is a coprecipitated A1 soap of naphthenic, oleic and palmitic acids in the approx ratio of 1 1 2. The three main disadvantages accruing from the properties of Ml thickener are (1) it is a critical wartime material since it is made from coconuts (2) it is extremely hygroscopic, and the presence of water decreases its ability to form a stable thickened fuel. In addition, the relatively large... 

Blood and urine 206Pb addition and sample acid digestion lead coprecipitation by addition of Ba(N03)2, IDMS No data 98-99 Manton and Cook 1984... 

Precipitation may also be employed in solvents other than water. For example, nanocrystals of ZnS Tb could be prepared by a coprecipitation was effected by addition of aqueous sodium sulfide to Tb(N03)3 and zinc acetate dissolved in methanol.114 ZnS TbF3 and ZnS Eu were prepared similarly. The photoluminescent intensities of the nanocrystals of ZnS Tb and ZnS Eu prepared in this way were 2.5 and 2.8 times stronger than those of bulk (conventionally prepared) phosphors, and these nanocrystals have been proposed for FED, EL, PDP, and CRT applications.114... 

Tsunogai and Nozaki [6] analysed Pacific Oceans surface water by consecutive coprecipitations of polonium with calcium carbonate and bismuth oxychloride after addition of lead and bismuth carriers to acidified seawater samples. After concentration, polonium was spontaneously deposited onto silver planchets. Quantitative recoveries of polonium were assumed at the extraction steps and plating step. Shannon et al. [7], who analysed surface water from the Atlantic Ocean near the tip of South Africa, extracted polonium from acidified samples as the ammonium pyrrolidine dithiocarbamate complex into methyl isobutyl ketone. They also autoplated polonium onto silver counting disks. An average efficiency of 92% was assigned to their procedure after calibration with 210Po-210Pb tracer experiments. 

In the oxalate coprecipitation method, oxalate acid reacts with metal cations to form the precipitate, which is subsequently calcined to obtain the products. The advantage of oxalate acid as a precipitant is that, unlike hydroxides, oxalates are less sensitive to the treatment conditions, such as washing and drying. In addition, the... 

Nevertheless, the oxalate coprecipitation method has some problems. For example, this method usually results in rodlike doped ceria particles, which are agglomerations of smaller particles with irregular shapes. Hence, the green density of the compact body is relatively low, so it is difficult to fabricate a dense electrolyte film or membrane. In addition, the poor flow of the rodlike powder makes forming difficult. 

By carrying out selective CO oxidation with some addition of C02 and H20 to the feed gas over a similar nanostructured CTy.Cej x02 catalyst prepared by a coprecipitation method [56], 15 vol% of C02 in the feed gas decreases the activity of the catalyst. Under these conditions the same values of activity and selectivity were obtained at temperatures 15 to 35°C higher. The addition of 10 vol% of H20 shifted the activity and selectivity curves to temperatures 20 to 40°C higher with respect to the curves where only CO, 02, H2, and He were used in the feed. 

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