Co-precipitation

Using a similar approach, Xu and co-workers also started from preformed metal particles to prepare Pd and Pt Ce02 samples. In their case, the preformed Pt and Pd colloids were protected by citrate and PVP, respectively, mixed with CeCl3 in an aqueous solution with urea and sutyected to a hydrothermal treatment. The hydrolysis of urea in warm water formed ammonia, turning the solution basic and precipitating Ce(OH) species around the preformed particles. The final material was then composed of Pd and Pt cores embedded in a ceria matrix (Fig. 7.3). 

The applicability of this technique is limited to metal hydroxides or carbonates that can be co-precipitated with Au(OH)g. Gold can be supported in the form of well-dispersed nanoparticles, by CP, on a-Fe203, C03O4, NiO and ZnO, but not on Ti02, Cr203, MnO, and CdO [28]. 

Supported metal oxides can be prepared through co-precipitation, in which metal precursors to both the support and the supported metal oxide are induced to form the support material and the supported layer simultaneously. The support and the supported metal oxide are more spatially distributed than materials derived from the various deposition methods, but a fraction of the supported metal oxide may be located below the surface, leading to overall lower metal oxide dispersion. 

Zuo et al. [87] prepared homogeneous 60-90 nm Bao.6Sro.4Ti03 nanoparticles with fine characteristics at a low synthesis temperature of 650°C by a citrate method with ammonium nitrate addition. 

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Taking francium as an example, it was assumed that the minute traces of francium ion Fr could be separated from other ions in solution by co-precipitation with insoluble caesium chlorate (VII) (perchlorate) because francium lies next to caesium in Group lA. This assumption proved to be correct and francium was separated by this method. Similarly, separation of astatine as the astatide ion At was achieved by co-precipitation on silver iodide because silver astatide AgAt was also expected to be insoluble. 

Initially, the only means of obtaining elements higher than uranium was by a-particle bombardment of uranium in the cyclotron, and it was by this means that the first, exceedingly minute amounts of neptunium and plutonium were obtained. The separation of these elements from other products and from uranium was difficult methods were devised involving co-precipitation of the minute amounts of their salts on a larger amount of a precipitate with a similar crystal structure (the carrier ). The properties were studied, using quantities of the order of 10 g in volumes of... 

The biogeochemical processes that generally describe the interaction of elements with particles are quite well known dissolution, flocculation, ion exchange, sorption, (co)precipitation, electron transfer, and biological uptake. In aquatic environments these reactions often occur simultaneously and competitively. In order to utilize marine tracers effectively, we must understand how elements are associated with particles and sediments. 

Aqueous hydrofluoric acid can be freed from lead by adding ImL of 10% strontium chloride per KXhnL of acid, lead being co-precipitated as lead fluoride with the strontium fluoride. If the hydrofluoric acid is decanted from the precipitate and the process repeated, the final lead content in the acid is less than 0.003 ppm. Similarly, lead can be precipitated from a nearly saturated sodium carbonate solution by adding 10% strontium chloride dropwise (l-2mL per lOOmL) followed by filtration. (If the sodium carbonate is required as a solid, the solution can be evaporated to dryness in a platinum dish.) Removal of lead from potassium chloride uses precipitation as lead sulfide by bubbling H2S, followed, after filtration, by evaporation and recrystallisation of the potassium chloride. 

Hydrofluoric acid [7664-39-3] M 20.0, b 112.2"(aq azeotrope, 38.2% HF), d 1.15 (47-53% HF), pK 3.21. Freed from lead (Pb ca 0.002ppm) by co-precipitation with Srp2, by addition of lOmL of 10% SrCl2 soln per kilogram of the cone acid. After the ppte has settled, the supernatant is decanted through a filter in a hard-rubber or paraffin lined-glass vessel [Rosenqvist Am J Sci 240 358 1942. Pure aqueous HF solutions (up to 25M) can be prepared by isothermal distn in polyethylene, polypropylene or platinum apparatus [Kwestroo and Visser Analyst 90 297 7965]. HIGHLY TOXIC. 

The usual means of identifying and quantifying the level of these additives in polymer samples is performed by dissolution of the polymer in a solvent, followed by precipitation of the material. The additives in turn remain in the Supernatant liquid. The different solubilites of the additives, high reactivity, low stability, low concentrations and possible co-precipitation with the polymer may pose problems and lead to inconclusive results. Another sample pretreatment method is the use of Soxhlet extraction and reconcentration before analysis, although this method is very time consuming, and is still limited by solubility dependence. Other approaches include the use of supercritical fluids to extract the additives from the polymer and Subsequent analysis of the extracts by microcolumn LC (2). 

The method based on the precipitation of peroxometalate precursors enables to achieve additional purification during the process. Thus, the addition of complexonates, such as OEDP or EDTA, which form stable complexes with some polyvalent metals, prevents co-precipitation of the main impurities such as Fe, Co, Ni, Mn, Mg, etc., which in turn significantly increases the purity of the final product. Enhanced purification can also be achieved by recrystallization of the precursor. Particularly, the precipitation of ammonium peroxofluorometalates, such as ammonium peroxofluoroniobate ((NH4)3Nb04F4), as a primary precursor, leads to significant reduction of the titanium contamination. Ammonium peroxofluoroniobate, (NH4)3Nb04F4, is... 

Potassium heptafluorotantalate, K2TaF7, or as it is called by its commercial name K-salt, is a starting material for tantalum metal production. K-salt is produced by adding potassium fluoride, KF, or potassium chloride, KC1, to a tantalum strip solution that results from a liquid-liquid extraction process. In order to prevent hydrolysis and co-precipitation of potassium oxyfluoro-tantalate, a small excess of HF is added to the solution [24]. Another way to avoid the possible formation and co-precipitation of oxyfluoride phases is to use potassium hydrofluoride, KHF2, as a potassium-containing agent. The yield of the precipitation depends mostly on the concentration of the potassium-containing salt and is independent of the HF concentration [535]. 

Potassium heptafluorotantalate, K2TaF7, precipitates in the form of transparent needles. The precipitated particles must not be too fine, since fine powder usually promotes co-precipitation and adsorption of some impurities from the solution. Even niobium can be adsorbed by the surface of K2TaF7 developed during precipitation, as shown by Herak et al. [535]. On the other hand, the precipitation of large K-salt crystals should not be strived for either. Laboratory and industrial experience indicates that excessively large crystals usually contain small drops of solution trapped within the crystals. This occluded solution can remain inside of the crystal until drying and will certainly lead the hydrolysis of the material. 

Re-crystallization also leads to a significant reduction in the level of carbon and oxygen impurities however, the reduction of cationic impurities is usually not efficient. To prevent co-precipitation of complex fluorotantalates of polyvalent metals, some suitable complexonates are added to the solution. 

Since, however, K2 is comparatively large, the solvent effect is relatively small this is why in the quantitative separation of barium sulphate, precipitation may be carried out in slightly acid solution in order to obtain a more easily filterable precipitate and to reduce co-precipitation (Section 11.5). 

Notes. (1) The usefulness of the HHSNNA indicator for the titration of calcium depends upon the fact that the pH of the solution is sufficiently high to ensure the quantitative precipitation of the magnesium as hydroxide and that calcium forms a more stable complex with EDTA than does magnesium. The EDTA does not react with magnesium [present as Mg(OH)2] until all the free calcium and the calcium-indicator complex have been complexed by the EDTA. If the indicator is added before the potassium hydroxide, a satisfactory end-point is not obtained because magnesium salts form a lake with the indicator as the pH increases and the magnesium indicator-lake is co-precipitated with the magnesium hydroxide. 

When a precipitate separates from a solution, it is not always perfectly pure it may contain varying amounts of impurities dependent upon the nature of the precipitate and the conditions of precipitation. The contamination of the precipitate by substances which are normally soluble in the mother liquor is termed co-precipitation. We must distinguish between two important types of co-precipitation. The first is concerned with adsorption at the surface of the particles exposed to the solution, and the second relates to the occlusion of foreign substances during the process of crystal growth from the primary particles. 

Post-precipitation differs from co-precipitation in several respects ... 

Precipitation should be carried out in dilute solution, due regard being paid to the solubility of the precipitate, the time required for filtration, and the subsequent operations to be carried out with the filtrate. This will minimise the errors due to co-precipitation. 

It must be pointed out that the above calculation is approximate only, and may be regarded merely as an illustration of the principles involved in considering the precipitation of sulphides under various experimental conditions the solubility products of most metallic sulphides are not known with any great accuracy. It is by no means certain that the sulphide ion S 2 is the most important reactant in acidified solutions it may well be that in many cases the active precipitant is the hydrogensulphide ion HS , the concentration of which is considerable, and that intermediate products are formed. Also much co-precipitation and post-precipitation occur in sulphide precipitations unless the experimental conditions are rigorously controlled. 

As CaC204,H20 by drying at 100-105 °C for 1-2 hours. This method is not recommended for accurate work, because of the hygroscopic nature of the oxalate and the difficulty of removing the co-precipitated ammonium oxalate at this low temperature. The results are usually 0.5-1 per cent high. 

Determination of zinc as 8-hydroxyquinaldinate Discussion. Zinc may be precipitated by 8-hydroxyquinaldine (2-methyloxine) in acetic acid-acetate solution it can thus be separated from aluminium and magnesium [see Section 11.11(G)]. It can be weighed as Zn(C10H8ON)2 after drying at 130-140 °C. The co-precipitated reagent is volatile at 130 °C. 

Determination of iodide as silver iodide Discussion. This anion is usually determined by precipitation as silver iodide, Agl. Silver iodide is the least soluble of the silver halides 1 litre of water dissolves 0.0035 mg at 21 °C. Co-precipitation and similar errors are more likely to occur with iodide than with the other halides. 

Barium sulphate exhibits a marked tendency to carry down other salts (see co-precipitation, Section 11.5). Whether the results will be low or high will depend upon the nature of the co-precipitated salt. Thus barium chloride and barium nitrate are readily co-precipitated. These salts will be an addition to the true weight of the barium sulphate, hence the results will be high, since the chloride is unchanged upon ignition and the nitrate will yield barium oxide. The error due to the chloride will be considerably reduced by the very slow addition of hot dilute barium chloride solution to the hot sulphate solution, which is constantly stirred that due to the nitrate cannot be avoided, and hence nitrate ion must always be removed by evaporation with a large excess of hydrochloric acid before precipitation. Chlorate has a similar effect to nitrate, and is similarly removed. 

In the presence of certain cations [sodium, potassium, lithium, calcium, aluminium, chromium, and iron(III)], co-precipitation of the sulphates of these metals occurs, and the results will accordingly be low. This error cannot be entirely avoided except by the removal of the interfering ions. Aluminium, chromium, and iron may be removed by precipitation, and the influence of the other ions, if present, is reduced by considerably diluting the solution and by digesting the precipitate (Section 11.5). It must be pointed out that the general method of re-precipitation, in order to obtain a purer precipitate, cannot be employed, because no simple solvent (other than concentrated sulphuric acid) is available in which the precipitate may be easily dissolved. 

Large amounts of chloride, cobalt(II), and chromium(III) do not interfere iron(III), nickel, molybdenum)VI), tungsten(VI), and uranium(VI) are innocuous nitrate, sulphate, and perchlorate ions are harmless. Large quantities of magnesium, cadmium, and aluminium yield precipitates which may co-precipitate manganese and should therefore be absent. Vanadium causes difficulties only... 

In view of the selective character of many colorimetric reactions, it is important to control the operational procedure so that the colour is specific for the component being determined. This may be achieved by isolating the substance by the ordinary methods of inorganic analysis double precipitation is frequently necessary to avoid errors due to occlusion and co-precipitation. Such methods of chemical separation may be tedious and lengthy and if minute quantities are under consideration, appreciable loss may occur owing to solubility, supersaturation, and peptisation effects. Use may be made of any of the following processes in order to render colour reactions specific and/or to separate the individual substances. 

Discussion. Minute amounts of beryllium may be readily determined spectrophotometrically by reaction under alkaline conditions with 4-nitrobenzeneazo-orcinol. The reagent is yellow in a basic medium in the presence of beryllium the colour changes to reddish-brown. The zone of optimum alkalinity is rather critical and narrow buffering with boric acid increases the reproducibility. Aluminium, up to about 240 mg per 25 mL, has little influence provided an excess of 1 mole of sodium hydroxide is added for each mole of aluminium present. Other elements which might interfere are removed by preliminary treatment with sodium hydroxide solution, but the possible co-precipitation of beryllium must be considered. Zinc interferes very slightly but can be removed by precipitation as sulphide. Copper interferes seriously, even in such small amounts as are soluble in sodium hydroxide solution. The interference of small amounts of copper, nickel, iron and calcium can be prevented by complexing with EDTA and triethanolamine. 

The co-precipitation technique starts with an aqueous solution of nitrates, carbonates, chlorides, oxychlorides, etc., which is added to a pH-controlled solution of NH4OH, allowing the hydroxides to precipitate immediately. This method requires water-soluble precursors and insoluble hydroxides as a final product. The hydroxides are filtered and rinsed with water when chlorides are employed as starting materials and chlorine is not desired in the final product. After drying the filtrate, it is calcined and sintered. This method is being applied very successfully for oxygen-ion conducting zirconia ceramics [30],... 

A promising approach toward controlling the static and impact sensitivity of initiators has consisted of co-precipitating the primary ex pi on a carrier or doping it in a manner which affects its solid state characteristics (Ref 97) Oscillatory Reactions... 

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