Coprecipitation steps

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... 

Cranston and Murray [171,188] took the samples in polyethylene bottles that had been precleaned at 20 °C for 4 days with 1% distilled hydrochloric acid. Total chromium (CrVI) + Cr111 + Crp (particulate chromium) was coprecipitated with iron (II) hydroxide, and reduced chromium (Cr111 + Crp) was coprecipitated with iron (III) hydroxide. These coprecipitation steps were completed within minutes of sample collection to minimise storage problems. The iron hydroxide precipitates were filtered through 0.4 pm nucleopore filters and stored... 

The eflBciency of these methods of co-precipitation was studied for plutonium, only, by comparing the chemical recovery of the original yield monitor with that of a second (and different) plutonium isotope yield monitor added to the acid solution of the co-precipitated hydroxide or oxalate (6). This second monitor shows all losses following the coprecipitation step. The eflBciency of the hydroxide precipitation for plutonium extraction was in the range 70-80%. That for the oxalate coprecipitation was typically 75-85%. 

Up to now several methods have been used to prepare iron molybdates, the most part of them based on coprecipitation techniques. Previous studies [11] have evidenced that the catalytic behavior of Mo-Fe oxides depends on many variables of the coprecipitation procedure starting compounds, concentration of parent solutions, pH and temperature of coprecipitation step, order of addition of parent solutions, ripening etc. In a typical preparation procedure iron molybdate is coprecipitated from solutions of ferric chloride or ferric nitrate and ammonium molybdate [8]. The control of all the above mentioned procedure variables, strongly difficult the preparation of these catalysts and deviations from the preparation recipe can have very adverse effects on the performances of the catalyst from the standpoint of activity, selectivity and stability. 

Many of the synthesization methods to produce nanoparticles are based on coprecipitation steps, or nucleation and growth in reactional media [98,108]. Precipitation reactions involve the simultaneous occurrence of these steps, as well as coarsening and agglomeration processes [122,123]. Because of the difficulties in isolating each process for independent study, the fundamental mechanisms of precipitation are still not entirely understood. However, a good understanding of the nucleation step is fundamental for grasping the nature of nanosize particles. 

Two samples of the same composition (50 Si02 50 AlPO ) have been prepared by the same route except the coprecipitation step. In one case, the ammonia solution is injected in the acid solution until the pH is equal to 8 (pH variable) and in the other case, the acidic solution is injected at the base of a reactor containing an ammonia solution kept at 0 C and pH 8 (constant pH). The second sample is more resistant (constant pH) to recrystallisation (fig 5). That behaviour can be understood in terms of a better homogeneneity of the second sample. 

The piehydiolysis of the TEOS can also improve the resistance to crystallization. A sample of chemical composition 50 Si02- 50 AIPO crystallizes at a higher temperature if the TEOS is hydrolysed during two hours at 60 C under acid catalysis than if no hydrolysis is made before the coprecipitation step (fig 6). Moreover, without the hydrolysis step, the precipitation of TEOS is not quantitative and the silica precipitation partly occurs after the precipitation of alumina or aluminophosphale. This causes inhomogeneity in the amorphous phase which eases the thomal recrystallization and decreases the useful temperature range. 

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. 

The sohds are treated with hydrochloric acid at 70°C at pH 3—4. The thorium hydroxide [13825-36-0] remains iasoluble and can be filtered off. Small amounts of trace contaminants that carry through iato solutioa, such as uranium and lead as well as some thorium, are removed by coprecipitation with barium sulfate ia a deactivatioa step. The resultiag product, after SX-removal of the heavy La fractioa, is a rare-earth/lanthanide chloride,... 

Note that during the cooling step the maximum amount of evaporation which is permitted by the material balance is 50 lb for the step shown. In an evaporative-cooling step, however, the actual evaporation which results from adiabatic cooling is more than this. Therefore, water must be added back to prevent the NaCl concentration from rising too high otherwise, coprecipitation of NaCl will occur. 

The presence of the ammonium chloride prevents the coprecipitation of zinc salts when sodium bicarbonate is added to the solution to precipitate the aminoguanidine as the bicarbonate. If the solution is not clear at this step, it should be filtered. 

Figure 15.4(A) shows the effect of the R = Zn2+/Al3+ ratio, which determines the charge density of the LDH layer, on the Freundlich adsorption isotherms. K values are far higher than those measured for smectite or other inorganic matrices. The increase in Kf with the charge density (Kf= 215, 228, 325mg/g, respectively, for R = 4, 3 and 2) is supported by a mechanism of adsorption based on an anion exchange reaction. The desorption isotherms confirm that urease is chemically adsorbed by the LDH surface. The aggregation of the LDH platelets can affect noticeably their adsorption capacity for enzymes and the preparation of LDH adsorbant appears to be a determinant step for the immobilization efficiency. [ZnRAl]-urease hybrid LDH was also prepared by coprecipitation with R = 2, 3 and 4 and Q= urease/ZnRAl from 1 /3 up to 2.5. For Q < 1.0,100 % of the urease is retained by the LDH matrix whatever the R value while for higher Q values an increase in the enzyme/LDH weight ratio leads to a decrease in the percentage of the immobilized amount.

The concentration of nickel in natural waters is so low that one or two enrichment steps are necessary before instrumental analysis. The most common method is graphite furnace atomic absorption after preconcentration by solvent extraction [122] or coprecipitation [518]. Even though this technique has been used successfully for the nickel analyses of seawater [519,520] it is vulnerable to contamination as a consequence of the several manipulation steps and of the many reagents used during preconcentration. 

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. 

Tsunogai [7] carried out a similar coprecipitation allowing a 20-hour standing period to ensure that iodide is fully recovered in the silver chloride coprecipitate. Again, the iodide is oxidised to iodate prior to spectrophotometric determination of the latter. This procedure also includes a step designed to prevent interference by bromine compounds. 

As the first step in the coprecipitation process, ferric hydroxide precipitates either from the effect of the changing pH on the solubility of ferric iron,... 

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. 

Fig. 3 TEM micrographs of Mg/Al - CO3 LDHs with different Mg /Al ratios prepared using the new method using rapid mixing and nucleation in a colloid mill followed by a separate aging step (a-c) and conventional coprecipitation at constant pH (d-f). The new method affords smaller crystallites with a much narrower range of length. Reprinted with permission from [20]. Copyright ACS Journal Archives...

This method has also been successfully applied in the synthesis of Cu(II)-containing LDHs, although well crystallized materials are difficult to prepare as a consequence of the Jahn-Teller distortion found in the coordination shell of Cu(II) [70,71]. Incorporation of Ni(II) in the layers was found to improve the crystallinity and structural stability of such LDHs. For the synthesis of Cu/Ni/Al - CO3 and Cu/Ni/Mg/Al - CO3 LDHs [70] by the method with separate nucleation and aging steps, LDHs with both smaller particle size and narrower distribution of particle size were obtained compared with those prepared using a conventional coprecipitation method, similar to the case for Mg/Al-C03 LDHs [20]. Well crystallized Cu/Ni/Cr-COs LDHs [71] were obtained when the Cu/Ni/Cr atom ratio ranged from 1 2 1 to 1 3 1 in the reaction mixture with hydrothermal aging conditions at 180 °C for 4 h. 

The catalysts can be obtained by a coprecipitation method consisting of two steps (Figure 6.2). In the first step, a stable suspension of protected metal nanoparticles is obtained according to the method reported by Schulz and co-workers [75-77]. The metal particles are prepared in the presence of a highly water-soluble ionic surfactant which is able, due to its nature, to modulate the particle size and to prevent their aggregation. Modifying parameters such as pH, temperature and surfactant concentration, it is possible to tune the metal particle size [71]. Moreover, the role of the... 

The uses of constant-current coulometry for the determination of drugs in biological fluids are few, basically due to sensitivity restriction. Monforte and Purdy [46] have reported an assay for two allylic barbituric acid derivatives, sodium seconal and sodium sandoptal, with electrogenerated bromine as the titrant and biamperometry for endpoint detection. Quantitative bromination required an excess of bromine hence back titration with standard arsenite was performed. The assay required the formation of a protein-free filtrate of serum with tungstic acid, extraction into chloroform, and sample cleanup by back extraction, followed by coulometric titration with electrogenerated bromine. The protein precipitation step resulted in losses of compound due to coprecipitation. The recoveries of sodium seconal and sodium sandoptal carried through the serum assay were approximately 81 and 88%, respectively. Samples in the concentration range 7.5-50 pg/mL serum were analyzed by this procedure. 

First, the RNA must be collected from the transcription mixture by precipitation. We use ammonium acetate in the ethanol precipitation step because this is the quickest method. Coprecipitated NH ions, which interfere with further enzymatic reactions, are removed in the next purification step. The RNA pellet is dissolved in an appropriate loading buffer for denaturing PAGE (7 M urea, 50 mM EDTA), incubated for 5 min at 65 °C, and then immediately loaded on the gel. The gel should be preequilibrated and preheated by preelectrophoresis for 15 min. 

---