Solvent precipitation techniques

The solvent precipitation technique is alto used to encapsulate material, ... 

Solvent precipitation techniques have been generally applied for hydrophobic polymers, except for dextran nanospheres. Several techniques described in the literature are based on the mechanism of polymer precipitation. 

A supercritical anti-solvent precipitation technique has been used to prepare a novel Titania catalyst support. The Titania precursor was prepared by precipitating TiO (acac)2 from a solution of methanol using supercritical carbon dioxide at 110 bar and 40°C. The surface area of the supercritical precursor was 160 m g and this decreased to 35 mV after calcination, although there was no significant reduction of particle size. The new titania support was used to prepare a supported gold catalyst and this was tested for ambient temperature carbon monoxide oxidation. The supercritical catalyst demonstrated notably high activity when compared with catalysts prepared by other nonsupercriticd methods. 

P. Caliceti et al.. Effective protein release from PEG/PLA nanoparticles produced by compressed gas anti-solvent precipitation techniques. /. Control Release, 94(1), 195-205 (2004). 

Solvent extraction techniques are useful in the quantitative analysis of niobium. The fluoro complexes are amenable to extraction by a wide variety of ketones. Some of the water-insoluble complexes with organic precipitants are extractable by organic solvents and colorimetry is performed on the extract. An example is the extraction of the niobium—oxine complex with chloroform (41). The extraction of the niobium—pyrocatechol violet complex with tridodecylethylammonium bromide and the extraction of niobium—pyrocatechol—sparteine complex with chloroform are examples of extractions of water-soluble complexes. Colorimetry is performed on the extract (42,43). Colorimetry may also be performed directly on the water-soluble complex, eg, using ascorbic acid and 5-nitrosahcyhc acid (44,45). 

Isolation. Isolation procedures rely primarily on solubiHty, adsorption, and ionic characteristics of the P-lactam antibiotic to separate it from the large number of other components present in the fermentation mixture. The penicillins ate monobasic catboxyHc acids which lend themselves to solvent extraction techniques (154). Pencillin V, because of its improved acid stabiHty over other penicillins, can be precipitated dkecdy from broth filtrates by addition of dilute sulfuric acid (154,156). The separation process for cephalosporin C is more complex because the amphoteric nature of cephalosporin C precludes dkect extraction into organic solvents. This antibiotic is isolated through the use of a combination of ion-exchange and precipitation procedures (157). The use of neutral, macroporous resins such as XAD-2 or XAD-4, allows for a more rapid elimination of impurities in the initial steps of the isolation (158). The isolation procedure for cephamycin C also involves a series of ion exchange treatments (103). 

An overview is presented of plutonium process chemistry at Rocky Flats and of research in progress to improve plutonium processing operations or to develop new processes. Both pyrochemical and aqueous methods are used to process plutonium metal scrap, oxide, and other residues. The pyrochemical processes currently in production include electrorefining, fluorination, hydriding, molten salt extraction, calcination, and reduction operations. Aqueous processing and waste treatment methods involve nitric acid dissolution, ion exchange, solvent extraction, and precipitation techniques. 

For the analysis of americium in water, there is a broad array of sample preparation and detection methodologies that are available (see Table 7-2). Many of the common and standardized analytical methodologies typically include the minimization of sample volume, purification through co-precipitation, anion exchange column chromatography, and solvent extraction techniques followed by radiochemical detection of americium in the purified sample. Gross alpha analysis or liquid scintillation are common... 

By taking into account the difficulties connected to the slow solvent evaporation technique, a new procedure for nanoparticle preparation by co-precipitation was set up. This technique does not use chlorinated solvents and energetic mixing, which are both known to cause appreciable protein denaturation[17,18]. 

In some cases the molecular-weight distribution can be determined by turbi-dimetric titration, a technique which is based on the fractional precipitation. A precipitant is added to a very dilute solution of the polymer, and the resulting turbidity is measured as a function of the amount of added precipitant the preparative separation of the fractions is thereby avoided. If the polymer is chemically homogeneous, the mass distribution function can then be calculated. Tur-bidimetric titration is also suitable as a means for establishing the best fractionation conditions (e.g., choice of solvent/precipitant combination, size of fractions, etc.), before carrying out a full-scale fractionation by precipitation. 

Fractional extraction is free from the disadvantages encountered in fractional precipitation. Here, the technique consists in extracting the polymer with a series of solvent/precipitant mixtures, the proportion of solvent being increased stepwise. Since one begins with the poorest solvent mixture - in contrast to fractional precipitation - the first fraction contains the low-molecular-weight com-... 

For mixed metal oxides obtained from their hydroxide or carbonate precursors after calcination, it is generally difficult to determine whether the as-prepared precursor is a single-phase or multiphase solid solution [35]. Non-aqueous solvents appear superior for achieving two dissimilar metal oxides such as MM Oz or MM 04 precipitates such reactions cannot be carried out simultaneously in aqueous solution due to the large variations in pH necessary to induce precipitations [41,42]. Table 6.1 summarizes some of the nanoparticulate semiconducting metal oxides and mixed metal oxides prepared via co-precipitation techniques. The general procedure of achieving metal loaded nanoparticles on an oxide support is shown in Fig.6.5. 

Various processes separate rare earths from other metal salts. These processes also separate rare earths into specific subgroups. The methods are based on fractional precipitation, selective extraction by nonaqueous solvents, or selective ion exchange. Separation of individual rare earths is the most important step in recovery. Separation may be achieved by ion exchange and solvent extraction techniques. Also, ytterbium may be separated from a mixture of heavy rare earths by reduction with sodium amalgam. In this method, a buffered acidic solution of trivalent heavy rare earths is treated with molten sodium mercury alloy. Ybs+ is reduced and dissolved in the molten alloy. The alloy is treated with hydrochloric acid, after which ytterbium is extracted into the solution. The metal is precipitated as oxalate from solution. 

Free radical polymerization processes [41] are carried out in bulk, solution, suspension, emulsion, or by precipitation techniques. In all cases the monomer used should be free of solvent and inhibitor or else a long induction period will result. In some cases this may be overcome by adding excess initiator. 

The classical solvent precipitation fractionation technique provides reproducible fractionations for determining molecular weight distributions of CTPB and almost 100% recovery of the sample from the column. A solvent-nonsolvent combination which has been used effectively is the toluene—acetone-methanol system, where acetone and methanol are used as the nonsolvents. The precipitating fractions are required to stand approximately 24 hours to ensure complete separation. Each fraction is vacuum stripped of solvent at approximately 30 °C., and the molecular weight of each fraction is then determined by either VPO or intrinsic viscosity. 

Figure 18. Reproducibility of the solvent precipitation fractionation technique for determining molecular weight distribution (molecular weights by intrinsic viscosity)...

Figure 19. Comparison of molecular weight distributions of carboxyl-terminated polybutadienes by the solvent precipitation-intrinsic viscosity technique...

The product structures available in this process are primarily determined by components in the paraffinic distillates used. The heterogeneity of virgin distillates has been illustrated by API sponsored work on distillate components 21). The preferred predominance of relatively straight-chain alkyl groups may be effected by combinations of selection of paraffin distillate source, and use of acid or solvent refining to effect enrichment of paraffinic components. With the advent of the urea precipitation technique of isolating the n-paraffins 26), particularly those in the decyl to hexadecyl range, it is now possible to produce products which are limited principally to the isomeric secondary phenyl alkanes a dodecyl benzene mixture prepared by this process from a 95+% n-dodecane would consist essentially of the 2-, 3-, 4-, 5-, and 6-phenyl-substituted dodec-anes. 

Amic acids (AA) were prepared by two different methods. Stirring appropriate mixtures of amine and anhydride overnight in NMP yielded solutions of the desired amic acid suitable for F-NMR studies, hi order to discern a crude isomer composition, -C-NMR was performed on compounds prepared by a literature procedure utilizing chloroform as a solvent." This technique gave very high yields of very high-purity monoamic acid as the materials precipitated from the reaction mixture. 

The chemical complexing-solvent extraction technique employed in this work involved the formation of a neutral complex in the aqueous phase between trialkyl lead chloride and a dithiocarbamate reagent such as sodium diethyl di-thiocarbamate. The complex was subsequently removed either as a precipitate or by extraction into an organic solvent. The extent of lead removal was traced by analysis of the aqueous phase for residual trialkyl lead using a Pye-Unicam 8000 spectrophotometer. 

Formation and subsequent extraction of a neutral species such as (C2H5)3PbCl° suggest that a combined chemical complexing-solvent extraction technique might be more effective in terms of a lower Cr/Cl ratio than direct precipitation. This is confirmed by the results of the chemical complexing solvent-extraction studies. 

One of the key steps in any isotope dilution analysis concerns the isolation and purification of the diluted activity, plus the measurement of its specific activity. Two techniques are usually preferred for the separation precipitation and solvent extraction. As a purification step, precipitation has the advantage that the precipitate can easily be weighed at the time of separation, thereby allowing a quick determination of the specific activity. The main problem with the use of precipitation techniques involves the occurrence of co-precipitation phenomena, in which unwanted materials are precipitated along with the desired substance, thus altering the sample specific activity. Precipitation techniques are used for the isolation of inorganic components. 

Even though in the THOREX process 233U can be preferentially recovered from irradiated thorium fuel by using an extraction flowsheet based on 5% TBP n-dodecane as the extractant, further lowering of the concentration of TBP in the solvent has certain advantages in terms of reduced co-extraction of thorium and fission products (195, 196). Ramanujam et al. reported a sequential precipitation technique... 

The most common protein precipitation technique involves the use of ammonium sulphate, which has been the subject of a recent review (3). The widespread use of ammonium sulfate can be ascribed to the fact that it is very soluble (saturated solutions have a concentration in the region of AM), the density of solutions do not compromise collection of precipitates by centrifugation and its use does not promote denaturation of proteins. The addition of ammonium sulfate will cause a neutralization of the surface charge of the protein and a decrease in the effective concentration of water leading to a decrease in protein solvent interactions. 

The theory behind such techniques is complex and not clearly understood, but the techniques themselves are straightforward. To say that an increase in protein-protein interactions leads to precipitation whereas protein-solvent interaction favors solubility is a simplistic but nonetheless useful paradigm. Precipitation techniques do not, by themselves, achieve a great increase in the purity of a protein solution, but generally they result in an increase in concentration and have a role to play in many protein purification protocols. 

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