Re-precipitation

The destiny of most biological material produced in lakes is the permanent sediment. The question is how often its components can be re-used in new biomass formation before it becomes eventually buried in the deep sediments. Interestingly, much of the flux of phosphorus is held in iron(lll) hydroxide matrices and its re-use depends upon reduction of the metal to the iron(ll) form. The released phosphate is indeed biologically available to the organisms which make contact with it, so the significance attributed to solution events is understandable. It is not clear, however, just how well this phosphorus is used, for it generally remains isolated from the production sites in surface waters. Moreover, subsequent oxidation of the iron causes re-precipitation of the iron(lll) hydroxide floes, simultaneously scavenging much of the free phosphate. Curiously, deep lakes show almost no tendency to recycle phosphorus, whereas shallow... 

Uranium, too, is widely distributed and, since it probably crystallized late in the formation of igneous rocks, tends to be scattered in the faults of older rocks. Some concentration by leaching and subsequent re-precipitation has produced a large number of oxide minerals of which the most important are pitchblende or uraninite, U3O8, and camotite, K2(U02)2(V04)2.3H20. However, even these are usually dispersed so that typical ores contain only about 0.1% U, and many of the more readily exploited deposits are nearing exhaustion. The principal sources are Canada, Africa and countries of the former USSR. 

If the precipitate is still appreciably contaminated as a result of coprecipitation or other causes, the error may often be reduced by dissolving it in a suitable solvent and then re-precipitating it. The amount of foreign substance present in the second precipitation will be small, and consequently the amount of the entrainment by the precipitate will also be small. 

Sometimes a film of the sulphide adheres to the glass vessel in which precipitation was carried out this can be dissolved in a little ammonia solution and the sulphide re-precipitated with the acid washing liquor. 

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. 

The PGA-GalAT assay precipitates HGA larger than a trimer (18), thus, any HGA product completely hydrolyzed by EPGase treatment would be lost when re-precipitated in... 

Enzymatic preparation with predominant content of pectinesterase (obtained from Penicillium fellutanum culture liquid by isolation by acetone was purified. Primary enzymatic preparation was re — precipitated by three volumes of ethyl alcohol and centrifuged (6000 rev/min.) during 20 min. The obtained precipitate of partially purified pectinesterase preparation was dried in vacuum — desiccator. Sephadexes G — 50, G-75, G-lOO, G-200 "LKB" (Sweden) and Toyopearl HW-55 (Japan) were used for separation of enzymatic complex by gel — filtration. 

The methods used for making pigments from these and other natural dyes were more like recipes than scientific procedures and were probably derived from the work of alchemists and herbalists. The former spent their lives trying to prepare gold by dissolving all kinds of cheap substances in acids and then re-precipitating them (hence their discovery of many precipitants), while the latter sought to extract compounds of medicinal value from plants and some of their extracts must have included natural dyes. 

The methods used to convert these vat dyes into a suitable physical form (and in some cases, crystal structure) for use as pigments have been carefully guarded industrial secrets, revealed only in patents. The general principles are clear, however. One method is to reduce the vat dye in the usual manner to bring it into solution and then to re-precipitate it under very carefully controlled conditions. The other is to subject the dye to a fine grinding operation. Whichever approach is used, the aim is to reduce the mean particle size to below 1 J,m (1000 nm). 

Preparative Photolysis. The preparative photolysis of an aqueous solution (pH=8.5) of AETSAPPE (2.5 M) was conducted in a 1-inch diameter quartz test tube in a Rayonet Reactor (Southern New England Radiation Co.) fitted with 254 nm lamps. Within two hours the solution gelled and the reaction was terminated. Upon acidification the solution cleared, and the product could be re-precipitated by addition of base. This indicates loss of the thiosulfate functionality. The product was dissolved in dilute HC1, precipitated with acetone, and filtered. This process was repeated three times, and the final precipitate was washed with water. The product (20 to 30 mg) was dried in vacuo for 24 hours and stored in a dessicator until use. Comparison of the13 C NMR spectrum of the product with the starting AETSAPPE 13C NMR spectrum clearly shows that the thiosulfate methylene peak shifted upfield, from 39 ppm to 35 ppm. The complete 13 C NMR and IR analysis of the product were consistent with the disulfide product. Further, elemental analysis of the product confirmed that the product was the desired disulfide product 2-amino (2-hydroxy 3-(phenyl ether) propyl) ethyl disulfide (AHPEPED) Expected C 58.39, H 7.08, N 6.20, S 14.18 actual C 58.26, H 7.22, N 6.06, S 14.28. 

The radiochemical assays were done as follows At the end of a polymerisation experiment, when the conductivity had become constant, a ten-fold excess of tritiated water was added from a burette (see Figure 1), the cell was warmed rapidly to room temperature, and any polymer which had been precipitated during the polymerisation was allowed to re-dissolve. It was always noted that no hydrolysis occurred until the solutions reached 0 °C. This could be seen from a rapid drop of conductivity to a very low value. The solvent and most of the tritiated water were then distilled out, within about 15 minutes. The polymer was then dissolved in toluene, also run from a burette into the reaction vessel, which was then cut from the vacuum line. The polymer was precipitated in methanol and prepared for the determinations of radioactivity and DP. For the radiochemical assay the polymers were dissolved in toluene, re-precipitated in methanol, dried, weighed, re-dissolved in toluene, and the activity determined. The processes of precipitation and dissolution were repeated until the activity of the polymer became constant, (up to 7 repetitions). It was assumed that when the activity had become constant, all the excess of tritium had been removed. 

Thorium is widely but rather sparsely distributed its only commercial sources are monazite (together with the rare earths) and uranothorite (a mixed Th, U silicate). Uranium is surprisingly common and more abundant than mercury, silver or cadmium in the earth s crust. It is widely distributed and it is found scattered in the faults of old igneous rocks. Concentration by leaching followed by re-precipitation has produced a number of oxide minerals of which the most important are uranite (also called pitchblende) U308 and carnotite, K UC HVO -SF O. 

Distillation, sublimation This can be used to remove the metals from Ta (from crucibles, etc. dissolved by the molten R and re-precipitated). The volatile R (Sm, Eu, Dy, Ho, Er, Tm, Yb, Sc) are also purified from C, N, O (present as carbides, nitrides, oxides). Gschneidner (1980) underlined that, in order to obtain high purity, a very high vacuum (10 8-10 9mmHg) and a slow sublimation rate must be maintained during the distillation process. 

Figure 12. Extent of dissolution and re-precipitation between aqueous Fe(III) and hematite at 98°C calculated using Fe-enriched tracers. A. Percent Fe exchanged (F values) as calculated for the two enriched- Fe tracer experiments in parts B and C. Large diamonds reflect F values calculated from isotopic compositions of the solution. Small circles reflect F values calculated from isotopic compositions of hematite, which have larger errors due to the relatively small shifts in isotopic composition of the solid (see parts B and C). Curves show third-order rate laws that are fit to the data from the solutions. B. Tracer experiment using Fe-enriched hematite, and isotopically normal Fe(lll). C. Identical experiment as in part B, except that solution Fe(lll) is enriched in Te, and initial hematite had normal isotope compositions. Data from Skulan et al. (2002).

Figure 7. Measured and corrected A Fe Fe(ni)-Hem.tite values ( and O, respectively) relative to average hematite precipitation rate for Experiments 5, 7, and 8 of Skulan et al. (2002). The A Fe jj(ni).Hem.tite values are defined as those measured at the termination of the experiments the corrected A Fe i,e(ni).Hem.tite values reflect the estimated correction required to remove any residual kinetic isotope fractionation that was produced early in experiments that was not completely removed hy dissolution and re-precipitation over the long term. Extrapolation of the corrected A Fe jj(ni).Hem.tite values to zero precipitation rates yields an estimate for the equilihrium Fe(III),q-hematite fractionation, A Fci,e(in).hem.tite,...

Effects of Flooding and Redox Conditions onfs. I know of no published data on this. Bnt it is likely that the natnre of particle surfaces in intermittently flooded soils wonld restrict snrface mobility. For ions to diffuse freely on the surface there must be a continuous pathway of water molecules over the surface and uniform cation adsorption sites. But in intermittently flooded soils the surface typically contains discontinuous coatings of amorphous iron oxides on other clay minerals, and on flooding reduced iron is to a large extent re-precipitated as amorphons hydroxides and carbonates as discussed above, resulting in much microheterogeneity with adsorption sites with disparate cation affinities. 

Figure 3.17 Calculated changes in a soil solution upon rednction of Fe(III) oxide coatings on soil surfaces and structural Fe(III) in clay lattices without re-precipitation of Fe(II). and are exchangeable cations. Param-...

The actual pattern in alkaline soils related directly to oxidizing sulfides should include two variables positive H" surrounded by an increase in Ca concentration where the mobilized calcite re-precipitates. Both of these variables can be detected by initially measuring the soil slurry pH, then by adding a drop of 10 % HCI, stirring the solution for about 10 seconds and taking the pH again. 

In the wetted condition, hydrothermal transformations of the waste form may occur at the temperature of service. Changes may occur either by dissolution and re-precipitation or by assimilation of water into the internal structures of the oxide matrices. The thermochemical essentials for hydrothermal recrystallisation have already been referred to previously in this paper. We also need to develop an adequate knowledge of the water-catalysed structural transformations. 

The polymer is visualized to continually re-precipitate and re-dissolve as the solvent front of a gradient repetitively overtakes and then loses the polymer. 

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