Homogenous precipitation

An increase in the time required to form a visible precipitate under conditions of low RSS is a consequence of both a slow rate of nucleation and a steady decrease in RSS as the precipitate forms. One solution to the latter problem is to chemically generate the precipitant in solution as the product of a slow chemical reaction. This maintains the RSS at an effectively constant level. The precipitate initially forms under conditions of low RSS, leading to the nucleation of a limited number of particles. As additional precipitant is created, nucleation is eventually superseded by particle growth. This process is called homogeneous precipitation. ... 

Two general methods are used for homogeneous precipitation. If the precipitate s solubility is pH-dependent, then the analyte and precipitant can be mixed under conditions in which precipitation does not occur. The pH is then raised or lowered as needed by chemically generating OH or H3O+. For example, the hydrolysis of urea can be used as a source of OH . 

In the second method of homogeneous precipitation, the precipitant itself is generated by a chemical reaction. For example, Ba + can be homogeneously precipitated as BaS04 by hydrolyzing sulphamic acid to produce S04 . 

Color Plate 5 shows the difference between a precipitate formed by direct precipitation and a precipitate formed by a homogeneous precipitation. 

The precipitated metallic hydroxides or hydrated oxides are gelatinous in character, and they tend to be contaminated with anions by adsorption and occlusion, and sometimes with basic salts. The values presented in Table 11.2 suggest that many separations should be possible by fractional precipitation of the hydroxides, but such separations are not always practical owing to high local concentrations of base when the solution is treated with alkali. Such unequal concentrations of base result in regions of high local pH and lead to the precipitation of more soluble hydroxides, which may be occluded in the desired precipitate. Slow, or preferably homogeneous, precipitation overcomes this difficulty, and much sharper separations may be achieved. 

Mercury(II) chloranilate 700 Mercury(II) nitrate standard soln. of, 359 Mercury/mercury( II )-EDTA electrode (mercury electrode) 586 potentiometric titration of metallic ions with EDTA and, 588 prepn. of, 587 Mercury thiocyanate 700 Metaphosphoric acid in homogeneous precipitation, 426 Metal apparatus 93 Metal ion buffer 53... 

Generally, the experimental results on electrodeposition of CdS in acidic solutions of thiosulfate have implied that CdS growth does not involve underpotential deposition of the less noble element (Cd), as would be required by the theoretical treatments of compound semiconductor electrodeposition. Hence, a fundamental difference exists between CdS and the other two cadmium chalcogenides, CdSe and CdTe, for which the UPD model has been fairly successful. Besides, in the present case, colloidal sulfur is generated in the bulk of solution, giving rise to homogeneous precipitation of CdS in the vessel, so that it is quite difficult to obtain a film with an ordered structure. The same is true for the common chemical bath CdS deposition methods. 

Yamaguchi K, YoshidaT, Lincot D, MinouraH (2003) Mechanistic study of chemical deposition of ZnS thin films from aqueous solutions containing zinc acetate and thioacetamide by comparison with homogeneous precipitation. J Phys Chem B 107 387-397... 

AE was purified from orange peels. After homogenization, precipitation with 30 - 60% (NH4)2S04 followed by dialysis, the sample was applied to a cation exchange column (CM-Sepharose CL-6B). AE binds strongly to a cation exchange column material at pH 6.8... 

Wagner, W.F. and Wuellner, J.A. (1952) Homogeneous precipitation of barium sulfate by hydrolysis of sulfamic acid. Anal. Chem., 24, 1031-1032. 

As these electrodes are in fact not produced from precipitates, it seems illogical to classify them as homogeneous precipitate electrodes. 

Pillai and Ganguly [286] have concentrated the nucleic acids from seawater by adsorption on homogeneously precipitated barium sulfate, then hydrolysed with 0.02 M hydrochloric acid and analysed for the constituents. 

By careful control of these parameters aluminium may be separated, and few interferences are observed if the precipitation is carried out from an ammoniacal-cyanide-EDTA solution. When large amounts of Ca or Mg are present the homogeneous precipitation procedure (Table 5.18) is usefully employed at pH = 5. The precipitate is readily filtered and may be weighed after drying at 150°C as the anhydrous compound. 

If a surface precipitate of metal hydroxy-polymer has formed on an adsorbent, the -pH relationship for the coated adsorbent should resemble closely that observed for particles consisting purely of the hydroxy-polymer or the hydrous oxide of the metal (15). This kind of evidence for Co(ll), La(lII), and Th(lV) precipitation on silica colloids was cited by James and Healy (15). It should be noted, however, that the increase in C toward a maximum value often occurs at pH values well below that required thermodynamically to induce bulk-solution homogeneous precipitation of a metal hydrous oxide (15, 16). If surface precipitation is in the incipient stage under these conditions, it must be a nucleation phenomenon. James and Healy (15) argue that the microscopic electric field at the surface of a charged adsorbent is sufficiently strong to lower the vicinal water activity and induce precipitation at pH values below that required for bulk-solution precipitation of a metal hydrous oxide. 

The numerical values of the distribution coefficients X and D have been derived from experimental data for a large number of systems (e.g. (10, 11, 13). From the constancy of either X or D values it can be determined whether or not the system yielded homogeneous precipitates. In either case, the numerical value of X or D should be equal to ... 

MgO- and Ag-modified membranes were obtained by homogeneous precipitation of the hydroxide from a typical solution consisting of 0.75 M urea and 0.2-0.5 M AgN03 or Mg(NOj)2 in water. The solution is introduced into the pores of the support and/or the y-alumina top layer by impregnation. An increase in temperature results in (I) evaporation of the solvent and concentration of the solution and (2) the decomposition of urea (at T > 90°C) resulting in the formation of NH3 and a decrease in the pH followed by precipitation of the metal hydroxide. The hydroxide is next converted to the oxide form at 350-450°C. 

Metal phosphates can be prepared by wet and dry methods. The homogeneous precipitation in aqueous systems is often employed to obtain uniformly sized and well-crystallized particles. The solid reactions can be used for preparation of some metal phosphates however, the particles with controlled morphology are difficult to synthesize. Moreover, the solid reaction consumes more energy than the reaction in aqueous system, except the hydrothermal reaction at elevated temperature. Recently the preparation from aqueous solution at low temperature received attention in view of saving energy and as an application for a wide variety of substances. Therefore the preparation of metal phosphate particles by the precipitation method is described next. Although phosphates include ortho-, pyro-, and polyphosphates, only orthophosphates are dealt with here. 

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