Precipitation from solution homogeneous

Nucleation Fischer found that, in PFHS, nucleation takes place early and only growth occurs thereafter. In the homogeneous precipitation of barium sulfate by sulfate generation from sulfamic acid in a solution containing 0.01 Af barium ion, he found (Section 8-2) that the number of particles was sensibly constant from the beginning to the end of precipitation. Evidently nucleation was complete within the first small fraction of the total precipitation time. From the rate of sulfate generation it 

PFHS often markedly improves the efficiency of separation. For example, in the precipitation of aluminum hydroxide by addition of ammonia, narrow pH limits must be maintained to get quantitative precipitation, because of the amphoteric properties of aluminum hydroxide. Consequently, it is impossible to regulate the ratio of ammonium ion and ammonia to attain minimum coprecipitation. Copper, 

The pH can also be controlled by evaporation of ammonia. For example, barium chromate can be precipitated in the presence of strontium and lead by first complexing the metal ions at a pH of 10.4, followed by evaporation of ammonia. The pH decreases slowly, and barium chromate precipitates since the barium ion is no longer complexed at the lower pH.  

Anion or cation generation Anions can be generated slowly in solution to bring about homogeneous precipitation. Swift and Butler reviewed precipitation of the metal sulfides by use of thioacetamide or thiourea. PFHS of sulfides of cadmium, mercury, zinc, and nickel have been studied more recently by Swift and others. Phosphate can be generated by hydrolysis of triethyl phosphate, oxalate by hydrolysis of methyl oxalate, and sulfate by hydrolysis of diethyl sulfate or sulfamic acid. 

Cations can be generated slowly in solution by releasing them from their EDTA complexes by oxidative destruction of the EDTA.  

Many different anions can be generated at a slow rate the nature of the anion is important in the formation of compact precipitates. It is convenient to deal with the subject under separate headings. 

Hydroxides and basic salts. The necessity for careful control of the pH has long been recognised. This is accomplished by making use of the hydrolysis of urea, which decomposes into ammonia and carbon dioxide as follows  

Urea possesses negligible basic properties (Kb = 1.5 x 10 l4), is soluble in water and its hydrolysis rate can be easily controlled. It hydrolyses rapidly at 90-100 °C, and hydrolysis can be quickly terminated at a desired pH by cooling the reaction mixture to room temperature. The use of a hydrolytic reagent alone does not result in the formation of a compact precipitate the physical character of the precipitate will be very much affected by the presence of certain anions. Thus in the precipitation of aluminium by the urea process, a dense precipitate is obtained in the presence of succinate, sulphate, formate, oxalate, and benzoate ions, but not in the presence of chloride, chlorate, perchlorate, nitrate, sulphate, chromate, and acetate ions. The preferred anion for the precipitation of aluminium is succinate. It would appear that the main function of the suitable anion is the formation of a basic salt which seems responsible for the production of a compact precipitate. The pH of the initial solution must be appropriately adjusted. 

The following are suitable anions for urea precipitations of some metals sulphate for gallium, tin, and titanium formate for iron, thorium, and bismuth succinate for aluminium and zirconium. 

the precipitation of barium as barium chromate in the presence of ammonium acetate  

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Gordon, L Salutsky, M. L. Willard, H. H. Precipitation from Homogeneous Solution, Wiley New York, 1959. 

A procedure which is commonly employed to prevent supersaturation from occurring is that of precipitation from homogeneous solution. This is achieved by generating the precipitating agent within the solution by means of a homogeneous reaction at a similar rate to that required for precipitation of the species. 

Calcium can be determined as the oxalate by precipitation from homogeneous solution by cation release from the EDTA complex in the presence of oxalate ion.28... 

Any strong acid that may be present is first neutralised. Then, by selecting an appropriate base, whose conjugate acid has a Ka of about 10 5, the equilibrium for the tripositive cations will be forced to the right the base is too weak, however, to remove the hydroxonium ions from the equilibrium of the dipositive cations. Since a large excess of the basic ion is added, a basic salt of the tripositive metal usually precipitates instead of the normal hydroxide. Acetate or benzoate ions (in the form of the sodium salts) are the most common bases that are employed for this procedure. The precipitation of basic salts may be combined with precipitation from homogeneous solution, and thus very satisfactory separations may be obtained. 

DETERMINATION OF ALUMINIUM AS THE 8-HYDROXYQUINOLATE, AI(C,H60N)3> WITH PRECIPITATION FROM HOMOGENEOUS SOLUTION... 

Procedure Precipitation from homogeneous solution (sulphamic acid method). The... 

Determination of chromium as lead chromate (precipitation from homogeneous solution) Discussion. Use is made of the homogeneous generation of chromate ion produced by the slow oxidation of chromium(III) by bromate at 90-95 °C in the presence of excess of lead nitrate solution and an acetate buffer. The crystals of lead chromate produced are relatively large and easily filtered the volume of the precipitate is about half that produced by the standard method of precipitation. 

Determination of iron as iron (III) oxide by initial formation of basic iron (III) formate Discussion. The precipitation of iron as iron(III) hydroxide by ammonia solution yields a gelatinous precipitate which is rather difficult to wash and to filter. Iron(III) can, however, be precipitated from homogeneous solution as a dense basic formate by the urea hydrolysis method. The precipitate obtained is more readily filtered and washed and adsorbs fewer impurities than that formed by other hydrolytic procedures. Ignition yields iron(III) oxide. 

Gordon, M., L. Salutsky and H.H. Willard, Precipitation from Homogeneous Solution , New York, John Wiley Sons, 1959. 

Fig. 1.1.2 Schematic presentation of the plug-flow type of a reactor for the continuous preparation of monodispersed colloids by precipitation from homogeneous solutions. (From Ref. 31.)...

L Gordon, ML Salutsky, HH Willard. Precipitation from Homogeneous Solutions. New York WUey, 1959. 

Conventional routes to ceramics involve precipitation from solution, drying, size reduction by milling, and fusion. The availability of well-defined mono-dispersed particles in desired sizes is an essential requirement for the formation of advanced ceramics. The relationship between the density of ceramic materials and the sizes and packing of their parent particles has been examined theoretically and modeled experimentally [810]. Colloid and surface chemical methodologies have been developed for the reproducible formation of ceramic particles [809-812]. These methodologies have included (i) controlled precipitation from homogeneous solutions (ii) phase transformation (iii) evaporative deposition and decomposition and (iv) plasma- and laser-induced reactions. 

The importance of polysulfides in the pyrite formation process was outlined by several studies (37, 38). Schoonen and Barnes (37) showed that no precipitation from homogeneous solution can be observed within a reasonable time scale, even in solutions highly supersaturated with respect to pyrite, unless pyrite seeds are already existing. Therefore future studies should address the role of ferric oxide surfaces in promoting the nucleation of pyrite. 

In order for a solid to precipitate from homogenous solution, first a nucleus has to form. The formation of a particle is governed by the free energy of agglomerates of the constituents of the solution. The total free energy change due to agglomeration, AG, is determined by... 

In many cases it has been found advantageous to work at low and relatively constant supersaturation which is achieved homogeneously in the whole solution (precipitation from homogeneous solution, PFHS). This can also be employed for deposition-precipitation processes, see Section A.2.2.1.5. This can be reached by using a precipitating agent which slowly decomposes to form the species active in the precipitation. The most commonly employed precursor for the liberation of... 

H., Synthesis of controlled spherical zinc sulfide particles by precipitation from homogeneous solutions, J. Am. Ceram. Soc., 81, 2699, 1998. 

With the exception of the alkali metals and the alkaline earth metals, most cations form sparingly soluble sulfides whose solubilities differ greatly from one another. Because it is relatively easy to control the sulfide ion concentration of an aqueous solution of H2S by adjustment of pH (see Section 11C-2), separations based on the formation of sulfides have found extensive use. Sulfides can be conveniently precipitated from homogeneous solution, with the anion being generated by the hydrolysis of thioacetamide (see Table 12-1). 

Precipitation from homogeneous solution Synonymous with homogeneous precipitation. 

Precipitation with oxalate at pH 3-4 separates calcium from metals which give soluble oxalate complexes [e.g., Fe(III), Al, and Ti], and also from phosphate. Calcium oxalate can also be precipitated from homogenous solution [11. 

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