Precipitation conditions, determining

Precipitation conditions determine crystal structure. Lemon chrome yellows are precipitated hot with excess lead to form the monclinic crystal. Primrose yellow is preciptated at a lower temperature to form the orthorhombic crystal. Chrome oranges are precipitated under alkaline conditions. The higher the alkalinity, the larger the particle. The precipitation of molybdate orange is complicated by the polymorphic nature of two of the components of the solid solution precipitate lead chromate and lead sulfate. As with primrose yellow, the crystal is stabilized after precipitation to prevent conversion. 

Precipitated Calcium Carbonate. Precipitated calcium carbonate can be produced by several methods but only the carbonation process is commercially used in the United States. Limestone is calcined in a kiln to obtain carbon dioxide and quicklime. The quicklime is mixed with water to produce a milk-of-lime. Dry hydrated lime can also be used as a feedstock. Carbon dioxide gas is bubbled through the milk-of-lime in a reactor known as a carbonator. Gassing continues until the calcium hydroxide has been converted to the carbonate. The end point can be monitored chemically or by pH measurements. Reaction conditions determine the type of crystal, the size of particles, and the size distribution produced. 

Temperature, pH, precipitation conditions, and the drying process determine the amount of water in the sohd. In addition the species Ce(OH)... 

Stripping voltammetry procedure has been developed for determination of thallium(I) traces in aqueous medium on a mercury film electrode with application of thallium preconcentration by coprecipitation with manganese (IV) hydroxide. More than 90% of thallium present in water sample is uptaken by a deposit depending on conditions of prepai ation of precipitant. Direct determination of thallium was carried out by stripping voltammetry in AC mode with anodic polarization of potential in 0,06 M ascorbic acid in presence of 5T0 M of mercury(II) on PU-1 polarograph. 

Sulphate process. The ilmenite is reacted with sulphuric acid giving titanium sulphate and ferric oxide. After separation of ferric oxide, addition of alkali allows precipitation of hydrous titanium dioxide. The washed precipitate is calcined in a rotary kiln to render titanium dioxide. The nucleation and calcination conditions determine the crystalline structure of titanium dioxide (e.g. rutile or anatase). 

Environmental conditions determine in large part the chemical reactions that will occur when waste is injected. For example, precipitation-dissolution reactions are strongly controlled by pH. Thus, iron oxides, which may be dissolved in acidic wastes, may precipitate when injection-zone mixing increases the pH of the waste. Similarly, redox potential (Eh) exerts a strong control on the type of microbiological degradation of wastes. 

Prepare 6-10 reservoirs with a solutions containing precipitant concentration that would result In producing a clear drop If crystallization drops were set up and left to Incubate under these conditions. Determine these concentrations from the region In the phase diagram that Is just under the supersolublllty curve (Fig. 3.3). 

The resulting glass-ceramics obtained at various experimental conditions consist of a crystalline phase and a residual glassy phase. The nature of the crystalline phase corresponding to different heat treatment and precipitation conditions is determined by X-ray diffraction This together with a detailed spectroscopic study of the steady state fluorescence, absorption, decay dynamics (by means of selective laser spectroscopy) as well as electron paramagnetic resonance reveals the detailed nature of the crystalline phases. 

Moreover, the structure of gels produced depends substantially on precipitation conditions and is determined by the precipitation temperature, pH of the medium, initial composition of solution, its concentration, of sequential mixing of solutions or conditions of introduction of precipitating agent [7]. It should be noted here that even in the production of one-component gel, periodicity of precipitation gives a product with a non-homogeneous structure which, as a rule, forms adsorbents with a set of pores different in size. Therefore, strict observance of all the conditions of sample synthesis enumerated above is absolutely necessary. 

In short, precipitates can consist of several polymorphic, amorphic and/or hybrid phases but in some applications, only a specific form is acceptable [52] it is therefore necessary to control the precipitation conditions in order to guarantee the formation of the desirable product. In addition, the repeatability of particle characteristics should be maintained from one determination to another [57] and this is efficiently achieved in flow analysis. 

Like in the preparation of single-walled carbon nanotubes, the chemical vapor deposition of MWNT consists in the generation of small carbon clusters or atoms from precursor compounds. The products precipitate in the shape of different carbon materials with the reaction conditions determining the specific stracture... 

Precvirsors obtained from the precipitation from both solutions of raw metal salts and alkali are composed of basic carbonates of copper and zinc such as malachite (MA) CU2CO3 (0H)2, aurichalcite (AU) (Zn,Cu)5(C03)2(OH)5, and hydrozincite (HZ) Zn5(C03)2(0H)g. For the ratio of Cu to Zn in industrial catalysts, the precursor would not exist as mono-phasic. Even if it seems to be monophasic, the atomic ratio of Cu to Zn in the double salts would be continuously variable. Since amorphous intermediates of the precursor are also known, the real phase cannot be well characterized by XRD measurements. Of course the structure and distribution of precursors are also very sensitive to the precipitation conditions (temperature, rate, pH, etc.) as well as precipitation agents. Consequently, the characteristics of precursors are not determined only by the starting composition. 

When creating supersaturation levels sufficient to induce particle formation, precipitation of sparingly soluble salts and sol-gel processes are viewed differently. Precipitation normally involves mixing a cation solution with a precipitant solution. For example, consider preparation of an oxalate precursor to a CoO- and MnO-doped ZnO powder. In this process, the Zn, Mn, and Co are coprecipitated with controlled stoichiometry and the precipitate is calcined to the oxide. To form the oxalate, a state of supersaturation is created by mixing an aqueous solution of the metal nitrates or chlorides with an oxalate precipitant solution. The system is supersaturated with respect to the different metal oxalate phases and a crystalline coprecipitate forms. Depending on precipitation conditions (pH, concentrations, temperature, etc.), different metal complexes are present in solution. The form and concentration of these complexes determine the phase, morphology, and particle size distribution of the resulting precipitate. 

Taking into account the steady state and Gibbs-Thomson conditions, the growth (or dissolution) rate for a spherical precipitate is determined by the following balance equation at the moving spherical boimdary ... 

Precipitation is a complex sequence of supersaturation, seeding, and coagulation. All three steps determine particle size and size distribution of the obtained particles. The process is strongly dependent on the exact precipitation conditions and is still difficult to model. Table 2.3.6 displays examples of technically important catalysts or catalyst supports produced by precipitation. 

These parameters and conditions determine complex interactions between Pt nanoparticles, carbon support, ionomer molecules, and solvent, which control the catalyst layer formation process. Self-organization of ionomer and carbon/Pt in the colloidal ink leads to the formation of phase-segregated and agglomerated morphologies. The choice of a dispersion medium determines whether ionomer exists in solubilized, colloidal, or precipitated form. This influences the microstructure and pore size distribution of the CL (Uchida et al., 1996). It is believed that mixing of ionomer with dispersed Pt/C catalysts in the ink suspension, prior to deposition to form a CL, enhances the interfacial area of Pt with water in pores and with Nation ionomer. 

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