Precipitation of metal ions

Addition of excess H ions to this solution will cause the equilibrium to move towards undissociated acid thereby decreasing the concentration of Ac . This effect is known as the common-ion effect and is of considerable practical importance. Thus, e.g. in the precipitation of metal ions as insoluble sulphides. 

Actually, it is recognized that two different mechanisms may be involved in the above process. One is related to the reaction of a first deposited metal layer with chalcogen molecules diffusing through the double layer at the interface. The other is related to the precipitation of metal ions on the electrode during the reduction of sulfur. In the first case, after a monolayer of the compound has been plated, the deposition proceeds further according to the second mechanism. However, several factors affect the mechanism of the process, hence the corresponding composition and quality of the produced films. These factors are associated mainly to the com-plexation effect of the metal ions by the solvent, probable adsorption of electrolyte anions on the electrode surface, and solvent electrolysis. 

The objective of this monograph is to include all major studies of metal ions in their aqueous solutions as well as some other important studies in their zerovalent metallic state or in alloys, since the nanoparticles of many of these metals have become too important. Besides, the study of the precipitation of metal ions in aqueous solutions, upon sonication, which has been carried out in our laboratory, would also be discussed. Some of such data include unpublished work. The sequence of metallic ions in this chapter are as they come in the sequence of wet chemical analysis of basic radicals, besides the cationic charge has been kept in mind to make groups and sequences that follow the detailed description. 

Selective precipitation of metal ions using hydrogen- or sodium sulphides, hydroxides or carbonates... 

When undesirable metal ions generally need to be removed from a solution without disturbing the other metal ions present, the process of selective precipitation comes into play. This step involves the precipitation of metal ion by using another solution whose anion forms an insoluble salt with the undesirable ion in the mixture. For example, if a NaCl solution is added to a solution containing both Ag" " and Mg2+ ions, AgCl will be precipitated as a white solid. Since MgCl2 is soluble, Mg2+ ions will remain in the solution. 

Among some kinds of reactions which are slow on a relevant time scale and in particular environments are certain metal-ion oxidations, oxidation of sulfides, sulfate reduction, various metal ion polymerizations (e.g., vanadium, aluminum), aging of hydroxide and oxide precipitates, precipitation of metal-ion silicates and carbonates (e.g., dolomites), conversions among aluminosilicates (e.g., feldspar-kaolinite), and solution or precipitation of quartz (9). Some of these reactions can be accelerated greatly by biological catalysis (e.g., sulfate reduction, metal ion oxidations) (7). 

Summary Ammonium chlorate is prepared in a similar manner as other inorganic ammonium salts, by the precipitation of metal ions with carbon dioxide. In thiscase, sodium chlorate istreated with ammonia, and carbon dioxide isthen passed there into. The carbon dioxide reacts with the water forming carbonic add. The carbonic add reacts with the sodium chlorate forming the water insoluble sodium bicarbonate. The ammonia molecules in solution take up an addic hydrogen from the carbonic add, and then unite with the chi orate ions forming ammonium chlorate in solution. The salt isthen easily recovered by recrystallization. 

Metal deactivator. Metal deactivator prevents precipitation of metal ion oxidation reactions and precipitation of insoluble metal compounds. Metal deactivator in combination with other antioxidants, shows strong synergistic effects. Oxygen and moisture present, diffuse through oil film and cause corrosion. Amine derivative, used in the additive has good water-displacing properties. They impede sludge formulation, disperse sediments and reduce corrosion in various fuel systems. 

Co-precipitation of metal ions on collectors can be attributed to several mechanisms, as follows ... 

A boundary layer with a gradient in caustic soda concentration also forms at the surface of the membrane facing the catholyte based on a similar principle, resulting in a caustic soda concentration on the membrane surface which is higher than that in the bulk phase. Since this tends to reduce the current efficiency and electric conductivity of the membrane, it is necessary to minimize the boundary layer thickness or reduce the caustic soda concentration in the bulk phase. It is also essential to purify the brine with ion-exchange resin of high selectivity, in order to prevent precipitation of metal ions as hydroxides in the membrane and the boundary layer (74). 

Precipitation of metal ions. Many metals can be removed from solution by adjusting the pH. Table 28.9 summarises the conditions under which various metal ions are precipitated [19.9, 19.10] (see sections 28.1.3, 28.3.3 and 28.4.3, as well as parts of chapters 31 and 32). 

The second self-healing effect, related to ECP doped with anions with inhibiting properties, is more general. It depends only on the formation of a thin adsorbed insoluble salt layer in the defect, and results from the precipitation of metal ions with the doping anion. In our opinion, this effect is similar to the ennobling effect, the difference being in the nature of the passivation of the metal, and in the fact that the kinetics of the reduction of the polymer is not as important as in the ennobling case. 

TABLE 29.2 The Optimum pH for the Precipitation of Metal Ions as Silicates... 

Precipitation of metal ions from waste waters as salts or hydroxides. 

The anode is the area where metal is lost. At the anode, the reactions which take place are oxidation reactions. It represents the entry of metal ion into the solution, by dissolution, hydration or by complex formation. It also includes precipitation of metal ions at the metal surface. For example Fe + -1- 20H -> Fe(OH)2- Ferrous hydroxide or rust formation on steel surface is a common example. Some more examples are ... 

Precipitation of metal ions, 16-1 to 12 Preparation of Special Analytical Reagents,... 

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