Zincation oxidative

A lithiation-zincation-oxidation sequence was valid and more effective for cyclization of tethered two bromothiophene rings than the lithiation-oxidation... 

The alkali metal hydroxides are also readily absorb CO2 and H2S to form carbonates (or hydrogencarbonates) and sulfides (or hydrogen-sulfides), and are extensively used to remove mercaptans from petroleum products. Amphoteric oxides such as those of Al, Zn, Sn and Pb react with MOH to form aluminates, zincates, stannates and plumbates, and even SiC>2 (and silicate glasses) are attacked. 

The majority of metals and alloys available depend for their resistance to corrosion on the properties of an oxide film or corrosion product which is formed initially by the corrosion process. In many cases the protectiveness of the oxide film is determined by its stability in aqueous solutions in a specific pH range, either chemically dissolving to form aquocations at lower pH values or complex anions (aluminate, ferroate, plumbate, zincate, etc.) at higher pH values (Fig. 2.22). An important property of the chemical is therefore the pH value that it develops when dissolved in water. For many materials and many chemicals this is the overriding factor and in many cases... 

Zincate-ions can dissociate and after the solubility exceeds the equilibrium level precipitation of zinc oxide proceeds ... 

The oxyanion containing Zn2+, Zn022-, is known as a zincate, and it equivalent to Zn(OII)42, which is named as the tetrahydroxozincate(II) anion (see Chapter 16). From these reactions, it is clear that ZnO can react as either an acidic or basic oxide and it is therefore known as an amphoteric oxide. In essence, there are some oxides that are clearly acidic, some that are dearly basic, and some that are in between. There is, in fact, a continuum of acid-base character for the oxides of elements that is shown in Figure 9.2. 

Zinc is a bluish-white metal which dissolves readily in strong acids. In nature it occurs as a sulfide, oxide, or carbonate. In solution, it is divalent and can form hydrated Zn2+ cations in acids, and zincated anions — probably Zn(OH)42 — in strong bases (USEPA 1980, 1987). Zinc dust and powder are sold commercially under a variety of trade names Asarco, Blue powder, Cl 77949, Cl pigment metal 6, Emanay zinc dust, granular zinc, JASAD Merrillite, LI 5, and PASCO (USPHS 1989). Selected physical and chemical properties of zinc, zinc chloride, and zinc sulfate are listed in Table 9.2. 

Incorporation of promoters can occur via two distinct mechanisms. A local pH drop at the leach front caused by the aluminum dissolution can cause a solvated promoter to deposit via a shift in the solubility equilibrium. Zincate shows this behavior, depositing as zinc oxide as the pH drops at the leach... 

Using the rotating disk electrode, Seliv-anov et al. [214] have investigated the zinc electrodeposition from zincate electrolyte containing polyethylene polyamine. The limiting current density of [Zn(OH)4] ion diffusion through a film of zinc oxides and hydroxides is shown to be responsible for the formation of dark zinc deposits in the potential range from —1.33 to —1.47 V. 

SCHEME 16. Preparation of ailyl- and benzylzinc derivatives by oxidative zincation... 

A. Baudrimont and J. Pelouze (1833) fused the sodium sulphate with galena or zinc blende and formed the alkali plumbate or zincate, and J. B. M. P. Closson boiled a soln. of sodium sulphate with milk of lime and lead oxide. The plumbate can be decomposed by sulphide, carbon dioxide, or by electrolysis. The St. Gobain Co. patented a process in which sand, coal, and sodium sulphate are heated together water-glass is formed and a soln. or suspension of that salt in water is decomposed by carbon dioxide or by milk of lime. J. Simpson (1890), J. C. Ody (1892), N. Basset and W. von Baranofi (1894) decomposed a soln. of sodium sulphate by calcium phosphate in dil. acid. The soluble sodium phosphate which is formed... 

Chemical Properties. Zinc oxide is amphoteric it reacts with organic and inorganic acids, and also dissolves in alkalis and ammonia solution to form zincates. It combines readily with acidic gases (e.g., C02, S02, and H2S). It reacts at high temperatures with other oxides to form compounds such as zinc ferrites. 

This type of cell is another variant on the basic Leclanche cell. In this case, the electrolyte is a concentrated aqueous solution of potassium hydroxide (about 30%), partly converted to potassium zincate by the addition of zinc oxide. The main advantage of alkaline manganese cells over Leclanche cells is their relatively constant capacity over a wide range of current drains and under severe service schedule conditions. Another feature of this system is that it can be the basis of a secondary battery system. The cell reaction may be written formally as... 

This system, commonly known as the mercury cell , is based on an amalgamated zinc anode, a concentrated aqueous potassium hydroxide electrolyte - saturated with zincate ion by zinc oxide - and a mercuric oxide/graphite cathode ... 

When the solution becomes saturated with zincate, zinc oxide is formed ... 

A slurry of zinc powder in aqueous KOH was pumped through the cyclindrical cell whose outer surface comprised the air electrode and for which the negative current collector was an axial brass rod. The slurry was recirculated through the cell several times until an acceptable percentage of the theoretical capacity had been utilized (i.e. for zinc oxidation to soluble zincate). For recharge, the zincate ion was reduced to zinc powder either externally or in an electrolysis cell connected to the system. In this context... 

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