Zincate ions

This results in high concentrations of zincate ions that are effective scavengers. However, high concentrations of zincate can lead to problems related to mud performance. 

The solution also contains a high concentration of zinc (as zincate), which is noble relative to aluminium. As metallic aluminium is exposed, it corrodes, reducing zincate ions and forming a coating of zinc ... 

The reason for this limited cycle life is the high solubility of the zinc electrode in alkaline electrolyte the zincate ions formed are deposited again during the subsequent charging in the form of dendrites, i.e., of fernlike crystals. They grow in the direction of the counterelectrode and finally cause shorts. 

Thus in this system, in addition to the usual requirements, the separator has the task of delaying penetration for as long as possible. A membrane would be regarded as perfect which lets hydroxyl ions pass, but not the larger zincate ions. This requirements is best met by regenerated cellulose ( cellophane ) [10,11], which in swollen condition shows such ion-selective properties but at the same time is also chemically very sensitive and allows only a limited number of cycles the protective effects of additional fleeces of polyamide or polypropylene have already been taken into account. 

Zinc and cadmium are both silvery, reactive metals that are similar to each other but differ sharply from mercury. Zinc is amphoteric (like its main-group neighbor aluminum). It reacts with acids to form Zn2+ ions and with alkalis to form the zincate ion, [Zn(OH)4]2 ... 

Alkaline manganese cell Consists of Zn anode and Mn02 cathode with KOH as the electrolyte is saturated with zincate ions Zn + 2 MnOj + 2 H20 —> Zn(OH)2 + 2 MnO (OH) 11 ZnO + H20... 

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

The first structurally characterized triarylzincate [Li(TMEDA)2][Zn 2,4,6-(Pr1)3G6H2 3] 59 was a byproduct of the lithiation of VC13(THF)3 with Li(2,4,6-(Pr1)3C6H2)3, in which the vanadium compound had been pre-reduced with metallic zinc.116 In the solid state, the compound consists of isolated, tetrahedrally coordinated lithium ions and trigonal-planar tris(tris(isopropyl)phenyl)zincate ions. The Zn-G bonds are equidistant (2.039(7) A), but the C-Zn-C bond angles are variable, ranging from 112.2(3)° to 125.4(3)°. 

In 62, two zincate ions of different composition, namely [Zn(CCPh)3] 62a and its THF adduct [Zn(CCPh)3(THF)] 62b, co-crystallized (Figure 31). Both, the coordination of only one zincate ion with THF and the trigonal-pyramidal structure of 62b, show that the interaction between zinc and the THF molecule in 62b must be weak. The zinc-carbon bonds in 62a and 62b are 1.967(7) and 2.003(5) A long, respectively. 

Four equivalents of lithium phenylacetylide reacted with bis bis(trimethylsilyl)amido zinc (Scheme 51) to form the ion-paired dilithiotetra(phenylacetylido)zincate 65, whose structure is shown in Figure 33.121 The zinc-carbon bonds (2.05 A) are somewhat longer than those observed in the tri(phenylacetylido)zincate 62a, due to the increase in both the coordination number and the negative charge on the ion. Each lithium ion is associated with the zincate ion through coordination to two of the triple bonds. 

The solid-state structure of 72 (Figure 38) consists of a polymer of alternating THF-solvated potassium cations and diamidomethylzincate anions. The alternating zincate ions, which are rotated by about 180° with respect to each other, sandwich the potassium ions. Each potassium ion is -coordinated to one methyldiamidozincate and -coordinated to the other. The zinc-methyl and zinc-nitrogen bonds of the zincate ion are 1.954(5) and 1.989(8) A, respectively. 

An excess of OH (the common ion) should shift the reaction to the right, i.e., to more complete precipitation of the Zn(OH)2. This effect is a general one, but the conclusions are not always vahd the example (deliberately) given here is one where it is not valid. The reason is that OH can form a complex with Zn (Zn(OH)4 — the zincate ion), thus removing free Zn from solution and reducing the degree of precipitation. For a sufficiently high concentration of OH , which can be calculated from the stability constant of the zinc-hydroxide (zincate) complex, the Zn(OH)2 will completely redissolve. 

Even when the exact composition is not known, this method can be of use. The number of ligands can then be omitted, as in hydroxozincate, or zincate ion, etc. 

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 ... 

In basic solution, unsaturated with zincate ions, the anode reaction may be written as... 

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... 

While mercury is inert to non-oxidizing acids and bases, both zinc and cadmium readily dissolve in simple acids to give the M2 ions. Zinc will dissolve in concentrated bases to form zincate ions ... 

The zinc ion Zn(H20)4 is a rolciless ion formed by solution of zinc in acid. It is poisonous to uian. and to bacteria, and is used as a disinfectant. It forms tetracomdmated complexes readily, such as Zn(NH d4+ b Zn(CN)r, and Zr (OH)4" ". The while precipitate of zinc hydroxide, Zn(OH)2, whidi foirn. when ammonium hydroxide is added to a solution containing zinc ion, dissolves ill excess ammonium hydroxide, forming the zinc ammonia complex. The zinc hydroxide complex, Zn(OH), which is called zincate ion, is similarly formed on solution of zinc hydroxide in an excess of strong base zinc hydroxide is amphoteric. 

Zn(OH)4 is called the zincate ion. The reactions above suggest that galvanized containers should not be used to transport either acids or alkalis. 

Cadmiate ions (analogous to zincate ions) are known, but Cd does not react with strong bases. 

Zinc Oxidation Mechanism. The oxidation reaction for the zinc anode (eq. 8) takes place in several steps (16,17), ultimately resulting in the zincate ions [16408-25-67, Zn(OH)2-4, that dissolves in the electrolyte. 

Both Zn and Cd react readily with non-oxidizing acids, releasing hydrogen and giving the divalent ions, whereas Hg is inert to non-oxidizing acids. Zinc also dissolves in strong bases because of its ability to form zincate ions (see below), commonly written ZnO ... 

Further, Zn(OH)2 readily dissolves in an excess of alkali bases to give zincate ions, probably of the type [Zn(0H)3(H20)] or [Zn(OH)4]2". At high hydroxide concentrations the only species observed in solution is the latter 9 solid zincates such as NaZn(OH)3 and Na2[Zn(OH)J can be crystallized from concentrated solutions. 

---