Amphoteric behaviour

If an oxide or hydroxide is able to act as either an acid or a base, it is said to be amphoteric. 

Some oxides and hydroxides are able to react with both acids and bases, thereby functioning as both bases and acids, respectively. Water is probably the most common example, but in this section we consider the amphoteric nature of metal oxides and hydroxides. The y-form of aluminium oxide, y-Al203, reacts with acids (equation 7.39) and with hydroxide ion (equation 7.40).  

The hexaaqua ion, 7.10, may be isolated as, for example, the sulfate salt after reaction with H2SO4. The ion [Al(OH)4], 7.11, can be isolated as, for example, the Na+ salt if the source of hydroxide is NaOH. 

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It shows some amphoteric behaviour, since it dissolves in alkali (concentrated aqueous or fused) to give a ferrate(lll) the equation may be written as... 

The amphoteric behaviour of Ga" salts parallels that of Al indeed, Gai03 is slightly more acidic than AI2O3 and solutions of gallates tend to be more stable than aluminales. Consistent with this, pKa for the equilibrium... 

Likewise, amphoteric behaviour can be observed. For example Zn(NH2)2 is insoluble in liquid NH3 (as is Zn(OH)2 in H2O), but it dissolves on addition of the solvo-base KNH2 due to the formation of K2[Zn(NH2)4] this in turn is decomposed by NH4+ salts (solvo-acids) with reprecipitation of the amide ... 

Fig.1 The amphoteric behaviour of the zwitterion [Co (T -C5H4COOH)(ri -C5H4COO)] depends on the presence of one -COOH group, which can react with bases, and one -COO group, which can react with acids...

Sodium hydroxide dissolves in water to give OH (aq) ions, the strongest base which can exist in an aqueous system. Chloric(VII) acid (perchloric acid) dissociates practically completely in dilute solution to give H30+(aq) ions, which represent the strongest acid which can exist in an aqueous system. The amphoteric behaviour of aluminium is noticed in a series of hydrated salts containing the Al3 + ion and in compounds such as NaA102, which contains the aluminate(III) ion, AIO2-. 

The corresponding As species in acid solution are less stable than those of P due to the effects of the 3d contraction. Further down the group, Sb and Bi show more metallic behaviour, with the two positive ions indicated in Table 6.11, and Sb shows amphoteric behaviour the + 3 oxide reacts with sodium hydroxide solution. 

The aqueous M2+ ions are fairly acidic, Sn2+ especially so and shows typical amphoteric behaviour, undergoing strong protolysis to form polymeric hydroxo species, which dissolve in alkali to form the pyramidal [Sn(OH)3]-. Pb2+ forms complexes with a class b pattern of stability analogous to that Cd2+... 

There are many indications in the literature that surface chemistry plays an important role in photoelectrochemical reactions at extended electrodes and at particles. There are, however, only a few quantitative investigations on this problem (see e.g. [12, 57, 58]), probably due to the lack of sufficiently sensitive methods. In the case of metal oxide particles, the adsorption of H2O plays already an important role. Due to the amphoteric behaviour of most metal hydroxides, two surface equilibria have to be considered [18] ... 

Behind the general parameters (viscosity, transmembrane pressure, temperature, flow velocity) which can influence cross-flow filtration with ceramic membranes two aspects must be considered to be more specific of this sort of membrane. One is related to the geometry (tubular multichannel or honeycomb) found for the major part of commercially available membranes, the other is the amphoteric behaviour of metal oxides used in the preparation of these ceramic membranes. 

The amphoteric behaviour of metal oxides in contact with water has thoroughly been described by many authors [22-24]. This basic property results in charged surfaces depending on pH condition. In a first approximation, connected porosity in ceramic membranes can be represented by an array of... 

Fig. 12.9. Mechanism of charged surface formation due to amphoteric behaviour of metal oxides.

Ag and Au are substitutional group-I elements in silicon. These atoms are amphoteric, giving an acceptor and a donor state. The absorption related to the donor state is discussed here and that of the acceptor state in Chap. 7. Pt is also included here as it displays the same amphoteric behaviour. 

Like Si, Sn also displays an amphoteric behaviour in GaAs and the ionization energy of Siias is rather large (117.1 meV) compared to those of the other group-IV acceptors [150]. 

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