Water amphoteric behaviour

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 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... 

A Bronsted acid is a proton donor, and a Bronsted base accepts protons. In aqueous solution, [H3O] is formed and in bulk water, self-ionization corresponds to the transfer of a proton from one solvent molecule to another (equation 8.11) illustrating amphoteric behaviour (see Section 6.8). 

Studies of the speciation of actinides in environmental waters are made difficult by the very low concentrations involved and the possibility that minor, undetected contaminants may dominate the binding of a particular metal ion. The environmental behaviour of the actinides has been reviewed. Americium and thorium exhibit simpler behaviour than other actinides since their oxidation states under such conditions are limited to Am and Th. Both are readily adsorbed by granitic rocks and tend to exhibit low solubilities, The thermodynamic solubility product of amorphous Am(OH)3 has been measured as log = 17.5 0.3 and no evidence for amphoteric behaviour or the formation of Am(OH)4 was found below pH 13. Stability constants for the binding of Am to humic acid have been found to vary with the degree of ionization, a, and were given by log = 10.58a -1-3.84 and log 2 = 5.32a -b 10.42. These were larger than the corresponding values for Eu. Humic acids also bind Th as described in Section 65.2.1. 

When PAM is washed with water, the reverse process takes place and it almost restores its initial crystallinity. It can be concluded that there is an equilibrium between the crystal and gel zones and the solution. When Li cations from the solution enter the gel zones (Fig. 2.34c), an equivalent amount of H" " ions should leave the particles in order to preserve their electroneutrality. CP anions from the solution have to be exchanged with OH ions from the gel (Fig. 2.34b). This means that PAM has an amphoteric behaviour. Hydrated lead dioxide is known to have amphoteric properties [95,96]. 

H bonds in the spatial water aggregates, favour the pure V (mss) process. For tin dioxide, its amphoteric behaviour towards water must lead to medium or long anion-cation H bonds and a partition of protons between anions and the water bed. The longer the anion-water bed H bonds, the more difficult is the proton transfer, and the remaining motion is Tj or, more probably, V. 

Amphoteric behaviour is observed in liquid ammonia just as in water. For example, Zn(OH)2 is amphoteric in water, and similarly zinc amide Zn(NH2)2 is amphoteric in ammonia. In both the cases, complex formation is observed. 

Ellis Wilson (1991, 1992) examined cement formation between a large number of metal oxides and PVPA solutions. They concluded that setting behaviour was to be explained mainly in terms of basicity and reactivity, noting that cements were formed by reactive basic or amphoteric oxides and not by inert or acidic ones (Table 8.3). Using infrared spectroscopy they found that, with one exception, cement formation was associated with salt formation the phosphonic add band at 990 cm diminished as the phosphonate band at 1060 cm" developed. The anomalous result was that the acidic boric oxide formed a cement which, however, was soluble in water. This was the result, not of an add-base readion, but of complex formation. Infrared spectroscopy showed a shift in the P=0 band from 1160 cm" to 1130 cm", indicative of an interaction of the type... 

Figure 35 combines the temperature-dependent and acetone-content-de-pendent behaviour of amphoteric gel APTAC-SA [155]. Equimolar polyampholyte gel undergoes a gradual volume change w ith an increase in acetone composition in an acetone/water mixture. When the content of the cationic groups increases, collapse of gel is observed. On the other hand, the same sample immersed in acetone/water mixtures of 60 and 65% acetone swells in a low... 

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