Amphoteric behavior

Copolymers can be used to introduce a mixture of chemical functionalities into a polymer. Acidic and basic substituents can be introduced, for example, through comonomers like acrylic acid and vinyl pyridine. The resulting copolymers show interesting amphoteric behavior, reversing their charge in solution with changes of pH. 

Table 1 Hsts many of acetamide s important physical properties. Acetamide, CH2CONH2, dissolves easily ia water, exhibiting amphoteric behavior. It is slow to hydroly2e unless an acid or base is present. The autodissociation constant is about 3.2 x 10 at 94°C. It combines with acids, eg, HBr, HCl, HNO, to form soHd complexes. The chemistry of metal salts ia acetamide melts has been researched with a view to developing electroplating methods. The hterature of acetamide melts and complexes, their electrochemistry and spectroscopy, has been critically reviewed (9).

Tlie amphoteric behavior of aluminum hydroxide, wliich dissolves readily in strong acids and bases, is shown in Figure 4. In the pH range of 4 to 9, a small change in pH towards the neutral value causes rapid and voluminous precipitation of colloidal hydroxide wliich readily fomis a gel. Gels are also fomied by the hydrolysis of organoaluminum compounds such as aluminum alkoxides (see Alkoxides, metal). 

Reaction (22) shows that Al(OH)3 has the properties of an acid, since it reacts with the base OH-. A substance that acts as an acid under some conditions and as a base under other conditions is said to be amphoteric. The electronic situation in Al(OH)3 is such that it can either accept a proton (act as a base) or react with OH- (act as an acid). We will see in Chapter 22 that several other hydroxides also show amphoteric behavior. 

Another reason chemists find the above complex ion picture of aqueous solutions useful is that it is easily extended to explain amphoteric behavior. Take the case of chromium hydroxide, Cr(OH)3, a good example of an amphoteric hydroxide. It dissolves very little in water, but is quite soluble both in acid and in base. Presumably it can react with either. How can this behavior be explained in terms of the complex ion picture ... 

Although this behavior has been illustrated starting with the metal ion, analogous equations can be written starting with the metal oxide as well as the hydroxide. Amphoteric behavior is exhibited in other solvents, as will be illustrated later. 

The basic species in liquid HF is the fluoride ion or the solvated fluoride ion, HF2 . As in the case of water in which OH forms hydroxo complexes, fluoride ion forms complexes in liquid HF. This behavior gives rise to amphoterism with metals ions such as Zn2+ and Al3 +. In the case of Al3+, A1F3 is relatively insoluble in liquid HF so the amphoteric behavior can be shown as follows. 

The amphoteric behavior of Zn2+ and Al3+ in some nonaqueous solvents has already been described. This behavior can also be demonstrated in liquid S02. For example, the aluminum compound containing the anion characteristic of the solvent forms a precipitate, which is then soluble in either the acid or base in liquid S02. This can be shown as... 

Write equations to show the amphoteric behavior ofZn2+ in 100% acetic acid. 

However, beryllium oxide is quite different, and it exhibits amphoteric behavior. 

The reactions with acid and base that show amphoteric behavior can be summarized as follows ... 

Attention was paid early on to solution pH, and in particular, to a surface — bulk proton balance. Various models of hydroxyl chemistry have been developed in colloid science literature [21], Perhaps the simplest and most common model assumes a single type of OH group and amphoteric behavior (i.e., one set of Kx and K2 from Figure 6.1). More complicated models invoke multiple OH groups and proton affinity distributions [22]. It will be demonstrated below that the simpler type has worked well for the revised physical adsorption (RPA) model. 

It is reasonable to assume that the surface of metal sulfides show amphoteric behavior and it has been shown that uptake of bivalent cations is pH-dependent. Metal sulfide precipitates are efficient scavengers for heavy metal ions. 

Alumina exhibits amphoteric behavior. It is soluble both in acids and bases. With acids, it produces their corresponding salts. It froms Al2(S04)3, AlCNOsls and AICI3 upon reactions with H2SO4, HNO3, and HCl, respectively. In acid medium, it exists as a solvated aluminum ion, in which water molecules are hexacoordinated to trivalent AP+, as shown below ... 

Its solution exhibits amphoteric behavior. It dissolves in aqueous bases to give arsenite ions that have formulas, [AsO(OH)2], [As02(0H)] and [AsOs] . 

Iodine pentafluoride similar to other halogen fluorides exhibits amphoteric behavior i.e., with strong Lewis acids, such as SbFs, it can form cation, IF4 ... 

Manganese(II) hydroxide is a base exhibiting weak amphoteric behavior. It reacts with acids forming the corresponding manganese(II) salt ... 

Problem 21.6 Write equilibria showing the amphoteric behavior of an H O solution of glycine. All monoaminomonocarboxylic acids are slightly acidic. M... 

One final comment on the Fe/E O system is needed if Fig. 15.3 were extended beyond pH 14, we would have to include the stability fields of Fe(OH)4 (aq) and Fe(OH)3 (aq).12 In other words, iron, like aluminum, chromium, zinc, and many other metals, exhibits amphoteric behavior (i.e., has both acidic and base like properties), but only if a sufficiently wide range of pH is considered. Amphoteric behavior, like many other chemical properties, is not so much something that a given element does or does not exhibit, but rather is a trait that different elements display to different extents. 

Furthermore, the amphoteric behavior of the aluminum ion can be shown in sulfur dioxide as readily as in water. Just as AI(OH), is insoluble in water but dissolves readily in either a strong acid or basic solution, AMS03)3 is insoluble in liquid sulfur dioxide. Addition of either base (SO2-) or acid (SO2 ) causes the aluminum sulfite to dissolve, and it may be reprecipitated upon neutralization. 

Furthermore, amphoteric behavior resulting from complex formation with excess amide also parallels that in water ... 

The laws of soil colloidal behavior VI. Amphoteric behavior. Soil Science, 32 343-365. 

Amphoteric Behavior of TMCQ and TECO in Alternating Copolymerization. 113... 

Amphoteric Behavior of TMCQ 97) and TECQ 103) in Alternating Copolymerization... 

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