Aluminum amphoteric behavior

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). 

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... 

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 ... 

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. 

Amphoteric behavior in liquid SO2 is similar to that in other solvents. In liquid S02, aluminum sulfite is relatively insoluble which leads to the reaction... 

Amphoteric behavior is also exhibited in the solid-state reactions of aluminum (Scheme 1). 

The aluminum oxyhydroxides, kaolinite, and halloysite dissolve to form cationic aluminum species at low pH and the anionic species (Al(OH)4) at high pH. The same amphoteric behavior is also true of the Fe(III) oxyhydroxides, although the latter are much less soluble, in general, under oxidizing conditions. We next compute the solubilities of the ferric oxyhydroxides as a function of pH. The approach is identical to that described above for the Al-oxyhydroxides. 

The presence of a protective oxide layer on aluminum is the main reason why aluminum alloys are so broadly used with success in indoor and outdoor environments provided they fall within the passivation potential/pH boundaries shown in Fig. 4.14. The aluminum susceptibility to corrode in both acidic and basic environments is referred to as an amphoteric behavior. While the aluminum oxide will form naturally on aluminum, it is common practice to produce this oxide in a controlled process called anodization. As described in Chap. 5, the quahty and properties of the protective oxide can thus be greatly enhanced, providing various finishes for a multitude of applications. 

The amphoteric behavior of AI2 (803)3 in sulfur dioxide is analogous to that of A1(0H)3 in water. Each contains the negative ion of the respective solvent. Aluminum sulfite, which is insoluble in sulfur dioxide, dissolves on addition of either thionyl chloride... 

The two preceding paragraphs have implied that many substances may act either as acids or as bases, depending upon the particular reaction imder consideration. Amphoteric behavior seems to be much more widespread than was previously supposed. Aluminum hydroxide... 

Certain alloys frequently used in cooling water environments, notably aluminum and zinc, can be attacked vigorously at high pH. These metals are also significantly corroded at low pH and thus are said to be amphoteric. A plot of the corrosion behavior of aluminum as a function of pH when exposed to various compounds is shown in Fig. 8.1. The influence of various ions is often more important than solution pH in determining corrosion on aluminum. 

The base anhydride of NaOH is Na20. Oxides of metals in the middle groups of the periodic table (III through V) lie on the border between ionic and covalent behavior and are frequently amphoteric. An example is aluminum oxide (AI2O3), which dissolves to only a limited extent in water but much more readily in either acids or bases ... 

Aluminum hydroxide is a typical amphoteric metal hydroxide. Its behavior as a base is illustrated by its reaction with nitric acid to form a normal salt. The balanced formula unit, total ionic, and net ionic equations for this reaction are, respectively ... 

Looking down Group 3A(13), we see a wide range of chemical behavior. Boron, the first metalloid we ve encountered, is much less reactive at room temperature than the other members and forms covalent bonds exclusively. Although aluminum acts physically like a metal, its halides exist in the gas phase as covalent dimers— molecules formed by joining two identical smaller molecules (Figure 14.4)—and its oxide is amphoteric rather than basic. Most of the other 3A compounds are ionic, but with more covalent character than similar 2A compounds because the 3A cations can polarize nearby electron clouds more effectively. 

A weakly acidic behavior is typical of the sdanol groups of silica as well as of the B—OH groups of boria-containing materials. Considering that aluminum is at the border between metal and semimetal character and that aluminum hydroxides are typically amphoteric, both bonding concepts may apply. In other words, the properties of bulk alumina are better explained in terms of ionicity of the Al —bonds, whereas the Al—OH groups at the surface may have a more covalent character. 

The iron(lll) oxides do not dissolve in the strongly basic solution. This difference in the behavior of the aluminum and iron compounds arises because Al is amphoteric, whereas Fe is not. (Section 17.5) Thus, the aluminate solution can be separated from the iron-containing solids by filtration. The pH of the solution is then lowered, causing the aluminum hydroxide to precipitate. 

Strontium aluminum polyphosphate. This pigment also has greater phosphate content than first-generation zinc phosphate. The solubihty behavior is further altered by inclusion of a metal whose oxides react basic compared to amphoteric zinc [38]. 

The effect of oxygen and pH on the corrosion rate of steel at two temperatures is shown in Fig. 8.7 [11]. In a broad range of about pH 5 to 9, the corrosion rate can be expressed simply in terms of the amount of DO present (e.g., micrometer per year per milliliter DO per liter of water). At about pH 4.5, acid corrosion is initiated, overwhelming the corrosion rate by DO. At about pH 9.5 and above, deposition of insoluble ferric hydroxide, FelOHlj, or magnetite, FejO, tends to slow down the corrosion attack. Amphoteric metals such as aluminum, zinc, and lead, are however additionally sensitive to high pH situations and show a corrosion rate increase in alkaline enviromnents. Figure 8.8 compares the behavior of steel and aluminum as a function of pH. 

In addition to providing information regarding the capacity of the solid surface for the liquid phase adsorbate, the adsorption isotherm can also provide valuable conformational and thermodynamic information. The data of O Fig. 10.14 presents the adsorption isotherms for poly(methylmethacrylate) (PMMA) on oxidized aluminum and silicon surfaces (Watts et al. 2000). O Figure 10.14a shows the behavior at low and medium concentrations, and this follows the expected form for chemisorption. The data at higher concentration, O Tig 10.14b, shows a sharp rise in adsorption that is consistent with multilayer rather than monolayer adsorption. The answer, however, lies in the conformation of the molecules at low concentrations they are in an extended form (illustrated by the schematic inset of Fig. 10.14a), while at higher concentrations they are in a more compact form and pack more efficiently on the surface (shown in the schematic of O Fig. 10.14b). Another usefiil feature that can be established qualitatively from inspection of the isotherms of O Fig. 10.14a is the heat of adsorption. The sharpness of the knee at low concentration provides an indication of this value, thus for the data of PMMA on aluminum and silicon the heat of adsorption for PMMA on aluminum is more exothermic than on silicon. This is as one would expect as the silicon surface will be rich in acidic silanol groups very receptive to the basic PMMA, whilst the aluminum oxide surface is amphoteric and will not react so readily with PMMA. 

The height of formalism was reached by Wickert in his definitions of acids and bases in terms of the solvent system. He overlooks such experimental behavior as amphoterism in order to state his definitions wholly in terms of ions. Shatenstein also has pointed out one of the several inconsistencies in Wickert s presentation. Although Wickert defines an acid as an ionic compound the cation of which has an incomplete electronic configuration, he recognizes that ammonium salts are acids in ammonia. Another contradiction of the experimental facts is his listing of antimony trichloride but not aluminum chloride as an acid. 

Alkoxides like Al(OR)3 will behave as fairly strong acid catalysts, owing to the tendency of the aluminum atom in these compounds to accept a share in a pair of electrons. In view of this fact it is to be expected that aluminum alkoxides will cause the amphoteric aldehydes to behave as bases, so that a simple ester results. As this behavior involves only the basic characteristics of the carbonyl group itself it makes little difference whether we have one, two, or no a-hydrogen atoms in the aldehyde an ester is the result. The first step is a typical acid-base neutralization reaction with the formation of a coordinate covalent bond ... 

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