Aluminium oxides, oxoacids, oxoanions and hydroxides

The catalytic and adsorbing properties of y-Al203, AIO(OH) and A1(0H)3 make this group of compounds invaluable commercially. One use of Al(OH)3 is as a mordant, i.e. it absorbs dyes and is used to fix them to fabrics. The amphoteric nature of y-Al203 and Al(OH)3 is illustrated in reactions 12.51-12.54 equation 12.53 shows the formation of an aluminate when Al(OH)3 dissolves in excess alkali. 

For use as the stationary phases in chromatography, acidic, neutral and basic forms of alumina are commercially available. 

The electrical and/or magnetic properties of a number of mixed oxides of A1 and other metals including members of the spinel family (see Box 12.6) and sodium P-alumina (see Section 27.3) have extremely important industrial applications. In this section, we single out Ca3Al205 because of its role in cement manufacture, and because it contains a 

The spinel lattice is not geometrically simple but can be considered in terms of a cubic close-packed array of ions with one-eighth of the tetrahedral holes occupied by A ions and half of the octahedral holes occupied by B ions. The unit cell contains eight formula units, i.e. [AB2X4]g. 

Some mixed metal oxides AB2X4 in which at least one of the metals is a i-block element (e.g. CoFe204) possess an inverse spinel structure which is derived from the spinel lattice by exchanging the sites of the A ions with half of the B ions. 

Aluminium oxide occurs in two main forms a-alumina corundum) and Y-AI2O3 activated alumina). The solid state structure of 0t-Al2O3 consists of an hep array of ions with cations occupying two-thirds of the octahedral interstitial sites. ot-Alumina is extremely hard and is relatively unreactive (e.g. it is resistant to attack by acids). Its density (4.0gcm ) exceeds that of Y-AI2O3 (3.5gcm ) which has a defect spinel structure (see Box 13.6 and Section 21.10). The a-form is made by dehydrating 

CHEMICAL AND THEORETICAL BACKGROUND Box 13.6 Normal spinel and inverse spinel lattices 

The occupation of octahedral sites may be ordered or random, and structure types cannot be simply partitioned into normal or inverse . A parameter A is used to provide information about the distribution of cations in the interstitial sites of the close-packed array of X ions A indicates the proportion of B ions occupying tetrahedral holes. For a normal spinel, A = 0 for an inverse spinel, A = 0.5. Thus, for MgAl204, A = 0, and for CoFc204, A = 0.5. Other 

Aluminium oxide occurs in two main forms a-alumina corundum) and Y-AI2O3 activated alumina). The solid 

21 The structures of selected borate anions trigonal planar and tetrahedral B atoms are present, and each tetrahedral B carries a negative charge. The [6405(011)4] anion occurs in the minerals borax and kernite. In the pyroborate ion, [6205] , the B-O-B btmd angle depends on the cation present, e.g. Z.B-0-B = 153° in C02B2O5, and 131.5° in Mg2B205. 

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