Aluminate ions

These equilibria give rise to an acidic solution in water, to the hexahydroxo-aluminate ion [Al(OH)f,]- in a strongly alkaline... 

Commercial grades of socbum aluminate contain both waters of hycbation and excess socbum hycboxide. In solution, a high pH retards the reversion of socbum aluminate to insoluble aluminum hycboxide. The chemical identity of the soluble species in socbum aluminate solutions has been the focus of much work (1). Solutions of sodium aluminate appear to be totaby ionic. The aluminate ion is monovalent and the predominant species present is deterrnined by the Na20 concentration. The tetrahydroxyaluminate ion [14485-39-3], Al(OH) 4, exists in lower concentrations of caustic dehydration of Al(OH) 4, to the aluminate ion [20653-98-9], A10 2) is postulated at concentrations of Na20 above 25%. The formation of polymeric aluminate ions similar to the positively charged polymeric ions formed by hydrolysis of aluminum at low pH does not seem to occur. Al(OH) 4 has been identified as the predominant ion in dilute aluminate solutions (2). 

Aluminum is the most abundant metallic element in the Earth s crust and, after oxygen and silicon, the third most abundant element (see Fig. 14.1). However, the aluminum content in most minerals is low, and the commercial source of aluminum, bauxite, is a hydrated, impure oxide, Al203-xH20, where x can range from 1 to 3. Bauxite ore, which is red from the iron oxides that it contains (Fig. 14.23), is processed to obtain alumina, A1203, in the Bayer process. In this process, the ore is first treated with aqueous sodium hydroxide, which dissolves the amphoteric alumina as the aluminate ion, Al(OH)4 (aq). Carbon dioxide is then bubbled through the solution to remove OH ions as HCO and to convert some of the aluminate ions into aluminum hydroxide, which precipitates. The aluminum hydroxide is removed and dehydrated to the oxide by heating to 1200°C. 

Sodium aluminate, NaAl(OH)4, is used along with aluminum sulfate in water purification. When mixed with aluminate ions, the acidic hydrated Al3+ cation from the aluminum sulfate produces aluminum hydroxide ... 

The soluble ion Al(OH) (aq) is known as the aluminate ion. The vigorous evolution of hydrogen gas helps to physically dislodge undissolved grease particles from the walls of plumbing. 

Aluminate cement, 2 415-416 Aluminate ions, silica sols and, 22 394s Aluminates, 2 273-279 analysis, 2 275-276 chemical reactions, 2 273-274 dispersants, 8 710t economic aspects, 2 275 health and safety factors, 2 276 manufacture, 2 274-275 physical properties of, 2 273-274 uses of, 2 276-277 Alumina trihydrate (ATH), 2 274 in synthetic fillers, 11 314-315 Alumina whisker reinforcement, 5 574t Alumina xerogels, X-ray diffraction of, 23 78... 

As noted in the earlier section Basics of Electrophilic Substitution Reactions, the loss of the hydrogen ion (H ) requires the presence of a strong base. The chloride ion (CL) is a base, but it isn t strong enough to accomplish this task. However, as shown in the mechanism, the tetrachloro-aluminate ion (A1C1 ) is a sufficiently strong base. This process also regenerates the catalyst so that it s available to continue the process. 

The development of ionic liquids dates to 1914. The first research efforts involved the synthesis of ethylammonium nitrate. Hurley and Wier at the Rice Institute in Texas, 1948, developed the first ionic liquids with chloro-aluminate ions as bath solutions for electroplating aluminum. These liquids were studied primarily for their applications as electrolytes in electrochemistry technologies such as electroplating, batteries and alloy preparations. 

If the aluminate ion is a component in the chemical affinity term, then the activity of dissolved silica, by itself, cannot describe the change in rate with changes in chemistry of the contacting fluid. A number of attempts have been made to explicitly include Al activity in the chemical affinity term. Gin (1996) suggested that glass dissolution could be modelled using a mixed Si/Al term for the ion activity product ((2) ... 

Many cements used today are composites of Portland cement and industrial waste materials that can enter into the hydration reactions and contribute to the strength of the hardened product. These substances include pulverized fuel ash (PFA) from burning of pulverized coal in thermal power stations, crushed blast-furnace slag (Section 17.7), and natural or artificial pozzolanas—that is, volcanic ash and similar finely particulate siliceous or aluminosilicate materials that can react with the Ca(OH)2 in Portland cement to form hydrated calcium silicates and aluminates. As noted earlier, the solubility of Ca(OH)2 is such that the pH of pore water in Portland cements will be about 12.7, at which the Si-O-Si or Si-O-Al links in the solid pozzolanas will be attacked slowly by OH- to form discrete silicate and aluminate ions and thence hydrated calcium silicate or aluminate gels. 

Metallic manganese certainly dissolves readily in 1 mol L-1 aqueous acid, as expected, but in aqueous base a coating of insoluble Mn(OH)2 would form immediately, stopping the predicted reaction. In cases such as aluminum [E° for A13+/A1(s) = -1.67 V], the product Al is soluble in alkali as the aluminate ion Al(OH)4+ but not in neutral water, in which the... 

An alternative hypothesis, developed from studies of the synthesis of Linde A zeolite carried out by Kerr (5) and Ciric (6), pointed to growth occurring from solution. The gel was believed to be at least partially dissolved in solution, forming active aluminosilicate species as well as silicate and aluminate ions. These species linked to form the basic building blocks of the zeolite structure and returned to the solid phase. Aiello et al. (7) followed the synthesis from a highly alkaline clear aluminosilicate solution by electron microscopy, electron diffraction, and x-ray diffraction. These authors observed the formation of thin plates (lamellae) of amorphous aluminosilicates prior to actual crystal formation. 

The product in this case is sodium aluminate, a compound that contains the aluminate ion, [Al(OH)4]. Because aluminum oxide reacts with both acids and bases, it is classified as amphoteric. Other main-group elements that form amphoteric oxides are shown in Fig. J.3. As you can see, these elements lie in a diagonal band across the table from beryllium to polonium. The acidic, amphoteric, or basic character of the oxides of the d-block metals depends on their oxidation state (see Chapter 16). 

Alumina, A1203, is obtain from the cleaned bauxite ore in the Bayer process. The ore is first treated with aqueous sodium hydroxide, which dissolves the amphoteric alumina as the aluminate ion, Al(OH)4 (aq) and the acidic silica, Si02, as the metasilicate ion, SiO,2. The insoluble iron and titanium oxides precipitate in a red mud, which is filtered off ... 

Carbon dioxide is then bubbled through the solution. It lowers the OH concentration, so causing a partial decomposition of the aluminate ions and the precipitation of aluminum hydroxide ... 

The high stability of the aluminate ion allows the production of concentrated solutions of aluminum with the virtual exclusion of the main metallic impurity, viz. iron as an oxide residue. The resultant impure aluminate solution is clarified and its temperature reduced when the reverse of the above reaction occurs with the formation of A1203,3H20 by a slow crystallization procedure. The high-purity alumina trihydrate product is calcined and then reduced electrochemically in a molten fluoride bath by the well-known Hall-Heroult process. The major problems in the Bayer process have their origin in the coordination chemistry of aluminum in alkaline solutions. The... 

Dissolution of Al(OH)3 in excess base is just a special case of the effect of complex-ion formation on solubility Al(OH)3 dissolves because excess OH - ions convert it to the soluble complex ion Al(OH)4- (aluminate ion). The effect of pH on the solubility of Al(OH)3 is shown in Figure 16.16. 

Sometimes, ores are concentrated by chemical treatment. In the Bayer process, for instance, the A1203 in bauxite (A1203 x H20) is separated from Fe203 impurities by treating the ore with hot aqueous NaOH. The amphoteric A1203 dissolves as the aluminate ion, Al(OH)4 , but the basic Fe203 does not ... 

Neptunium and plutonium sorption behaviors were remarkably similar, implying that they had similar sorption reactions and solution species. Both NaOH and NaA102 decreased neptunium and plutonium sorption. Several explanations can be offered to rationalize this behavior. First, NaOH and NaAlO, may have reacted with the sediment minerals to yield solids of lower sorptive capacity. Aluminate ion, as an anionic species, also may have competed with the similar neptunate and plutonate anions for sorption sites. Finally, sodium hydroxide may have stabilized the hydrolyzed Np02(0H) and Pu02(0H)2" species in solution, as was shown in the solubility tests, and prevented sorption. Explanation of the effect of NaOH and NaA102 on neptunium and plutonium sorption will require further investigation. 

It is important to know what species are present at the beginning of the reaction. Silicate and aluminate solutions have been well studied so that one can be reasonably sure what species are present in a given solution of known concentration and pH. Aluminate solutions have been shown to contain only one type of ion at high pH the tetrahedral AI(OH)4 ion (1). It is only when the pH drops towards neutral that other, polymeric ions appear which ultimately give way to AI(H20) 3+ in acid conditions. The tetrahedral aluminate ion is the important species for normal zeolite synthesis. 

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