Aluminum hydroxide , solubility

White precipitate of aluminum hydroxide soluble in excess sodium hydroxide... 

One may plot Equations 2.69, 2.71, and 2.73 as pAlhydroxy species versus pH. This will produce three linear plots with different slopes. Equation 2.69 will produce a plot with slope 3, whereas Equations 2.71 and 2.73 will produce plots with slopes 2 and -1, respectively. The sum of all three aluminum species as a function of pH would give total dissolved aluminum. This is demonstrated in Figure 2.12, which describes the pH behavior of eight Al-hydroxy species. The following three points can be made based on Figure 2.12 (1) aluminum-hydroxide solubility exhibits a U-shaped behavior, (2) aluminum in solution never becomes zero, and (3) different aluminum species Predominate at different pH values. 

A study of the chemistry of a small California stream that receives acid mine water (27) showed that above pH 4.9 the dissolved aluminum concentration appeared to be controlled by aluminum hydroxide solubility equilibria, but below pH 4.6 aluminum hydroxy-sulfates apparently predominated. This study also cited data for acid mine drainage in the Appalachian region and lakes affected by acid precipitation that showed a similar pattern of aluminum behavior. 

The third process involves careflil addition of aluminum hydroxide to fluorosiUcic acid (6) which is generated by fertilizer and phosphoric acid-producing plants. The addition of Al(OH)2 is critical. It must be added gradually and slowly so that the siUca produced as by-product remains filterable and the AIF. -3H20 formed is in the soluble a-form. If the addition of Al(OH)2 3H20 is too slow, the a-form after some time changes into the insoluble P-form. Then separation of siUca from insoluble P-AIF. -3H20 becomes difficult. 

The FD C certified colors are all water-soluble dyes, but can be transformed into insoluble pigments known as lakes by precipitating the dyes with alurninum, calcium, or magnesium salts on a substrate of aluminum hydroxide. The lakes are useful in appHcations that require color whereas in dry form, such as cake mixes, or where water may be present and bleeding is a problem, such as food packaging. FD C Red Lake No. 3 was deHsted in Febmary... 

The hexahydrate of aluminum chloride is a deUquescent, crystalline soHd soluble in water and alcohol and usually made by dissolving aluminum hydroxide [21645-51 -2] A1(0H)2, ia concentrated hydrochloric acid. When the acid is depleted, the solution is cooled to 0 °C and gaseous hydrogen chloride is introduced. Crystalline aluminum chloride hexahydrate, AIQ 6H20, is precipitated, filtered from the Hquor, washed with ethyl ether, and dried. 

We can tell from the ionization energy of aluminum that this atom holds its second and third valence electrons rather firmly. With this fact in mind, we can see why aluminum hydroxide, Al(OH)3, would not be as strongly basic as are the hydroxides, NaOH and Mg(OH>2. Aluminum hydroxide has extremely low solubility in neutral aqueous solutions but does react with strong acids according to the reaction... 

Aluminum sulfate, A12(S04)3 H20, is the commonest alum used. Hydration is typically 14 to 16 H20. It hydrolyzes and polymerizes in water and typically is used within a narrow window of pH levels of 5.5 to 6.5 to minimize the solubility of aluminum in the treated water. If alkalinity is present (say, due to calcium bicarbonate), the following reaction occurs, producing insoluble aluminum hydroxide [Al(OH)3]. 

Use the data in Table 11.4 to calculate the solubility of each of the following sparingly soluble substances in its respective solution aluminum hydroxide at (a) pH = 7.0 ... 

Scale prevention methods include operating at low conversion and chemical pretreatment. Acid injection to convert COs to CO2 is commonly used, but cellulosic membranes require operation at pH 4 to 7 to prevent hydrolysis. Sulfuric acid is commonly used at a dosing of 0.24 mg/L while hydrochloric acid is to be avoided to minimize corrosion. Acid addition will precipitate aluminum hydroxide. Water softening upstream of the RO By using lime and sodium zeolites will precipitate calcium and magnesium hydroxides and entrap some silica. Antisealant compounds such as sodium hexametaphosphate, EDTA, and polymers are also commonly added to encapsulate potential precipitants. Oxidant addition precipitates metal oxides for particle removal (converting soluble ferrous Fe ions to insoluble ferric Fe ions). 

The enhanced aqueous solubility of aluminum hydroxide in the presence of an acid or alkali is an example where neutralization reaction plays a part. In pure water, the solubility of aluminum hydroxide is low which is very well reflected in the following equation ... 

This way a neutralization reaction can lead to an enhancement of the solubility of an otherwise insoluble or sparingly soluble material. Because of its ability to dissolve both in acid and alkali, aluminum hydroxide is known as an amphoteric hydroxide. 

Coprecipitation is a partitioning process whereby toxic heavy metals precipitate from the aqueous phase even if the equilibrium solubility has not been exceeded. This process occurs when heavy metals are incorporated into the structure of silicon, aluminum, and iron oxides when these latter compounds precipitate out of solution. Iron hydroxide collects more toxic heavy metals (chromium, nickel, arsenic, selenium, cadmium, and thorium) during precipitation than aluminum hydroxide.38 Coprecipitation is considered to effectively remove trace amounts of lead and chromium from solution in injected wastes at New Johnsonville, Tennessee.39 Coprecipitation with carbonate minerals may be an important mechanism for dealing with cobalt, lead, zinc, and cadmium. 

Problem 27 Determine the solubility in grams per liter for aluminum hydroxide, Al(OH)3. 

The solubility of aluminum hydroxide is 0.00843 gram per liter. The A1(0H)3 dissociates to 4 ions with the concentration of OH being 3 times that of Al. ... 

Rubidium acid salts are usually prepared from rubidium carbonate or hydroxide and the appropriate acid in aqueous solution, followed by precipitation of the crystals or evaporation to dryness. Rubidium sulfate is also prepared by the addition of a hot solution of barium hydroxide to a boiling solution of rubidium alum until all the aluminum is precipitated. The pH of the solution is 7.6 when the reaction is complete. Aluminum hydroxide and barium sulfate are removed by filtration, and rubidium sulfate is obtained by concentration and crystallization from the filtrate. Rubidium aluminum sulfate dodecahydrate [7488-54-2] (alum), RbA SO 12H20, is formed by sulfuric acid leaching of lepidolite ore. Rubidium alum is more soluble than cesium alum and less soluble than the other alkali alums. Fractional crystallization of Rb alum removes K, Na, and Li values, but concentrates the cesium value. Rubidium hydroxide, RbOH, is prepared by the reaction of rubidium sulfate and barium hydroxide in solution. The insoluble barium sulfate is removed by filtration. The solution of rubidium hydroxide can be evaporated partially in pure nickel or silver containers. Rubidium hydroxide is usually supplied as a 50% aqueous solution. Rubidium carbonate, Rb2C03, is readily formed by bubbling carbon dioxide through a solution of rubidium hydroxide, followed by evaporation to dryness in a fluorocarbon container. Other rubidium compounds can be formed in the laboratory by means of anion-exchange techniques. Table 4 lists some properties of common rubidium compounds. 

Sucralfate is a salt of sucrose complexed to sulfated aluminum hydroxide. In water or acidic solutions it forms a viscous, tenacious paste that binds selectively to ulcers or erosions for up to 6 hours. Sucralfate has limited solubility, breaking down into sucrose sulfate (strongly negatively charged) and an aluminum salt. Less than 3% of intact drug and aluminum is absorbed from the intestinal tract the remainder is excreted in the feces. 

Alternatively, several workers have shown that not only is the soluble, zero-charged hydrolysis product considerably more surface active than the free (aquo) ion but also a polymeric charged or uncharged hydrolysis product may be formed at the solid-liquid interface at conditions well below saturation or precipitation in solution. Hall (5) has considered the coagulation of kaolinite by aluminum (III) and concluded that surface precipitates related to hydrated aluminum hydroxide control the adsorption-coagulation behavior. Similarly Healy and Jellett (6) have postulated that the polymeric, soluble, uncharged Zn(OH)2 polymer can be nucleated catalytically at ZnO-H20 interfaces and will flocculate the colloidal ZnO via a bridging mechanism. 

Acetate Aluminum acetate A1(C2H302)3, white crystals, soluble, by reaction of aluminum hydroxide and acetic acid and then crystallizing. Used (1) as a mordant in dyeing and printing textiles, (2) in die manufacture of lakes, (3) for fireproofing fabrics, (4) for waterproofing cloth. 

Aluininatcs Sodium aluminate NaAlOi, white solid, soluble, (1) by reaction of aluminum hydroxide and NaOH solution. (2) by fusion of aluminum oxide and sodium carbonate the solution of sodium aluminate is reactive with CO2 to form aluminum hydroxide. Used as a mordant in the textile industry, in the manufacture of artificial zeolites, and in the hardening of budding stones. See silicates below and calcium aluminates. 

Chlorides Aluminum chloride AlClj 6H20. white crystals, soluble, by reaction of aluminum hydroxide and HC1. and then crystallizing anhydrous aluminum chloride A1C1-, white powder, fumes in air, formed by reaction of dry aluminum oxide plus carbon heated with chlorine in a furnace, used as a reagent in petroleum refining and other organic reactions. 

Fluoride Aluminum fluoride AIF3, white solid, soluble, by reaction of aluminum hydroxide plus hydrofluoric acid and then crystallizing 2AIF3 7H20, used in glass and porcelain ware. 

Hydroxide Aluminum hydroxide Al(OH>3, white gelatinous precipitate, by reaction of soluble, aluminum salt solution and an alkali hydroxide, carbonate or sulfide (sodium aluminate is formed with excess NaOH but no reaction with excess NH4OH). upon heating aluminum hydroxide the residue formed is aluminum oxide. Used as intermediate substance in transforming bauxite into pure aluminum oxide. 

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