Amorphous precipitation

Silver f) oxide, Ag20, is obtained as a brown amorphous precipitate by the action of alkali on AgN03 solution. It cannot be obtained pure and is soluble in ammonia solution. 

Kea.tlte, Keatite has been prepared (65) by the crystallisation of amorphous precipitated silica ia a hydrothermal bomb from dilute alkah hydroxide or carbonate solutions at 380—585°C and 35—120 MPa (345—1180 atm). The stmcture (66) is tetragonal. There are 12 Si02 units ia the unit cell ttg = 745 pm and Cg = 8604 pm the space group is P42. Keatite has a negative volumetric expansion coefficient from 20—550°C. It is unchanged by beating at 1100°C, but is transformed completely to cristobahte ia three hours at 1620°C. 

Applications. In 1994, approximately 675,000 metric tons of amorphous precipitated sihca were manufactured for sale (38,62). Degussa, J. M. Huber, and PPG in the United States and in Europe, and Akzo (Germany), Aluflour (Sweden), Crosfield (United Kingdom), Nippon Sihca (Japan), Rhc ne-Poulenc (France), Shionogi (Japan), Tokuyama Soda (Japan), and Vitro PQ (Mexico) are the primary producers. 

These probably form the basis of the amorphous precipitates formed on cooling. The more soluble resins produced on continuation of the reaction probably contain pendant methylol groups formed by reactions of the NH groups with free formaldehyde Figure 24.3 I). 

During precipitate ageing, a gradual transformation of an initially precipitated metastable phase into a final crystalline form often occurs. The metastable phase may be an amorphous precipitate, a polymorph of the final material, a hydrated species or some system-contaminated substance (Mullin, 2001). In 1896, Ostwald promulgated his rule of stages which states that an unstable... 

Add to a little of the solution a few drops of iodine solii tion a brown amorphous precipitate is formed. This reai lion is given by many of the alkaloids. 

There are other soUd states which sometimes confuse the measurement and definition of solubiUty. The dmg may crystaUize as a hydrate, i.e. under inclusion of water molecules. If the hydrate form is more stable than the pure form it may be difficult to measure the intrinsic solubility of the drug at all. Often drugs tend to precipitate in an amorphous form, often under the inclusion of impurities. As with metastable polymorphs, such amorphous precipitates may lead to erroneously high solubility measurements. CommerciaUy, drugs are often crystallized in salt form, e.g. as the hydrochloride salt, a cation with a chloride anion. In these co-crystallized salts, a much lower solubility than the intrinsic solubility will typi-... 

Ageing of polynuclears eventually leads either to an amorphous precipitate or to a variety of crystalline compounds. The molar ratio of excess base added per Fe(III), a, determines both the precipitation time and whether the precipitate is amorphous or crystalline. A crystalline precipitate forms in minutes or hours at a critical value of a, amin which lies between 0.5 and 1.0 (Figure 1.5). At values... 

Oxyhydroxides Amorphous precipitates of oxides and hydroxides that form in alkaline solutions such as seawater. These precipitates usually contain a variety of caUons, such as trace metals. 

The phases of the hB elements are largely unused, e.g. NaSi etc. with the tetrasilatetrahedrane Si, " unit or LiiaSiv and LiizGev with the trigonal planar Sii and Gez stars (28). We were already able to observe reactions of these compounds, but upto now the products have not been characterized. Reactions of NasSijte, Naj Siiae (x=3-12) and LiaSns with benzophenone proceed in the way described above, but so far only amorphous precipitations of silicon and tin have been obtained. 

When supersaturated fresh gallbladder bile (or model bile) is centrifuged to remove solid crystalline and amorphous precipitates, and supernatant vesicles, and the resultant isotropic (one phase) solution maintained in a dust-free environment at 37 °C and examined daily by light microscopy, cholesterol crystals can be observed to precipitate. The time taken for these solid cholesterol... 

For removing low levels of priority metal pollutants from wastewater, using ferric chloride has been shown to be an effective and economical method [41]. The ferric salt forms iron oxyhydroxide, an amorphous precipitate in the wastewater. Pollutants are adsorbed onto and trapped within this precipitate, which is then settled out, leaving a clear effluent. The equipment is identical to that for metal hydroxide precipitation. Trace elements such as arsenic, selenium, chromium, cadmium, and lead can be removed by this method at varying pH values. Alternative methods of metals removal include ion exchange, oxidation or reduction, reverse osmosis, and activated carbon. 

Native RNase is quite resistant to digestion with trypsin, even at a w/w ratio of 1 20, but small or unfolded fragments would be expected to be digested. When the synthetic enzyme was treated with trypsin, a 70% recovery of protein with a specific activity of 61% was obtained. Treatment of this material with saturated ammonium sulfate (diluted 16 26), pH 4.6, gave 33% of amorphous precipitate and 66% of soluble RNase A. The overall yield from the first Val residue was only 3%, but the specific activity was quite high at 78%. This is as far as the purification was carried out at that time. 

At room temperature CuCN is not soluble in most solvents. Therefore, the direct reaction of CuCN with PPh3 in DMF results in the formation of an amorphous precipitate that is not soluble in any solvent. But when 2,2 -bipyridine is introduced into the reaction system of CuCN and PPh3 in DMSO, a red solution is obtained. By slowly diffusing diethyl ether into this solution, red block crystals of the complex (CuCN)2(2,2-bipyridine)(PPh3) are obtained. In contrast, when pyridine-2-thiolate replaces 2,2 -bipyridine, the complex CuCN(PPh3)2 6 is formed instead. ... 

When calcium phosphate is precipitated from aqueous solutions of high supersaturation and pH values above 7, the solid phase appearing initially is an ACP with the formula Ca9(P04)61U u2 - If this amorphous precipitate is allowed to remain in contact with the solution, it transforms to crystalline HA through a process of dissolution, nucleation and crystal growth77, unless stabilized in some manner. 

Stannic sulfide may be prepared as a yellow amorphous precipitate by double decomposition of a stannic salt with hydrogen sulfide. The crystalline modification, known as mosaic gold, may be prepared by sulfurizing stannous sulfide in the following way ... 

The thiocyanates are generally soluble in water, the exceptions being those of lead, silver, mercury and copper. Most of them dissolve also in alcohol and ether. Aqueous solutions of the alkali thiocyanates undergo atmospheric oxidation under the influence of sunlight with solutions of medium concentration this change takes place rapidly, with separation of a yellow, amorphous precipitate consisting of pseudocyanogen sulphide, (CNS)3 (cf. p. 236). The concentration of thiocyanate most favourable to the separation of this sulphide is about 50 per cent, in summer and 10 per cent, in winter. In addition to this substance the products of the photochemical oxidation of potassium thiocyanate include hydrocyanic acid, sulphate, carbon dioxide, ammonia and ammonium salts ... 

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