Ammonia stability

The ammonia stabilizes Cu11 relative to Cu1 allowing it to be regenerated in the presence of air,52 and although the reduction of Cu11 by thiosulfate is normally rapid in a pure aqueous solution, in the presence of ammonia in these systems it proceeds much more slowly,43... 

Solutions of alkali metals in ammonia have been the best studied, but other metals and other solvents give similar results. The alkaline earth metals except- beryllium form similar solutions readily, but upon evaporation a solid ammoniste. M(NHJ)jr, is formed. Lanthanide elements with stable +2 oxidation states (europium, ytterbium) also form solutions. Cathodic reduction of solutions of aluminum iodide, beryllium chloride, and teUraalkybmmonium halides yields blue solutions, presumably containing AP+, 3e Be2, 2e and R4N, e respectively. Other solvents such as various amines, ethers, and hexameihytphosphoramide have been investigated and show some propensity to form this type of solution. Although none does so as readily as ammonia, stabilization of the cation by complexation results in typical blue solutions... 

Sheldon and co-workers have circumvented this problem to some extent by three approaches 46 the use of sulfuric acid to reduce the pH, by addition of ammonium fluoride and by addition of ammonia. Ammonia stabilizes monomeric chromium(III) species via the formation of amine complexes, and the fluoride effects dissolution of the silica at near-neutral pH. The three catalysts that were synthesized were evaluated in the oxidative cleavage of styrene with 35% m/m hydrogen peroxide in 1,2-dichloroethane at 70 °C (Table 4.4). The... 

The addition of ammonia stabilizes the Cu(II) catalyst by the formation of stable Cu(II) amine complexes, preventing the formation of Cu(OH)2, which has the potential to passivate the surface of gold (Marsden House, 2006). Ammonia is additionally beneficial because it maintains a high pH in solution, which can help limit the decomposition of thiosulfate. At low pH,thiosulfate becomes unstable and converts into tetrathionate, elemental sulfur and/or sulfide (Baron et al., 2013). 

One way to minimize the problem is to remove as much of the sodium hydroxide as possible and replace it with ammonium hydroxide to maintain the desired pH. When the product is dried the ammonia goes off with the water and leaves a purer form of amorphous silica behind. With less sodium content to catalyze the conversion to cristobalite the useful temperature range of the insulation can be increased, perhaps as much as 200 degrees. The use of mixed bed ion exchange resins in the elimination of sodium content and the manufacture of the ammonia-stabilized grades is discussed in the Manufacturing section above. 

Since sodium often acts as a catalyst poison, ammonia stabilized versions of colloidal silica are most often used in the preparation of catalysts. In some cases freshly deionized silicate is used as the silica source. This form of active silica is much higher in surface area and is therefore a more efficient binder. However, its use causes the... 

A 320-g quantity of an ammonia-stabilized silica sol (30.9% silica) containing 15 nm silica particles prepared as described in United States Patent 2,574,902, issued November 12, 1951, to Bechtold and Snyder, is deionized with a sulfonic acid-type cationic resin in the hydrogen form and then with a polyamine-type anionic resin in the hydroxyl form. This cationic resin is filtered from the sol before the sol is treated with the anionic resin the pH of the sol after double deionization is about 3. 

Eive liters of an ammonia-stabilized silica sol of the type described in Example 1, containing 5% silica, is deionized as described in Example 1. Concentrated nitric acid is added to the sol until the pH drops to 1.5. The acidified sol is allowed to stand overnight. An anionic exchange resin in the hydroxyl form is then added to the sol until the pH is raised to 2. The resin is filtered from the sol. A 50-50 by volume mixture of cationic and anionic exchange resins is added to the sol until the pH cannot be raised any more. The mixture is stirred for twenty-five minutes. The pH at this point is 3.5. The resins are filtered from the sol. This procedure removes the alkali from the interior of the silica particles. Eour cubic centimeters of concentrated nitric acid is added to the purified sol to increase its stability toward gelation. 

Six liters of an ammonia-stabilized silica sol of the type described in Example 1, containing 10% silica, is deionized as described in Example 1. The sol is then spray-dried in the drier described in Example 1. 

A modification of the preparation of sol from gel is described by Ahlberg and Simpson (lOS), who formed the gel under alkaline conditions by incompletely neutralizing the alkali of a silicate such as sodium silicate with less than the equivalent acid, then washing out the salts and heating the wet gel to peptize it. Much higher conversion of gel to sol is claimed than when an acidic gel.is first made. Characteristics of sols made by this process are not available, but probably sols 15-45 nm in diameter were produced. A similar-process had been described by Legal (106). Conversion of gel to sol in an autoclave to obtain a 30% ammonia-stabilized sol was patented by Mertz (107). The effect of ultrasonic dispersion of silfca gel was examined by Bubyreva and Bindas (108). 

The presence of ammonia stabilizes Nla-NHa, because ammonia retards the decomposition of Nla-NHa to pure Nla. In a concentrated ammonia solution, Nla -NHa cannot be ignited even with a very strong flash. Its stability and sensitivity are also reduced in an ammonia atmosphere. Removal of the NHa causes spontaneous explosion of nitrogen triiodide [11] and it therefore explodes immediately in high vacuum when dry [64]. Under pure water local explosions of Nla-NHa do occur, but the explosion does not spread to the surrounding material [11, 58],... 

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