Ammonia evaporation, deposition-precipitation

Ammonium Uranyl Carbonate, (NH4)4U0,(C03)3.2H20, is obtained by dissohnng freshly precipitated ammonium uranate in a solution of ammonium carbonate at 60° to 70° C. or by treating a soluble uranyl salt with ammonium carbonate and ammonia. In either case a yellow solution is formed which on evaporation deposits yellow monoclinic ery> stals of the double carbonate. The same compound results on mixing concentrated solutions of ammonium carbonate (3 molecules) and uranyl nitrate (1 molecule) at 50° to 55° C.,... 

It had been mentioned by Geus (8) that deposition-precipitation by evaporation of solvent is another way to gradually increase the concentration of a metal precursor in solution, but he also reported that this leads to an inhomogeneous distribution of the active material. However, ammonia evaporation of solutions containing ammine or ammine carbonate metal complexes has been reported as a way to gradually decrease the pH of the solution to induce precipitation of hydroxy carbonate compound on alumina support and to prepare Co/AljOj (43, 44), Ni/Al203 (45, 46), CU/AI2O3 catalysts (47). The preparation method was very broadly described in the patent references. It consists of a suspension of alumina... 

Finally, new alternative methods of deposition-precipitation have been published involving sonolysis techniques and decrease in pH by ammonia evaporation. 

Thanks to Prof. Krijn de Jong for his reading of and comments on this review. Thanks also to Dr. Lok for his information on deposition-precipitation by ammonia evaporation. 

Silver solutions used in photography can become explosive under a variety of conditions. Ammoniacal silver nitrate solutions, on storage, heating or evaporation eventually deposit silver nitride ( fulminating silver ). Silver nitrate and ethanol may give silver fulminate, and in contact with azides or hydrazine, silver azide. These are all dangerously sensitive explosives and detonators [1], Addition of ammonia solution to silver containing solutions does not directly produce explosive precipitates, but these are formed at pH values above 12.9, produced by addition of alkali, or by dissolution of silver oxide in ammonia [2]. 

ZnO films for use as buffer layers in photovoltaic cells (see Chap. 9) have been chemically deposited from aqueous solutions of ZnS04 and ammonia [57]. The solution was heated to 65°C, and adherent, compact Zn(OH)2 + ZnO films were formed after one hour. Low-temperature annealing converted the hydroxide to oxide. The solution composition will be important in this deposition. On one hand, increased ammonia concentration will increase the pH and therefore the homogeneous Zn(OH)2 precipitation in solution. However, further increase in ammonia concentration will redissolve the hydroxide as the ammine complex. There will clearly be an optimum ammonia (and zinc) concentration where Zn(OH)2 does form, but slowly enough to prevent massive homogeneous precipitation. The use of ammonia in (hydr)oxide deposition derives, in part at least, from its gradual loss by evaporation if the system is not closed [58], Any open solution of an ammonia-complexed metal ion (which forms an insoluble hydroxide or hydrated oxide) should eventually precipitate the (hydr)oxide for this reason alone. 

Diammino-osmo-hydroxide, [OsO(NH3)2](OH)2, is prepared from osmium tetroxide and excess of concentrated aqueous ammonia. The yellow liquid obtained is heated in a closed vessel at 50° C. till it becomes dark brown in colour and a black powder is deposited. It is then exposed to the atmosphere and evaporated at low temperature, when a blackish-brown powder is deposited. The dry powder decomposes explosively on heating, with evolution of nitrogen. It dissolves in acids yielding the corresponding salts, and from these the base may be precipitated on the addition of aqueous alkali hydroxide. On boiling with a solution of alkali, however, the base is decomposed, ammonia is liberated, and a precipitate of osmium dioxide remains.1... 

A white crystalline precipitate of the ammonium salt of methyl-nitramine is deposited. This is dissolved in alcohol, and the solution is boiled—whereby ammonia is driven off—and concentrated to a small volume. The product is procured by completing the evaporation in a vacuum desiccator over sulfuric acid. 

Silver hyponitrite is bright yellow, and if pale in colour it generally contains a trace of ammonia, or some silver oxide. If black silver suboxide is present, the colour may be greyish-yellow, and with other impurities present, the colour may be bright green. There is no evidence that the difference in colour is due to the presence of allotropic forms. When precipitated from strongly alkaline soln., or from cone. soln. of the sodium and silver salts, the precipitate is compact, and dense, but if precipitated from dil. soln., it is flocculent and bulky. A. Kirschner, and C. Paal and F. Kretschmer, obtained crystals of silver hyponitrite by deposition from the ammoniacal soln., either by evaporation or by dilution with water. 

Well-cooled sulphuryl chloride, diluted with 15-20 vols. of chloroform, is sat. with dry ammonia, the precipitated product dissolved in water, the soln. acidified with nitric acid, and the whole of the chlorine precipitated with silver nitrate the filtered soln. is neutralized with alkali, treated with silver nitrate, and the crystalline precipitate, which consists of silver sulphimide and some other silver derivative, separated by filtration. After adding a further quantity of silver nitrate, the clear soln. is treated with alkali, when silver sul-phamide is obtained as a colourless, amorphous precipitate, which, however, is invariably mixed with some other silver compound, probably the same as that present in the silver sulphimide precipitate. In order to remove this impurity, the well-washed precipitate is treated with hydrochloric acid in quantity exactly sufficient to convert the silver into chloride, and the strongly acid soln. is neutralized with ammonia and mixed with silver nitrate, when only the silver compound of unknown composition is precipitated pure silver sulphamide can now be precipitated by adding silver nitrate and excess of ammonia, and on decomposing this compound with the necessary quantity of hydrochloric acid, a neutral or feebly acid soln. of sulphamide is obtained. Sulphamide is deposited in large, colourless crystals when its aq. soln. is evaporated over sulphuric acid under reduced press. 

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