Ammonium salt decompositions sublimation

NH4)2S03 sublimes unaltered [946] but the monohydrate undergoes partial decomposition to H20, NH3 and (NH4)2S207. Decomposition of ammonium thiosulphate is inhibited by the addition of other ammonium salts, e.g. (NH4)2C03, and by briquetting [947]. 

The baking process, particularly the batch variety, presents a number of serious disadvantages. Not only does the reaction produce solid urea decomposition products, but it also releases large amounts of ammonia and ammonium salts which escape by sublimation. The foam which is thus formed makes for a porous reaction mixture, which in turn even prevents heat conduction. Moreover, the reaction mixture tends to adhere to the surface of the reaction vessel and the stirring unit a phenomenon which adds to the complexity of the problem. 

How can the different nature of thermal decomposition of ammonium salts be explained What ammonium salts sublime How can one prove experimentally that the salts used are ammonium ones ... 

Each of the ammonium salts, and even hydroxylammonium nitrate, initiate decomposition in the same fashion—loss of amine. Indeed, ammonium nitrate, perchlorate, and dinitramidate all sublime. 

Some textbooks recommend the removal of ammonium salts simply by sublimation. This can be done by first evaporating the acidified filtrate on a water bath, and then heating the dry residue at a higher temperature, until the white fumes of ammonium salts cease to appear. It is much easier however to remove ammonium salts with concentrated nitric acid, as described in Table V.12, when a chemical decomposition takes place ... 

A common feature of the decompositions of many ammonium compounds is the identification of the first step in reaction as proton transfer. The consequent accumulation of protons with the residual oxy-anions may be followed by the elimination of water, accompanied by condensation (or continued condensation) of the anions, see chromates and phosphates above, ultimately leading to residual oxide formation. Alternatively the acid may be volatilized, many ammonium salts sublime... 

Many of these salts melt or sublime before or during decomposition and reaction temperatures generally increase with molar mass. Thermal analyses for a selection of ammonium carboxylates have been given by Erdey et al. [915] who conclude that the base strength of the anion increases with temperature until it reaches that of NH3. Decompositions of ammonium acetate (>333 K) and ammonium oxalate (>473 K) proceed through amide formation. Ammonium benzoate and ammonium salicylate sublime (>373 K) without decomposition but ammonium citrate decomposes (>423 K) to yield some residual carbon. 

Ammonium chloride is also formed by the action of hydrochloric acid on a soln. of ammonia or ammonium carbonate J. G. Qentele 5 made it by the double decomposition of ammonium bicarbonate and sodium, magnesium, calcium, and other chlorides H. J. E. Hennebutte and E. Mesnard, and A. Dubose and M. Heuzey, made it by the action of ammonium bicarbonate or sulphate on the double chloride of iron and calcium and it is made by the action of soln. of ammonium sulphate and sodium chloride when the soln. is cone, the crystals of sodium sulphate separate out and they are removed by suitable shovels the cone. soln. of ammonium chloride which remains is purified by crystallization. Ammonium chloride can also be obtained by sublimation from a dry intimate mixture of the same two salts. A. French made it by the joint action of air and steam on a mixture of salt, pyrites, and carbon or organic matter 2NaCl+4H20-j-S02+C-j-N2=2NH4Cl-i-Na2S04-i-C02. 

H. St. C. Deville (1863) found that when the two gases are mixed at 350°, the temp, rises to 395°, showing that probably some combination occurs. J. A. Wanklyn (1865) estimated from the observed vapour density, 29 04 at 350°, that about 17 per cent, of the gases unite to form ammonium chloride. J. C. G. de Marignac (1868) noticed that the heat absorbed in the sublimation of ammonium chloride is much greater than that usually observed with other substances, and is almost eq. to the heat of decomposition of the salt in question, and hence concludes It is very probable that sal ammoniac—in great part at least—is decomposed on volatilization. ... 

Ammonium iodide sublimes when heated, and the sublimate is freed from decomposition products only when hir is excluded, otherwise it is coloured yellow by iodine or polyiodides. The vapour density of the salt corresponds with its decomposition into ammonia and hydrogen iodide, and of hydrogen iodide into... 

Kummer [19], Friedman and Biegeleisen [20] studied the decomposition of ammonium nitrate labelled withI5N [15N]H4 [14N]03. The completely anhydrous salt did not decompose until 300°C. Only sublimation took place. But a trace of water initiated decomposition at 180°C. The main products of the reaction were NzO and water (according to the eqn. (4)) and with the labelled salt, nitrous oxide [14N] [15N]0 was produced showing that the gas was formed by the interaction of the two ions in the molecule. They also showed that with ordinary ammonium nitrate and H2180, no N2I80 was formed, indicating that the water had a purely catalytic effect. 

Only a few solids have vapor pressures near atmospheric at safe temperatures, among them COz, UF , ZrCL(, and about 30 organics. Ammonium chloride sublimes at 1 atm and 350°C with decomposition into NH3 and HC1, but these recombine into pure NH4C1 upon cooling. Iodine has a triple point 113.5°C and 90.5 Torr it can be sublimed out of aqueous salt solutions at atmospheric pressure because of the entraining effect of vaporized water. 

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