Oxysalt Decomposition

Amongst the studies of oxysalt decomposition has been one on oxalates. A number of metal oxalates were subjected to a thermogravimetric run in air and nitrogen up to 1000 °C. The object was to see if there were any differences in behaviour between metals, to determine the end product, to see if the atmosphere made any difference and to check the experimental mass losses against the theoretical ones to confirm the reaction mechanism. The shapes of the curves produced fell into 5 types as shown in Figure 7. 

Decomposition of the rare earth nitrates proceeded [821] through the intermediate formation of oxysalts of the form MON03 and E values were low Nd(N03)3, 33 kJ mole 1, 663-703 K Dy(N03)3, 23 kJ mole 1, 583—633 K Yb(N03)3, 46 kJ mole 1, 563—598 K. Thermogravimetric curves showed that the formation of anhydrous salts was possible, in contrast to observations by Wendlandt and Bear [826]. In a similar study [827] of the reaction of Pr(N03)3 at 558—758 K, the intermediate formation of a nitrite is postulated during decomposition to a non-stoichiometric residual oxide, Pr0li83 (the actual composition depends on temperature). 

Atomic and ionic radii affect the attraction, for electrons and anions, and govern such properties as basicity. Basicity differences affect in the hydrolysis of ions, the solubilities of salts, the thermal decomposition of oxysalts and the formation of complex species. 

Stem, K. H., High temperature properties and decomposition of inorganic salts. Part 4, Oxysalts of the halogens, J. Phys. Chem. Ref. Data, 3, 481, 1974. 

The quantitative high-temperature chemistry of chlorine oxysalts is rather underdeveloped. There are very few thermodynamic data for these compounds above 298 K. Even when they exist, they must be applied cautiously, since there may be kinetic rather than thermodynamic factors that determine decomposition behavior. Although the thermal decomposition of a few compounds has been studied very carefully (e.g., the KC104 literature extends back for more than a hundred years because of the compound s use in explosives), the bulk of the available information is qualitative or semiquantitative. In recent years this has changed somewhat with increasing use of automated techniques such as DTA and TGA. Many of the reactions are complex, with mechanisms frequently controversial and not completely worked out. Decomposition products may depend on experimental conditions e.g., salts are frequently prepared by dehydration of their hydrates, and residual water may affect the course of the decomposition. 

The decomposition of other oxysalts, such as nitrates and sulfates, follows the same rules. Barium sulfate can be heated to bright redness without decomposition, as the analytical chemist knows calcium sulfate loses some SOj... 

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