Sulfate decompositions, kinetics

A feature of the literature on sulfate decompositions is the diversity of reported values of E. Mihalik and Horvath [95] found that the decompositions of Zn, Cu, Mg, Fe, Ni, Co and Mn sulfates could be described by the zero-order kinetic equation and that , was about 250 kJ mol. Kolta and Askar [96], however, described the decompositions of Zn, Fe, Co, Ni and Cu sulfates as being slightly deceleratory with , in the range 96 to 125 kJ mol. ... 

A review by Brandt and van Eldik provides insight into the basic kinetic features and mechanistic details of transition metal-catalyzed autoxidation reactions of sulfur(IV) species on the basis of literature data reported up to the early 1990s (78). Earlier results confirmed that these reactions may occur via non-radical, radical and combinations of non-radical and radical mechanisms. More recent studies have shown evidence mainly for the radical mechanisms, although a non-radical, two-electron decomposition was reported for the HgSC>3 complex recently (79). The possiblity of various redox paths combined with protolytic and complex-formation reactions are the sources of manifest complexity in the kinetic characteristics of these systems. Nevertheless, the predominant sulfur containing product is always the sulfate ion. In spite of extensive studies on this topic for well over a century, important aspects of the mechanisms remain to be clarified and the interpretation of some of the reactions is still controversial. Recent studies were... 

Polyelectrolyte multilayer microspheres, prepared by alternating adsorption of dextran sulfate and protamine on melamine formaldehyde cores followed by the partial decomposition of the core, were used to immobilise the peroxidase and glucose oxidase. Retention of enzymic activity of the peroxidase/glucose oxidase system incorporated into the microspheres was demonstrated. These bienzyme system immobilised in the microspheres can be applied for kinetic glucose assays [ 156]. 

However, in decomposing salts such as sulfates where stable oxysulfates exist, traces of sulfur can persist at temperatures even above 1200°C. Decomposition reactions often follow kinetic models of a sphere or cylinder, where the reaction interface moves at constant velocity from the spherical or cylindrical surfaces inward. 

Oxygen in the reaction vessel may influence kinetic behaviour by participating in these equilibria. There is also the possibility that changes in oxidation state of the cation may occur during the decomposition (e.g. FeS04 and CrSOJ. A number of sulfates (e.g. CaS04 and rare earth sulfates) decompose to yield intermediate oxysulfates. The possibility of melt formation before or during reaction cannot always be excluded and is a further factor that complicates the kinetic interpretations. 

These include the decompositions of many sulfates and nitrates. Measurements of rates and other kinetic parameters for reversible reactions may be subject to the same problems as ttiose described for CaCOj (Chapter 12) unless experimental conditions are carefiiUy designed to ensure that inifluence of the reverse reaction is minimised, as described for the dissociation of BaS04 [93]. 

Selected kinetic characteristics for the decompositions of metal sulfates... 

Thermal decomposition of the preceding two basic sails finally yields foundry quality Al203. Using TG and other TA techniques, Pysiak and Glinka (681 studied the thermal decomposition of basic aluminum potassium sulfate and determined the intermediate products and the reaction kinetics. 

The new methods of measurement and calculation proposed have radically improved the precision and accuracy of determining the thermochemical parameters and, in this way, expanded considerably the potential use of TA in kinetics studies. The methods have been employed to obtain, for the first time, reliable kinetic characteristics and to uncover the mechanism of decomposition of several tens of substances belonging to different classes of inorganic compounds (crystalline hydrates, oxides, hydroxides, nitrides, azides, nitrates, sulfates, carbonates, and oxalates). The results obtained in these studies have been published in the literature in various journals spanning the period from 1981 to 2006). This book draws the work together to make it more accessible for researchers confronted with these issues. 

Li and coworkers [50] studied the zinc hydroxide carbonate precursor, Zii4C03(0H)6-H20, synthesized from zinc sulfate using ammonium carbonate as a precipitating agent. TG), DSC, transmission electronic microscopy (TEM), infiared spectroscopy (IR) and XRD were used to characterize die precursor and the decomposition product, while the non-isothermal kinetics of the thermal decomposition of zinc hydroxide carbonate were studied in nitrogen. The kinetic parameters were obtained using a model-free method and the reaction model was then derived by means of non-linear regression. The results showed that the decomposition of zinc hydroxide carbonate is a two-step reaction a reversible reaction of two-dimensional diffusion (D2), followed by an irreversible one of... 

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