Dissolution of Magnesium Oxide

MgO single crystals and powder samples with regard to H and Mg concentrations. Several pH-dependent rate-controlling processes were found to be present at room temperature. 

At pH 5 the rate-controlling step was H+ attack followed by desorption of Mg2+ and OH-. The rate was proportional to either —pH or 

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Figure 8.2 Representation of the dissolution of magnesium oxide in acid solution. (Reproduced by permission of the Royal Australian Chemical Institute.)...

Magnesium Oxide In some cases, metal oxides are converted to a more soluble chemical and dissolve more rapidly. For example, magnesium oxide is relatively insoluble in water, but is converted to a more soluble carbonate form by exposure to body fluids whose buffer is bicarbonate (25). This would be expected to enhance the dissolution of magnesium oxide nanoparticles (25). However, titanium dioxide s most soluble form is the acid chloride. If the particles were porous, the porosity (access to the internal surface area) could enhance solubility stiU further for MgO. Empirical evidence suggests that the aggregated-agglomerated metal oxide nanoparticles have minimal reduction of measured surface area, or solubUity properties that parallel such surface areas. 

We have reported that dissolution of magnesium oxide nanoparticles was more rapid in a bicarbonate buffer than in distilled water (26) rate of solubility was proportionate to bicarbonate quantity in HBSS and DMEM. The concentrations of DMEM and HBSS were approximately those anticipated in lung ELF at the end of expiration and the end of inspiration, respectively. In vitro solubility studies revealed a uniform pH increase (pH change 1 to 2 units data not shown). This increase is consistent... 

Elemental composition Mg 60.32%, O 39.68%. The oxide can be identified nondestructively by x-ray methods. Oxygen content may be determined by elemental microanalysis. Magnesium may be analyzed by AA or ICP following dissolution of the oxide in nitric acid and appropriate dilution with water. 

The overall electrochemical reaction cannot explain its corrosion rate in frequent conditions. The electrochemical dissolution of magnesium can be carried out through two successive steps the electrochemical formation of Mg " and its oxidation to a divalent ion through hydrogen ion reduction (chemical reaction) or the electrochemical formation of divalent ion directly or both. The mechanism of dissolution can depend also on the stability of magnesium dihydride as a functiou of pH at the interface. 

Mitrovic-Scepanovic and Brigham (1990) considered a schematic and theoretical approach for the dissolution of magnesium in de-aerated alkaline solution at pH 13. In this approach the divalent state dissolution has been considered the dissolution of magnesium in the monovalent state accompanied by the oxidation-reduction reaction of hydrogen ions leading to the negative difference effect on the anodic curve and the formation and stability of magnesium hydride at this pH are not considered. 

The electrochemical effects of slowly and erratically thickening oxide films on iron cathodes are, of course, eliminated when the film is destroyed by reductive dissolution and the iron is maintained in the film-free condition. Such conditions are obtained when iron is coupled to uncontrolled magnesium anodes in high-conductivity electrolytes and when iron is coupled to aluminium in high-conductivity solutions of pH less than 4-0 or more than 12 0 . In these cases, the primary cathodic reaction (after reduction of the oxide film) is the evolution of hydrogen. 

Both humic acids and fulvic acids have a strong affinity for particulate and crystalline substances possessing oxygen atoms at their surfaces and they have been reported to bring about the dissolution of iron phosphate, calcium phosphate (61), uranium dioxide (65), hydrated magnesium alumino-silicates (66) and limonite, a complex mixture of hydrated ferric oxides (67). 

The sample is brought into solution by wet digestion or by dry ashing followed by acid dissolution. The concentrations of magnesium and calcium are determined by flame AAS using an air—acetylene and nitrous oxide-acetylene flame for magnesium and calcium, respectively. 

Sclar and Kilpatrick (43) studied the dissolution of Mg cylinders in several acids in absolute ethanol. They found the process to be autocatalytic the rate goes through a maximum and dissolution continues after the acid is neutralized, the solution becoming alkaline. Small amounts of water retard the reaction. The authors ascribe the behavior to the film of oxide which is always present on magnesium when in contact with air. In aqueous acid this film dissolves immediately, but in ethanol it dissolves slowly and the dean metal can then react with the solvent until and after the acid is neutralized. Eventually the water present or formed by reaction forms an inhibiting film again. 

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