Carbonate Solubility and Water Stability

CO3 species was formed and the X-ray structure solved. It is thought that the carbonate species forms on reaction with water, which was problematic in the selected strategy, as water was produced in the formation of the dialkyl carbonates. Other problems included compound solubility and the stability of the monoalkyl carbonate complex. Van Eldik and co-workers also carried out a detailed kinetic study of the hydration of carbon dioxide and the dehydration of bicarbonate both in the presence and absence of the zinc complex of 1,5,9-triazacyclododecane (12[ane]N3). The zinc hydroxo form is shown to catalyze the hydration reaction and only the aquo complex catalyzes the dehydration of bicarbonate. Kinetic data including second order rate constants were discussed in reference to other model systems and the enzyme carbonic anhy-drase.459 The zinc complex of the tetraamine 1,4,7,10-tetraazacyclododecane (cyclen) was also studied as a catalyst for these reactions in aqueous solution and comparison of activity suggests formation of a bidentate bicarbonate intermediate inhibits the catalytic activity. Van Eldik concludes that a unidentate bicarbonate intermediate is most likely to the active species in the enzyme carbonic anhydrase.460... 

In the wet oxidation process, materials partially or completely dissolve into a homogeneous, condensed-phase mixture of oxygen and water, and chemical reactions between the material and oxygen take place in the bulk water phase. This condensed-phase makes wet oxidation an ideal process to transform materials which would otherwise be non-soluble in water to a harmless mixture of carbon dioxide and water. Since oxidation reactions are also exothermic, the high thermal mass of supercritical water makes this reaction medium better suited for thermal control, reactor stability, and heat dissipation. The purpose of this research was to establish a new method for selectively oxidizing waste hydrocarbons into new and reusable products. 

The effect of gas solubility on the rate of foam destruction is of major practical importance. For example, in the production of firefighting foams for underground use in coal mines, it is advisable to use exhaust gases as a disperse phase. However, they contain a considerable amount of carbon dioxide and water vapour that sharply decrease the expansion ratio and stability of the foam produced. 

Thorium. Experimental and theoretical studies of thorium speciation, solubility, and sorption in low-ionic-strength waters are described by Langmuir and Herman (1980), Laflamme and Murray (1987), Osthols et ai (1994), Osthols (1995), and Quigley et al. (1996). Langmuir and Herman (1980) provide a critically evaluated thermodynamic database for natural waters at low temperature that is widely used. However, it does not contain information about important thorium carbonate complexes, and the stability of phosphate complexes may be overestimated (US EPA, 1999b). 

Thermoanalytical methods have been frequently employed in the characterization of complex samples such as minerals and clays [6] and carbonate stones used in the construction of monuments [7]. They have become a routine analytical tool for the characterization of new compounds in the pharmaceutical industry, where molecules are frequently prepared as hydrates or pseudosolvates to ensure good water solubility and good stability in moist environments [8]. Biochemical and biological applications of thermal analysis are also the subject of a recent review [9]. 

In Figure 2 the solubility and speciation of plutonium have been calculated, using stability data for the hydroxy and carbonate complexes in Table III and standard potentials from Table IV, for the waters indicted in Figure 2. Here, the various carbonate concentrations would correspond to an open system in equilibrium with air (b) and closed systems with a total carbonate concentration of 30 mg/liter (c,e) and 485 mg/liter (d,f), respectively. The two redox potentials would roughly correspond to water in equilibrium wit air (a-d cf 50) and systems buffered by an Fe(III)(s)/Fe(II)(s)-equilibrium (e,f), respectively. Thus, the natural span of carbonate concentrations and redox conditions is illustrated. 

The hydrothermal carbons obtained in the end from soluble, non-structural carbohydrates are micrometer sized, spherically shaped particle dispersions, containing a sp2 hybridized backbone (also responsible for the brown to black color) decorated with a dense layer of polar oxygenated functionalities still remaining from the original carbohydrate. The presence of these surface groups offers the possibility of further functionalization and makes the materials more hydrophilic and well-dispersible in water. The size of the final particles depends mainly on the carbonization time and precursor concentration inside the autoclave, as well as additives and stabilizers potentially added to the primary reaction recipe. An SEM image of a model reaction illustrating this dispersion state is shown in Fig. 7.1. 

For recently prepared terpolymers of PO/CO2 noticeable improvements in thermal stability were thus achieved. The rate of decomposition decreases with the termonomer content. Part of the effect obviously originates from the lower concentration of propylene carbonate entities and another part from the lower flexibility of the main chain. In addition, another explanation for the increase in thermal stability with termonomers (which is, however, not substantiated) could be found in the solubility of water in the product because every recent study indicates that hydrolysis is a major cause of the initiation of thermal decomposition [23]. Also, additives have been identified as slowing down thermal degradation. 

Other Coordination Complexes. Because carbonate and bicarbonate are commonly found under environmental conditions in water, and because carbonate complexes Pu readily in most oxidation states, Pu carbonato complexes have been studied extensively. The reduction potentials vs the standard hydrogen electrode of Pu(VI)/(V) shifts from 0.916 to 0.33 V and the Pu(IV)/(III) potential shifts from 1.48 to -0.50 V in 1 M carbonate. These shifts indicate strong carbonate complexation. Electrochemistry, reaction kinetics, and spectroscopy of plutonium carbonates in solution have been reviewed (113). The solubility of Pu(IV) in aqueous carbonate solutions has been measured, and the stability constants of hydroxycarbonato complexes have been calculated (Fig. 6b) (90). 

Acetaldehyde Cyanohydrin. This cyanohydrin, commonly known as lactonilrilc, is soluble in water and alcohol, but insoluble in diethyl ether and carbon disulfide. Lactonilrilc is used chiefly to manufacture lactic acid and its derivatives, primarily ethyl lactate. Lactonilrilc is manufactured from equimolar amounts of acetaldehyde and hydrogen cyanide containing 1.5% of 20% NaOH at -t0-20°C. The produci is stabilized with sulfuric acid. 

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