Supercritical aqueous solution

A further problem is that ion association, that is, the tendency of oppositely charged ions to form pairs or larger aggregates in solution, becomes increasingly important as the temperature rises unless the density is kept constant this is because ion association is inversely related to the dielectric constant (relative permittivity) of the medium, which is correlated with density for a given solvent. Helgeson and co-workers have attacked these problems theoretically for aqueous solutions up to 1000 °C.28 For our purposes, it is enough to note that quantitative treatment of ionic reactions in sub- and supercritical aqueous solutions is extremely difficult at present, and likely to remain so for some time. 

Friedrich, C. Kritzer, P. Franz, G. Boukis N. and Dinjus E. (1999). The corrosion of tantalum in oxidizing sub- and supercritical aqueous solutions of HCl, H2SO4 and H3PO4. J. Material Science 34, 3137-3141. 

Kritzer, P. Boukis, N. Franz, G. Dinjus, E. (1999). The Corrosion of Niobium in Oxidizing Sub- and Supercritical Aqueous Solutions of HCl and HjSO J. Material Science Letter 18, 25-27. 

Electrochemical Techniques for Studying High-temperature Subcritical and Supercritical Aqueous Solutions... 

While all necessary thermodynamic properties for calculating g/AgCl are available, no reliable high-temperature internal reference electrode has been developed for a temperature range above 300 °C, apparently because of a chemical degradation process of the Ag/AgCl electrochemical couple in a hydrothermal environment. Therefore, no reliable studies have been carried out to find a suitable internal reference electrode that can be employed in high-temperature subcritical and supercritical aqueous solutions. 

In conclusion, while a reliable flowthrough external reference electrode has been successfully developed, there is still the pressing need for the development of a robust internal reference electrode, which could reliably operate in high-temperature subcritical and supercritical aqueous solutions. 

The usefulness of a number of metal/metal-oxide (e.g. Ir/IrC>2, Zr/ZrC>2, W/WO2, etc.) electrodes and the glass electrode has been tested over a wide range of temperatures. However, the existence of the Nernstian behavior has not been well demonstrated yet. The glass electrode can probably be employed at temperatures up to about 200 °C but was found to be impractical owing to an inconvenient design for high-temperature subcritical and supercritical aqueous solutions. 

For assessing the viability and accuracy of high-temperature potentiometric measurements, the reference systems should be used. If Pt(H2) or YSZ(Hg/HgO) electrodes are used as the indicator electrodes, an aqueous solution with well-known activity of H+ (aq) should be used as the reference systems. At temperatures below 250 °C, the dilute aqueous solutions of strong acids and bases, such as HCl(aq) or NaOH(aq), can be employed to precisely calculate the activity of H+ (aq) so that the measured potential can be compared with the calculated one within a few millivolts or less. If HCl(aq) or NaOH(aq) is to be used at temperatures above 250 °C, the association constants of the electrolytes should be taken into account. Furthermore, at these temperatures, the precision of the calculated activities of H+ (aq) can be decreased. However, even in the low-density, supercritical aqueous solution, a reference system, which consists of a couple of three-component aqueous solutions, can be found to test the accuracy of the Pt(H2) or YSZ(Hg/HgO) electrodes within about 3 mV. Each of the three-component aqueous solutions consists of NaCl and either HC1 or NaOH... 

Advances in electrochemical techniques for studying high-temperature subcritical and supercritical aqueous solutions with emphasis on new cell configurations and electrodes have been reviewed in this chapter. 

Consider the issue of designing a recycle reactor to accomplish the degradation of 99.99% of the phenol present at a concentration of 1.1 x 10 M in supercritical water at 380°C and 278 atm. This destruction level is selected on the basis of Environmental Protection Agency standards for thermal treatment of hazardous organic wastes. The corresponding inlet concentration of oxygen is 0.08 M. Prepare a plot of the reactor volume required to process 8 m /h of the supercritical aqueous solution versus the recycle ratio for values of this ratio between 0 and 20. Comment. 

Supercritical aqueous solutions have become the subject of wide-ranging interest because of their potential applications in emerging new technologies [165]. Progress has been made toward the understanding of the fundamental role played by the solute-induced structural changes of the solvent in the vicinity of a species in solution... 

Cununings, P. T. Cochran, H. D. Simonson, J. M. Mesmer, R. E. Karabomi, S. (1991) Simulation of Supercritical Water and of Supercritical Aqueous Solutions, Journal of Chemical Physics 94, 5606-5621... 

Cummings, P. T. Chialvo, A. A. Cochran, H. D. (1994) Molecular Simulation Study of Solvation Structure in Supercritical Aqueous Solutions, Chemical Engineering Science 49, 2735-2748... 

Oelkers, E. H. Helgeson, H. C. (1993) Multiple Ion Association in Supercritical Aqueous Solutions of Single Electrolytes, Science 261, 888-891... 

Kritzer, P. et al. (1998) Degradation of reactor materials in sub- and supercritical aqueous solutions, this meeting, poster session. 

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