Acidity and Thermochemical Stability of Molten Sodium Nitrate and Nitrite Mixtures

Abstract

The influence of acidity on the thermochemical stability of molten sodium nitrate-nitrite mixtures used as heattransfer fluids (as in the case of the French electrosolar power system called Themis) has been demonstrated by considering the potential-pO 2-diagram relative to redox systems corresponding to these mixtures and been established from thermochemical data. The importance of fixing the pO 2-value with a buffer has been clarified. The couple H20(g)/ OH-, whose acid-base constant was previously determined, appears as the most efficient for this purpose. The potential-pO 2-diagram could be set up experimentally by studying the disproportionation of nitrite ion into nitrate and nitrogen monoxide and by determining the standard potential of the electrochemical system NO3-/NO2-by means of potentiometric measurements with a platinum electrode and with an yttria-stabilized zirconia membrane electrode indicator of pO 2-. This diagram has allowed us to specify, as a function of temperature, the boundary values of pO 2-which should not be gone beyond in order to avoid the decomposition of the mixtures as a result of the nitrite disproportionation.Molten alkali nitrate-nitrite mixtures are known for their high heat capacities and their low melting points, which have led to their use as heat-transfer fluids at temperatures not greater than 723 K. The use of the mixture called HITEC [KNO~53-NaNO240-NaNO37 weight percent (w/o)] in the French electrosolar power system Themis, built by Electricit6 de France, makes necessary that this liquid can circulate in a thermal loop where the extreme working temperatures are 523 and 773 K. In view of this use, we have decided to study the chemical factors influencing the thermal stability of molten NaNO~-NaNO2 mixtures.The thermochemical decomposition of nitrates (1, 2) and nitrites (3-5) has been the subject of many studies, but the authors do not agree about the reactions which take place. It seems, nevertheless, well established that decomposition is accompanied by the release of nitrogen, nitrogen oxides, and oxygen, with the production of alkali oxide. Because of the complexity of these reactions, we have undertaken a preliminary study based on the existing thermochemical data (6). Reactions in these media put into play the redox systems of nitrogen which depend on the oxide anion activity. So, the results have been expressed in the form of a potential-(oxo)acidity diagram. This sort of presentation has already been used for alkali nitrates and for alkali nitrites by Conte and Casiado (7), Marchiado and Arvia (8), and by Haan and Van Der Porten (9), but not for their mixtures.The calculated diagram has subsequently been specified by experimental determinations, and the results obtained have. finally permitted the definition of the conditions which would be favorable to the thermochemical stability of the heat-transfer fluid.We present here a part of our investigation concerning the effect of acidity, in studying especially the sodium nitrate-sodium nitrite-nitrogen monoxide syst...

Corrosion of high temperature alloys in solar salt at 400, 500, and 680ÀC.

“…These similarities motivated the use of accelerated aging criteria, thus determining activation energies. Molten chemistry of nitrate salts is quite complex[22,23] with changes in ullage gas resulting in a variety of different melt chemistries. Presence of an O 2 cover gas is speculated to influence relative concentrations of nitrite, nitrate, oxide, peroxide, superoxide, hydroxide[22]…”

Lux-Flood Basicity of Mixed La2O3-Alkali Molten Carbonates Determined by Analysis of their Oxygen Solubility Properties

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Investigation of Fluoroacidity in Molten Fluorides by the Combination of High Temperature NMR and Molecular Dynamics

“…In all these applications, one of key parameter is the acidity of the molten salts mixture, because it influences directly the major part of the physicochemical properties and can have a strong influence in any electrochemical process. Some acidity scales have already been built up by classifying the strengths with which different acids react with the different bases for a wide variety of molten salts (1) except in molten oxides and fluorides. In these latter, the exchanged species that defines the acid-base systems is the anion, O 2in molten oxides and Fin fluorides.…”