Stability of Hydrazine, Morpholine and Ethanolamine at 275 A°C and In Situ Measurement of Redox and Acid-Base Properties

The hydrothermal reduction of nitrogen is important in several fields. Hydrazine and amines are injected in the secondary circuit of the pressurized water reactors (PWR) nuclear plants to minimize materials corrosion. Amines are also possible reactants in the nitrogen chemistry of submarine hydrothermal vents. To better evaluate the chemical properties of N-bearing hydrothermal solutions, experiments have been carried out at 275 A degrees C using a hydrothermal reactor equipped for a potentiometric monitoring, allowing in situ pH and Eh measurements. The thermal decomposition of hydrazine in simulated feed waters steam generators of PWR plants was monitored and appeared to be catalyzed by the surface of the titanium vessel or by magnetite particles, present as a model of the usual corrosion products. Rates are also dependent on pH or, keeping in mind that it is a catalyzed reaction, on the increasing proportion of N2H4 relative to N2H (5) (+) , the hydrazinium cation being less sorbed under acidic conditions. Ethanolamine appears to be stable for over a month while morpholine is decomposed in few days in contact with magnetite. The in situ pH measurements are consistent with theoretical predictions using acidity constants of morpholine and ethanolamine from the limited literature data. The redox potential imposed by hydrazine was measured and was between -0.45 V (pH(T) = 5) and -0.65 V (pH(T) = 7) provided that the concentration of hydrazine exceeds the threshold concentration of 20-50 mu g center dot kg(-1). Above this concentration, the potential was poorly dependent on the hydrazine concentration. The observed pH-Eh relationship reflects proton production during the oxidation of N2H4 to N-2. From the Eh values, a standard potential of -1.1 V at 275 A degrees C has been calculated for this reaction.