Experimental Determination of the Ionization Constant of N,N‑diethylhydroxylamine (DEHA) from 25 to 250 °C at 15 MPa.

Ionization constants, K a,H, for N,N-diethylhydroxylamine (DEHA) have been measured under hydrothermal conditions, from 25 to 250 °C at 15 MPa, using the thermally stable colorimetric pH indicator acridine. The measurements were carried out by UV-visible spectroscopy using a high-temperature, high-pressure titanium flow cell with sapphire windows. The use of flow minimized the effects of thermal decomposition because the data could be collected rapidly with short equilibration times. NMR and Raman spectroscopic analysis of the quenched experimental solutions from 250 °C showed that the extent of DEHA decomposition in the experimental solution was minimal at the highest temperature studied. These are the first experimental values for the ionization constant of DEHA above 25 °C that have been reported. The experimental values of pK a,H for DEHA were fitted using the simplified form of the extended van't Hoff equation for an isocoulombic reaction and with Mesmer's "Density" model to generate thermochemical parameters suitable for the EPRI MULTEQ chemical equilibrium model and other similar software used to model water chemistry in nuclear reactor secondary coolant systems and in fossil steam generating systems.