Thermodynamic study on redox reactions of boiling nitric acid solutions

Kato, Chiaki   ; Kiuchi, Kiyoshi; Sugimoto, Katsuhisa*

It is necessary to know the generation mechanism of high equilibrium potential in the solutions. Existing nitrogen oxides in nitric acid solutions were first analyzed by Raman spectroscopy and then existing amount of nitrogen oxides were examined by thermodynamic calculation using the SOLGASMIX software. The Raman spectroscopic analysis showed that the existing amount of un-dissociated HNO increased with increasing nitric acid concentration and solution temperature. The thermodynamic calculation showed that the important nitrogen oxides in nitric acid solutions are HNO, NO, HNO, NO, and NO. The equilibrium potential of nitric acid solutions is, however, mainly decided by the HNO/HNO equilibrium. The thermodynamic calculation also suggested that the increased oxidization potential on the heat-transfer surface is attributed to the reduction of nitrous acid concentration by the thermal decomposition of nitrous acid on the surface and the continuous removal of decomposition product from the solutions by boiling bubbles.