Equilibria Between Gas and Liquid Phases for Concentrated Aqueous Solutions of Nitric Acid

Abstract

Calculated diagrams, representing chemical and electrochemical properties of concentrated aqueous nitric acid mixtures, have been drawn for temperatures of 25 °C and 100 °C using vapor pressures and thermochemical data of related gaseous species. Electrochemical measurements have permitted the establishment of an experimental potential scale (referenced to the saturated mercurous sulfate electrode at 25 °C) incorporated in the diagram at 100 °C. These measurements also allowed the determination of the Gibbs free energy of formation of dissolved nitrous acid for various nitric acid solutions at 100 °C, leading to a value of ΔGf(HNO2) = –36.31 kJ mol−1, for the whole nitric acid concentration range.

The influence of nitrate concentration and acidity on the electrocatalytic reduction of nitrate on platinum

“…There is a distinction in the literature between two nitrate reduction mechanisms on platinum: a direct reduction mechanism which develops at all concentrations of nitrate [2][3][4][5][6][7][8][9] and an indirect reduction mechanism that develops only at high concentrations of nitric acid (>1 M) in the presence of nitrite [11][12][13][14][15][16][17][18].…”

“…Apart from the direct nitrate reduction there is a second nitrate reduction mechanism [11][12][13][14][15][16][17][18][19]. This mechanism develops at much lower overpotentials (0.8-0.9 V vs. SHE), but only at high nitrate concentrations (>1 M) in highly acidic environments (>1 M) in the presence of nitrite.…”

“…Although the present study does not investigate the effect of adding nitrite to concentrated nitrate solutions, it is to be expected that the indirect nitrate reduction develops anyway at sufficiently high nitric acid concentrations, because nitric acid starts to decompose at concentrations above 4 M [10,11]. At these concentrations the equilibria given below will be shifted to the right:…”

“…Data are only available on the reduction of nitric acid at 100°C [11,18] and nitric acid in the presence of nitrous acid [12][13][14][15][16][17]. Under these conditions the indirect nitrate reduction needs only a small overpotential and is mainly selective towards HNO 2 , NO and NO 2 [11][12][13][14][15][16][17][18].…”

“…Among the different ways to reduce nitrate [1], a large number of papers [2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19] has been devoted to the electrocatalytic reduction on platinum in acidic solution.…”