Nitric Acid-Water Equilibrium

Nitric acid is one of the most water-soluble atmospheric gases with a Henry s law constant (at 298 K) of Hhno, = 2.1 x 105 Matm-1. After dissolution 

FIGURE 7.9 Ammonium concentration as a function of pH for a gas-phase ammonia mixing ratio of 1 ppb at 298 K. 

FIGURE 7.10 Equilibrium fraction of total ammonia in the aqueous phase as a function of pH and cloud liquid water content at 298 K. 

The total dissolved nitric acid [HN03 ] will then be 

Substituting this into the dissociation equilibrium equation, we have 

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Figure 5. Magnesium nitrate-nitric acid-water equilibrium vapor composition...

Carbon Dioxide/Water Equilibrium 345 Sulfur Dioxide 348 Ammonia/Water Equilibrium 353 Nitric Acid/Water Equilibrium 355 Equilibrium of Other Important Atmospheric Gases Aqueous-Phase Reaction Rates 361 S(IV) to S(VI) Transformation and Sulfur Chemistry 363... 

If, for example, a mixture of ethanol and water is distilled, the concentration of the alcohol steadily increases until it reaches 96 per cent by mass, when the composition of the vapour equals that of the liquid, and no further enrichment occurs. This mixture is called an azeotrope, and it cannot be separated by straightforward distillation. Such a condition is shown in the y — x curves of Fig. 11.4 where it is seen that the equilibrium curve crosses the diagonal, indicating the existence of an azeotrope. A large number of azeotropic mixtures have been found, some of which are of great industrial importance, such as water-nitric acid, water-hydrochloric acid, and water-alcohols. The problem of non-ideality is discussed in Section 11.2.4 where the determination of the equilibrium data is considered. When the activity coefficient is greater than unity, giving a positive deviation from Raoult s law, the molecules of the components in the system repel each... 

Tao, F.-M., K. Higgins, W. Klemperer, and D. D. Nelson, Structure, Binding Energy, and Equilibrium Constant of the Nitric Acid-Water Complex, Geophys. Res. Lett., 23, 1797-1800 (1996). 

Many papers concerning salt effect on vapor-liquid equilibrium have been published. The systems formed by alcohol-water mixtures saturated with various salts have been the most widely studied, with those based on the ethyl alcohol-water binary being of special interest (1-6,8,10,11). However, other alcohol mixtures have also been studied methanol (10,16,17,20,21,22), 1-propanol (10,12,23,24), 2-propanol (12,23,25,26), butanol (27), phenol (28), and ethylene glycol (29,30). Other binary solvents studied have included acetic acid-water (22), propionic acid-water (31), nitric acid-water (32), acetone-methanol (33), ethanol-benzene (27), pyridine-water (25), and dioxane-water (26). 

For a continuous extractive distillation process to be possible there must be adequate enhancement of the nitric acid-water relative volatility, and a system equilibrium which permits virtually complete separation of nitric acid from magnesium nitrate, the latter taking up the water content of the weak acid feedstock. This requires addition to the weak nitric acid of solutions of magnesium nitrate usually containing 60 wt% or more of Mg(NC>3)2. Under these conditions a nitric acid-water relative volatility of greater than 2.0 is obtained at the low end of the liquid phase concentration at a nitric acid mole fraction below 0.05 (4, 7). 

Since in an extractive distillation process based on this ternary system the extractive agent is nonvolatile and remains in the liquid phase, and since because of the similarity of the molar latent heats of nitric acid and water there is substantially constant molar liquid overflow, the mole fraction of magnesium nitrate remains almost constant throughout the process. It is appropriate to represent the equilibrium situation as a pseudo-binary system for each magnesium nitrate concentration, and Figure 7 shows vapor-liquid equilibria on a nitric acid-water basis at a series of magnesium nitrate concentrations from zero to 0.25 mole fraction in the liquid phase. 

Figure 7. System magnesium nitrate-nitric acid-water, liquid-vapor equilibrium, pseudo binary basis...

It is convenient to use phase diagrams [46] to represent the thermodynamic properties that determine the stability and equilibrium composition of water-containing aerosols. The properties of interest are the temperature, the vapour pressure and composition of the various components in the condensed phases. This is particularly important with respect to the composition and stability of the various hydrates formed at low temperature in the nitric acid-water [47] and sulfuric acid-water binary systems [48], and the ternary systems HjSO/HNOj/HjO and HjSO/HCl/HjO [49],... 

The vapor-liquid equilibrium for the nitric acid / water system at atmospheric pressure is shown in Figure 9.4. This figure shows that a concentration of 68.4 weight % nitric acid is the maximum (i.e., the azeotropic point) that can be obtained by simple distillation of the weak acid220. 

Figure 9.4. Vapor-liquid equilibrium of nitric acid - water system. (Reproduced by permission of Heraeus Nitric Acid Conference, November, 2001)...

W. Davis, Jr., J. Mrochek, and C. J. Hardy. 1966. The system tri- -butyl phosphate (TBP)-nitric acid-water I activities of TBP in equilibrium with aqueous nitric acid and partial molar volumes of the three components in the TBP phase. J. Inorg. Nucl. Chem. 28 2001-2014. 

In the right column the mixture nitric acid/water is shown. For this the explanations given for the mixture acetone/chloroform hold as principal. Taking into account that there are extremely strong interaction energies between nitric acid and water molecules the described effects are very distinct in this case. There are, for example, very negative deviations from Raoult s law and the distance of the equilibrium curve to the diagonal is considerable. Solutions of small amounts of nitric acid in water show very small partial pressures of the nitric acid. This effect is even more... 

Tabulated below are vapor-liquid equilibrium data for nitric acid/water mixtures at 1.0 atm. Use these data to plot a phase diagram (suitable for analyzing a flash drum) and an x-y liquid-vapor diagram (suitable for McCabe-Thiele analysis). Label the axes and, where appropriate, label the regions of the plot (liquid, vapor, etc.). ... 

Fig. 10-6. Vapor-liquid equilibrium system, nitric acid-water-sulfuric acid.

The state of aqueous solutions of nitric acid In strongly acidic solutions water is a weaker base than its behaviour in dilute solutions would predict, for it is almost unprotonated in concentrated nitric acid, and only partially protonated in concentrated sulphuric acid. The addition of water to nitric acid affects the equilibrium leading to the formation of the nitronium and nitrate ions ( 2.2.1). The intensity of the peak in the Raman spectrum associated with the nitronium ion decreases with the progressive addition of water, and the peak is absent from the spectrum of solutions containing more than about 5% of water a similar effect has been observed in the infra-red spectrum. ... 

Considering first pure nitric acid as the solvent, if the concentrations of nitronium ion in the absence and presence of a stoichiometric concentration x of dinitrogen tetroxide are yo and y respectively, these will also represent the concentrations of water in the two solutions, and the concentrations of nitrate ion will be y and x- y respectively. The equilibrium law, assuming that the variation of activity coefficients is negligible, then requires that ... 

NOj ions/ Addition of water to nitric acid at first diminishes its electrical conductivity by repressing the autoprotolysis reactions mentioned above. For example, at -10° the conductivity decrea.ses from 3.67 x 10 ohm cm to a minimum of 1.08 x 10" ohm" cm at 1.75 molal H2O (82.8% NjOs) before rising again due to the increasing formation of the hydroxonium ion according to the acid-base equilibrium... 

Similar to the alkylation and the chlorination of benzene, the nitration reaction is an electrophilic substitution of a benzene hydrogen (a proton) with a nitronium ion (NO ). The liquid-phase reaction occurs in presence of both concentrated nitric and sulfuric acids at approximately 50°C. Concentrated sulfuric acid has two functions it reacts with nitric acid to form the nitronium ion, and it absorbs the water formed during the reaction, which shifts the equilibrium to the formation of nitrobenzene ... 

Nitrogen dioxide, N02 (oxidation number -t-4), is a choking, poisonous, brown gas that contributes to the color and odor of smog. The molecule has an odd number of electrons, and in the gas phase it exists in equilibrium with its colorless dimer N204. Only the dimer exists in the solid, and so the brown gas condenses to a colorless solid. When it dissolves in water, NOz disproportionates into nitric acid (oxidation number +5) and nitrogen oxide (oxidation number +2) ... 

Watson, J. G., J. C. Chow, F. W. Lurmann, and S. P. Musarra, Ammonium Nitrate, Nitric Acid, and Ammonia Equilibrium in Wintertime Phoenix, Arizona, J. Air Water Manage. Assoc., 44, 405-412 (1994). 

Koop and Carslaw (1996) have shown that as solid SAT is cooled in the presence of gaseous water and nitric acid, the SAT deliquesces. That is, SAT takes up water as well as nitric acid and forms an equilibrium with water-nitric acid-sulfuric acid solutions (Fig. 12.21). 

Figure 12.22 shows the composition in terms of the weight percent HNO, and H2S04 as a function of temperature as solid SAT is cooled from 194 K under conditions corresponding to a pressure of 50 rnbar in an atmosphere containing 5 ppm HzO and an HNO, concentration of 10 ppb (Koop and Carslaw, 1996). Under these particular conditions, as the temperature falls below 192 K, the SAT is in equilibrium with a liquid film on the particle containing both HN03 and H20. The particular temperature at which SAT deliquesces is a function of the water vapor and gaseous nitric acid concentrations as shown in Fig. 12.23. As the temperature falls further and more HNO, and HzO are taken up into the liquid, the solid SAT dissolves completely, forming a ternary solution of the two acids and water. This solution can then act again to nucleate PSCs.

The effect of the magnesium nitrate on the vapor-liquid equilibrium of nitric acid and water can be seen in Figure 3. As the concentration of magnesium nitrate in the liquid increases, the volatility of nitric acid also increases. 

In the system nitric acid-nitroglycerine-water, a labile equilibrium is established, determined by the coefficient of phase separation. Thus under certain conditions as much as 8% nitric acid passes into the water phase whereas the nitroglycerine does not contain more than 0.01% HNO3. A thorough study of the partition of HNO3 and dinitroglycerine between nitroglycerine and aqueous solutions was made by Oehman [108]. 

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