3.5.2
In the nitration process, nitro groups are introduced into the nitrified compound. The essence of this process is realized by the mutual transfer of nitrating agents and nitrified compounds under certain conditions. A nitrating agent is a macroscopic concept, and there are numerous ions and groups in its system. For instance, there are , and unionized, molecular nitric acid in mixed acids.37-39 It can be seen from the structural formula of nitric acid that the most easily broken bonds are the H-O bond and N-O bond, by which H+ and are generated via acidic ionization and basic ionization, respectively. Sulfuric acid is referred to as a nitration activator. It has a greater acidity than nitric aicd and can donate protons to nitric acid, resulting in further basic ionization of nitric acid and the formation of . Therefore, a nitrating agent composed of nitric acid and sulfuric acid is a much stronger nitrating agent than nitric acid alone and is more applicable in industrial production.
Since is the substance that actually plays a role in nitration,4 an accurate estimation of its concentration is crucial to understanding the underlying reaction mechanism and kinetics. The concentration of molecules and ions in nitric acid and aqueous sulfuric acid has been extensively studied by various spectroscopy techniques, such as Raman,28,40UV29,41 and NMR42 spectroscopy. Several studies28,36,42 have reported values at different temperatures (mainly 23, 40, and 60 °C) and sulfuric acid concentrations. A plot of versus the mass fraction of sulfuric acid was prepared by summarizing these data. The fitting curve exhibited a good linear correlation, as shown in Figure 7a . According to the fitted model, the values of within the investigated sulfuric acid concentration range (85~95 wt%) can be calculated at temperatures of 23, 40 and 60 °C. By plotting versus 1/T , a series of straight lines at different sulfuric acid concentrations can be obtained, as shown in Figure 7b . In this way, equilibrium data for nitric acid in a sulfuric acid solution can be obtained at other temperatures.