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.