3.3 Effect of sulfuric acid concentration
Numerous related studies19,26,27 have shown that the rate of nitration is strongly related to the concentration of sulfuric acid. Within a certain range, the higher the concentration of sulfuric acid is, the faster the rate of aromatic nitration. The effect of the mass fraction of sulfuric acid (85~95 wt%) on CB conversion was investigated, as shown in Figure 4 . When the temperature was 15 °C and the mass fraction of sulfuric acid was 95%, the conversion of CB reached 25% in less than 8 s. When the mass fraction of sulfuric acid was 90%, the conversion of CB was less than 10% under the same conditions. For the lower sulfuric acid concentration (85 wt%), when the residence time was 22 s, the reaction conversion was less than 5%, indicating that the reaction rate was greatly reduced. As shown in Scheme 1 , sulfuric acid provides the necessary environment for the generation of . The experimental results of Deno28 and Marziano29showed that as the mass fraction of sulfuric acid was reduced below 90%, the proportion of in nitric acid decreases exponentially, resulting in a decrease in the amount of the effective nitration agent and a significant reduction in the reaction rate. Variation in the concentration of in the mixed acid will be discussed in Section 3.5.2.