Investigation the solubility difference of sodium 4-nitrotoluene-2-sulfonate and 4-nitrotoluene-2-sulfonic acid in (sulfuric acid + water) system

• The solubilities of NTS and NTSNa in sulfuric acid aqueous solution were determined.
• Solubility of NTS is the lowest in the 0.6 mass fraction sulfuric acid aqueous solution.
• Solubility discrepancy of NTS and NTSNa declines with increase of H2SO4 content until reaching 50%.
• The modified Apelblat model fit the experimental data well.

The solubility data of sodium 4-nitrotoluene-2-sulfonate (NTSNa) and 4-nitrotoluene-2-sulfonic acid (NTS) in (sulfuric acid + water) mixture solvents were measured at temperatures ranging from 285 K to 340 K using a dynamic method. The solute-free mass fraction of sulfuric acid (w30) in solvent mixtures ranged from 0 to 0.70. NTS showed the lowest solubility when w30 is 0.60. After the sulfonation reaction, the reaction solution should be diluted to the concentration 0.60 mass fraction of sulfuric acid to make NTS precipitate as much as possible. Solubility of NTS is much larger than that of NTSNa when w30 is smaller than 0.20, but no obvious solubility difference can be detected when w30 is larger than 0.50. This result shows that only when w30 is smaller than 0.20, the recycle process which was designed by our previous study can be carried out. The solubility data were correlated by the modified Apelblat model, the calculated (solid + liquid) equilibrium curves are in good agreement with the experimental data.