Raman spectroscopy and atomic force microscopy of the reaction of sulfuric acid with sodium chloride

The reaction of NaCl with H2SO4 is investigated using Raman spectroscopy; atomic force microscopy (AFM), and gravimetric analysis. Raman spectra are consistent with the formation of NaHSO4 with no evidence for Na2SO4. The spectra indicate that the phase of NaHSO4 varies with the amount of H2O in the H2SO4 At low H2O concentrations, the reaction produces anhydrous beta -NaHSO4, which undergoes a phase change to anhydrous alpha -NaHSO4 over the course of 18 h. At higher H2O concentrations, anhydrous alpha -NaHSO4 is formed with small amounts of NaHSO4 H2O. AFM measurements on NaCl (100) show the formation of two distinct types of NaHSO4 structures consistent in shape with alpha -NaHSO4 and beta -NaHSO4. The beta -NaHSO4 structures are mobile and move along the NaCl (110) plane until they encounter existing stationary alpha -NaHSO4 structures whereupon the two forms coalescence to form larger alpha -NaHSO4 structures. Gravimetry was used to determine the amount of HCl evolved upon exposure to aqueous H2SO4 solutions modeling atmospheric aerosols. At low H2SO4 concentrations, a large percentage of the HCl formed remains dissolved in the H2O. These results indicate that for conditions simulating relative humidities above 40%, the fraction of HCl released from this reaction may be as low as 0.40 +/- 0.11.