Equilibrium Water Partial Pressures and Salt Solubilities in Aqueous NH4HSO4 to Low Temperatures

Abstract

Equilibrium water partial pressures (pH2O) above 3.695, 5.672, and 8.651 mol kg-1 NH4HSO4(aq), and 10.19 mol kg-1 H2SO4(aq), have been determined using a capacitance manometer over the temperature ranges 288.11−239.56 K and 298.12−238.52 K, respectively. Measured pH2O above the aqueous H2SO4 solution agree with values calculated using the model of Clegg et al. (J. Phys. Chem. A 1998, 102, 2137−2154) to within +5.3% to −2.8% at all temperatures. Results for the NH4HSO4(aq) solutions are also consistent with calculated pH2O, with deviations of +0.45% to +6.27% (measured − calculated) for all data. The formation of solids was observed in the NH4HSO4(aq) solutions at the following temperatures: 258.06 ± 0.1 K [3.695 mol kg-1], 246.47 ± 0.1 K [5.672 mol kg-1], and 261.01 ± 0.1 K [8.651 mol kg-1]. The data are consistent with predictions of ice precipitation in the 3.695 mol kg-1 test solution, letovicite ((NH4)3H(SO4)2(cr)) in the 8.651 mol kg-1 solution, and both ice and letovicite in the 5.672 mol kg-1 solution. The liquid-phase compositions of the solutions containing precipitates are calculated for different temperatures. The results of the experiments are compared with solubility data from other studies and with model predictions of solid/liquid equilibrium for NH4HSO4−H2SO4−H2O mixtures to low temperature over a wide range of composition.