Equilibrium water partial pressures and salt solubilities in aqueous NH₄HSO₄ to low temperatures

Equilibrium water partial pressures (pH₂O) above 3.69₅, 5.67₂, and 8.65₁ mol kg^-1 NH₄HSO_4(aq), and 10.19 mol kg^-1 H₂SO_4(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 NH₄HSO_4(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 NH₄HSO_4(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.