Experimental Determination of the H₂SO₄/(NH₄)₂SO₄/H₂O Phase Diagram

Abstract

We have experimentally investigated the water and sulfuric acid-rich regions of the H₂SO₄/(NH₄)₂SO₄/H₂O\nternary liquid/solid phase diagram using differential scanning calorimetry (DSC) and infrared spectroscopy\nof thin films. We present the liquid/solid ternary phase diagram for temperatures below 373 K and H₂SO₄\nconcentrations below 60 wt %. We have determined two ternary eutectics and two tributary reaction points\nfor this system in the regions studied. It is also seen that sulfuric acid tetrahydrate (SAT) forms as a metastable\nsolid over a large concentration range. Two true binary systems have been identified: ice/letovicite and SAT/ammonium bisulfate. Finally, we have compared our results to the predictions of the aerosol inorganics model\nand have found significant differences both in the final melting points and in the location of some of the\nphase boundaries including a significant discrepancy in the invariant points predicted versus those observed.