Solubilities and Glass Formation in Aqueous Solutions of the Sodium Salts of Malonic Acid With and Without Ammonium Sulfate

Abstract

The solubility of sodium hydrogen malonate and sodium malonate in water both with and without ammonium sulfate present has been studied using differential scanning calorimetry and infrared spectroscopy. The crystals that form from sodium hydrogen malonate/water solutions were determined to be sodium hydrogen malonate monohydrate by single-crystal X-ray diffractometry. The crystals formed in sodium malonate/water solutions were determined to be sodium malonate monohydrate, a compound whose structure had not been previously known. When ammonium sulfate is added to these respective aqueous systems, the precipitation solids contain sodium sulfate decahydrate under low to moderate ammonium concentrations and lecontite (NaNH4SO4·2H2O) under high ammonium concentrations, which can be found under dry atmospheric conditions. Thus, it appears the presence of malonate and hydrogen malonate ions does not significantly affect the precipitation of inorganic salts in these systems. The glass transition temperatures of all solutions were also determined, and it was observed that the addition of ammonium sulfate slightly lowers the glass transition temperature in these solutions.

Deliquescence Relative Humidities of Organic and Inorganic Salts Important in the Atmosphere

“…Now, we can contrast the DRH values with the solubility of each salt over the same temperature range. We have plotted the temperature-dependent solubility of each salt as the mole fraction of water present at saturation (solid symbols in Figure ) for malonic acid, NaHC 3 H 2 O 4 ·H 2 O, and Na 2 C 3 H 2 O 4 ·H 2 O . First, we notice that for each compound the mole fraction of water present is higher than the DRH (thus activity value) at all temperatures.…”

“…DRH (open symbols with scale on left) as a function of temperature for the following: (blue) NaHC 3 H 2 O 4 ·H 2 O, (black) H 2 C 3 H 2 O 4 from Beyer et al, (red) Na 2 C 3 H 2 O 4 ·H 2 O. Solute solubilities represented by mole fraction of water in a saturated solution (solid symbols with scale on right) as a function of temperature: (blue) NaHC 3 H 2 O 4 ·H 2 O from Kissinger et al, (black) malonic acid from Apleblat and Manzurola, (red) Na 2 C 3 H 2 O 4 ·H 2 O from Kissinger et al Lines are fits to the DRH data.…”

“…We also measured the onset DRH of the crystals resulting from a completely dried mixture of 20/25% (NH 4 ) 2 SO 4 /NaHC 3 H 2 O 4 solution, which has a dry mole ratio (NH 4 ) 2 SO 4 /NaHC 3 H 2 O 4 of 0.76. Kissinger et al determined that these crystals are a mixture of lecontite (NaNH 4 SO 4 ·2H 2 O) and NaHC 3 H 2 O 4 ·H 2 O. They also determined Na 2 SO 4 ·10H 2 O is the least soluble solid at low mole ratio (NH 4 ) 2 SO 4 /NaHC 3 H 2 O 4 (0.32), while ammonium sulfate is the least soluble solid at high mole ratio (NH 4 ) 2 SO 4 /NaHC 3 H 2 O 4 (2.95).…”

“…Crystal structures were determined at the Molecular Structure Laboratory in the Chemistry Department at the University of Wisconsin−Madison. Details of how these experiments are performed can be found in the Supporting Information of Kissinger et al 25 Briefly, a single crystal is removed from a saturated solution and placed in the instrument for analysis. The identity of the solid is determined by a match of the crystallographic data in the Cambridge Crystallographic Database (CSD).…”

“…Once NaHC 3 H 2 O 4 •H 2 O has also precipitated (consuming the remaining sodium ions) there will be an excess of ammonium and hydrogen malonate ions. 25 We also attempted to measure the DRH of crystals from dried (NH 4 ) 2 SO 4 /Na 2 C 3 H 2 O 4 solutions with concentrations 30/20 and 20/20 wt % (NH 4 ) 2 SO 4 /Na 2 C 3 H 2 O 4 . The first solution was dried in a vacuum oven at 323 K for several days, and the latter solution was air-dried at room temperature for 2 weeks, and then dried in a vacuum oven at 323 K for several days.…”