Solubilities in aqueous solutions of the sodium salts of succinic and glutaric acid with and without ammonium sulfate

https://doi.org/10.1016/j.jct.2018.06.002Get rights and content

Highlights

  • The most common precipitate is in this ternary system is lecontite (56% of samples).

  • In 19% of samples the organic salt was the least soluble compound and in 6% it was (NH4)2SO4.

  • Salts made up of ions in atmospheric aerosols must be considered when predicting insoluble salts.

Abstract

The aqueous solubility of the least soluble salts in mixtures of sodium hydrogen succinate, sodium succinate, sodium hydrogen glutarate, and sodium glutarate with and without ammonium sulfate present has been studied. Solubility temperatures were determined in solutions of known concentration using Differential Scanning Calorimetry. The identity of the least soluble solid was determined by a combination of X-ray crystallography and infrared spectroscopy. The identified solids included ammonium sulfate, sodium sulfate, sodium ammonium sulfate dihydrate (lecontite), sodium hydrogen succinate, sodium succinate hexahydrate, sodium hydrogen glutarate dihydrate, and ammonium hydrogen glutarate. Of the 16 series of solutions studied, lecontite was most frequently the least soluble solid.

Introduction

Field measurements have shown that the major chemical components of aerosols in the free and upper troposphere (UT) include organic and inorganic compounds and mineral dust [1], [2], with the most abundant inorganic components being ammonium and sulfate [3]. With respect to the organic fraction, dicarboxylic acids (DCA) have been found in a range of environments, particularly for aerosols that have undergone chemical aging [4]. Both primary organic and secondary organic aerosols have been found to contain DCA [3], and their concentration in aerosols is increasing [5]. Field measurements have shown that succinic and glutaric acids are among the most abundant DCA in atmospheric aerosols [5], [6], [7]. Mineral dust has also been observed in aerosols, especially under acidic conditions, where mineral dust components can be reacted to the aqueous phase through chemical aging [8], [9] The aqueous phase chemistry of aerosols can be enhanced by metals at the surface or reacted into the aerosol interior [8], [10]. The presence of organics, metals, and metal salts have been shown to be present in large numbers of aerosols in field studies with sources of sea spray (Na, Mg) [4], [11], [12], [13], biomass burning (K) [14], [15], and mineral dusts and meteoritic material (Na, K, Ca, Fe) [1], [16], [17], [18], [19]. Metal ions can displace hydrogen ions from organic acids to form carboxylate salts in atmospheric aerosols as shown by field and lab studies [11], [13], [20], [21]. As an example, a particle that experienced UT temperatures or dry conditions and contained dissolved NaCl and succinic acid could undergo the following:nNaClaq+H2C4H4O4aqNanH2-nC4H4O4s+nHClaq,gwhere n equals one or two (corresponding to the number of acidic hydrogens present in the acid). According to recent studies the HCl product is highly volatile, and the organic salt will remain in the particle since it has a much lower vapor pressure [13], [20]. The sodium oxalate salt could precipitate when the particle experiences either cold or dry conditions in the atmosphere [21]. While the solubilities of NaHC4H4O4 and Na2C4H4O4 in water are well known [22], the impact of ammonium sulfate on solubility in solutions of the sodium succinates has not been investigated. The solubility of NaHC5H6O4 in water has not been previously investigated to our knowledge, and that of Na2C5H6O4 has only recently been investigated [23]. In both cases the effect of ammonium sulfate on solubility in water has not been investigated. In particular, salts other than the sodium oxalates and ammonium sulfate could form from these complex solutions of ions as has been found for similar systems in our lab [24], [25].

Section snippets

Sample preparation

Solutions studied in our experiments were made by mixing chemicals as listed in Table 1 with deionized water. All samples were made such that NaOH was in very slight excess. For NaHC4H4O4 and Na2C4H4O4 samples, 1.005 ± 0.004 and 2.004 ± 0.007 NaOH/H2C4H4O4 mole ratio solutions were made, respectively. For NaHC5H6O4 and Na2C5H6O4 samples, 1.001 ± 0.001 and 2.002 ± 0.003 NaOH/H2C5H6O4 mole ratio solutions were made, respectively. (Throughout this paper values reported as a ± b are mean values (a)

NaHC4H4O4/H2O

The solubility of NaHC4H4O4 in water has been known for some time and values appear in standard compilations [22], [35], though these are based on the work of Marshall and Bain [36]. Two crystals form from solution dependent on the concentration of the precipitating solution. For solutions with sodium hydrogen succinate concentration w ≤ 0.39 (where w is defined as mass fraction solute), NaHC4H4O4·3H2O is the stable solid. At concentrations of w ≥ 0.39 NaHC4H4O4, NaHC4H4O4 solid forms from

Discussion and atmospheric implications

The solubilities of the sodium salts of oxalic and malonic acid have previously been studied in our lab [24], [25]. In both of those cases, we determined the order of solubility to be H2A > Na2A > NaHA, where A represents the organic anion. In this work we find glutaric acid and its sodium salts follow the same pattern, whereas succinic acid and its sodium salts follow the reverse ordering. We have compared the data sets in Fig. 6 where we plot the various solubilities. As an example, if we

Conclusions

We have studied the solubility of the sodium salts of succinic and glutaric acids with and without ammonium sulfate present. As we have observed in other systems, we determined that the least soluble solids are most often not the organic sodium salts or ammonium sulfate, but rather a salt made up of the ions present, namely lecontite. This is a mineral that has been generally overlooked in the literature in terms of its presence in atmospheric aerosols. However, we have determined in this

Acknowledgements

We wish to thank Anastasiya Vinokur and Dr. Ilia Guzei at the University of Wisconsin-Madison for running and analyzing the X-ray crystallography experiments.

Funding

This work was supported by the National Science Foundation (USA) Atmospheric Chemistry Program (AGS-1361592)

Declaration of interests

None.

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