Solubilities of magnesium-l-ascorbate, calcium-l-ascorbate, magnesium-l-glutamate, magnesium-d-gluconate, calcium-d-gluconate, calcium-d-heptagluconate, l-aspartic acid, and 3-nitrobenzoic acid in water

Ran

2014

Aqueous solubility of calcium l-lactate, calcium d-gluconate, and calcium d-lactobionate increases with temperature (10-30 °C investigated), most significantly for the least soluble d-gluconate, while the calcium ion activity of the saturated solutions decreases with temperature, as measured electrochemically, most significantly for the most soluble d-lactobionate. This unusual behavior is discussed in relation to dairy processing and explained by endothermic binding of calcium to hydroxycarboxylate anions determined to have ΔH°ass = (31 ± 3) kJ·mol(-1) for l-lactate, (34 ± 2) kJ·mol(-1) for d-gluconate, and (29 ± 3) kJ·mol(-1) for d-lactobionate in 1:1 complexes with thermodynamic binding constants at 25 °C of Kass = 49 (l-lactate), 88 (d-gluconate), and 140 (d-lactobionate). Quantum mechanical calculations within density functional theory (DFT) confirm the ordering of strength of binding. The complex formation is entropy driven with ΔS°ass> 0, resulting in decreasing calcium ion activity in aqueous solutions for increasing temperature, even for the saturated solutions despite increasing solubility.

Section: Resultssupporting

confidence: 88%

Thermodynamics of Dissolution of Calcium Hydroxycarboxylates in Water

Ran

2014

“…In the temperature interval investigated, from 10 to 50 °C, it was observed that the total calcium concentration, determined by EDTA titration, increased more than 3.5 times, while the free calcium concentration, determined electrochemically as calcium ion activity (see Figure ) and corrected to concentration, increased 4.5 times, as may be seen from Figure . The increasing solubility with the temperature is in agreement with the temperature effect on solubility of other calcium salts of organic acids previously reported in the literature ,,− and shows that the overall dissolution of calcium d -saccharate (reactions of eqs and ) is an endothermic process.…”

Section: Resultssupporting

confidence: 90%

Calcium d-Saccharate: Aqueous Solubility, Complex Formation, and Stabilization of Supersaturation

Garcia

2016

Molar conductivity of saturated aqueous solutions of calcium d-saccharate, used as a stabilizer of beverages fortified with calcium d-gluconate, increases strongly upon dilution, indicating complex formation between calcium and d-saccharate ions, for which, at 25 °C, Kassoc = 1032 ± 80, ΔHassoc° = −34 ± 6 kJ mol–1, and ΔSassoc° = −55 ± 9 J mol–1 K–1, were determined electrochemically. Calcium d-saccharate is sparingly soluble, with a solubility product, Ksp, of (6.17 ± 0.32) × 10–7 at 25 °C, only moderately increasing with the temperature: ΔHsol° = 48 ± 2 kJ mol–1, and ΔSassoc° = 42 ± 7 J mol–1 K–1. Equilibria in supersaturated solutions of calcium d-saccharate seem only to adjust slowly, as seen from calcium activity measurements in calcium d-saccharate solutions made supersaturated by cooling. Solutions formed by isothermal dissolution of calcium d-gluconate in aqueous potassium d-saccharate becomes spontaneously supersaturated with both d-gluconate and d-saccharate calcium salts, from which only calcium d-saccharate slowly precipitates. Calcium d-saccharate is suggested to act as a stabilizer of supersaturated solutions of other calcium hydroxycarboxylates with endothermic complex formation through a heat-induced shift in calcium complex distribution with slow equilibration upon cooling.

“…9,23 Equilibrium is established after a few hours for all three salts, as evident from Figure 1. The free calcium ion concentrations as measured by a calcium-ion-selective electrode and also presented in Table 1 became constant during this period of time.…”

Section: ■ Results and Discussionmentioning

confidence: 68%

Aqueous Solubility of Calcium l-Lactate, Calcium d-Gluconate, and Calcium d-Lactobionate: Importance of Complex Formation for Solubility Increase by Hydroxycarboxylate Mixtures

Munk

2013

Among the calcium hydroxycarboxylates important for cheese quality, D-lactobionate [Ksp = (7.0 ± 0.3) × 10(-3) mol(3) L(-3)] and L-lactate [Ksp = (5.8 ± 0.2) × 10(-3) mol(3) L(-3)] were found more soluble than D-gluconate [Ksp = (7.1 ± 0.2) × 10(-4) mol(3) L(-3)], as indicated by the solubility products determined electrochemically for aqueous 1.0 M NaCl at 25.0 °C. Still, solubility of calcium L-lactate increases by 45% in the presence of 0.50 M sodium D-gluconate and by 37% in the presence of 0.50 M sodium D-lactobionate, while solubility of calcium D-gluconate increases by 66 and 85% in the presence of 0.50 M sodium L-lactate and 0.50 M sodium D-lactobionate, respectively, as determined by complexometric titration. Sodium L-lactate and sodium D-gluconate have only little influence on solubility of calcium D-lactobionate. The increased solubility is described quantitatively by calcium binding to D-gluconate (K1 = 14 ± 3 mol(-1) L) in 1.0 M NaCl at 25 °C, D-lactobionate (K1 = 11 ± 2 mol(-1) L), and L-lactate (K1 = 8 ± 2 mol(-1) L), as indicated by the association constants determined electrochemically. In mixed hydroxycarboxylate solutions, calcium binding is quantitatively described by the geometric mean of the individual association constants for both aqueous 1.0 and 0.20 M NaCl, indicating a 1:1 stoichiometry for complex formation.