Thermodynamic properties of citric acid and the system citric acid-water

Abstract

The binary system citric acid-water has been investigated with static vapour pressure measurements, adiabatic calorimetry, solution calorimetry, solubility measurements and powder X-ray measurements.

Properties of Citric Acid and Its Solutions

“…2.2). This temperature is higher than that mentioned above from the literature ~ 309.7 K but probably it indicates that the formation of the monohydrate from the high-temperature solid phase was not complete [8]. As can be seen in Fig.…”

“…15 temperature. ■, ■, ■ - [8]; ■, ■, ■ - [58] [60] using the enzymatic equilibrium data, gives a slightly higher value for the Gibbs free energy of formation of the crystalline monohydrate ΔG f (s, 298.15 K) = − 1168.8 ± 6.3 kJ mol −1 . The Wilhoit and Shiao value ΔH f (s, 298.15 K) = − 1543.9 kJ mol −1 and the Korchergina et al [61] value ΔH f (s, 298.15 K) = − 1551.7 ± 1.3 kJ mol −1 for the enthalpy of formation are lower than these given above because they used in calculations the heat of formation of the standard substance for CO 2 (gas) and not for C(graphite) as in [58,59].…”

“…Anhydrous citric acid ■ - [8]; citric acid monohydrate - [58]; - [8] Thermodynamic functions ΔG, ΔH and T ⋅ ΔS of citric acid monohydrate as a function of temperature were determined by Evans et al [58] and De Kruif et al [8] and they are plotted in Fig. 2.3.…”

“…Bennett and Yuill also found that the transition from citric acid monohydrate to anhydrous citric acid occurs between 36.15 and 36.45 °C with the mean value of 36.3 °C. The Marshall results [7] are slightly higher, from 36.35 to 36.6 °C and he proposed the transition temperature of 36.5 °C when De Kruif et al [8] gave the value of 36.0 ± 0.5 °C based on the X-ray powder diffraction patterns. Oechler [9] based on solubility and vapour pressure measurements reported the value of 36.7 °C.…”

“…From thermodynamic properties of solid citric acid monohydrate, the heat capacities, enthalpies and entropies were determined by Evans et al [58] in the 20-300 K temperature range. De Kruif et al [8] reported the heat capacities, enthalpies, entropies and the Gibbs free energies from 120 to 300 K for monohydrate, and the corresponding values of the thermodynamic functions from 90 to 330 K for anhydrous citric acid. They observed a slightly superheated large transition at 312.1 K and above this transition, a very large molar heat capacities with a significant temperature dependence ( Fig.…”