Heat capacity, enthalpy and entropy of citric acid monohydrate

Abstract

The heat capacity of citric acid monohydrate has been measured in the range 20" to 300°K. A value of 67.74f0.14 cal deg.-1 mole-1 has been calculated for the entropy of the monohydrate at 25°C and with existing published data leads to the free energy of formation of the aqueous citrate ion = -277.69f0.34 kcal mole-1.Thermodynamic data for citric acid monohydrate and the aqueous citrate ion are of biochemical interest in connection with the energy changes of metabolic processes.We have measured the heat capacities of the crystalline monohydrate in the range 20-300°K. From these data an accurate value for the entropy of formation at 25°C has been calculated and used in conjunction with the heat of formation 1 and data for the solution 2 and ionization 3 to obtain an accurate value for the free energy of formation of the aqueous citrate ion. EXPERIMENTALThe apparatus and method used will be described elsewhere. MAmruALS.-The citric acid monohydrate was kindly given by Prof. Sir Hans Krebs and was found to contain 0.16 % adsorbed water. The weight of the sample used was 53.119 g.

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

“…The experimental results are also given in Table2, and the calculated thermodynamic values and the smoothed heat capacity values from 90 K are given in Table4. Specific heat measurements of the monohydrate have been reported by Evans et al [3] ranging from 10 to 298 K. Comparing our data from 90 to 298 K shows no significant difference. Our data measured with automated apparatus are more smooth, but the overall difference in enthalpy increment TABLE 4 Calculated thermodynamic values of citric acid After pumping a few seconds at the hydrate sample, it took several hours before the equilibrium pressure was re-established.…”

Properties of Citric Acid and Its Solutions

“…As can be observed, the Gibbs free energy is negative ΔG < 0 and the enthalpy and entropy are positive and they have similar values with ΔH < T ⋅ ΔS. The absolute values of all thermodynamic functions increase with increasing of temperature T. Using the Evans et al [58] results which cover a more extended temperature range 0 < T < 300 K, the thermodynamic functions of citric acid monohydrate are (…”

“…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.…”

“…For temperatures below 22 K, Evans et al [58] obtained the molar heat capacities using the Debye function with T D = 150 K. In the case of anhydrous citric acid, in the 84 K < T < 330 K temperature interval, the molar heat capacities can be expressed by (2.4) …”

“…The studies of the water-citric acid-electrolyte aerosols in atmosphere are also important because they provide significant information about activities, solubilities, surface tension and other properties of aqueous solutions of citric acid [46,[49][50][51][53][54][55][56]. 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.…”