The heat of combustion of citric acid monohydrate

Abstract

A bomb calorimeter and associated apparatus to determine the heats of combustion of organic substances is described. It hEs been employed to obtain the heat of formation of monohydrate citric acid. The value found was AH; = 1837.6 abs. W/mole, = 439.2 kcal/mole.A bomb calorimeter and associated apparatus has been designed and constructed to determine the heats of combustion of organic substances. The calorimeter was calibrated by burning benzoic acid, and has been employed to determine the heat of combustion of monohydrate citric acid. Thermodynamic data for citric acid are of biochemical interest in connexion with the energy changes of metabolic processes. The free energy of formation of the aqueous citrate 3-ion has been evaluated with the aid of biochemical data to be -279 kcal/mole at 25O.1 This is consistent with the previously available combustion measurements for the crystalline monohydrate 2 and data for the solution,3 and ionization 4 only if the entropy has the unlikely value of 88 cal/mole deg. at 25". The combustion measurements in this paper are consistent with a more reasonable value for the entropy of about 70 cal/mole deg. for the crystalline monohydrate.

Temperature Profile Analysis of the Citrate−Nitrate Combustion System

“…The activation energy of the reaction (generalized in eq ) was obtained using various approaches and was estimated to be within the range of 60−66 kJ/mol, which is of the same order of magnitude as the value established for the combustion synthesis of γ-lithium aluminate by Kim et al (54 kJ/mol) . The activation energy estimated for the present system is significantly lower than that of diffusion (several hundreds of kilojoules per mole) in a normal solid-state reaction, but somewhat higher than those for the combustion reaction of fuel (20−30 kJ/mol) and the evaporation of water (about 40 kJ/mol) . The activation energy, 60−66 kJ/mol, is thought to be that of product formation (eq ).…”

Properties of Citric Acid and Its Solutions

“…However, in the case of more sensitive quantity, the apparent osmotic coefficients as can be observed in Fig. 2.31, the agreement is less satisfactory [59].…”

“…■, ■, ■ - [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].…”

Screening for Pharmaceutical Cocrystal Hydrates via Neat and Liquid-Assisted Grinding

“…Theophylline and caffeine form a stoichiometric and a nonstoichiometric hydrate, respectively, and have previously been used as model APIs in cocrystal formation. We decided to explore citric acid as the cocrystal former in our hydrate formation studies, since it forms a stable monohydrate at room temperature. , In addition, citric acid is an attractive pharmaceutical cocrystal former due to physiological acceptability and the presence of four hydrogen bond donor sites . In principle, each site could be utilized to form a hydrogen bond to a different API molecule.…”