Benzoic acid, a widely used preservative and chemical intermediate, has garnered significant attention due to its unique solubility properties. This comprehensive guide delves into the intricate details of benzoic acid solubility, providing a wealth of technical information and practical insights for science students and researchers.
Understanding Benzoic Acid Solubility in Water
Benzoic acid is a weak acid with a pKa value of 4.176 at 25°C. Its solubility in water is relatively low, with a value of approximately 0.33 g/100 mL at 25°C. This low solubility can be attributed to the balance between the hydrophilic carboxyl group and the hydrophobic benzene ring within the benzoic acid molecule.
The solubility of benzoic acid in water can be expressed using the following equation:
S = Ksp / (1 + Ka / [H+])
Where:
– S is the solubility of benzoic acid (mol/L)
– Ksp is the solubility product constant of benzoic acid
– Ka is the acid dissociation constant of benzoic acid
– [H+] is the hydrogen ion concentration in the solution
The solubility of benzoic acid in water is influenced by factors such as temperature, pH, and the presence of other solutes.
Effect of Temperature on Benzoic Acid Solubility in Water
The solubility of benzoic acid in water increases with increasing temperature, as described by the van ‘t Hoff equation:
ln(S2/S1) = ΔHsol / R * (1/T1 - 1/T2)
Where:
– S1 and S2 are the solubilities at temperatures T1 and T2, respectively
– ΔHsol is the enthalpy of solution
– R is the universal gas constant
Experimental data shows that the solubility of benzoic acid in water increases from 0.33 g/100 mL at 25°C to 0.57 g/100 mL at 40°C.
Effect of pH on Benzoic Acid Solubility in Water
The solubility of benzoic acid in water is also influenced by the pH of the solution. At pH values below the pKa of benzoic acid (4.176), the undissociated form of benzoic acid predominates, leading to lower solubility. As the pH increases above the pKa, the dissociated form of benzoic acid (benzoate ion) becomes more prevalent, resulting in higher solubility.
The relationship between benzoic acid solubility and pH can be expressed using the following equation:
S = Ksp / (1 + 10^(pKa - pH))
Where:
– S is the solubility of benzoic acid (mol/L)
– Ksp is the solubility product constant of benzoic acid
– pKa is the acid dissociation constant of benzoic acid
Enhancing Benzoic Acid Solubility through Salt Addition
The solubility of benzoic acid in water can be significantly increased by the addition of salts, such as sodium chloride (NaCl). This phenomenon is known as the “salt effect” or the “salting-in” effect.
The presence of salts in the solution alters the water-water interactions, leading to changes in the solute-solute correlations. This, in turn, affects the activity coefficients of the solute and the solvent, ultimately increasing the solubility of benzoic acid.
The relationship between the solubility of benzoic acid and the salt concentration can be described by the following equation:
ln(S/S0) = k * m
Where:
– S is the solubility of benzoic acid in the presence of the salt
– S0 is the solubility of benzoic acid in pure water
– k is the salting-in coefficient
– m is the molality of the salt
Experimental data shows that the addition of 1 M NaCl can increase the solubility of benzoic acid in water from 0.33 g/100 mL to approximately 0.55 g/100 mL at 25°C.
Benzoic Acid Solubility in Organic Solvents
Benzoic acid exhibits higher solubility in various organic solvents compared to water. The solubility of benzoic acid in common organic solvents at 25°C is as follows:
| Solvent | Solubility (g/100 mL) |
|---|---|
| Acetic acid | 6.0 |
| Acetone | 3.5 |
| 2-Propanol | 2.9 |
| Cyclohexane | 0.34 |
The increased solubility in organic solvents can be attributed to the enhanced solute-solvent interactions, such as hydrogen bonding and dipole-dipole interactions, between benzoic acid and the organic solvent molecules.
Benzoic Acid Solubility in Supercritical Fluids
Supercritical fluids, such as supercritical carbon dioxide (scCO2), have gained attention as alternative solvents for the extraction and purification of benzoic acid. The solubility of benzoic acid in scCO2 is higher than in water and can be tuned by adjusting the temperature and pressure.
The solubility of benzoic acid in scCO2 at 308 K can be expressed using the following equation:
ln(x) = A + B/T + C*ln(P)
Where:
– x is the mole fraction solubility of benzoic acid in scCO2
– A, B, and C are empirical constants
– T is the absolute temperature (K)
– P is the pressure (MPa)
Experimental data shows that the solubility of benzoic acid in scCO2 at 308 K increases from 0.0012 mole fraction at 10 MPa to 0.0045 mole fraction at 30 MPa.
Thermodynamic Properties of Benzoic Acid
The thermodynamic properties of benzoic acid, such as the dissociation constant (pKa) and the enthalpy of solution (ΔHsol), provide valuable insights into its behavior in various solvents.
The pKa of benzoic acid in water has been determined to be 4.176 at 25°C. The value of pKa is inversely proportional to temperature, indicating that the dissociation of benzoic acid is less favored at lower temperatures.
The enthalpy of solution (ΔHsol) for benzoic acid in water has been reported to be around 17.1 kJ/mol, suggesting that the dissolution process is endothermic.
Practical Applications and Future Perspectives
The understanding of benzoic acid solubility has numerous practical applications, including:
- Food and beverage preservation: Benzoic acid is widely used as a preservative in various food and beverage products due to its antimicrobial properties.
- Pharmaceutical formulations: Benzoic acid and its salts are used in pharmaceutical preparations, such as ointments and creams, to enhance the solubility and stability of active ingredients.
- Chemical synthesis: Benzoic acid is an important chemical intermediate in the production of various compounds, and its solubility plays a crucial role in the efficiency of these synthetic processes.
- Environmental remediation: The solubility of benzoic acid in supercritical fluids and organic solvents has been explored for the extraction and purification of this compound from contaminated environments.
While benzoic acid solubility has been extensively studied, there are still opportunities for further research and development. Exploring the solubility of benzoic acid in emerging solvents, such as ionic liquids and deep eutectic solvents, could lead to the development of more sustainable and environmentally friendly extraction and purification methods. Additionally, investigating the effects of nanoparticles and other additives on benzoic acid solubility could provide new strategies for enhancing its solubility in various applications.
References
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- Blas, F. J., & Vega, L. F. (1997). Thermodynamic behaviour of homonuclear and heteronuclear Lennard-Jones chains with association sites from simulation and theory. Molecular Physics, 92(1), 135-150.
- Deng, T., Xie, Y., & Wang, J. (2015). Solubility of benzoic acid in supercritical carbon dioxide: Experiment and correlation. The Journal of Supercritical Fluids, 104, 71-77.
- Lide, D. R. (Ed.). (2004). CRC handbook of chemistry and physics (Vol. 85). CRC press.
- Ruelle, P. (2000). The n-octanol-water partition coefficient of problematic molecules. Validation of the calculated log P by the cleared log P (Clog P) approach. Perspectives in Drug Discovery and Design, 19(1), 59-84.
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