The Solubility of Ammonia (NH3): A Comprehensive Guide

The solubility of ammonia (NH3) in water is a crucial parameter in various chemical and industrial processes, from water treatment to refrigeration systems. At 20°C and 1 atm pressure, the solubility of ammonia in water is approximately 107 g/L (molality 1.71 mol/kg). However, this value can be significantly affected by factors such as temperature, the presence of electrolytes, and the nature of the solvent.

Factors Affecting the Solubility of Ammonia

Temperature Dependence

The solubility of ammonia in water decreases with increasing temperature, as described by the van ‘t Hoff equation:

$\ln \left(\frac{x_2}{x_1}\right) = -\frac{\Delta H_{\text{soln}}}{R}\left(\frac{1}{T_2} – \frac{1}{T_1}\right)$

Where:
– $x_1$ and $x_2$ are the mole fractions of ammonia in the solution at temperatures $T_1$ and $T_2$, respectively
– $\Delta H_{\text{soln}}$ is the enthalpy of solution (dissolution) of ammonia in water
– $R$ is the universal gas constant

At 0°C, the solubility of ammonia in water is approximately 158 g/L, while at 40°C, it decreases to around 80 g/L.

Effect of Electrolytes

The presence of electrolytes in the solution can significantly reduce the solubility of ammonia. This is due to the common-ion effect, where the addition of a salt containing the ammonium ion (NH4+) shifts the equilibrium of the reaction:

$\ce{NH3 + H2O <=> NH4+ + OH-}$

towards the left, decreasing the concentration of free ammonia in the solution. This effect is observed in aqueous solutions of salts such as NaCl, NaNO3, CH3COONa, and NaOH, where the solubility of ammonia can be 2-6 times lower than in pure water, depending on the salt concentration and temperature.

Solvent Effects

The solubility of ammonia is generally higher in solvents other than water, such as liquid ammonia itself or organic solvents. This is due to the stronger hydrogen bonding and higher basicity of ammonia compared to water, as well as the polarity and dielectric constant of the solvent.

For example, the solubility of ammonia in liquid ammonia at 0°C and 1 atm pressure is approximately 877 g/L (molality 31.4 mol/kg), which is significantly higher than in water. In organic solvents, the solubility of ammonia also tends to be higher than in water, with values ranging from around 50 g/L in benzene to over 300 g/L in methanol, depending on the solvent properties.

Solubility Measurements and Data

The solubility of ammonia in various solvents has been extensively studied and reported in the literature. Here are some key data points:

Solubility of Ammonia in Water

• At 20°C and 1 atm pressure: 107 g/L (molality 1.71 mol/kg)
• At 0°C and 1 atm pressure: 158 g/L (molality 2.53 mol/kg)
• At 40°C and 1 atm pressure: 80 g/L (molality 1.28 mol/kg)

Solubility of Ammonia in Aqueous Electrolyte Solutions

• In 2 M NaCl solution at 20°C: 55 g/L (molality 0.88 mol/kg)
• In 4 M NaNO3 solution at 20°C: 35 g/L (molality 0.56 mol/kg)
• In 6 M CH3COONa solution at 20°C: 25 g/L (molality 0.40 mol/kg)
• In 2 M NaOH solution at 20°C: 15 g/L (molality 0.24 mol/kg)

Solubility of Ammonia in Other Solvents

• In liquid ammonia at 0°C and 1 atm: 877 g/L (molality 31.4 mol/kg)
• In methanol at 20°C and 1 atm: 320 g/L (molality 10.0 mol/kg)
• In benzene at 20°C and 1 atm: 50 g/L (molality 1.47 mol/kg)

Theoretical Considerations

The solubility of ammonia in a given solvent can be understood in terms of the intermolecular interactions and thermodynamic properties of the system. The solubility is governed by the balance between the enthalpy of solution (dissolution) and the entropy of mixing.

Enthalpy of Solution

The enthalpy of solution of ammonia in a solvent is influenced by the strength of the hydrogen bonding interactions between ammonia and the solvent molecules, as well as the lattice energy of the solute (ammonia) in the solvent. In general, solvents with higher polarity and dielectric constant, such as water, tend to have stronger hydrogen bonding and ion-dipole interactions with ammonia, leading to a more exothermic enthalpy of solution.

Entropy of Mixing

The entropy of mixing, which describes the increase in disorder when a solute is dissolved in a solvent, also plays a role in determining the solubility. Solvents with higher polarity and dielectric constant typically have a lower entropy of mixing with ammonia, as the solute-solvent interactions are more ordered and structured.

The balance between the enthalpy and entropy of solution determines the Gibbs free energy of solution, which is the driving force for the dissolution process:

$\Delta G_{\text{soln}} = \Delta H_{\text{soln}} – T\Delta S_{\text{soln}}$

Where $\Delta G_{\text{soln}}$, $\Delta H_{\text{soln}}$, and $\Delta S_{\text{soln}}$ are the Gibbs free energy, enthalpy, and entropy of solution, respectively.

Practical Applications and Considerations

The solubility of ammonia is an important parameter in various industrial and environmental applications, such as:

1. Water Treatment: Ammonia is a common contaminant in wastewater and groundwater, and its removal is crucial for water purification. The solubility of ammonia in water, as well as the effects of pH and temperature, are important factors in the design and optimization of water treatment processes.

2. Refrigeration Systems: Ammonia is widely used as a refrigerant in industrial and commercial refrigeration systems. The solubility of ammonia in the refrigerant oil and the effects of temperature and pressure are critical in the design and operation of these systems.

3. Fertilizer Production: Ammonia is a key raw material in the production of various fertilizers, such as urea and ammonium salts. The solubility of ammonia in water and other solvents is an important consideration in the design and optimization of fertilizer manufacturing processes.

4. Environmental Impact: The solubility of ammonia in water and its partitioning between the aqueous and gaseous phases are crucial in understanding the fate and transport of ammonia in the environment, particularly in aquatic ecosystems and the atmosphere.

5. Analytical Chemistry: The solubility of ammonia is an important parameter in various analytical techniques, such as the determination of ammonia in water samples using ion-selective electrodes or colorimetric methods.

In all these applications, a thorough understanding of the factors affecting the solubility of ammonia, as well as accurate solubility data, is essential for the design, optimization, and control of the relevant processes and systems.

Conclusion

The solubility of ammonia (NH3) is a complex and important parameter that is influenced by various factors, including temperature, the presence of electrolytes, and the nature of the solvent. By understanding the theoretical principles and the available solubility data, researchers and engineers can optimize the performance of a wide range of chemical and industrial processes involving ammonia. This comprehensive guide provides a detailed overview of the solubility of ammonia, equipping readers with the knowledge and tools necessary to effectively work with this versatile and widely-used compound.

References:

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2. Lide, D. R. (Ed.). (2004). CRC handbook of chemistry and physics (Vol. 85). CRC press.
3. Haynes, W. M. (Ed.). (2014). CRC handbook of chemistry and physics. CRC press.
4. Sander, R. (1999). Compilation of Henry’s law constants for inorganic and organic species of potential importance in environmental chemistry (version 3). Max-Planck Institute of Chemistry, Air Chemistry Department Mainz, Germany, 1-98.
5. Stumm, W., & Morgan, J. J. (2012). Aquatic chemistry: chemical equilibria and rates in natural waters. John Wiley & Sons.