Summary
Magnesium hydroxide, Mg(OH)2, is a sparingly soluble compound with a solubility of 1.6 x 10^-4 mol/L at 298 K. This means that for every liter of solution, only 1.6 x 10^-4 moles of magnesium hydroxide can be dissolved before reaching saturation. The solubility of magnesium hydroxide is influenced by various factors, including temperature, pH, and the presence of other ions in the solution. Understanding the solubility of magnesium hydroxide is crucial in various applications, such as in the pharmaceutical industry, water treatment, and soil remediation.
Magnesium Hydroxide Solubility
Solubility of Magnesium Hydroxide
Magnesium hydroxide, Mg(OH)2, is considered a sparingly soluble compound in aqueous solutions. The solubility of magnesium hydroxide at 298 K (25°C) is 1.6 x 10^-4 mol/L. This means that for every liter of solution, only 1.6 x 10^-4 moles of magnesium hydroxide can be dissolved before reaching saturation.
The solubility of magnesium hydroxide can be expressed using the following equilibrium equation:
Mg(OH)2 (s) ⇌ Mg^2+ (aq) + 2 OH^- (aq)
The solubility product constant (Ksp) for magnesium hydroxide is 1.6 x 10^-11, which represents the equilibrium constant for the dissociation of magnesium hydroxide in water.
Factors Affecting Magnesium Hydroxide Solubility
The solubility of magnesium hydroxide can be influenced by various factors, including:
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Temperature: The solubility of magnesium hydroxide generally increases with increasing temperature. This is due to the endothermic nature of the dissolution process, which is favored by higher temperatures.
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pH: Magnesium hydroxide is a weak base, and its solubility is affected by the pH of the solution. In acidic environments (low pH), the solubility of magnesium hydroxide increases due to the formation of soluble magnesium ions (Mg^2+). Conversely, in basic environments (high pH), the solubility decreases as the concentration of hydroxide ions (OH^-) increases, shifting the equilibrium towards the precipitation of magnesium hydroxide.
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Presence of other ions: The presence of other ions in the solution can also affect the solubility of magnesium hydroxide. For example, the addition of chloride ions (Cl^-) can increase the solubility of magnesium hydroxide by forming soluble magnesium chloride complexes.
Solubility Product Constant (Ksp)
The solubility product constant (Ksp) for magnesium hydroxide is 1.6 x 10^-11 at 298 K (25°C). The Ksp is a fundamental thermodynamic property that represents the equilibrium constant for the dissociation of a sparingly soluble compound in water.
The Ksp for magnesium hydroxide can be expressed as:
Ksp = [Mg^2+] × [OH^-]^2
where [Mg^2+] and [OH^-] are the equilibrium concentrations of the respective ions in the solution.
The Ksp value can be used to calculate the solubility of magnesium hydroxide in water or to determine the conditions under which precipitation or dissolution will occur.
Absorption and Bioavailability of Magnesium Hydroxide
Absorption of Magnesium from Magnesium Hydroxide
When magnesium hydroxide, Mg(OH)2, is ingested, it can dissociate in the stomach acids, releasing Mg^2+ cations and OH^- anions. The Mg^2+ cations can then be absorbed through the epithelial lining of the small intestine.
Studies have shown that the absorption of Mg^2+ cations from magnesium hydroxide is typically in the range of 5-15% of the dissociated Mg^2+ ions. This relatively low absorption rate is due to the limited solubility of magnesium hydroxide and the competition with other ions for absorption sites in the intestine.
Tolerable Upper Limit (TUL) for Magnesium Intake
The equivalent tolerable upper limit (TUL) for the intake of Mg^2+ from nonfood sources is 5 mg/kg-d (0.21 mmol/kg-d) for all population groups 1 year old and older. Based on the differences in molecular weight, the equivalent TUL for Mg(OH)2 is estimated to be about 2.4 times that of Mg^2+, or 12 mg/kg-d.
The TUL represents the maximum daily intake of a nutrient that is unlikely to pose any risk of adverse health effects for most individuals in the general population.
Absorption of Magnesium from Other Sources
In a study of human volunteers, the oral absorption of Mg^2+ cations from magnesium lactate or magnesium citrate was found to be:
- Magnesium lactate: 34.5% ± 18.8% (mean ± standard deviation)
- Magnesium citrate: 39.8% ± 24.3% (mean ± standard deviation)
The absorption of Mg^2+ was also estimated to be 25.6% ± 34.5% based on fecal excretion.
These results indicate that the bioavailability of magnesium from different sources can vary, with magnesium citrate and lactate showing higher absorption rates compared to magnesium hydroxide.
Equilibrium and Solubility Product Constant (Ksp)
Equilibrium Reaction of Magnesium Hydroxide
As mentioned earlier, the equilibrium reaction for the dissolution of magnesium hydroxide in water can be represented as:
Mg(OH)2 (s) ⇌ Mg^2+ (aq) + 2 OH^- (aq)
At equilibrium, the concentrations of the ions in the solution are related to the solubility product constant (Ksp) through the following equation:
Ksp = [Mg^2+] × [OH^-]^2
Solubility Product Constant (Ksp)
The solubility product constant (Ksp) for magnesium hydroxide is 1.6 x 10^-11 at 298 K (25°C). This value represents the equilibrium constant for the dissociation of magnesium hydroxide in water.
The Ksp can be used to calculate the solubility of magnesium hydroxide in water or to determine the conditions under which precipitation or dissolution will occur. For example, if the ion product [Mg^2+] × [OH^-]^2 is greater than the Ksp, the solution will be supersaturated, and magnesium hydroxide will precipitate. Conversely, if the ion product is less than the Ksp, the solution will be undersaturated, and magnesium hydroxide will dissolve.
Relationship between Solubility and Ksp
The solubility of a sparingly soluble compound, such as magnesium hydroxide, is directly related to its Ksp value. The higher the Ksp, the more soluble the compound is in water.
For magnesium hydroxide, the solubility (S) can be calculated from the Ksp using the following equation:
Ksp = [Mg^2+] × [OH^-]^2 = S × (2S)^2 = 4S^3
Rearranging this equation, we can solve for the solubility (S):
S = (Ksp/4)^(1/3) = (1.6 x 10^-11/4)^(1/3) = 1.6 x 10^-4 mol/L
This calculated solubility value matches the experimentally determined solubility of magnesium hydroxide at 298 K (25°C), which is 1.6 x 10^-4 mol/L.
Conclusion
Magnesium hydroxide, Mg(OH)2, is a sparingly soluble compound with a solubility of 1.6 x 10^-4 mol/L at 298 K (25°C). The solubility of magnesium hydroxide is influenced by various factors, including temperature, pH, and the presence of other ions in the solution. Understanding the solubility of magnesium hydroxide is crucial in various applications, such as in the pharmaceutical industry, water treatment, and soil remediation.
When magnesium hydroxide is ingested, it can dissociate in the stomach acids, releasing Mg^2+ cations that can be absorbed through the epithelial lining of the small intestine. The absorption of Mg^2+ from magnesium hydroxide is typically in the range of 5-15% of the dissociated Mg^2+ ions.
The solubility product constant (Ksp) for magnesium hydroxide is 1.6 x 10^-11 at 298 K (25°C), which represents the equilibrium constant for the dissociation of magnesium hydroxide in water. The Ksp can be used to calculate the solubility of magnesium hydroxide and to determine the conditions under which precipitation or dissolution will occur.
Overall, the comprehensive understanding of magnesium hydroxide solubility and its related properties is essential for various applications and research in the fields of chemistry, materials science, and environmental science.
References
- National Institutes of Health. (2021). Magnesium. Retrieved from https://www.ncbi.nlm.nih.gov/books/NBK225636/
- Socratic. (n.d.). Solubility of Mg(OH)2 is 1.6 x 10^-4 mol/L at 298 K. What is its solubility product? Retrieved from https://socratic.org/questions/solubility-of-mg-oh-2-is-1-6-x-10-4-mol-l-at-298-k-what-is-its-solubility-produc
- CourSidekick. (n.d.). Magnesium Hydroxide Solubility. Retrieved from https://www.coursesidekick.com/chemistry/1228273
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