What pH level is required to precipitate magnesium hydroxide?

What pH level is required to precipitate magnesium hydroxide?

• A. 8.3
• B. 9.8
• C. 10.6
• D. 11.3

Answer: (C)

Magnesium hydroxide will form as a solid when the concentration of hydroxide ions is high enough that the solubility product of Mg(OH)2 is exceeded. The solubility product is very small, so only at sufficiently high [OH-] does Mg2+ pair with OH− to precipitate as Mg(OH)2. As pH rises, [OH−] increases (pOH = 14 − pH), and at about pH 10.6 this threshold is reached for typical magnesium levels in water. Below that pH, the ionic product is not yet above Ksp, so precipitation doesn’t start; above it, precipitation begins and becomes more pronounced. Therefore, the pH around 10.6 is the characteristic point at which Mg(OH)2 begins to precipitate.

Magnesium hydroxide will form as a solid when the concentration of hydroxide ions is high enough that the solubility product of Mg(OH)2 is exceeded. The solubility product is very small, so only at sufficiently high [OH-] does Mg2+ pair with OH− to precipitate as Mg(OH)2. As pH rises, [OH−] increases (pOH = 14 − pH), and at about pH 10.6 this threshold is reached for typical magnesium levels in water. Below that pH, the ionic product is not yet above Ksp, so precipitation doesn’t start; above it, precipitation begins and becomes more pronounced. Therefore, the pH around 10.6 is the characteristic point at which Mg(OH)2 begins to precipitate.