What pH is needed to form the magnesium hydroxide precipitate?

What pH is needed to form the magnesium hydroxide precipitate?

• A. above 9.0
• B. above 9.2
• C. above 9.5
• D. above 10.6

Answer: (D)

Magnesium hydroxide precipitation is governed by the solubility product. For Mg(OH)2, precipitation will occur when the ion product [Mg2+][OH−]^2 exceeds Ksp. Raising the pH increases [OH−], pushing the system toward precipitation. Because the exact pH where precipitation starts depends on how much Mg2+ is present, designers choose a pH high enough to ensure Mg(OH)2 forms across typical water chemistries. In many cases, that conservative target is around the basic end of the scale, so a pH above 10.6 is used to guarantee formation of the precipitate. If magnesium concentration is relatively high, precipitation can begin at somewhat lower pH values; with lower Mg2+, you’d need a higher pH. The smaller pH values listed (around 9.0–9.5) may permit some precipitation in favorable conditions but aren’t reliable across common water chemistries, whereas 10.6+ provides a more dependable threshold.

Magnesium hydroxide precipitation is governed by the solubility product. For Mg(OH)2, precipitation will occur when the ion product [Mg2+][OH−]^2 exceeds Ksp. Raising the pH increases [OH−], pushing the system toward precipitation. Because the exact pH where precipitation starts depends on how much Mg2+ is present, designers choose a pH high enough to ensure Mg(OH)2 forms across typical water chemistries. In many cases, that conservative target is around the basic end of the scale, so a pH above 10.6 is used to guarantee formation of the precipitate.

If magnesium concentration is relatively high, precipitation can begin at somewhat lower pH values; with lower Mg2+, you’d need a higher pH. The smaller pH values listed (around 9.0–9.5) may permit some precipitation in favorable conditions but aren’t reliable across common water chemistries, whereas 10.6+ provides a more dependable threshold.