Magnesium Hydroxide
Properties
| State | Solid (white powder or suspension) |
| Color | White |
| Solubility | Sparingly soluble in water (0.012 g/L at 25 °C) |
| Melting Point | 350 °C (decomposes) |
| Boiling Point | Decomposes before boiling |
About Magnesium Hydroxide
Magnesium hydroxide is the textbook example of a base that is strong on paper (it dissociates fully into Mg2+ and 2 OH- on the bit that does dissolve) but mild in practice, because its solubility in water is just 0.012 g/L — 0.0002 M — pinned by a Ksp of 5.6 × 10^-12. That low solubility is exactly why Charles Henry Phillips marketed it as 'milk of magnesia' in 1873: an antacid that neutralizes stomach HCl on contact (Mg(OH)2 + 2 HCl -> MgCl2 + 2 H2O) without producing the violent pH overshoot that a soluble strong base like NaOH would cause. The pH of saturated Mg(OH)2 sits around 10.5, comfortably below tissue-damaging territory. In industry, the compound is a major halogen-free flame retardant: when wire jacket compounds and engineering plastics filled with 50 to 65 wt% Mg(OH)2 reach about 330 °C, the hydroxide undergoes endothermic decomposition (Mg(OH)2 -> MgO + H2O, dH about +1450 kJ/kg of water released), pulling heat out of the polymer and diluting combustible volatiles with steam. The natural mineral form is brucite, which occurs in serpentinite belts and is mined as a magnesium ore. Wastewater treatment plants use Mg(OH)2 slurries to neutralize acidic effluent precisely because the buffering plateau near pH 9 is hard to overshoot.
Where you'll encounter it
If you have ever poured chalky milk of magnesia for heartburn, stripped insulation off a halogen-free LSZH cable in a server room, or dosed an acid-mine-drainage stream to lift the pH without flash-precipitating heavy metals, you have used Mg(OH)2. In a wastewater plant the operator picks magnesium hydroxide slurry over caustic soda specifically because the saturated suspension self-buffers near pH 9 — overdose is essentially impossible, where a NaOH overshoot could push effluent past pH 12 and trigger fines. In a polymer compounding line, the 50-65 wt% Mg(OH)2 filler is what lets a halogen-free cable jacket pass the IEC 60332 vertical-tray flame test without dripping burning droplets. Brush a Mg(OH)2 paste into a stomach acid model on the bench and you watch the pH rise stop at 9, never overshoot.
Common Uses
- Antacid for heartburn, sold as milk of magnesia (8% w/v aqueous suspension)
- Osmotic laxative at multi-gram doses
- Halogen-free flame retardant for LSZH cable jackets and engineering plastics at 50-65 wt% loading
- Acidic-effluent neutralization in wastewater and acid-mine-drainage treatment
- Pulp and paper bleaching pH buffer for hydrogen peroxide stages
- Magnesium dietary supplement and laxative in elemental Mg-deficient soils
Safety Information
Very low acute toxicity; ingested doses up to several grams clear as MgCl2 in stomach acid. Excess oral consumption is its own self-limiting feature — strong osmotic laxation. Renal-impaired patients accumulate Mg2+ and risk hypermagnesemia (cardiac and respiratory depression), so chronic high-dose use needs medical supervision. Inhaled dust mildly irritates the upper airway. Not classified hazardous under GHS. Industrial flame-retardant grades occasionally contain trace nickel or iron from the ore source; check the COA for medical-grade applications.
This safety summary is for educational reference only and may not be complete. It is not a substitute for Safety Data Sheets (SDS), medical advice, or professional chemical safety guidance. Always consult appropriate SDS and qualified professionals before handling chemicals.