Chemistry · Mg / Mg(OH)₂

Magnesium–Seawater Precipitation (2D)

Raise the pH of seawater with lime (Ca(OH)₂) or caustic soda and watch dissolved Mg²⁺ convert to solid Mg(OH)₂ once the ion product exceeds the solubility product K_sp.

Undersaturated

Dissolved Mg²⁺ Mg(OH)₂ floc

Process controls

Dosing target pH

Seawater Mg²⁺ (initial)

Stirring / mixing rate

Alkaline reagent

Solution state

pH—target

[OH⁻]—mol/L

[Mg²⁺] remaining—mol/L

Precipitated—of initial Mg

Saturation index—log(IAP/K_sp)

Reagent dose (est.)—g/L

Model: K_sp[Mg(OH)₂] = 5.6×10⁻¹² at 25℃. [OH⁻] = K_w/[H⁺] with K_w=1.0×10⁻¹⁴. Equilibrium dissolved [Mg²⁺]_eq = K_sp/[OH⁻]²; anything above that precipitates as Mg(OH)₂ floc. Reagent dose is stoichiometric (2 mol OH⁻ consumed per mol Mg²⁺ precipitated, plus the free [OH⁻] left in solution at the target pH), a simplification that ignores seawater's carbonate/borate buffering.