Magnesium Chloride
Properties
| State | Solid (crystalline, deliquescent) |
| Color | Colorless to white |
| Solubility | Highly soluble in water (543 g/L at 20 °C) |
| Melting Point | 714 °C |
| Boiling Point | 1412 °C |
About Magnesium Chloride
Magnesium chloride is a deliquescent ionic salt that pulls water out of the air so aggressively that anhydrous MgCl2 left on an open balance pan turns into a puddle within hours — which is why the laboratory rarely sees pure anhydrous material and almost always works with the hexahydrate MgCl2.6H2O (bischofite) or solutions thereof. The Mg2+ ion is small and high-charge-density, so it sits in seawater as the second-most-abundant cation after sodium (1.28 g/L Mg2+, mostly paired with chloride and sulfate) and is the largest mineral reserve of magnesium on Earth. Industrially the compound is the feedstock for primary magnesium metal: anhydrous MgCl2 is electrolyzed at roughly 700 °C in a molten KCl/NaCl/CaCl2 bath — the IG Farben process and its descendants. The chemistry tradeoffs are interesting: making the anhydrous salt is harder than the electrolysis itself, because heating the hexahydrate just produces MgO and HCl gas (pyrohydrolysis) unless the dehydration is run under a stream of dry HCl. In Japan and Korea, the bittern left after evaporating seawater for table salt is sold as nigari and used to coagulate soy proteins into tofu, where Mg2+ bridges adjacent glycinin molecules. In North American winter operations, MgCl2 brine sprayed at about 30 wt% out-performs rock salt below -9 °C and stays liquid on the road longer because of its hygroscopicity.
Where you'll encounter it
If you have ever pressed your own tofu with nigari, watched a winter highway crew apply pre-storm brine to the asphalt, or set up a Grignard reaction and worried about Mg-OH side products from trace water, you have been managing the chemistry of MgCl2. A home tofu-maker stirring 2 g of nigari into a liter of warm soy milk at 75 °C watches glycinin curds form within seconds as Mg2+ bridges adjacent protein subunits — the textbook silken-tofu coagulation. Highway DOT crews spraying 30 wt% MgCl2 brine ahead of a forecast snow event get a film that stays liquid down to -15 °C of pavement temperature, well past the -9 °C limit where rock salt stops working. Synthetic chemists weighing anhydrous MgCl2 for a Ziegler-Natta catalyst support do the entire transfer inside a glovebox because a single deliquescent gram pulled from open air is already wet.
Common Uses
- Molten-salt feedstock for primary magnesium metal production by electrolysis
- Road de-icing brine effective down to about -15 °C, less corrosive than NaCl
- Tofu coagulant (nigari) bridging soy glycinin molecules into curds
- Sorel cement (MgO + MgCl2 brine) for industrial flooring and fireproof panels
- Catalyst support for Ziegler-Natta polypropylene polymerization
- Oral magnesium supplement at 200 to 400 mg elemental Mg per dose
- Saltwater fish-tank trace mineral replenishment
Safety Information
Low acute toxicity; oral LD50 in rats is around 2.8 g/kg. Ingestion of multi-gram doses produces strong osmotic diarrhea and can disturb electrolyte balance — not a medication for routine self-supplementation above label doses. Solid MgCl2 dust mildly irritates eyes and respiratory mucosa. Concentrated brines are corrosive to galvanized steel and to unsealed concrete (chloride attack on rebar passivation), which is the main reason municipalities still mix it with corrosion inhibitors before road application. Not classified under GHS for general handling.
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.