Magnesium chloride

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Magnesium chloride
Magnesium chloride hexahydrate flakes sample.jpg
A sample of hygroscopic food-grade MgCl2·6H2O
Names
IUPAC name
Magnesium chloride
Systematic IUPAC name
Magnesium chloride
Other names
E511
Nigari
Nigari salt
Identifiers
Jmol-3D images Image
Properties
MgCl2 (anhydrous)
MgCl2·6H2O (hexahydrate)
Molar mass 95.211 g/mol (anhydrous)
203.31 g/mol (hexahydrate)
Appearance White or colorless crystalline hygroscopic solid
Density 2.32 g/cm3 (anhydrous)
1.569 g/cm3 (hexahydrate)
Melting point 714 °C (1,317 °F; 987 K)
117 °C (243 °F; 390 K) (hexahydrate)
Boiling point 1,412 °C (2,574 °F; 1,685 K)
anhydrous
52.9 g/100 mL (0 °C)
54.3 g/100 mL (20 °C)
72.6 g/100 mL (100 °C)
hexahydrate
167 g/100 mL (20 °C)
Solubility Soluble in conc. HCl, methanol
Slightly soluble in acetone, ethanol, pyridine
Solubility in ethanol 3.61 g/100 mL (0 °C)
7.4 g/100 mL (30 °C)
15.89 g/100 mL (60 °C)
Solubility in methanol 15.5 g/100 mL (0 °C)
20.4 g/100 mL (60 °C)
Vapor pressure ~0 mmHg
Thermochemistry
89.88 J·mol-1·K-1
-641.1 kJ/mol
Hazards
Safety data sheet ICSC 0764
Flash point Non-flammable
Lethal dose or concentration (LD, LC):
2,800 mg/kg (oral, rat)
Related compounds
Related compounds
Calcium chloride
Strontium chloride
Barium chloride
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Infobox references

Magnesium chloride, also known as Nigari, is a chemical compounds with the formula MgCl2.

Properties

Chemical

Magnesium chloride will react with calcium hydroxide to give magnesium hydroxide and calcium chloride:

MgCl2(aq) + Ca(OH)2 → Mg(OH)2(pp) + CaCl2(aq)

Physical

Magnesium chloride is a white hygroscopic solid, soluble in water but poorly soluble in organic solvents, such as ethanol.

Availability

Magnesium chloride is available as tofu coagulant, more exactly as "Nigari flakes" or "Nigari salt". Some formulations also have magnesium sulfate.

Magnesium chloride sometimes occurs naturally as the mineral bischofite.

Preparation

Magnesium chloride can be prepared by reacting hydrochloric acid with magnesium metal, though this reaction is highly exothermic and magnesium metal is expensive.

Mg + 2 HCl → MgCl2 + H2

Magnesium carbonate or magnesium hydroxide are much safer to use and much cheaper.

MgCO3 + 2 HCl → MgCl2 + H2O + CO2
Mg(OH)2 + 2 HCl → MgCl2 + 2 H2O

A cheaper way involves the reaction of magnesium sulfate (Epsom salt) with calcium chloride.

MgSO2 + CaCl2 → MgCl2 + CaSO2

The resulting calcium sulfate is filtered off and the magnesium chloride solution is gently heated at 100 °C to remove the water. Since a small amount will hydrolyze, adding hydrogen chloride gas/conc. hydrochloric acid will regenerate the magnesium chloride. Magnesium carbonate can also be used instead of Epsom salt, but since it's very poorly soluble, the reaction needs lengthy boiling under reflux (similarly to converting barium sulfate to carbonate).

On heating, the hexahydrate loses water in several steps, but it will decompose before becoming anhydrous. To prepare anhydrous magnesium chloride, the hexahydrate is heated in a stream of dry hydrogen chloride. Alternatively, the hydrated magnesium chloride can be dissolved in dimethylformamide, and a fractional distillation apparatus used to remove the water and DMF, leaving behind an anhydrous mass consisting mainly of magnesium chloride with some minor decomposition products from the DMF.

Projects

  • Make elemental magnesium (electrolysis of molten MgCl2 at temperatures over 714 °C, or of an eutectic mixture of MgCl2-KCl-NaCl above 475 °C)
  • Make Rieke magnesium
  • Home-made desiccator
  • Make tofu

Handling

Safety

Magnesium chloride is deliquescent and its anhydrous form may cause irritations on contact with skin, eyes or mouth.

Storage

Because of its hygroscopicity, magnesium chloride must be stored in an air-tight container.

Disposal

Magnesium chloride can be safely poured down the drain.

References

Relevant Sciencemadness threads