Magnesium nitrate

Magnesium nitrate is a hygroscopic salt with the formula Mg(NO₃)₂. In air, it quickly forms the hexahydrate with the formula Mg(NO₃)₂.6H₂O. It is very soluble in both water and ethanol.

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Uses

Magnesium nitrate occurs in mines and caverns as nitromagnesite. This form is not common, although it may be present where guano contacts magnesium-rich rock. It is used in the ceramics, printing, chemical and agriculture industries. Its fertilizer grade has 10.5% nitrogen and 9.4% magnesium, so it is listed as 10.5-0-0 + 9.4% Mg. Fertilizer blends containing magnesium nitrate usually have ammonium nitrate, calcium nitrate, potassium nitrate and micronutrients; these blends are used in the greenhouse and hydroponics trade.

Reactions

The magnesium nitrate used in commerce is a man-made product. It can be synthesized in a variety of ways. The reaction between nitric acid and magnesium metal or magnesium oxide results in magnesium nitrate.

2HNO₃ + MgO → Mg(NO₃)₂ + H₂O

Magnesium hydroxide and ammonium nitrate also react to form magnesium nitrate as ammonia is released as a by-product.

Mg(OH)₂ + 2NH₄NO₃ → Mg(NO₃)₂ + 2NH₃ + 2H₂O

Since magnesium nitrate has a high affinity for water, heating the hexahydrate does not result in the dehydration of the salt. Instead, magnesium nitrate hexahydrate decomposes into magnesium oxide, oxygen, and nitrogen oxides.

2Mg(NO₃)₂ → 2MgO + 4NO₂ + O₂

The absorbption of these nitrogen oxides in water is one possible route way to synthesize nitric acid. Although it is inefficient, it does not require the use of another strong acid.

Anhydrous magnesium nitrate is also used to increase the concentration of nitric acid past its azeotrope of approximently 68% nitric acid and 32% water. It is also occasionally used as a desiccant.

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