Hi all,
I've read that magnesium nitrate exists in its hydrated form at room temperature, and only at 100 degC can you remove the water molecules. In this way, a molten salt can be formed. In saying this, it is important not to hit the decomposition temperature, which is ~290 degC, if avoiding the formation of MgO. Is this correct? I was wondering if there was any overlap between the dehydration temperature, the temperature at which it can become a molten salt, and the decomposition temperature.
Thank you so much,
Alyssa
Also in my experience, when I tried TGA experiments to estimate the number of water molecules, I verified that the water release occurs together the initial decomposition, so it is difficult to estimate the hydratation level from thermogravimetric experiments
Hello Alyssa Ishchenko ,
Magnesium nitrate exists in two forms, the solid dihydrate, Mg(NO3)2•2H2O, and the solid hexahydrate, Mg(NO3)2•6H2O. The melting points of the dihydrate and hexahydrate are 129°C and 89°C, respectively. No boiling point is listed for the dihydrate, but under the boiling point for the hexahydrate it is listed that it decomposes at 330°C into MgO, NO2, and O2. Note, the hexahydrate is a deliquescent, crystalline solid, i.e., a solid that absorbs water from the atmosphere and becomes a liquid. Magnesium oxide, MgO, is a refractory, crystalline solid that melts at 2,800°C, and boils at 3,600°C. When mixed with a stoichiometrically equivalent amount of water, magnesium oxide becomes a crystalline solid, magnesium hydroxide, Mg(OH)2, which is almost insoluble in water.
Re your question, it looks as if the only way to get anhydrous magnesium nitrate is to start with the hexahydrate, and heat it up until just below its decomposition temperature.
Regards,
Tom Cuff
Hi Alyssa Ishchenko
Most of your questions can be answered using the TGA plot for Mg(NO3)2•6H2O given by Prieto et al (2019) Figure 2Article Effect of the impurity magnesium nitrate in the thermal deco...
For a heating rate of 2C/min, air purge at 50 ml/min and atmospheric pressure:1. Conversion from hexahydrate to dihydrate occurs gradually between 50 and 188C. The measured 31.97% weight loss corresponds to 4.6 moles H2O.
2. Conversion from dihydrate to anhydrous occurs gradually between 187 and 336C. The measured 11.18% weight loss corresponds to 1.6 moles H2O.
3. The decomposition of anhydrous Mg(NO3)2 to MgO occurs between 336 and 498C. The measured 40.51% weight loss is close to the expected weight loss (148.3 - 40.3)/256.4 = 42.12%
Therefore it is possible to make anhydrous Mg(NO3)2 at 336C at atmospheric pressure, but 2C/min is an impractically slow heating rate for an industrial process. At higher heating rates, the reaction temperatures will increase and dehydration and decomposition temperatures will start to overlap.
In an industrial setting, the evaporator usually dehydrates to the water content corresponding to the eutectic point between hexahydrate and dihydrate, to minimize the chance of precipitating any magnesium nitrate hydrates. Also, the evaporator is operated under vacuum to lower the boiling temperature and decrease the chance of premature decomposition.
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