The magnesium nitrate - water binary system exhibits a particular solubility behavior encountered in many aqueous salt systems: multiple solubilities at the same temperature. Between 50 and 90°C the solubility of magnesium nitrate can be one of three values, as it appears from the phase diagram below. The figure also shows that the Extended UNIQUAC model is able to reproduce this type of solubility behavior quite accurately. Three eutectic points and one peritectic point are found in this binary system.
The eutectic point where ice and magnesium nitrate nona-hydrate precipitate simultaneously is at -27°C and 33 % Mg(NO3)2. This is the cryohydratic point. A peritectic point appears at -21°C and 36 % Mg(NO3)2 marking the transition between the nona-hydrate and the hexa-hydrate of magnesium nitrate.
The hexa-hydrate and the di-hydrate of magnesium nitrate form a simple eutectic system with a eutectic point at 50°C and 67 % Mg(NO3)2. Finally magnesium nitrate di-hydrate and anhydrous magnesium nitrate form a simple eutectic system with an eutectic point at 129°C and 82 % Mg(NO3)2.
The fact that the transition at 129°C is a eutectic rather than a peritectic point has been documented in a number of experiments as for example the measurements carried out by Ewing, W. W., Brandner, J. D., Slichter, C. B., Griesinger, W. K. "The temperature-composition relations of the binary system magnesium nitrate-water", J. Am. Chem. Soc. 55(1933)4822-24.