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So I wanna do an experiment using manganese nitrate and I bought manganese (II) nitrate hexahydrate $\ce{Mn(NO3)2.6H2O}$. But when I received it, it was kinda watery already (most part is still crystallized but there was also liquid in the bottle). After inspecting the bottle, it says that I need to refrigerate at 5 degree Celsius. But after I refrigerate it becomes icy hard, I cannot take anything out. So I am now wondering how to handle this compound. I am afraid if I use the liquid part it will have various concentration, and it is barely impossible to use the solid part because it's so hard. Thank you.

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    $\begingroup$ As long as you can you DON't use nitrates of multicharged cations, period. Most of them are extremely hygroscopic and cannot be weighted easily. If you really need to use nitrate solution, the best way is to prepare a solution and obtain the exact concentration using titration. Than said solution with known concentration can be stored in a closed bottle/flask and used as needed. $\endgroup$ – permeakra Sep 3 '18 at 15:05
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Manufacturers of chemicals can make life interesting for first-time users. What a boon the CRC Handbook is! Without even listing $\ce{Mn(NO3)2.6H2O}$, the CRC Handbook can solve your problem. It gives the melting point of $\ce{Mn(NO3)2.4H2O}$ as $\pu{25.8^\circ C}$. Undoubtedly, the melting point of the hexahydrate is even lower, but maybe not by much. In hot water, the tetrahydrate is "infinitely" soluble.

Heat your jar of hexahydrate to 30C in warm water until all the crystals are melted and withdraw what you need with a pipette. You might plan ahead and fill several small bottles with the hexahydrate for later use. I doubt that the material needs to be stored cold.

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Manganese nitrate hydrate can be converted into anhydrous manganese nitrate by using a dehydrating agent like phosphorus pentoxide or dinitrogen pentoxide. First gently heat the hexahydrate around 100-110°C in vacuum to form the dihydrate and then heating around 80-90°C with the dehydrating agent to get the anhydrous product.

\begin{align} \ce{Mn(NO3)2•6H2O &->[110°C,vacuo] &&Mn(NO3)2•2H2O + 4H2O}\\ \ce{Mn(NO3)2•2H2O + N2O5 &->[80-100°C,vacuo] &&Mn(NO3)2 + 2HNO3 + H2O} \end{align}

The mechanism of this reaction is found in this e-book:

Anhydrous nitrate is mostly prepared by dehydration of solid hydrate in vacuum dessicator at room temperature over phosphorus pentoxide or dinitrogen pentoxide.[...], a solvate is formed $\ce{Mn(NO3)2.N2O4}$ and by heating at 90°C, the dinitogen tetroxide can be removed to give the unsolvated compound. Thermal decomposition of the hydrated manganese(II) nitrate in vacuo also yields the anhydrous salts[...]

But you should be careful if you want to direct heat the hexahydrate. Overheating may cause the hexahydrate to fully decompose into manganese dioxide.

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