What solvent dissolves calcium nitrate but not ammonium nitrate?

Question

I am experimenting with a precipitation reaction between ammonium sulfate solution and calcium nitrate. Calcium sulfate precipitates and can be filtered out easily.

I am wondering if there's a way I can separate other compounds from the solution. If I simply boil away water, I will be left with the mixture of all three solids: $\ce{NH4NO3},$ $\ce{(NH4)2SO4},$ and $\ce{Ca(NO3)2}.$

I suppose the easiest way to extract ammonium nitrate from the mixture would be to find a solvent that can dissolve every other compound except the ammonium nitrate. Is there a way to find solvents that won't dissolve ammonium nitrate, but will dissolve calcium nitrate, for example?

I found comprehensive tables with the data for aqueous solubility, but only a few references about specific substance's solubility/insolubility in non-aqueous media.

I do not have a background in university level chemistry, nor is this question for any publication purposes. I am purely asking from my curiosity.

Answer 1

Both $\ce{Ca(NO3)2}$ and $\ce{NH4NO3}$ are highly soluble in water, but at 80°C or higher, $\ce{NH4NO3}$ is more soluble. (not too much higher, or the ammonium salt would decompose.)

What might be the easiest way to separate them is to evaporate water from the mixture at ~80°C until the $\ce{CaSO4}$ and most of the $\ce{Ca(NO3)2}$ precipitate, decant the $\ce{NH4NO3}$ solution, and evaporate water from it. This would still be impure if done rapidly, but performed slowly enough, the ammonium salt could be further purified by crystallization.

Answer 2

Among other solvents that might prove possible, methanol is stated to have some solubility for NH₄NO₃ and Ca(NO₃)₂. (Ref: CRC Handbook) However, there is a very good reason to NOT use methanol with soluble metallic nitrates, especially when the nitrates are acidic (like NH₄NO₃). The mixture would be highly flammable and very likely susceptible to self-ignition if you stirred it too hard or heated it near any source of sparks or flame. The safety hazards are overwhelming. Any other replacement solvent that is flammable or contains carbon (even if not very flammable) could be a serious hazard.

However, there is a way to use water and get a fairly pure solution of ammonium nitrate. You can calculate the exact amount of NH₄NO₃ needed to react with a given amount of Ca(NO₃)₂. You will, of course, need to know the exact concentration of your ammonium sulfate solution and the exact amount of water in the Ca(NO₃)₂ (because it is generally supplied as a tetra hydrate, but that varies, depending on whether you use a research grade or a technical (fertilizer) grade).

Now the problem is that poorly soluble calcium salts are generally more soluble in solutions of ammonium salts, and here we have one of the reactants and one of the products being an ammonium salt - you can't get away from it! Or can you? The reason Ca(SO₄)₂ is (somewhat) soluble in ammonium salts is that the acidity of the ammonium salt converts some of the SO₄$^{2-}$ to bisulfate and some more of the calcium sulfate can dissolve. If you add NH₄OH to reduce the acidity, you can eliminate the solvent power of the ammonium salt. In this way, you can complete the precipitation of the calcium sulfate.

There will be a price to pay: now the solution is going to smell like ammonia. However, if you do the addition of ammonia carefully, to about pH 7-8, the odor should be manageable. Use good ventilation. Filter off your calcium sulfate precipitate and evaporate your ammonium nitrate solution with minimal heat but plenty of ventilation.

Further purification of the calcium sulfate could be done by simple washing; the ammonium nitrate could be redissolved, filtered and evaporated to produce crystals as opposed to evaporation to dryness.