Chemgineer
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Drying compounds
People tell me I shouldn't be boiling things to dryness..... If I am reacting a metal with an acid to the point where it has more or less stopped
reacting, what is the usual process to dry the results and crystallise the salt?
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Elemental Phosphorus
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It depends. If the product is very heat stable, like if you were making calcium chloride from CaCO3 and HCl, then it's fine to boil to dryness. If
not, it depends on the compound. You might choose to place a concentrated solution in the sun and let it evaporate, or perhaps evaporate using a warm
plate (below the boiling point of water). Sometimes, if you don't care about recovering all of your product, you can crystallize some of it by putting
it in the freezer. For some compounds that are both heat-sensitive and hygroscopic, the best option might be to put it in a vacuum desiccator with
some phosphorus pentoxide (if you have a desiccator, I don't).
For the most part, boiling alkali metal salts of mineral acids to dryness is fine, same with alkali earth metal salts. Salts with organic anions,
though, require more care. Transition metal (or rare earth metal) salts may also require care. For example, cobalt nitrate will decompose at boiling,
so you can't isolate it that way.
Some salts can't be made in their anhydrous form from solutions or hydrated forms. Aluminum chloride, TiCl4 and Ti(NO3)4 simply can't be made from
hydrates or solutions. Some salts can't even be made from anhydrous acids, and are instead produced by something like the reaction of metal oxides
with acid anhydrides. Vanadyl nitrate is made with V2O5 and N2O5.
So essentially: sometimes boiling to dryness is fine.
Sometimes it's wise to simply evaporate rather than boil.
Sometimes a desiccator will do the trick
And sometimes it's impossible to make an anhydrous salt from a solution.
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