Meltonium
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Zinc Chloride Tetrahydrate Decomp.
I have made a solution of ZnCl2 from dissolving zinc metal in hydrochloric acid. While researching about zinc chloride, I came across info that
suggests the hydrate of ZnCl2 decomposes upon heating.
ZnCl2 4H2O -> ZnClOH + HCl + 3H2O
Obviously, I don't want this to happen, but I do want solid zinc chloride tetrahydrate. Currently, I have been heating my solution and evaporating off
the water. I have been keeping the temp ~70C and things look good. When the solution gets low enough, will this temperature be enough to decompose the
zinc chloride?
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blogfast25
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Quote: Originally posted by Meltonium  | I have made a solution of ZnCl2 from dissolving zinc metal in hydrochloric acid. While researching about zinc chloride, I came across info that
suggests the hydrate of ZnCl2 decomposes upon heating.
ZnCl2 4H2O -> ZnClOH + HCl + 3H2O
Obviously, I don't want this to happen, but I do want solid zinc chloride tetrahydrate. Currently, I have been heating my solution and evaporating off
the water. I have been keeping the temp ~70C and things look good. When the solution gets low enough, will this temperature be enough to decompose the
zinc chloride? |
It's true you can't dehydrate ZnCl2 hydrate by merely heating it: it will then hydrolyse.
But gently boiling in a solution shouldn't be a problem. Just make sure the solution contains a small excess of HCl all the way to the end.
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AJKOER
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Yes, I would boil with periodic additions of HCl through a single vent hole in a cover that insulates the solution of any interplay with air.
If you do run into trouble latter on, likely due to the presence of transition metal impurities (check your HCl source and if produced from cheap
H2SO4 acting on NaCl, not the best choice in my opinion).
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Meltonium
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So finally it's gotten down from 400ml to about 120 mL. As the concentration increased, a straw yellow color came to the solution. I attribute this to
possible iron impurities. When the solution got down to 120mL after a total of 30 hours on heat, it became cloudy like a suspension. I turned off the
heat and let everything cool. The cloudiness was due to some white particles precipitating out of the solution. These are so small that I can't
determine if they are crystals, or if they are the decomposed zinc chloride hydrate. I suspect that these are the decomposition because the solubility
of zinc chloride is, according to wikipedia, over 400g/100mL at 25C and I did not dissolve enough zinc to yield that much.
I would suspect that nothing would precipitate until there was nearly no liquid left, correct? I have been adding more HCl periodically just as you
guys suggested.
Also, looking closely at the solution, it looks like a thin layer has formed on the top. What could this be?
P.S. My source of my HCl is hardware store concrete cleaner.
[Edited on 28-10-2016 by Meltonium]
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ave369
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Yeah, the cloudiness when you crystallize salts of very weak bases usually means hydrolysis. You need more excess acid to suppress it.
Smells like ammonia....
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