TheChemistryShack
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Wet Copper (II) Hydroxide Turning Black
I recently made a small amount of Cu(OH)2 by adding an excess of CuSO4 solution to some 1 M NaOH solution. I decanted the solution and let the wet
precipitate dry. However, about thirty minutes later, some of the wet goop had turned black. I initially assumed that this is due to the formation of
copper (II) oxide, but upon doing some research I found that reaction requires heat to catalyze it. Does anyone know if this is copper oxide, and if
it is, how it could have formed?
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WGTR
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Watts, Henry (1872). A Dictionary of Chemistry and the Allied Branches of Other Sciences, Vol 2. Longmans, Green, and Co. (p 69).
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Amos
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All I had to do was check wikipedia, which says that moist copper(II) hydroxide slowly converts to the oxide in air. This is further aggravated by
strongly basic starting solutions in my experience. Properly prepared and well-dried copper(II) hydroxide can withstand this reaction, but not when
fresh and wet.
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HgDinis25
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Copper(II) Hydroxide is unstable in strongly alkaline conditions and oxygen. Heat also catalyzes the reaction as you stated.
If you want to avoid this problem I recomend vacuum filtering the Copper(II) Hydroxide. After vacuum filtering, add a volatile solvent to it, while
still running vacuum. Acetone is the easiest to come by, ether is usually the best. One of the most simple yet effective methods of drying Copper(II)
Hydroxide.
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The Volatile Chemist
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I could've sworn a document of woelen's stated Cu(OH)2 dehydrated, even in water. But I guess that would mean it would still be forming
CuO.
Sorta spit-balling, sorry.
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WGTR
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Aww, heck. You gave away my secret. That's how I found the reference.
Quote: Originally posted by No Tears Only Dreams Now  | | This is further aggravated by strongly basic starting solutions in my experience. Properly prepared and well-dried copper(II) hydroxide can withstand
this reaction, but not when fresh and wet. |
I've noticed the same thing. Copper hydroxide is easy to prepare when it is precipitated with a stoichiometric amount of sodium hydroxide. I filter
and wash it on a Buchner funnel, and have no problem working with it while wet. It doesn't seem to decompose at all.
If however, too much sodium hydroxide is added; the precipitate turns dark green to black even as it stirs in the beaker. Heat will do the same
thing.
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Amos
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It is quite easy to prepare if done right; Theoretically I believe the hydroxide solution should be dilute and should be poured into the copper salt
solution, not the other way around. I then filter and squeeze the filter paper of any remaining water(since I don't have a vacuum filtration system)
and put it in my food dehydrator, which operates at about 80 C. My product has always been a chalky blue powder, no sign of decomp.
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Brain&Force
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Copper(II) is amphoteric, maybe this has something to do with the easier dehydration in a more strongly basic solution?
At the end of the day, simulating atoms doesn't beat working with the real things...
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