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Will Co(OH)2 solid dissolve in NH3?

My Chemistry Professor said that cobalt (II) hydroxide solid will not dissolve in ammonia, but during the experiment I observed that there was a reaction taking place (the clear ammonia solution turned brown, like the precipitate).

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1 Answer

Oh, the unpredictability of chem lab!

Tell me a few things: Was your Co(OH)2 in water?

Were you using dilute NH3 or concentrated?

Co(OH)2 in water complexes with 4 H2O molecules, so makes: [Co(OH)2(H2O)4

If you add dilute NH3, the ammonia base with only interact with the water molecules surrounding the Cobalt complex. There are just not enough NH3 molecules around in dilute solution to fight off all the water molecule protecting their precious Co(OH)2.

If you add concentrated NH3 to the aqeuous mixture, the NH3 can win the battle, fighting off the water molecules, kicking out the OH groups, and claiming Cobalt. You get [Co(NH3)6]+2

Knowing the possible reactions listed above, I have one more thing to add: Cobalt(II) compounds, if left sitting, will oxidize to form Cobalt(III) complexes, which are brown colored.

What you need to think about is:

Was my NH3 solution dilute or concentrated?

Did I see solid Co(OH)2 dissolve in the NH3 solution? Did it then turn brown after sitting?

If I had a pretty pink Co(OH)2 precipitant reaction, did the colors start to change when I added the NH3 solution, or did I leave it sitting at my lab bench for a while and come back to a brown precipitant?


What I think, since your instructor told you it would not dissolve, is that you added a dilute NH3 solution, saw no immediate change, went to go do some other steps, and came back to see the ugly brown precipitant. All this would mean is that your Co(II) oxidized to Co(III).

If this does not seem right, please let me know! I will try to help you brain storm some other possiblities.