[VorticalAbyss]

When approx. 4 ml of 4 M CH₃COOH is added to Cu(NH₃)₄SO₄.2H₂O a precipitate initially forms. Upon further addition the precipitate dissolves.

What is the precipitate? Why does it form, and why does it dissolve upon further addition of the acetic acid?
I'm really not too sure what is causing this, ordinarily I would presume something along the lines of Copper acetate or Copper hydroxide is formed which is insoluble and falls out of solution. The fact that further addition of acid redissolves the compound would make me think this is wrong then. Have we maybe affected solubility by changing hydration of the complex by abstracting water molecules? Oh, and if memory serves me correctly when we did the experiment the precipitate was darkly colored (dark blue/purple).

All help would be much appreciated.

[Borek]

When approx. 4 ml of 4 M CH₃COOH is added to Cu(NH₃)₄SO₄.2H₂O

Please clarify. You add acetic acid to the solid and you observe solid forming?

[VorticalAbyss]

No, sorry for lack of clarity. The copper complex is in a 25 ml solution (water is the solvent). Upon addition of the acid a precipitate is formed. By the stage all 4 ml of acid have been added the precipitate has dissolved again.

[unsu]

Interesting question.
We asked for clarification because you wrote the formula of the solid hydrated salt. If it is dissolved in water, you can't write the water of crystallization in the formula.

[VorticalAbyss]

Oh, sorry for the confusion.

[unsu]

Are you sure you added the acetic acid to [Cu(NH₃)₄]SO₄ and not to [Cu(NH₃)₄](OH)₂?

[VorticalAbyss]

Well the complex, Cu(NH₃)₄SO₄.2H₂O, is dissolved in water. This solution has ethanoic acid added to it. So maybe [Cu(NH3)₄](OH)₂ has already been formed?

[unsu]

The first thing came to my mind is the reaction between the acetic acid and the ammonia. The copper complex can dissociate giving some NH₃. Addition of acetic acid would shift the equilibrium:
4CH₃COOH + [Cu(NH₃)₄]²⁺ 4CH₃COONH₄ + Cu²⁺(aq)

The presence of the ammonia acetate could change the pH. But I checked pKb of the acetate is 9.24 and pKa of NH₄⁺ is 9.25. So... the expected pH would be somewhere about neutral.

So, maybe somebody else would share their ideas..

And yes.. if the initial copper solution was concentrated, it is possible that it could precipitate and then dissolves, because it is going to be a different salt

[VorticalAbyss]

Well, cheers for trying anyway bud.