quick question about Copper Sulfate preparation...

[Arthur Dent]

Hi all,

 

I just prepared a batch of Copper Sulfate... I have prepared a solution of 60ml distilled water, 6ml HN03 and 10ml H2S04 and put a big ball of fine telephone copper wire (unsheated). The reaction was energetic and produced a beautiful blue solution. Now it has settled and there is a bit of copper left, but the solution is deep blue.

 

Even though the HNO3 is the oxidant and most of it escapes as it turns into nitrous oxide, can there be some Copper Nitrate impurities in my CuSO4 ?

 

And now that the solution is settled, I know that there is a bit of sulfuric acid left in the solution, what's the easiest way to get rid of the H2SO4 to get clean crystals?

 

Thanks

 

Robert

[UC]

There is still nitrate present in your solution. As the concentration of nitric acid goes down, it's oxidizing power drops off and dilute acid will fail to attack additional copper.

 

For the record, the endproduct of nitric acid oxidation of metal is usually NO2 (nitrogen dioxide).

 

However, copper nitrate is much more soluble in water than the sulfate. Reduce down your solution and chill it and the crystals that form will be largely acid and nitrate free. Rinse them with a little cold water after removing them from the solution. To remove any trapped acid or nitrate (somewhat minimal, but possibly problematic), dissolve them in hot water and add a small amount of Copper (II) hydroxide, oxide, or carbonate (in reality, a basic carbonate since AFAIK you cannot prepare pure CuCO3). These will react with any excess acid. Filter the excess (insoluble) copper compound from the solution and chill to form very pure crystals of CuSO4*5H2O. Since the solution is fairly pure, you can boil it down and repeat to collect further crops of crystals. It's good practice to discard the last small amount of solution in any case since it contains the concentrated impurities (if any were present).

 

An easier way to go about this whole process is to just dissolve the copper in moderately strong nitric acid (~30% works well, IIRC), neutralize and precipitate all the copper as basic carbonates with an excess of NaCO3 solution. Filter and wash the solid repeatedly with clean water to remove contaminants. Then slowly add dilute sulfuric acid to this until you just have a tiny bit of solid remaining. Filter this off, boil down and cool to crystallize. Repeat as above to collect copper sulfate pentahydrate.

[Arthur Dent]

Oh yeah, NO2... sorry I confused with N2O.

 

Good advice!, i'll chill the solution and retrieve the crystals as they form. I don't have any Copper Hydroxide or Carbonate but I guess I can sacrifice a little bit of the solution to have the purest CuSO4 possible. Crystals are already starting to grow on the lattice of copper wire (I left the solution outside overnight so it gasses off).

 

Many thanks!

 

Robert