Copper forms many stable complexes. Common ones are tetraamminecopper(II) sulphate , tetraamminecopper(II) nitrate, etc. Copper persulphate has not been isolated. But complex persulphates can be isolated, when suitable ligands are present. Examples of ligands that can stabilise copper persulphate are ammonia, ethyleendiamine and pyridine. In this experiment, tetraaminecopper(II) persulphate is made. Used equipment -beakers -filtering papers -an evaporating dish or petri dish to dry the crytals. Paper can also be used, wich absorbs moisture. -Ice bath or freezer Used chemicals -Copper sulphate pentahydrate -Ammonia 25% -Acetone -Sodium persulphate (Ammonium salt is also suitable. Potassium-salt is not very soluble, so this is not preferred.)
 acetone, sodium persulphate, copper sulphate pentahydrate and ammonia 25% Safety -Sodium persulphate is a very strong oxidiser. However, it’s not a dangerous substance, because of it’s relatively slow oxidising action. However, if you give the persulphate enough time, it will attack almost anything. -Soluble copper salts are moderately toxic -Concentrated ammonia is corrosive to the skin, and extremely corrosive to the eye. Eye contact will lead to serious eye injury or blindness! Ammonia gives off loads of pungent gas. -Acetone is highly flammable. -Tetraamminecopper(II) persulphate is mild explosive. it’s sensitive to heat, but not friction. Small amounts are quite harmless. However, don’t scale up! Disposal -Copper salts are bad for the environment, because copper is very toxic to aquatic life. Dispose of them as hazardous waste. I plate copper waste on iron nails. This is quite effective, as the only waste what will be created will be iron(II) or iron(III). Iron is non-toxic to the environment. Experiment: Synthesis. Weigh off an amount of copper sulphate pentahydrate, and dissolve this in as little water. I used approx 10mL of water for 2.50 grams copper sulphate pentahydrate. Make a concentrated solution of persulphate. When usingsodium persulphate, take the same amount (by mass) as the amount of copper persulphate used. I took 2.50 grams of sodium persulphate. Take 2 mL of concentrated ammonia, for every gram of copper sulphate. I took 6mL, wich is a little bit more. This is an exces of ammonia.
the solutions Add the liquids to eachother and shake well. This results in a deep blue, clear liquid. Cool the solution to 0C. If crystallisation does not start, scratch the walls of the beaker with a spatula. Leave the beaker at 0C for about 20 minutes. Needle-like crystals will have formed. Decant as much liquid as possible. Rinse with a few mL of very cold water, followed by a rinse with acetone. Let the crystals dry. My yield was only 1.75 grams. In theory 3.24 grams would be possible. I had a dissapointing yield of only 54%.
product This is probably due to the fact that I was in a hurry. While I decanted, quite some product was lost. I also took too little time to let the compound crystallise out. I left it at 0C for about 5 minutes. A wash with diethyl ether would make decanting and thus drying much easier. Heating the solid results in a ‘poof sound’. This ‘explosion’ is far from violent.
When a solution containing both the Cu(NH3)42- -ion as the S2O82- -ion is cooled, tetraamminecopper(II) persulphate crystallises out: Cu(NH3)4S2O8. I do not know what is exactly happening when the compound decomposes. I do know persulphate is oxidising the NH3-groups, but I think there are many side reactions taking place. |
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