Letters - September 2006

The reverse addition approach

From Dr Noel Dickson

Here is an alternative approach to Colin Baker's recent Exhibition chemistry demonstration (see Educ. Chem., 2006, 43(3), 66). Starting with the light blue copper sulfate solution in a large conical flask, add slowly and cautiously concentrated ammonia. The first product to form is a precipitate of the copper hydroxide which is a light, creamy blue. On further addition of the concentrated ammonia, the light blue precipitate dissolves and the deep blue cationic copper tetrammine complex ion forms.  

Next add concentrated hydrochloric acid. If you do this very slowly and cautiously a heady mist (ammonium chloride) forms in the head space of the flask. Over the years I have found that there is a mid-point where a 'cr?me de menthe' effect occurs: a deep blue top layer, some re-precipitated copper hydroxide at the interface leading to the intensely green bottom layer, which is the anionic tetrachlorocuprate(ii) complex. If done correctly, this looks great. 

On further addition of the concentrated acid with swirling, the solution turns green, which is because of the formation of the tetrachlorocuprate(ii) complex. Finally, on addition of excess water, the green solution reverts back to the light blue of the cationic copper hexaqua complex. 

Dr Noel Dickson, University of South Florida, US