| MadSci Network: Chemistry |
I have read over both the original question and the response - and did some digging as well (including trying the reaction out myself in one of our labs). I think that what we have, in this case, is a matter of semantics.
Copper(II) does appear to form a complex upon addition of citrate after a few moments. It is notable by the colour change that occurs in the solution. However, it is an equilibrium (as such reactions tend to be) that is both dependent upon the concentration of the citrate and the pH of the solution. The original answer said that the "citrate" is responsible for the hydroxide ions in solution and that is not correct as the amount of hydroxide would be minimal from hydrolysis induced by the acid. Further, addition of carbonate does result in the evolution of carbon dioxide, suggestive of an acidic solution. It would appear that, in the reaction, the basic character results in the citrate anion which complexes the copper(II) (possibly changing its redox potential) with the resulting solution then being the active species for the generation of copper oxide. At least, that is my reading of what is going on.
Technically, though, it is the copper that is generating the positive result and therefore, the citrate is not involved. Semantics.
Hope this helps.
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