In this article we will understand how to set up tests to identify mercury when this had been previously precipitated as Hg2Cl2, mercurous chloride.
During group 1 analysis mercury (I), if present, is indeed precipitated as Hg2Cl2 after the treatment with diluted HCl. If this is the first time you face cations analysis don't worry! You can find a brief summary at the following link.
Confirmation tests for mercury (I)
Suppose you are in front a white precipitate; considering that we are dealing with group 1 analysis, this has to be made up with chlorides of the group 1 cations. These are silver (Ag+), lead (Pb2+) and finally mercury (I).
Our goal is to perform some sort of reaction that without any doubt certificate the presence of mercury (I) in such a precipitate. Let's get this done!
- Reaction with ammonia (NH3)
This is a very interesting reaction that will certainly identify mercury (I), where present, in a precipitate. When treated with ammonia, mercurous chloride undergoes a particular redox reaction called "dismutation". At the same time Hg22+ is oxidized (to Hg2+) and reduced (to elementary mercury Hg°)
In particular the products of the dismutation will be metallic mercury Hg ° and mercuric amidochloride, HgNH2Cl (white precipitate).
- Treatment with aqua regia
This test is actually a further confirmation of the previous one. Treating the precipitate of mercuric amidochloride (white) and metallic mercury with aqua regia (a mixture of concentrated HCl and concentrated HNO3 ratio 3:1) all the mercury will be unequivocally oxidized to Hg(II). Alkalinizing this solution with diluted NaOH we will get a yellow precipitate of mercuric oxide, HgO.