Silver identification (group 1)

Silver (Ag+) is identified as one of the group 1 cations together with lead and mercury (I).

If this is the first time you hear about the analysis of group 1 cations don't worry! You can find a simple summary at the following link.

We are going now to find out how to set up specific assays to highlight the presence of silver in an acqeous solution.

 

Just take in account that all these confirmatory tests are performed on an ammonia solution (check group 1 flow chart) where silver is dissolved as [Ag(NH3)2]+.

 

1) Confirmation with diluted nitric acid HNO3

In order to understand how this test work, first consider that silver is dissolved in water as diamine silver [Ag(NH3)2]+ . You should not forget also that the same solution is rich in Cl- ions. What happens if I add some drop of HNO3 to such a solution? A simple acid-base reaction takes place. NH3 is the base and HNO3 is the acid. Protonation of NH3 result in the dissolution of the complex (diamine silver) temporary freeing Ag+. Since the solution contains Cl- ions, as soon as the concentration of free Ag+  provoke the exceeding of the solubility product (Ksp = [Ag+][Cl-]we will get a white caseous precipitate of silver chloride!

If we draw a simple scheme of the two equilibria involved, the identification test gets even easier.

 

2) Reaction with potassium iodide (KI)

This test is (not surprisingly) the same that we performed for lead identification. On the same solution on whom we performed the confirmation test above, we add potassium iodide; if the solution contained silver (as diamine silver) we get a precipitate of silver iodide, very similar to lead iodide but even less solubleThanks its great insolubility (very low Ksp, around 10-16) we get the precipitate even if silver is in the form of diamine complex.

 

Chemically speaking, the concentration of Ag+ freed from the complex dissociation equilibria is enough to exceed the value of Ksp for silver iodide. Furthermore, as soon as AgI begins to precipitate, the dissociation of the complex is even more favored!  That's why we are removing Ag+ from the equilibria, and according to Le Chatelier principle, the equilibria will shift to the right (free Ag+). 

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