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Question: My question deals with qualitative analysis.  In the reaction of
sulfide with Fe@e2+, my text gives the instructions:  "Mix a few drops
of 0.65M FeSO4, 2ml of 6M acetic acid, and 5ml water; saturate with
H2S.  Then add about 5ml of 6M ammonia NH3."  I do not understand the purpose
of adding the acetic acid, and then counteracting the acid with the
ammonia.  It seems to me that FeS would precipitate out without the
addition of the acid.
Thanks,
Ray McKinney, TAMU

Answer:
I believe iron(II) will form a hydoxy complex with water:
        Fe2+  +  H2O  <=====>  [FeOH+] +  H+
That complex in the brackets can't react with the sulfide ion (S2-).
LeChatlier's Principle then comes in when you add the acetic acid.  By
increasing the H+ on the right side, the reaction shifts left leaving
free Fe2+.  You then saturate with H2S and add strong ammonia.  The
ammonia is basic enough to pull the proton off the H2S giving you free
S2-.  Even though the solution is now basic, the
above complex won't form because the equilibrium constant for the
formation of FeS is on the magnitude of 10^19.
-Joe Schultz