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Interfering ions in a half cell

  1. Jan 19, 2012 #1
    1. The problem statement, all variables and given/known data

    The electrochemical cell:

    SCE // CrO4 2- (x M) / Ag2CrO4 (s) / Ag (s)

    is used for the determination of CrO4 2- . The standard reduction potential for the reaction:

    Ag2CrO4 (s) + e = 2 Ag + CrO42-

    is +0.446 V vs. NHE.

    (b) Which of the following ions would be expected to interfere with CrO4 2- quantitation if added to the analyte solution:

    NO3 - , Br - , S 2- , K + , Pb 2+

    3. To solve this I guess I need to understand how the half cell is working. So I think the electrons coming from the salt bridge react with Ag2CrO4 (s) at the surface of the Ag electrode to produce a deposit of Ag on the metal and CrO42- which stays in solution. However I am not sure where the interference will take place. I am not sure if adding K and Pb cations will just take in electrons and reduce current or if some of the anions interfere at the electrode surface. Any help would be greatly appreciated!
     
  2. jcsd
  3. Jan 20, 2012 #2

    Borek

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    Staff: Mentor

    Can you write Nernst equation for the measuring electrode?
     
  4. Jan 21, 2012 #3
    S.C.E: 1/2Hg2Cl2 (s) + e-⇌Hg(l)+Cl-

    Emeas=0.242V-0.05916*log[Cl-]

    Note: the half cell rxn in the original post should be
    Ag2CrO4 (s) + 2e- = 2 Ag(s) + CrO4 2- (aq)

    I don't see how the reference electrode matters in this case, since the interfering ions are in the other half cell.
     
  5. Jan 21, 2012 #4

    Borek

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    Staff: Mentor

    You are right, reference doesn't matter.

    Hint: note that while you wrote reaction as the one between silver chromate and metallic silver, it is in fact Ag+ that gets reduced, just its concentration near the electrode is controlled by the Ksp of Ag2CrO4.
     
  6. Jan 21, 2012 #5
    So, should I consider the Ksp values of the interfering ions with Ag+ and CrO42-

    Then the nitrate ion won't interfere since it is highly soluble and will stay in solution. However, the Ksp of S2- and Br- with Ag+ is less than the Ksp of the silver chromate reaction. Does this mean that these ions will mostly form a solid product when interacting with Ag+ effectively interfering with the half-cell reaction?

    When considering the cations, could I look at their standard reduction potentials and compare them to Ag+? I see that they are both lower and therefore, the reduction of Ag+ will be favoured, but lead chromate Ksp is also low, and will precipitate some CrO42- out of solution.

    So the ions that interfere will be S2-, Br- and Pb2+?
     
  7. Jan 21, 2012 #6

    Borek

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    Staff: Mentor

    That's what I would expect.

    Sorry, no time for more detailed discussion, and I won't be able to read the forum for a week.
     
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