Interfering ions in a half cell

[SebPT]

Homework Statement

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 + CrO₄^2-

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 CrO₄^2- 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!

 

[Borek]

Can you write Nernst equation for the measuring electrode?

 

[SebPT]

S.C.E: 1/2Hg₂Cl_{2 (s)} + e^-⇌Hg_(l)+Cl^-

E_meas=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.

 

[Borek]

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.

 

[SebPT]

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 S^2- 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 S^2-, Br^- and Pb²⁺?

 

[Borek]

So the ions that interfere will be S^2-, Br^- and Pb²⁺?

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.