Why does the Persulfate-Sulfate Couple not reach a potential of 2V in the lab?

[1question]

Hi. I have a lab that concerns this reaction as follows:

S₂O₈ ^2- + 2e^- = 2SO₄ ^2-

Can anyone give me a hint as to why this reaction would NOT (and indeed does not as found in the lab) have a potential of ~ 2V from the H+/H2 reference? What is preventing the reaction from reaching its maximum?

Attempts:

1) I've tried looking at http://hyperphysics.phy-astr.gsu.edu/hbase/tables/electpot.html and figuring something out but nothing seems to work. No extra information is given.
2) Is equilibrium reached?

Thank you.

 

[Borek]

2.1 V is a normal potential, what did you put in the solution and in what concentrations?

 

[1question]

Hi Borek. The ions above are added via 0.1M K₂S₂O₈ and 0.1M K₂SO₄. Thanks.

 

[Borek]

And what is the measured potential?

 

[1question]

1.7V relative to Cu/CuSO4.

 

[Borek]

And what is the standard potential for the Cu/Cu²⁺ system?

 

[1question]

It has a standard potential of +0.34V.

 

[Borek]

So what is the potential against hydrogen electrode?

 

[1question]

It is 2.1V

 

[Borek]

So where is the problem you've mentioned in the very first post?

 

[1question]

My question was "Can anyone give me a hint as to why this reaction would NOT (and indeed does not as found in the lab) have a potential of ~ 2V from the H+/H2 reference? What is preventing the reaction from reaching its maximum?" Perhaps I wrote it in a "too-wordy" style. I simply meant that I need help understanding why this reaction does NOT have a potential of ~ 2V from the H+/H2 reference (as measured, it actually has a potential of 0.5V).

 

[Borek]

Data you have listed so far suggest something else, I don't understand what you are talking about.

What was the procedure you followed? What have you really measured?

If you have measured the potential to be around 1.7 V against Cu/Cu²⁺, it is equivalent of measuring it as around 2 V against hydrogen electrode. Now you are telling us it had 0.5 V against hydrogen electrode. For me that means that either the signal hydrogen electrode is broken, or you are not telling us everything.

 

[1question]

Whoops, think I messed it up. Let me try again:
Given: rxn has a potential of ~2V relative to standard H⁺/H₂, Cu/CuSO4 reference electrode has a potential of about
+0.34 V relative to standard H+/H₂, so persulfate-sulfate solution should have a potential of around 1.7 V relative to the Cu/CuSO4 reference.
Measured: ~0.5V relative to Cu/CuSO4 reference.

 

[Borek]

OK, so you measured around 0.8 V against hydrogen electrode.

Honestly - no idea. Can't think of any reasonable chemical explanation, looks like some fubar case ;)

 

[1question]

Fair enough, I'll ask my Prof. Thanks anyway.