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

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The discussion centers on the unexpected low potential of the persulfate-sulfate couple in a lab setting, which is measured at approximately 0.5V against a Cu/CuSO4 reference, despite theoretical expectations of around 2V. The participants explore possible reasons for this discrepancy, including issues with the measurement setup or the reference electrode. There is confusion regarding the relationship between the measured potentials and the standard hydrogen electrode, with clarifications sought on experimental procedures and conditions. Ultimately, the conversation reveals a lack of consensus on the cause of the low potential, leading to the suggestion of consulting a professor for further insight. Understanding the limitations of the experimental setup is crucial for resolving the issue.
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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.
 
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2.1 V is a normal potential, what did you put in the solution and in what concentrations?
 
Hi Borek. The ions above are added via 0.1M K2S2O8 and 0.1M K2SO4. Thanks.
 
And what is the measured potential?
 
1.7V relative to Cu/CuSO4.
 
And what is the standard potential for the Cu/Cu2+ system?
 
It has a standard potential of +0.34V.
 
So what is the potential against hydrogen electrode?
 
It is 2.1V
 
  • #10
So where is the problem you've mentioned in the very first post?
 
  • #11
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).
 
  • #12
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.
 
  • #13
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.
 
  • #14
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 ;)
 
  • #15
Fair enough, I'll ask my Prof. Thanks anyway.
 

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