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

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