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

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Discussion Overview

The discussion revolves around the observed electrochemical potential of the persulfate-sulfate couple in a laboratory setting, specifically questioning why it does not reach the expected potential of approximately 2V relative to the hydrogen electrode. Participants explore various aspects of the experimental setup and measurements.

Discussion Character

  • Technical explanation
  • Debate/contested
  • Experimental/applied

Main Points Raised

  • One participant inquires about the reasons for not achieving a potential of ~2V for the reaction S2O8 2- + 2e- = 2SO4 2- based on lab findings.
  • Another participant asks for details about the solution composition and concentrations used in the experiment.
  • It is noted that the measured potential against a Cu/CuSO4 reference electrode is 1.7V, which raises questions about the accuracy of the measurements.
  • Participants discuss the standard potential of the Cu/Cu2+ system, which is +0.34V, and its implications for calculating the potential against the hydrogen electrode.
  • Confusion arises regarding the reported potential of 0.5V against the hydrogen electrode, leading to speculation about possible errors in measurement or equipment malfunction.
  • One participant expresses uncertainty about the chemical explanations for the observed results, suggesting a lack of clarity in the experimental outcomes.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the reasons for the discrepancy in the expected and measured potentials. There are multiple competing views regarding the accuracy of measurements and potential equipment issues.

Contextual Notes

Participants highlight potential issues with the reference electrode and the accuracy of the measurements, but do not resolve these uncertainties. The discussion reflects a complex interplay of experimental conditions and theoretical expectations.

Who May Find This Useful

This discussion may be of interest to those involved in electrochemistry experiments, particularly in understanding the behavior of redox couples and the factors influencing measured potentials.

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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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