Concentration cells

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Proton concentration cell is made by connecting with Pt wire two acid-base solutions and measuring emf. but wire allows redox reaction, changing conc of H+ in each solution. how is original pH found- with initial emf reading?
 

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  • #2
Borek
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Wire doesn't "allow redox reaction". For the reaction to take place you need a closed circuit. Then, speed of the reaction (and concentration changes) depend on the charge that flowed through the circuit. Controlling the charge you can control changes in the concentration.
 
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ok, closed circuit allows transfer of electrons between the solutions so redox reaction occurs. this changes conc of species, so how does emf not change down to 0. as, won't it disturb emf reading, if you control or interfere the natural redox flow of electrons? but if you dont redox equilibrium will reached then conc measurement was pointless...
 
  • #4
Borek
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If you allow the current to flow long enough, you will reach equilibrium, and the EMF will go down to zero. But it is not something that happens instantly.

To measure EMF you use voltmeter - do you know how it works?
 
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If you allow the current to flow long enough, you will reach equilibrium, and the EMF will go down to zero. But it is not something that happens instantly.

To measure EMF you use voltmeter - do you know how it works?
so statement "Fast electrode kinetics is desirable" is incorrect, we desire to see the initial emf. also isn't this destructive, we our method for reading concentrations is changing them...

I think I have grasped voltmeter concept from my book.
 
  • #6
Borek
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so statement "Fast electrode kinetics is desirable" is incorrect
Depends on the context. But generally speaking it is perfectly correct.

You are still missing the point. Just because the reaction CAN run fast, doesn't mean it WILL.

If you know how the voltmeter works, think what kind of currents flow through the voltmeter. Try to estimate concentration changes related to the current flowing through the voltmeter. How long will it take for the concentration change - and EMF drop - to be noticeable?
 
  • #7
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Depends on the context. But generally speaking it is perfectly correct.

You are still missing the point. Just because the reaction CAN run fast, doesn't mean it WILL.

If you know how the voltmeter works, think what kind of currents flow through the voltmeter. Try to estimate concentration changes related to the current flowing through the voltmeter. How long will it take for the concentration change - and EMF drop - to be noticeable?
Currents very tiny, so would take long for equilibrium to be reached. but still why fast electrode kinetics are desirable then?
 
  • #8
Borek
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Reaction creates charge on the electrodes. This charge is a source of potential difference. If the reaction is not fast enough to create enough charge in time (and while voltmeter draws a very tiny current, it still draws one, so the charge slowly disappears) the measured EMF will be lower than the real one (which should be measured using ideal voltmeter of infinite resistance - but such a device doesn't exist).

Technically what we measure is not the cell potential, but kind of a steady state potential of the cell under load. The higher the resistance the lower the load, the faster the reaction, the faster the charge buildup - the better the result.
 

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