[Big-Daddy]

This is coming off the other thread at http://www.chemicalforums.com/index.php?topic=74324, I thought it is too off-topic, but it is troubling me.

I want to ask in relation to the galvanic cell Pt (m) | H2(g) (1 bar) |NH3 (0.0212M), NH4Cl (aq) (0.0212M) | AgCl (s), Ag (m) :

Why is the reaction so slow - because of the voltmeter and set-up, or just because it's a really slow redox reaction? I know for redox reactions that are fast in solution we can use a salt bridge to separate the cells and then it will be slow for our measurement. But in the case above, there's no salt bridge - the reaction happens in one solution, no need for current to flow across the voltmeter because electrons can be exchanged between the ions directly, so why is it slow enough that we don't bother correcting for it or separating the solutions by salt bridge? Because it's a naturally slow redox reaction - or have we done something in the set-up of a galvanic cell to slow it down?

[Borek]

Even if there is no salt bridge mentioned, reference electrodes (And Ag/AgCl electrode is a reference) are always isolated from the solution to not get contaminated and to not react directly. In tjhsi particular case it is not reduction of AgCl with hydrogen that is the most likely problem, but rather dissolution of AgCl with ammonia.

[Big-Daddy]

I see. So the reduction of AgCl with H₂, would it happen reasonably fast if you dipped the AgCl into water without isolation? Or is the reaction slow enough of itself that, if it was the only reaction of the AgCl, you would not need to worry. I know the complex formation of Ag(NH₃)₂⁺ happens fast enough.

How is this isolation done? After all I would have thought the electrode still needs to be in contact with the solution otherwise there is no way of building up the potential difference being measured. Also, why only for reference electrodes - it seems like this would be necessary for all electrodes (unless the cell reaction would be really slow anyway) being used to get EMF readings?

[Borek]

No idea about how fast the hydrogen reduction would be.

Reference electrodes are often kept in separate cells, and connected with some kind of a salt bridge. Salt bridge can be realized in many ways, in combined pH electrodes AgCl/Ag wire is just kept in a gel, so the diffusion is very slow and there is no mixing: http://www.ph-meter.info/pH-electrode-flowing-gel

I never said other electrodes are not isolated - when necessary they can be, with a salt bridge. All I said was that reference electrodes should always be. Otherwise they can't be trusted. They should be isolated and never overloaded (or more precisely: never loaded at all, the lower the current flowing through teh reference electrode, the better; that's why we often use three electrodes setup in electrochemistry).

[Big-Daddy]

Ah I see, think I got it.

Can you explain what my book means by the following statement:

"It is also desirable that the potential term for {E of the reference electrode} [paraphrased by me] attains its thermodynamic equilibrium value rapidly. In other words the potential determining equilibrium should display fast electrode kinetics [sic]."

I thought fast electrode kinetics would lead to high current and if equilibrium was reached then E=0, which we definitely don't want because our measurement is then useless ...

[Borek]

I thought fast electrode kinetics would lead to high current

Looks like you still don't understand how and when the cell works. There will be no current when the circuit is open (or closed, but through a high resistance; technically on open circuit is equivalent to the one closed through infinitely high resistance). Still, to get to the equilibrium potential electrodes have to be charged, and that means both half reaction have to go - not far, just to charge the electrodes to the equilibrium potential. We want them to reach that point fast.

[Big-Daddy]

I'm being confused by your meaning by "equilibrium potential". I appreciate that the electrodes need to be charged to register a potential difference. So is this "equilibrium potential" the point of reaction extent at which the electrodes will be charged enough to register the potential difference, so that the EMF measured is at "equilibrium" (Nernst equation can be used I guess)?

[Borek]

Equilibrium potential means: all reaction went to (possible*) completion, all electrodes are charged and in equilibrium with their redox systems.

*if you short circuit the cell, reaction can proceed further, but we don't allow it.

Fast kinetics in this context is connected with the fact when you disturb the cell (for example by shorting it for a limited amount of time)), it has to reach a new equilibrium state. If the system is fast, this equilibrium can be reached quickly, if it is not, we have to wait before being able to take the new measurement. Otherwise we will see a drifting potential, which will converge to the new equilibrium value.