Reference Probes in pH and other electrochemical measurements

[fsonnichsen]

I am amused by how skillfully most of the texts I have avoid discussing reference electrodes in electro-chemical measurement-They simply start out by discussing the H2 and Calomel electrodes with no further explanation. Both have certain hazards and seem overly complicated.

So--why are these electrodes so commonly specified in measurements? I have some ideas on the constant replenishment inherit in bubbled H2 or liquid Hg but in a bigger picture-- let us take this back further a bit--Why not just have a gold wire as the reference?

Fritz

 

[Borek]

And what would be the voltage of the gold wire?

 

[fsonnichsen]

The voltage is relative to an arbitrary number, the H2/Pt electrode being defined as zero. After that it is highly empirical-I would assume a polarization error would be incurred in the gold electrode for the example I mention so it would be referenced to a "standard electrode" first. Disregarding this offset the voltage I would assume that the potential you ask for would be the electromotive potential for the "other" half-cell reaction. Thus if one had an AgCl/Ag electrode in AgCl solution on the positive side of a driving battery the potential would be that for 2Ag+ + 2Cl- --> 2Ag0 + Cl2, the chlorine be dispelled at the electron bearing Au electrode.. I am assuming here that one does not need to have two counter-opposed chemical reactions to complete the cell. I have thought some about this and believe it is correct.

Of more interest in my case is the pH glass electrode. Here there is no defined electrochemical reaction at the glass membrane (this whole thing 110 years after its invention is still highly controversial). When I studied (40 years ago) there was allusion to conduction in the glass. That has been proven wrong with tritium tracers. I think the current dogma is that it the potential is due to a polarization Helmholtz layer and a layer in the glass with an associated capacitive effect. The troublesome reference electrode in a commercial pH cell is often a KCl/Ag type. I assume there is something wrong in my reasoning that has led the industry to ignore using a direct noble metal electrode to supplant this but I cannot quite see why.

Thanks for you response
Fritz

 

[Borek]

am assuming here that one does not need to have two counter-opposed chemical reactions to complete the cell. I have thought some about this and believe it is correct.

You already have two opposite reactions (silver reduction, chloride oxidation), how can it be that they are both on the "positive side"?

But you have not answered my question - what would be the voltage of the golden wire (and why)?

 

[fsonnichsen]

Regarding my quoted comment I was not clear-there are two half-cell reactions but not necessarily separated in a conventional "reference probe" configuration like that typical in a pH sensor. They are not both on the positive side. The reactions are:
2Cl- -> Cl2 + 2e -1.36 V
Ag+ + e -> Ag 0.8 V(stp)
So you would need a potential of -0.56V on the gold wire to reduce the silver.

In the case of a pH probe the accepted thermodynamic interpretation is applied to the membrane potential-in that case E=E0+(RT/nF)log(C1/C2) where the H+ concentrations are C1,C2. E0 is empirically determined.

I am talking about 2 different processes here which is confusing but I think they both allude to the puzzle for me--of why reference electrodes more complex than an Au wire are used.

thanks for your response
Fritz

 

[Borek]

There is no Cl₂ produced. AgCl/Ag electrode works by reducing silver, there are no other reactions involved.

It is obvious you are missing something about how the whole thing works, but it is not clear where the problem is, you are trying to catch too many things at once.

Let's start with the more simple thing. Say, how are you going to measure the standard potential of Cu/Cu²⁺ reaction? Don't bother about membranes, doubts about how the pH electrode works, we are talking the very basic setup - Cu wire in a 1 M Cu²⁺ solution. How are you going to measure the potential of the Cu wire?

 

[fsonnichsen]

Well--I agree with that Let's forgo the pH probe for now-Sorry for the delay-busy here----at any rate:

If Ag is reduced only--where is the charge balance?

Regarding the Cu/Cu++ setup--one needs to drive it with 0.34V(stp). Plus an offset for the "complimentary" electrode. If I apply this potential to the Cu wire via a battery, (+) terminal to the Cu wire, then another wire would be applied to the solution as "reference", placing for example, a more noble wire in the solution and attaching to the (-) terminal. This reference could of course be a calomel or H2/Pt electrode. In the latter case the reference potential is arbitrarily defined as zero. I am suggesting referencing the an Au electrode. Is there a problem with this?

Thanks
Fritz

 

[Borek]

If Ag is reduced only--where is the charge balance?

You have a reaction taking place at the other electrode, don't you? You need some wires to close the circuit, but Ag⁺ is getting reduced and Cu is being oxidized, that's where the charges balance.

And (now I think that can be the part you are missing) you are measuring the voltage difference between potentials at which these two reactions take place.

Regarding the Cu/Cu++ setup--one needs to drive it with 0.34V(stp).

No need to apply any voltage for electrolysis, we are just talking about measuring the voltage at which the reaction takes place, so it is a galvanic cell ATM (but once we solve that, the same principle will apply to the electrolytic cell).

 

[fsonnichsen]

Yes-we have come full circle-I think we are agreeing on the same thing up to this point--"if one wants to measure a half-cell reaction in order to maintain charge balance a complementary reaction must be paired with it---if ions are removed from the solution by reduction, for example, some other ion must be oxidized and introduced into the solution".

My question is basically why the rather complex H2/Pt electrode, the poisonous calomel electrode, or the expensive silver/AgCl electrode are used for this? Would it not be just as effective use a simple copper(or other metal) wire as you suggest?

 

[Borek]

Would it not be just as effective use a simple copper(or other metal) wire as you suggest?

We are back at point one - what (and why) would be the potential of the wire? (and it was never my suggestion to use it!) What would be the reaction taking place?

 

[fsonnichsen]

Sorry for the misinterpretation - I thought you were suggesting the copper wire when you first mentioned it in this string.
The reaction is Cu++ + 2e- ---> Cu at 0.34 V

Tell me if am understanding you wrongly but I think your point is that for measuring a half-cell reaction, you need to have some counteropposed one? I agree with this-I am just questioning the choice of electrodes-

thanks
fs

 

[Borek]

The reaction is Cu++ + 2e- ---> Cu at 0.34 V

That's true for the standard state, yes - but how are you going to make sure that you have a standard state solution (with a well defined activity of copper ions)?

Tell me if am understanding you wrongly but I think your point is that for measuring a half-cell reaction, you need to have some counteropposed one?

Yes, but that's just an effect of the charge balance, nothing else.

I agree with this-I am just questioning the choice of electrodes-

It wasn't clear from your earlier posts (at least to me, English is my second language so I could misunderstand some nuances of what you wrote). If it is just about choice of electrodes the answer is simple and will follow from the question I posted above

 

[fsonnichsen]

Borek
I thought I had replied to you some time ago but I see it never posted. Resending:
The Cu++ reaction may not be the best choice for a reference electrode. It is probably compromised by an ill-defined activity coefficient especially if other ions are present. I expect that the poisonous Hg based electrode, the cumbersome H2 electrode and the noble Ag/AgCl electrodes are possessed with some type of resilience against drift and other effects that makes them more accurate than a plain wire one like the Cu/Cu++. However I have never read any elaboration on this in any of my books here.

thanks
fritz

P.S. your English is quite good!

 

[Borek]

I expect that the poisonous Hg based electrode, the cumbersome H2 electrode and the noble Ag/AgCl electrodes are possessed with some type of resilience against drift and other effects that makes them more accurate than a plain wire one like the Cu/Cu++.

Exactly - they are designed to be as stable as possible, as repeatable as possible and as easy to prepare as possible (even if they don't always look this way, others are even worse).

Hydrogen electrode is rarely used because it is difficult to maintain (requiring gaseous hydrogen of known pressure) and costly (platinum electrode).