[Gargamel]

Hello

What I know:
A pH-meter is a precise voltmeter that measures the difference in charges across a membran (glass electrode), a so called potential difference. My issue is how to actually measure that?

My own explanation based on what I've read:
Somewhere Ive read that the reference electrode has a well defined pH, which I think of as a constant pH. So this electrode just works a set-point (zero) and completes the electrical circuit when combined with a glass electrode (the measuring electrode)
When the glass electrode is put into an solution of for example pH=4, H+ will start to adsorb to the glass membrane and Na+ or some other cat-ion will start to leave the innerside of the glass membrane until the equilibrium is achieved in the internal solution. By that I mean no more H+ gets adsorbed to the outerside of the membrane and no more cat-ions leave the innerside of the membrane.  So the change in equilibrium in the internal solution (must be affected by the cat-ions) is transformed to a change in pH by using Nernst equation.

Then I've read this:
http://www.ph-meter.info/pH-Nernst-equation

It says that pH is constant  on the inner side of the membrane (internal solution). If this is true how is H+ measured on the outerside of the membrane???

Question 2:
How does a voltmeter actually works? I thought is measures the speed of the electrons (Volt=Joule/Coulomb) between two points A and B. If that’s the case, where is the electrons in the pH-meter?

I get a bit confused when involving activity and chemical potential (SO abstract to me), so please if it can be explained without I appreciate it.
Please keep your answer as simple as possible, I have read some different articles and I still don’t get it.
If you have a link for "dummies" feel free to tell. I try and read that first, because my books doesn’t explain it very well or its just my brain...

Thanks for reading.

[Borek]

Ive read that the reference electrode has a well defined pH, which I think of as a constant pH

Then I've read this:
http://www.ph-meter.info/pH-Nernst-equation

It says that pH is constant  on the inner side of the membrane

Sounds the same to me.

As H⁺ ad/desorbe on the outer surface they change the charge density of the outer electrode surface - thus the potential difference between inside (constant) and outside (changeable) builds up. This potential difference is a function of pH.

How does a voltmeter actually works? I thought is measures the speed of the electrons (Volt=Joule/Coulomb) between two points A and B. If that’s the case, where is the electrons in the pH-meter?

No, it measures current that flows thorugh it - just like ammeter does. But voltmeter has a very high internal resistance (the higher the better), and the current flowing is very tiny (the smaller the better).

[Gargamel]

You wrote:
As H⁺ ad/desorbe on the outer surface they change the charge density of the outer electrode surface - thus the potential difference between inside (constant) and outside (changeable) builds up. This potential difference is a function of pH.


Ok. So charges will build up on the outer side of the membrane and pH will be constant on the innerside (is charges on the innerside also constant?, Please confirm 1). But there must be a physical device that measures the charges on the outerside of the membrane?

Look at this picture nr. 2 (combination electrode).
http://www.ph-meter.info/pH-electrode-construction

There is nothing here that measures the build up of charges on the outerside of the membrane. Apparently I just don’t get it 

The reference electrode (to the right inside of the combination electrode, Ag-wire) is in contact with the solution which we want to measure, because KCl will flow out of the junction (to the right). I thought this right electrode was just constant in pH? Please confirm 2

The glass membrane is also in contact with the solution we want to measure. If there is no change close to the Ag-wire (the left one) and we are not in contact with the outerside of the membrane how can measure a potential difference?

[Borek]

is charges on the innerside also constant?

Yes.

But there must be a physical device that measures the charges on the outerside of the membrane?

You have two charges separated by glass - that means there is a potential difference. You don't measure charge - you measure potential difference. Device you are looking for is called voltmeter.

Look at this picture nr. 2 (combination electrode).
http://www.ph-meter.info/pH-electrode-construction

I draw it myself.

There is nothing here that measures the build up of charges on the outerside of the membrane.

Yes there is - just not drawn. There is voltmeter attached to the wires that go up.

The reference electrode (to the right inside of the combination electrode, Ag-wire) is in contact with the solution which we want to measure, because KCl will flow out of the junction (to the right). I thought this right electrode was just constant in pH? Please confirm 2

It probably is, but it doesn't matter. Much more important is the solution in the glass electrode. Note, that these are separated solutions, and they are separated not by thin membrane, but by thick glass that acts as an insulator.

The glass membrane is also in contact with the solution we want to measure. If there is no change close to the Ag-wire (the left one) and we are not in contact with the outerside of the membrane how can measure a potential difference?

Because the circuit is closed - externally - through the voltmeter.

[Gargamel]

I wrote:
There is nothing here that measures the build up of charges on the outerside of the membrane.

You answer:
Yes there is - just not drawn. There is voltmeter attached to the wires that go up.

Yes, the voltmeter is attached to the 2 Ag-wires that goes up, 1 from glass electrode (left) and 1 from reference electrode (right). This potential difference between those 2 wires cannot be proportional with pH, because the pH is contant in the inner solution of the glass electrode and you wrote probably also contant at the reference electrode (right).
From your last answer I understood that when charges build up on the outerside of the membrane it doesn’t affect the Ag-wire in the inner solution of the glass electrode (please confirm)

In my head there must be a wire that is close to the outerside of the membrane, and a voltmeter that measures the difference between charges build up on the outer side (the not drawn wire?) of the membrane compared to charges on the inner side of the membrane (the left Ag-wire). This wire placed on the outerside of the membrane is that what you say is missing on the drawing? (Please confirm)
(I have not seen such a wire on any of the figures I have looked at, so I guess im wrong about it?)

If this is the case, there is 3 wires in total…

Oh, this annoys me.

Thanks for taking time to explain, I really appreciate it!

[Borek]

Don't think about the combination electrode for now, what you have is electrode that looks like that:

-----Ag-wire-covered-with-Ag-Cl------ internal solution | external solution -----Ag-wire-covered-with-Ag-Cl------

where | stands for glass membrane and ---- means wire. You have two wires sticking outside that you attach to voltmeter. Do you see how it works now?

Wire doesn't have to close to the membrane, it has to be connected through the conductor - that is, conducting solution. When it is close to the membrane, solution resistance is lower so it is easier to measure potential difference, but as long as the solution resistance is not unreasonably high, there is no problem with measurements.

[Gargamel]

It helps a bit.

I understand that solutions containing ions conduct electricity.

In your example there is 2 Ag-wires, 1 in internal solution and 1 in the solution we want to measure (you call it external solution). Connecting those 2 wires to a voltmeter will measure the potential difference across the membrane. THIS I understand.

I have tryed to show what I think happens on your drawings (hope its okay without interrupting copyrights). The Ag-wire in the glass electrode measures the H+ on the innerside of the membrane and the Ag-wire in the reference electrode measures H+ on the outer side of the membrane, because the solution which we want to measure conducts electricity. Is it correct?

[Gargamel]

Please can someone confirm my thinking

[Borek]

The Ag-wire in the glass electrode measures the H+ on the innerside of the membrane and the Ag-wire in the reference electrode measures H+ on the outer side of the membrane, because the solution which we want to measure conducts electricity. Is it correct?

You have correctly marked the circuit - the same one I have shown schematically earlier. Now you should try to understand what is potential in each point of the circuit - in some places it goes up, in some places it goes down. Basically there are three places where the potential changes - twice on the Ag/AgCl interface, and once on the glass membrane.

And Ag-wire doesn't measure H⁺.