This question was asked here: pH probe bulb - what is happening within the glass? and How does a pH glass electrode work?.
But, the mechanism remains unanswered. How does a potential difference between the inner and outer surfaces of the glass bulb result in a measurable difference between the two electrodes? These are two different electrical systems. I am unable to find in the literature how the the two systems are connected.
- There is an electrolytic solution connecting the electrodes through openings in the tubes housing each electrode.
- A very small electric current flows between the reference electrode to the pH electrode through a high value resistor for voltage measurement. The electrodes maintain ion balance through the chemical reactions occurring at each electrode and the movement of ions through the openings.
From a-cyclohexane-molecule answer in pH probe bulb - what is happening within the glass?:
A few side remarks.
- Equilibrium is reached when the favorable binding of protons to the glass surface is balanced by the unfavorable electrostatic repulsion and chemical potential gradient that result from diffusion into the hydrated gel layer. This provides an equation relating the potential difference to the pH of the solution and allows for pH measurement.
- Something has to be able to move through the inner layer of the glass membrane to conduct a current and hence allow for a measurement of the potential difference. It turns out that sodium ions can move through this inner layer, but only sluggishly---the resistance of the glass membrane is about 108Ω.
A hypothesis is:
- When H+ invades the porous glass, the side with the more H+ can repulse H+ out of the opposite side. An acidic solution being measured, ejects H+ out of the inner bulb surface and an alkaline solution allows more H+ to enter into the inner glass surface.
- The positive ion concentration near the electrode next to the inner glass surface changes.
- This change in positive ion concentration can not be compensated by the chemical reactions at each electrode that are maintaining a electrically neutral solution.
- The change in positive ion concentration near the pH electrode tip is causing a change in the pH meter potential.
- This change in the pH meter potential results in a measurable difference in voltage between the electrodes.