Understanding the Role of a Voltmeter in Electrode Potential Measurement

[Dario56]

TL;DR Summary

How Does Voltmeter Sense/Measure All Potential Differences Present in an Electrochemical Cell?

Voltmeter is an instrument which measures electric potential difference between two points.

When measuring electrode potential of some redox system (vs SHE for example), it is said that voltmeter reading contains sum of all potential differences present in a cell. This includes all electrode/electrolyte potential differences, contact potential differences between electrodes and probes of voltemter and possibly liquid junction potential. Probes of voltmeter are connected with two electrodes.

However, since voltmeter measures potential difference between two points, how can it sense sum of all potential differences in a cell if it is connected only between two electrodes and not all potential differences are created at these two points?

 

[gleem]

Although I am not an expert on such measurements, I know one does not use a voltmeter since first a voltmeter draws current from a cell introduces the voltage drop due to internal resistance and second, you cannot measure a contact potential by using a contact type probe. There is a probe called a Kelvin probe that can be used to measure contact potential since it does not need to make physical contact. As far as the overall emf of a cell one uses a potentiometer circuit that does not draw current from the cell.

 

[.Scott]

When you connect a voltmeter to the battery terminals, you get a single voltage.
Assuming that the battery cells are arrange in series, that voltage is the some of the voltages from each of those cells. Each cell may in turn consist of several layers and interfaces arranged in series. The sum of those components will be the total voltage from that cell.

 

[Dario56]

Although I am not an expert on such measurements, I know one does not use a voltmeter since first a voltmeter draws current from a cell introduces the voltage drop due to internal resistance and second, you cannot measure a contact potential by using a contact type probe. There is a probe called a Kelvin probe that can be used to measure contact potential since it does not need to make physical contact. As far as the overall emf of a cell one uses a potentiometer circuit that does not draw current from the cell.

Thank you. Voltmeter isn't a best option and we can say that some other voltage measuring device can be used like an potentiometer as you mentioned.

 

[Dario56]

When you connect a voltmeter to the battery terminals, you get a single voltage.
Assuming that the battery cells are arrange in series, that voltage is the some of the voltages from each of those cells. Each cell may in turn consist of several layers and interfaces arranged in series. The sum of those components will be the total voltage from that cell.

Thank you. I am not sure if all potential differences created are in fact arranged in series. How do we know that?

 

[.Scott]

Most common batteries (AAA-D cells and Auto/Marine batteries) are arranged in series.
You need a large cell for current, but cell size is not a practical limit - some are hundreds of pounds.
But for more than just a couple of volts, you need a series arrangement of cells.

 

[hutchphd]

Assuming that the battery cells are arrange in series, that voltage is the some of the voltages from each of those cells. Each cell may in turn consist of several layers and interfaces arranged in series. The sum of those components will be the total voltage from that cell.

One should be too sanguine about knowing what goes on inside a sealed battery. Typically it is useful and sufficient to model a voltage source by its Thevenin equivalent: a perfect voltage source in series with a resistor. To some degree this seems to match the physics because the the Thevenin voltage aligns with thee electrochemistry involved. But don't get to fat and happy.