MadSci Network: Chemistry
Query:

Re: on reduction potentials of cells

Date: Fri Aug 6 18:16:52 1999
Posted By: William Beaty, Electrical Engineer / Physics explainer / K-6 science textbook content provider
Area of science: Chemistry
ID: 933599165.Ch
Message:

Hi Uday! Are you asking how the half-cell potentials can be observed directly, rather than measuring the potential across the entire cell?

The half-cell potentials have physical reality. If a metal is placed in contact with an electrolyte, a static charge-imbalance appears, and an electrostatic field arises in the space around the objects. But how can we connect the metal wires of a voltmeter to the electrolyte without creating an unwanted half-cell potential at the contact point? One possible method would be to use an electrostatic voltmeter and to measure the electrostatic field via tight capacitive coupling, without making contact with the electrolyte.

I believe that a more common method is to construct a reference half-cell, determine its potential (i.e. calibrate it), and then use it to touch the electrolyte of the half-cell being measured. If the reference half-cell is within an ion-permeable container, then the container can be placed into the electrolyte of the half-cell being measured. A conventional voltmeter can then be connected to the metal plate of the reference half-cell and to the plate of the half-cell being studied. To discover the half-cell potential, subtract the potential of the reference half-cell from the reading on the voltmeter.

A similar problem arises in measuring the true potential across the dissimilar metals of a thermocouple. If we simply connect a voltmeter to the two wires of the thermocouple, the reading will be zero, since the metals of the voltmeter leads create opposing potentials and gives a reading of zero, even though the work-functions of the different metals cause a true difference of potential.


Current Queue | Current Queue for Chemistry | Chemistry archives

Try the links in the MadSci Library for more information on Chemistry.



MadSci Home | Information | Search | Random Knowledge Generator | MadSci Archives | Mad Library | MAD Labs | MAD FAQs | Ask a ? | Join Us! | Help Support MadSci


MadSci Network, webadmin@www.madsci.org
© 1995-1999. All rights reserved.