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Radhakrishnamurty Padyala's picture
Radhakrishnamur... | Tue, 08/05/2014 - 01:36

Topic for Discussion: Transport number

The concept of transport number says: t+ + t- =1. where t+ and t- are respectively the fractions of the current carried by the positive and negative ions. It is another form of the statement: I+ + I- = I, yet another form of which is: Q++ Q- = Q, where I and Q represent current and charge carried by the ions represented by the subscripts. The last equation may be stated in verbal form as: Q+ equivalents of positive ion combines with Q- equivalents of negative charge to give Q equivalents of charge.

The above equations of transport numbers violate the fundamental principle of chemistry, that states:

'one equivalent of any substance combines with one equivalent of any other substance to give one equivalent of a product'. This fundamental principle, therefore, demands: t+ = t- =1 or I+ = I- = I or Q+= Q- = Q.

In view of the above, the equations of transport number given above are untenable. The concept of transport number and mechanism of ionic conduction of electic currents needs a fresh look and thorough discussion by teachers of chemistry.

At a more fundamental level we will be dealing with the concept of dynamic equilibrium that is taught in chemistry curricula, during this discussion.

Joseph Lomax's picture
Joseph Lomax | Thu, 08/07/2014 - 11:20

t+ + t- =1 does not imply t+ = t- =1 (sic) (period, full-stop)

An immobile charge solid framework (glassy, semi-crystalline or crystalline; organic, inorganic or hybrid; stoichiometric or non-stoichiometric) and a mobile ion (cation or anion) will give t+ not equal t-.  For example in a lithium ion battery, the negative charge lies on the carbon in the anode which releases electron chage to the wire, then through the load to the cobalt oxide cathode.  The lithium ion goes through a electron insulating framework (a solid state lithium ion conductor, the separator) and balances the charge of the reduced cobalt oxide framework (intercalation).  The wire, cathode, separator and anode all rely on essentially unity-value transport numbers.  My article "Conducting Midshipmen", J.Chem.Educ, 1992, 69 (10), p 794-5  discusses a hopping mechanism for discussion of semiconductors, but it would work just as well for a description of solid state ion conductors. 

Radhakrishnamurty Padyala's picture
Radhakrishnamur... | Fri, 08/08/2014 - 00:25

yes, Certainly, t+ + t- =1 does not imply t+ = t- =1, we don't say it does, either. Yes, solid state ionics mostly use unit transport numbers as I noticed. There, the problem under discussion does not arise! Iwould like to see your JCE (1992) paper if you can send a pdf copy along with your other publications on the subject. We can then take this discussion forward in a more meaningful and useful (to others as well) fashion.

Joao Poco | Fri, 08/08/2014 - 14:45

This year I am teaching Reactor Engineering.

1) I miss some ilustrative movies about reactions occuring in research laboratory with the purpose, for example, of finding kinetics of the reaction in diferent kind of reactor.

2) I found a movie about Brigss - Rauscher, but it has more explanation of what is happening than reactions demonstration. I intend to propose a chalenge and I would like to have more image to show in class since I could not everyone in the lab to show this reaction.


Thank you

Joao Guilherme