A direct ionic current definitely requires electrochemical reactions at the electrodes. However, most slag conductivity measurements are done with AC, in which case charge is stored in and released from the interfacial capacitance of the electrodes, without need for an electrochemical reaction.

Also, the voltage for electrochemical reactions is rather modest. For example, for CaO = Ca + 0.5 O2, the standard potential is -2.2 V at 1873 K - so even with a DC measurement, the current can be Faradayic (i.e. involving electrochemical reactions at the electrodes).

If the applied voltage is less than the voltage required for electrochemical reactions, the apparent conductivity would be low - but only if a two-electrode method is used. With a four-electrode method, the central two electrodes measure the voltage drop in the slag only; this voltage drop does not include the polarization at the electrodes that pass the current into and out of the slag. Finally (I think), many slags contain some dissolved water, with a much lower voltage required - the standard potential for H2O = H2 + 0.5O2 is only -0.74 V at 1873 K.

I hope this helps. The original paper by Bockris et al. is definitely worth reading.

Here is the reference:

Bockris, J. M., Kitchener, J. A., Ignatowicz, S., & Tomlinson, J. W. (1952). Electric conductance in liquid silicates. Transactions of the Faraday Society, 48, 75-91.

Thank you for your reply P. Christiaan Pistorius, I was hoping that my question would catch your eye. My question relates to DC, so thank you for the insights. I will get the Bockris article. It's in my "to read" list, but now it jumped to the top.

Dear Johan,

my background are transport measurements in solids. There I have to do with electrons and holes. The role of the holes take in your case the ions.

The conductivity sigma consists of 2 contributions:

sigma = 2e*µion*p + e*µelectron*n

with µ the mobility of ion and electron ,resp., and p or n the concentration of ions or electrons.

The mobility of ions probably depends on the electric field and the mass of the ion or electron (µ ~ 1/m). Due to the fact, that ions are at least 3 orders in magnitude heavier than electrons, the current probably is determined by electrons. Nevertheless, the concentration of ions or their lifetime could be much higher.

Therefore, I propose to decide this question experimentally. You carry out a Hall measurement. Therefore are needed two additional electrodes and a magnet. The electrodes (plates) are introduced in the liquid vertical to the field plates. The magnetic field must be orientated vertical to all the plates. For the first time a permanent magnet is sufficient to decide qualitatively the sign of the Hall voltage (compare to the tacho sensor at bycicle).

The sign of the Hall voltage decides whether you have ion or electron conductivity. Probably, a reversion of the Hall voltage at low fields can be observed. In this case, your question is answered.

With Regard

R. Mitdank

Dear Rüdiger Mitdank

Thank you for your reply. I will study it further and incorporate it in my investigation. Could you perhaps recommend books or articles on this topic that I can refer to as well?

Thanks again

Johan

Dear Johan Zietsman ,

both mechanisms, ionic and/or electronic are possible in your case.

The interpretation[1] of your results(?) depend on many experimental parameters, we need for our basic understanding. So, let me ask some of the major experimental details:

1. what are the electrodes' materials[2] ?

2. The DC-measurements are at steady state (or short time, transient) ?

3. What is the time-line of your signal(s) and your i-V(s) ?

4. What are the OCV(s) values before and after your DC-measurements ?

1. Ion_transport_number https://en.wikipedia.org/wiki/Ion_transport_number

2. Ion-selective electrodes https://en.wikipedia.org/wiki/Ion-selective_electrode

Dear Ioannis Samaras

Thank you for your reply. I mostly do computational modelling on industrial processes, and no laboratory experiments. My question was not related to experiments, but to an electrical furnace of several MW power rating. I therefore cannot answer any of your questions.

Thank you for the useful links.

All the best

Johan