Ionic and Electronic DC Conduction

An electrolyte is a substance with ionic DC conductivity. Intracellular and extracellular liquids contain ions free to migrate. In pure electroljrtes, the charge carriers are ions, and there is no separate flow of electrons—they are all bound to their respective atoms. Therefore, tissue DC currents are ionic currents, in contrast to the electronic current in metals. This is not contradictory to a possible local electronic conductance due to free electrons (e.g., in the intracellular DNA molecules). New solid materials such as organic polymers and glasses may contain an appreciable amount of free ions with considerable mobility therefore, the materials of an electrolytic measuring cell are not limited to liquid media. Some of these solid media show a mixture of ionic and electronic conductivity. 

Two current-carrying electrodes in an electrolyte are the source and sink of electrons—from electrons of the metal to ions or uncharged species of the electrolyte. 

The electrode is the site of a charge carrier shift, or a charge exchange between electrons and ions. 

At the very low migration velocities, there are no collision phenomena when charge carriers are stopped. The electronic conduction in the vacuum of a cathode ray tube (CRT) is very different. Friction is low and electron velocity is very high—on the order of thousands of meters per second (but with much fewer electrons engaged). When these fast electrons are stopped, there is a collision (e.g., with the phosphor plate that lights up in a CRT or the anode of an X-ray tube, which emits X rays). 

Electric current flow in an ionic solution is a more complex event than in a metal. Electron current implies no transport of substance an externally applied DC current can flow forever without changing the conductor. However, ion current implies a transport of substance. Therefore, an externally applied DC current cannot flow forever without changing the conductor. At first, changes will occur near the electrodes however, in a closed electrolytic cell with sufficiently long time, the change will spread to the bulk of the electrolyte. Accordingly, electrolytic long-duration DC conductivity is a difficult concept in a closed system. 

---