Passage of direct current in an aqueous solution containing dissolved mineral salts leads to a set of reactions and modifications that are referred to as electrolysis. The phenomenon of electrolysis consists in chemical decomposition of some substances in a solution thanks to the passage of the electric current.

The study of electrolysis makes it possible to understand the reactions that are produced under the electrodes applied to the skin.

This is because the skin is constantly in contact with a saline aqueous solution, which is the product of perspiration.

When both terminals of a source of current are immersed in a recipient containing absolutely pure water, i.e. water free of any dissolved substance (distilled water), there is no current flow. Pure water does not conduct current; it is an insulator. If a substance such as sugar is added to the water, current still does not flow. On the other hand, if salt (sodium chloride NaCl) is added, current is conducted. Some substances, of which salt is the typical example, make water that they are dissolved in a conducting medium. These substances are called electrolytes and they allow current flow because they dissolve in water as ions. This dissociation into ions is called ionization. The dissolved ions will be attracted by the pole of the opposite sign; this is ionic migration. It is ionic migration that explains current flow.

Positive ions are attracted to the negative pole, the cathode, and are called cations. Negative ions are attracted to the positive pole, the anode, and are called anions. On contact with the cathode, cations will participated in chemical changes; the same is true for the anions in contact with the anode.

NaCl dissolved in water is ionized into Na+ and Cl-. The Na+ is attracted to the cathode and the Cl- to the anode.

Overall, the cathode has given up an electron and the anode has gained an electron, in other words there has been a flow of electric current. An alkaline reaction occurs (NaOH sodium hydroxide is produced) in the cathode with release of hydrogen. And an acid reaction occurs (hydrochloric acid HCI is produced) in the anode with release of oxygen.

For the therapist, it is this alcaline reaction at the level of the cathode that is most important. This is because the sodium hydroxide that accumulates under the negative electrode may produce a chemical burn on the skin in contact with this electrode.

Therefore, the burn that may occur in iontophoresis treatment is above all a chemical burn due to the sodium hydroxide that accumulates at cathode level. The quantity of sodium hydroxide that accumulates depends on the density of the current (intensity divided by the surface area of the electrode) and on how long it is applied.