Electrolyte

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In chemistry, an electrolyte is any substance that can transport electric charge between two electrodes that have a voltage difference between them. Most electrolytes are aqueous (watery) solutions of easily dissociating salts, for instance table salt (NaCl) in water. In water NaCl dissociates into the cation Na+ and the anion Cl and these two kinds of ions take care of the charge transport between the electrodes. The negative ion (anion) moves to the positive electrode (the anode) and the positive ion (cation) moves to the negative electrode (the cathode). At their respective electrodes the ions will usually discharge, giving the corresponding uncharged substance. When the ions are atomic the corresponding substance is elemental. Most free elements are either gaseous and escape from the solution, or are solid and are deposited on the electrode. This possibility of discharge distinguishes electrolytes from the more common conductors of electric currents, such as copper wires, where electrons, not ions, transport charge between a voltage difference.

Bases, as NaOH (sodiumhydroxide) and acids, as H2SO4 (sulfuric acid), split into positive and negative ions in water—their solutions are also electrolytic, meaning that they can transport charge. Note that ions are not necessarily singly charged, i.e., they can contain more than one positive or negative elementary charge. For instance, sulfuric acid splits into 2H+ and SO42−.

Other polar solvents, as for instance methanol, containing dissolved ions, but also molten salts, can transport charge as well. These substances are then also named electrolytes. An example of a solid electrolyte is AgI (silver iodide).

Electrolytic solutions can result from the dissolution of some biological (e.g., DNA, polypeptides) and synthetic polymers (e.g., polystyrene sulfonate), these compounds are termed polyelectrolytes.

Applications

Electrolytes are used in devices where the chemical reaction at the interface of an electrode and the electrolyte yields useful effects.

  • In batteries, two metals with different electron affinities are used as electrodes; electrons flow from one electrode to the other outside of the battery, while inside the battery the circuit is closed by the electrolyte's ions. Here the electrode reactions convert chemical energy to electrical energy.
  • In some fuel cells, a solid electrolyte connects the plates electrically while keeping the hydrogen and oxygen fuel gases separated.
  • In electroplating, the electrolyte deposits metal onto the object to be plated.
  • In electrolytic capacitors the chemical effect is used to produce an extremely thin dielectric or insulating coating, while the electrolyte layer behaves as one capacitor plate.
  • In some hygrometers the humidity of air is sensed by measuring the conductivity of a nearly dry electrolyte.
  • Hot, softened glass is an electrolytic conductor, and some glass manufacturers keep the glass molten by passing a large current through it.