engleski

Electrolytes

An electrochemical cell consists of two electronic conductors (electrodes) connected via the external circuit (metallic conductor) and separated by an ionic conductor that is called the electrolyte. While the electrodes can be either pure metallic conductors, or mixed electronic and ionic conductors, the separator must be an electronic insulator to prevent a short circuit between the electrodes. In principle, electrolytes can be used in all three physical states: solid, liquid and gas. Solid electrolytes, e.g., RbAg4I5, are confined to special studies using solid-state electrochemical cells and sensors for gases such as oxygen, hydrogen, sulfur dioxide and carbon dioxide as well as for ion-selective electrodes. The most common solid electrolyte sensor is a pH electrode in which a glass membrane is an ionic conductor with sodium ions as charge carriers. Liquid electrolytes are either solutions of ionic salts, acids and bases, or molten ionic salts. Gaseous electrolytes are some ionized vapors, but the gases that exhibit mixed electronic and ionic conduction are not good separators. In analytical electrochemistry the most common electrolytes are solutions. The basis of ionic conduction is the mobility of ions. In liquid electrolytes it is the consequence of a three-dimensional random movement of ions. The characteristic of the random walk is that the mean distance <x> traveled by the ion is zero, but the mean square distance <x2> is proportional to time. Because of this movement, the concentration of ions is uniform throughout the volume of the electrolyte, in the absence of an electric field. Under the influence of a certain force, e.g., in an electric field, the ions acquire a non-random component of velocity in the direction of the force. The velocity developed under unit applied force is called the absolute mobility of the ion. The conventional, or electrochemical, mobility is the velocity of ions in a unit electric field. The relationship between the absolute and conventional mobility is: uconv = uabszie0, where zie0 is the charge on the ion. The electrolyte contains at least two types of ions with opposite charge. In liquids, all ions are mobile and contribute to the conductivity, provided that no ion pairing occurs. In solid electrolytes often only one of the ions is mobile. Ion conductivity of solid materials can result from a variety of different intrinsic and extrinsic defects. Details of these processes can be found in the specialized literature.

electrolytes

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