Electrolyte

An electrolyte is a medium in which electric current is carried by the movement of ions, or a substance that supplies ions when dissolved in a solvent or melted. Electrolytes include aqueous solutions, molten salts, and ion-conducting solids. Their defining feature is ionic transport, rather than conduction primarily by electrons. In physiological usage, “electrolytes” commonly denotes charged mineral species in blood and other body fluids. (goldbook.iupac.org)

Formation and classification

Many ionic compounds become electrolytes when dissolved in water. Solvent molecules stabilize the separated ions, allowing them to move independently. Molecular substances can instead generate ions through ionization involving a reaction with the solvent. Hydrogen chloride, for example, reacts with water to form hydronium and chloride:

Thus, an electrolyte need not consist of ions before dissolution. (openstax.org)

A strong electrolyte produces ions essentially completely from its dissolved material; examples include soluble ionic salts and strong acids and bases. A weak electrolyte produces ions only partially, leaving a mixture of ions and un-ionized species. Weak acids and bases belong to this category. Their proportions are governed by chemical equilibrium. A nonelectrolyte, such as ethanol, dissolves without appreciable production of ions. Pure water conducts very poorly because its own ionization is slight. (openstax.org)

Electrolyte strength should not be confused with solubility or concentration. Solubility determines how much material enters solution, whereas strength describes ion formation from the dissolved material. A sparingly soluble salt can therefore supply only a small concentration of ions. Silver chloride illustrates this limitation: its dissolution equilibrium strongly restricts the amount of dissolved silver and chloride. Low conductivity alone does not establish partial ionization. (openstax.org)

Ionic transport and conductivity

Under an electric field, positive and negative ions migrate in opposite directions, both contributing to the transport of electric charge. An ion’s contribution depends on its charge and electrical mobility. IUPAC defines its ionic conductivity by

where $z_i$ is the ion’s charge number, $F$ is the Faraday constant, and $u_i$ is its electrical mobility. The ionic species and normalization must be specified when comparing reported values. (openstax.org)

The conductivity of a solution depends on the identities and concentrations of its ions, not merely on whether the dissolved substance is classified as strong or weak. Temperature also affects the measurement. Consequently, conductivity values require specified conditions, and comparisons between solutions must account for both composition and temperature. In water analysis, specific conductance is commonly reported in microsiemens per centimetre at a reference temperature of 25 °C. (water.usgs.gov)

Conductivity meters provide an aggregate measurement rather than an identification of individual ions. In natural waters, conductivity can serve as a proxy for dissolved-solids content, but the conversion depends on ionic composition and is not universal. Calibration, equipment maintenance, and temperature compensation are therefore important parts of laboratory and field measurement. Continuous conductivity records are also used to monitor changing water conditions. (pubs.usgs.gov)

Liquid and solid electrolytes

Electrolytic conduction is not restricted to water-based solutions. Molten salts conduct through mobile ions without a molecular solvent. Ionic liquids are salts that are liquid below 100 °C; they constitute a distinct class of electrolytes with applications in electrochemical analysis. Their properties differ from those of conventional aqueous or organic-solvent solutions. (goldbook.iupac.org)

A solid electrolyte is a solid whose predominant charge carriers are ions. Its structure can remain solid while selected ions move through it. Superionic compounds exhibit particularly rapid ionic motion, making them relevant to solid-state batteries and other electrochemical devices. Ionic conductivity in a solid therefore does not imply that the entire material has melted or that all its constituent ions are equally mobile. (goldbook.iupac.org)

Electrochemical applications

In an electrochemical cell, the electrolyte provides the internal path for ionic transport between electrodes. In a battery, electrons move through the external circuit while ions move through the electrolyte. These coupled movements permit charge balance during charging and discharging. The electrolyte is consequently an essential component of the cell, not simply a passive container for its other materials. (energy.gov)

Electrolyte composition influences ion mobility and interactions within the cell. Research on lithium-ion batteries examines how solvent coordination around lithium ions affects their movement. Other studies investigate reactions between liquid electrolytes and lithium metal, which can produce an interfacial layer and affect battery performance. Electrolyte selection thus involves both transport properties and chemical compatibility. (arxiv.org)

In analytical chemistry, a supporting electrolyte is added so that its contribution to conductivity and ionic strength is much greater than that of the electroactive substance under investigation. Its constituents are chosen to remain electrochemically inactive over the applied potential range. This definition is conditional: a substance suitable for one measurement may not remain inactive under different experimental conditions. (goldbook.iupac.org)

Physiological usage

In physiology, common electrolytes include sodium, potassium, calcium, magnesium, chloride, bicarbonate, and phosphate. These charged species participate in fluid balance, regulation of pH, and nerve and muscle function. Electrolyte balance is part of bodily homeostasis, and depends on the relationship between intake, losses, and physiological regulation. The term here usually refers to particular dissolved ions rather than to the entire conducting fluid. (medlineplus.gov)

An electrolyte imbalance means that the level of one or more electrolytes is too high or too low. Such changes can accompany altered water balance, gastrointestinal losses, certain medicines, or organ disorders. Laboratory tests can measure individual electrolytes or groups of them in blood and urine; different tests cover different sets of substances. (medlineplus.gov)