Electrochemistry Dictionary - M

m: The letter "m" when used as a prefix before a unit symbol indicates a multiplier of 10^-3. Symbol of "milli". E.g., mV = 10^-3 volt, one millivolt, one thousandth of a volt.
M: The letter "M" when used as a prefix before a unit symbol indicates a multiplier of 10⁶. Abbreviation of "meg" or "mega". E.g., MΩ = 10⁶ ohm, one megohm, one million ohms. (The symbol is the letter "M" followed by the "Greek capital omega" letter, some browsers unfortunately do not support this.)
μ: The letter "μ" when used as a prefix before a unit symbol indicates a multiplier of 10^-6. Abbreviation of "micro". E.g., μV = 10^-6 volt, one microvolt, one millionth of a volt.
mA: Symbol and abbreviation of milliampere (= 10^-3 ampere, one thousandth of an ampere).
μA: Symbol and abbreviation of microampere (= 10^-6 ampere, one millionth of an ampere).
Magnetoelectrochemistry: Electrochemical phenomena occurring under the influence of magnetic field.
Maintenance: The procedures which are required in order to keep a battery in proper operating conditions. They may include trickle-charging to compensate for self-discharge, addition of water to the electrolyte, etc.
Maintenance-free battery: A rechargeable battery which does not require periodic "topping up" (addition of water) to maintain electrolyte volume. See also sealed battery.
Mass transport: The phenomenon of movement (transportation) of mass (e.g., chemical compounds, ions) from one part of the system to another. Most of the time this occurs through diffusion and convection. Under special circumstances it can also occur through electromigration since the movement of the electrically charged ions also carries mass with them (e.g., ionic current through an ion-exchange membrane. The mass transport and the charge transport are typically "decoupled" in electrochemistry; that is, the reacting species and the charge carrying species are not necessarily identical. E.g., one would intuitively assume that during electroplating of copper from a solution of copper sulfate all the mass and charge required for the electrode reaction at the cathode would be carried by the copper cations in the solution. That is not the case at all. In the presence of a large excess of supporting electrolyte, all the current is carried by electromigration of the ions of the supporting electrolyte (both in the bulk solution and in the diffusion layer) while all the mass is carried by the copper ions by convection in the bulk solution and by diffusion in the diffusion layer. In the absence of any supporting electrolyte, all the mass is still carried by the copper ions, while the current is divided between the copper cations and the sulfate anions according to the ratio of their transport numbers (both in the bulk electrolyte and in the diffusion layer). The situation is maybe more self-explanatory in the case of an electrically neutral molecule reactant that cannot carry any current.
Mass-transport overpotential (polarization): See concentration overpotential.
Measuring electrode: See working electrode.
Mediated reaction: See indirect electrolysis.
Mediator: See indirect electrolysis.
Meg or mega: When used as a prefix before a unit name it indicates a multiplier of 10⁶. E.g., megohm = 10⁶ ohm, one million ohms. Symbol: "M".
Megohm: 10⁶ ohm, symbol: "MΩ" (one million ohms). (The symbol is the letter "M" followed by the "Greek capital omega" letter, some browsers unfortunately do not support this.)
Membrane: See separator.
Membrane electrode: An ion-selective electrode assembly terminating in an ion permeable (e.g., ion-exchange) membrane sensing element. The membrane separates the internal filling solution (that contains a fixed concentration of the ion to be detected) and the test solution. The potential across the membrane depends on the concentration ratio of the ion in the two solutions. The assembly also contains an internal reference electrode immersed in the filling solution, serving as an electrical contact with a stable potential. The potential of this assembly is then measured against an external reference electrode immersed in the test solution. See also Donnan potential.
Membrane potential: See Donnan potential.
Metal deposition/dissolution: A class of electrode reactions involving oxidation/reduction of a solid metal and its dissolved ion, e.g., if a copper metal rod is immersed in a copper sulfate solution, the copper cations can be cathodically reduced to copper metal, or the copper metal can be anodically oxidized to copper ions. Compare with a redox reaction where both the oxidized and the reduced species are in solution. The terms "electrodeposition" and "electrodissolution" are often used to describe these reactions. These reactions are used in many technologies, such as electroplating, electrowinning, and electrorefining. And also in electrogravimetry.
Metal displacement reaction: See cementation.
Metal plating: See electroplating.
Metal refining: See electrorefining.
mho: Alternative name of siemens.
micro: When used as a prefix before a unit name it indicates a multiplier of 10^-6. E.g., microvolt = 10^-6 volt, one millionth of a volt. Symbol: "µ".
microampere: 10^-6 ampere, symbol: "μA" (one millionth of an ampere).
microelectrode: A small electrode, with dimensions not larger than a few millimeters, and typically with dimensions of a small fraction of a millimeter.
microvolt: 10^-6 volt, symbol: "μV" (one millionth of a volt).
Migration: See electromigration.
milli: When used as a prefix before a unit name it indicates a multiplier of 10^-3. E.g., millivolt = 10^-3 volt, one thousandth of a volt. Symbol: "m".
milliampere: 10^-3 ampere, symbol: "mA" (one thousandth of an ampere).
millivolt: 10^-3 volt, symbol: "mV" (one thousandth of a volt).
Mixed potential: The electrode potential when two electrode reactions occur on the same electrode surface. The mixed potential has a value in between the equilibrium potentials of the two electrode reactions. The mixed potential is a steady-state phenomena, with the corrosion potential being a good example.
MΩ: Symbol and abbreviation of megohm (= 10⁶ ohm, one million ohms). (The symbol is the letter "M" followed by the "Greek capital omega" letter, some browsers unfortunately do not support this.)
Mobility, ionic: See ionic mobility.
mol: The measurement unit and symbol of mole.
mole: See gram-mole.
Molar concentration: See concentration.
Molarity: See concentration.
Molecular weight: The weight of a molecule of a compound that may be calculated as the sum of the atomic weights of its constituent atoms.
Molecule: The smallest physical unit of a substance that retains all the physical and chemical properties of that substance. It may consist of a single atom or of a group of atoms bonded together chemically.
Molten-carbonate fuel cell: A fuel cell that employs a molten, ionically conductive salt (carbonate) as electrolyte. Due to the high melting point of these salts, these fuel cells must operate at high temperatures.
Multimeter: Instrument that can be used for the measurement of more than one parameter. Typically, it can be used to measure current, potential, and resistance.
mV: Symbol and abbreviation of millivolt (= 10^-3 volt, one thousandth of a volt).
μV: Symbol and abbreviation of microvolt (= 10^-6 volt, one millionth of a volt).