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In electrochemistry, the Nernst equation is an equation that relates the reduction potential of a half-cell (or the total voltage, i.e. the electromotive force, of the full cell) at any point in time to the standard electrode potential, temperature, activity, and reaction quotient of the underlying reactions and species used. When the reaction quotient is equal to the equilibrium constant of the reaction for a given temperature, i.e. when the concentration of species are at their equilibrium values, the Nernst equation gives the equilibrium voltage of the half-cell (or the full cell), which is zero; at equilibrium, Q=K, ΔG=0, and therefore, E=0. It is named after the German physical chemist who first formulated it, Walther Nernst. The Nernst equation gives a formula that relates the numerical values of the concentration gradient to the electric gradient that balances it. For example, if a concentration gradient is established by dissolving KCl in half of a divided vessel that is full of H2O, and then a membrane permeable to K+ ions is introduced between the two halves, then after a relaxation period, an equilibrium situation arises where the chemical concentration gradient, which at first causes ions to move from the region of high concentration to the region of low concentration, is exactly balanced by an electrical gradient that opposes the movement of charge. ==Expression== The two (ultimately equivalent) equations for these two cases (half-cell, full cell) are as follows: : (half-cell reduction potential for ) : (total cell potential) where * is the half-cell reduction potential at the temperature of interest * is the ''standard'' half-cell reduction potential * is the cell potential (electromotive force) at the temperature of interest * is the ''standard'' cell potential * is the universal gas constant: * is the absolute temperature * is the chemical activity for the relevant species, where is the reducing agent and is the oxidizing agent. , where is the activity coefficient of species X. (Since activity coefficients tend to unity at low concentrations, activities in the Nernst equation are frequently replaced by simple concentrations.) * is the Faraday constant, the number of coulombs per mole of electrons: * is the number of moles of electrons transferred in the cell reaction or half-reaction * is the reaction quotient. At room temperature (25 °C), may be treated like a constant and replaced by 25.693 mV for cells. The Nernst equation is frequently expressed in terms of base 10 logarithms (''i.e.'', common logarithms) rather than natural logarithms, in which case it is written, ''for a cell at 25 °C'': : The Nernst equation is used in physiology for finding the electric potential of a cell membrane with respect to one type of ion. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Nernst equation」の詳細全文を読む スポンサード リンク
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