|
The Standard hydrogen electrode (abbreviated SHE), is a redox electrode which forms the basis of the thermodynamic scale of oxidation-reduction potentials. Its absolute electrode potential is estimated to be 4.44 ± 0.02 V at 25 °C, but to form a basis for comparison with all other electrode reactions, hydrogen's standard electrode potential (''E''0) is declared to be zero at all temperatures.〔(IUPAC Gold Book )〕 Potentials of any other electrodes are compared with that of the standard hydrogen electrode at the same temperature. Hydrogen electrode is based on the redox half cell: :2H+(aq) + 2e− → H2(g) This redox reaction occurs at a platinized platinum electrode. The electrode is dipped in an acidic solution and pure hydrogen gas is bubbled through it. The concentration of both the reduced form and oxidised form is maintained at unity. That implies that the pressure of hydrogen gas is 1 bar and the activity of hydrogen ions in the solution is unity. The activity of hydrogen ions is their effective concentration, which is equal to the formal concentration times the activity coefficient. These unit-less activity coefficients are close to 1.00 for very dilute water solutions, but usually lower for more concentrated solutions. The Nernst equation should be written as: : or : where: * ''a''H+ is the activity of the hydrogen ions, ''a''H+=fH+ CH+ /C0 * ''p''H2 is the partial pressure of the hydrogen gas, in pascals, Pa * ''R'' is the universal gas constant * ''T'' is the temperature, in kelvins * ''F'' is the Faraday constant (the charge per a mole of electrons), equal to 9.6485309 *104 C mol−1 * ''p''0 is the standard pressure 105 in Pa ==Relationship between the normal hydrogen electrode (NHE) and the standard hydrogen electrode (SHE) == During the early development of electrochemistry, researchers used the normal hydrogen electrode as their standard for zero potential. This was convenient because it could ''actually be constructed'' by having "() a platinum electrode into a solution of 1N strong acid and () hydrogen gas through the solution at about 1 atm pressure". However, this electrode/solution interface was not entirely reproducible, so the standard for zero potential was later changed. What replaced it was a theoretical electrode/solution interface, where the concentration of H+ was 1''m'', but the H+ ions were assumed to have no interaction with other ions (a condition not physically attainable at those concentrations). To differentiate this new standard from the previous one it was given the name 'Standard Hydrogen Electrode'. 〔Ramette, R. W. "Outmoded terminology: The normal hydrogen electrode." Journal of Chemical Education, 1987; 64, 10, page: 885. , http://pubs.acs.org/doi/abs/10.1021/ed064p885 http://pubs.acs.org/doi/pdf/10.1021/ed064p885〕 In summary, NHE: potential of a platinum electrode in 1N acid solution SHE: potential of a platinum electrode in a theoretical solution (the current standard for zero potential) 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「standard hydrogen electrode」の詳細全文を読む スポンサード リンク
|