Mutlak elektrot potansiyeli

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Mutlak elektrot potensiyeli, bir metalin evrensel bir referans sistemine göre (herhangi bir metal-solüsyon ara yüzeyi olmadan) ölçülen elektrokimyasal elektrik potensiyelidir.[1]

Tanim[değiştir | kaynağı değiştir]

Trasatti tarafından sunulan daha spesifik bir tanıma göre,[2] mutlak elektrot potansiyeli, elektrot bir metalin içinde bir noktada bulunan elektronik enerji ile (Fermi seviyesi) bu elektrodun içine daldırıldığı bir elektrolitin bir noktasında bulunan elektronik enerji arasındaki farka denir.

Mutlak elektrot potansiyelinin hassas düzeyde ölçümü zordur. Bu nedenle, standart hidrojen elektrodu (SHE) genel olarak referans potansiyeli olarak kullanılır. SHE'nin mutlak potansiyeli 4.44 ± 0.02 V (25 °C'de). Dolayısıyla, 25 °C'de herhangi bir elektrot için:

anlamları:

E: elektrot potansiyeli
V: birim volt
M: elektrot metal türü
(abs): mutlak potansiyel
(SHE): elektrot potansiyeli (standart hidrojen elektroduna görece)

Mutlak elektrot potansiyeli için literatürde alternatif bir tanım da önerilmiştir (tanım, mutlak yarı-hücre potansiyeli ve tek elektrot potansiyeli olarak da bilinmektedir).[3]


In this approach, one first defines an isothermal absolute single-electrode process (or absolute half-cell process.) For example, in the case of a generic metal being oxidized to form a solution-phase ion, the process would be

M(metal) → M+(solution) +
e-
(gas)

For the hydrogen electrode, the absolute half-cell process would be

1/2H2 (gas)H+(solution) +
e-
(gas)

Other types of absolute electrode reactions would be defined analogously.

In this approach, all three species taking part in the reaction, including the electron, must be placed in thermodynamically well-defined states. All species, including the electron, are at the same temperature, and appropriate standard states for all species, including the electron, must be fully defined. The absolute electrode potential is then defined as the Gibbs free energy for the absolute electrode process. To express this in volts one divides the Gibbs free energy by the negative of Faraday's constant.

Rockwood's approach to absolute-electrode thermodynamics is easily expendable to other thermodynamic functions. For example, the absolute half-cell entropy has been defined as the entropy of the absolute half-cell process defined above.[4] An alternative definition of the absolute half-cell entropy has recently been published by Fang et al.[5] who define it as the entropy of the following reaction (using the hydrogen electrode as an example):

1/2H2 (gas) → H+(solution) +
e-
(metal)

This approach differs from the approach described by Rockwood in the treatment of the electron, i.e. whether it is placed in the gas phase or in the metal. The electron can also be in another state, that of solvated electron in solution, as studied by Alexander Frumkin and B. Damaskin[6] and others.

Saptama/Olcum (determination)[değiştir | kaynağı değiştir]

The basis for determination of the absolute electrode potential under the Trasatti definition is given by the equation:

where:

EM(abs) is the absolute potential of the electrode made of metal M
is the electron work function of metal M
is the contact (Volta) potential difference at the metal(M)–solution(S) interface.

For practical purposes, the value of the absolute electrode potential of the standard hydrogen electrode is best determined with the utility of data for an ideally-polarizable mercury (Hg) electrode:

where:

is the absolute standard potential of the hydrogen electrode
σ = 0 denotes the condition of the point of zero charge at the interface.

The types of physical measurements required under the Rockwood definition are similar to those required under the Trasatti definition, but they are used in a different way, e.g. in Rockwood's approach they are used to calculate the equilibrium vapour pressure of the electron gas. The numerical value for the absolute potential of the standard hydrogen electrode one would calculate under the Rockwood definition is sometimes fortuitously close to the value one would obtain under the Trasatti definition. This near-agreement in the numerical value depends on the choice of ambient temperature and standard states, and is the result of the near-cancellation of certain terms in the expressions. For example, if a standard state of one atmosphere ideal gas is chosen for the electron gas then the cancellation of terms occurs at a temperature of 296 K, and the two definitions give an equal numerical result. At 298.15 K a near-cancellation of terms would apply and the two approaches would produce nearly the same numerical values. However, there is no fundamental significance to this near agreement because it depends on arbitrary choices, such as temperature and definitions of standard states.

Ayrıca bakınız[değiştir | kaynağı değiştir]

Kaynakça[değiştir | kaynağı değiştir]

  1. ^ IUPAC Gold Book - absolute electrode potential
  2. ^ Trasatti, Sergio (1986). "The absolute electrode potential: an explanatory note (Recommendations 1986)" (PDF). Pure and Applied Chemistry. 58 (7). ss. 955-966. 
  3. ^ Rockwood, Alan L. (1986-01-01). "Absolute half-cell thermodynamics: Electrode potential". Physical Review A. American Physical Society (APS). 33 (1): 554–559. Bibcode:1986PhRvA..33..554R. doi:10.1103/physreva.33.554. ISSN 0556-2791. PMID 9896642. 
  4. ^ Rockwood, Alan L. (1987-08-01). "Absolute half-cell entropy". Physical Review A. American Physical Society (APS). 36 (3): 1525–1526. Bibcode:1987PhRvA..36.1525R. doi:10.1103/physreva.36.1525. ISSN 0556-2791. PMID 9899031. 
  5. ^ Fang, Zheng; Wang, Shaofen; Zhang, Zhenghua; Qiu, Guanzhou (2008). "The electrochemical Peltier heat of the standard hydrogen electrode reaction". Thermochimica Acta. Elsevier BV. 473 (1–2): 40–44. doi:10.1016/j.tca.2008.04.002. ISSN 0040-6031. 
  6. ^ J. Electroanal. Chem., 79 (1977), 259-266