Standard hydrogen electrode


Standard hydrogen electrode

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 (E0) is declared to be zero at all temperatures.[1] 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 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 1 molar. The activity of hydrogen ions is their effective concentration, which is equal to the formal concentration times the activity coefficient. Activity coefficients are close to 1.00 for very dilute water solutions, but are usually lower for more concentrated solutions. The Nernst equation should be written as:

E={RT \over F}\ln {a_{H^+} \over (p_{H_2}/p^0)^{1/2}}

or

E=-{2.303RT \over F}pH - {RT \over 2F}\ln {p_{H_2}/p^0}

where:

  • aH+ is the activity of the hydrogen ions, aH+=fH+ CH+ /C0
  • pH2 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
  • p0 is the standard pressure 105 in Pa

Contents

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 is could actually be constructed by having "[immersing] a platinum electrode into a solution of 1N strong acid and [bubbling] 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 1m, but the H+ ions were assumed to have no interaction with other ion (a condition which is not physically attainable at those concentrations). To differentiate this new standard from the previous one it was given the name 'Standard Hydrogen Electrode'. [2]

In summary,

NHE: potential of a platinum electrode in 1N acid solution (historical standard, no longer in use)

SHE: potential of a platinum electrode in a theoretical solution (the current standard for zero potential)

Choice of platinum

The choice of platinum for the hydrogen electrode is due to several factors:

  • inertness of platinum (it does not corrode).
  • the capability of platinum to catalyze the reaction of proton reduction
  • a high intrinsic exchange current density for proton reduction on platinum
  • excellent reproducibility of the potential (bias of less than 10 μV when two well-made hydrogen electrodes are compared with one another)[3]

The surface of platinum is platinized (i.e., covered with platinum black) because of:

  • necessity to employ electrode with large true surface area. The greater the electrode true area, the faster electrode kinetics
  • necessity to use electrode material which can adsorb hydrogen at its interface. Platinization improves electrode kinetics

Nevertheless, other metals can be used for building electrodes with a similar function, for example, palladium-hydrogen electrode.

Interference

Because of the high adsorption activity of the platinized platinum electrode, it's very important to protect electrode surface and solution from the presence of organic substances as well as from atmospheric oxygen. Inorganic ions that can reduce to a lower valency state at the electrode also have to be avoided (e.g., Fe3+, CrO42-). A number of organic substances are also reduced by hydrogen at a platinum surface, and these also have to be avoided.

Cations that can reduce and deposit on the platinum can be source of interference: silver, mercury, copper, lead, cadmium and thallium.

Substances that can inactivate ("poison") the catalytic sites include arsenic, sulfides and other sulfur compounds, colloidal substances, alkaloids, and material found in living systems.[4]

Construction

Standard hydrogen electrode 2009-02-06.svg

The scheme of the standard hydrogen electrode:

  1. platinized platinum electrode
  2. hydrogen blow
  3. solution of the acid with activity of H+ = 1 mol dm−3
  4. hydroseal for prevention of the oxygen interference
  5. reservoir through which the second half-element of the galvanic cell should be attached. The connection can be direct, through a narrow tube to reduce mixing, or through a salt bridge, depending on the other electrode and solution. This creates an ionically conductive path to the working electrode of interest.

See also

References

  1. ^ IUPAC Gold Book
  2. ^ Ramette, R. W. "Outmoded terminology: The normal hydrogen electrode." Journal of Chemical Education, 1987; 64, 10, page: 885. doi: 10.1021/ed064p885, http://pubs.acs.org/doi/abs/10.1021/ed064p885 http://pubs.acs.org/doi/pdf/10.1021/ed064p885
  3. ^ D.T. Sawyer, A. Sobkowiak, J.L. Roberts, Jr., "Electrochemistry for Chemists, 2nd edition", John Wiley and Sons, Inc., 1995.
  4. ^ D.J.G. Ives, G.J. Janz, "Reference Electrodes. Theory and Practice", Academic Press, 1961.

Wikimedia Foundation. 2010.

Look at other dictionaries:

  • standard hydrogen electrode — noun An electrode with a standard electrode potential of zero; used as a standard against which other electrodes are measured. Syn: normal hydrogen electrode …   Wiktionary

  • hydrogen electrode — noun : an electrode composed typically of platinum black on platinum over which a stream of hydrogen is bubbled and that under specified conditions serves as the standard electrode with an assigned potential of zero to which all other electrode… …   Useful english dictionary

  • hydrogen electrode — a standard reference electrode with a potential of zero, used in pH measurements, consisting of a platinum black surface covered with hydrogen bubbles. [1895 1900] * * * …   Universalium

  • Palladium-hydrogen electrode — The Palladium Hydrogen electrode (Abbreviation: Pd/H2) is one of the common reference electrodes used in electrochemical study. Most of its characteristics are similar to the standard hydrogen electrode (with platinum). But palladium has one… …   Wikipedia

  • normal hydrogen electrode — noun Alternative name for the standard hydrogen electrode; an electrode with a standard electrode potential of zero; used as a standard against which other electrodes are measured. Syn: standard hydrogen electrode …   Wiktionary

  • Dynamic hydrogen electrode — A dynamic hydrogen electrode (DHE) is a reference electrode, more specific a subtype of the standard hydrogen electrodes for electrochemical processes by simulating a reversible hydrogen electrode with an approximately 20 to 40 mV more negative… …   Wikipedia

  • Hydrogen technologies — are technologies that relate to the production and use of hydrogen. Hydrogen technologies are applicable for many uses.Some hydrogen technologies are carbon neutral and could have a role in preventing climate change and a possible future hydrogen …   Wikipedia

  • Standard electrode potential — In electrochemistry, the standard electrode potential, abbreviated Eo, E0, or EO (with a superscript plimsoll character, pronounced nought), is the measure of individual potential of a reversible electrode (at equilibrium) at standard state,… …   Wikipedia

  • Standard electrode potential (data page) — The standard electrode potentials are electrode potentials of half cells at equilibrium. They can be used to determine the potential of an electrochemical cell or galvanic cell, or a position of equilibrium for an electrochemical (redox) reaction …   Wikipedia

  • electrode — noun /əˈlɛk.trəʊd,iˈlɛk.trəʊd/ a) the terminal through which electric current passes between metallic and nonmetallic parts of an electric circuit b) a collector or emitter of electric ch …   Wiktionary