Standard hydrogen electrode, definition

Hydrogen Electrode an electrode at which the equilibrium (aq.) + jHj, is established. By definition, at unit activity of hydrogen ions and unit fugacity of hydrogen gas the potential of the standard hydrogen electrode h+/y//2 =... 

The most widely used reference electrode, due to its ease of preparation and constancy of potential, is the calomel electrode. A calomel half-cell is one in which mercury and calomel [mercury(I) chloride] are covered with potassium chloride solution of definite concentration this may be 0.1 M, 1M, or saturated. These electrodes are referred to as the decimolar, the molar and the saturated calomel electrode (S.C.E.) and have the potentials, relative to the standard hydrogen electrode at 25 °C, of 0.3358,0.2824 and 0.2444 volt. Of these electrodes the S.C.E. is most commonly used, largely because of the suppressive effect of saturated potassium chloride solution on liquid junction potentials. However, this electrode suffers from the drawback that its potential varies rapidly with alteration in temperature owing to changes in the solubility of potassium chloride, and restoration of a stable potential may be slow owing to the disturbance of the calomel-potassium chloride equilibrium. The potentials of the decimolar and molar electrodes are less affected by change in temperature and are to be preferred in cases where accurate values of electrode potentials are required. The electrode reaction is... 

Eq. (8)] represents by definition the zero point of the electrochemical potential scale (standard hydrogen electrode, often denoted SHE). 

The standard cell potential for the reduction of hydrogen ions to hydrogen gas is, by definition, 0.00 V. This potential is for the standard hydrogen electrode, SHE, which is the reference to which we compare all other cell potentials. All metals above hydrogen on the Activity Series will displace hydrogen gas from acids. (See Chapter 4) Metals below hydrogen will not displace hydrogen gas. 

Fig.1 Calculated free energy diagram for hydrogen evolution at a potential U = 0 V relative to the standard hydrogen electrode at pH = 0. The free energy of H+ + e is by definition the same as that of j - i at standard conditions. The free energy of H atoms bound to different catalysts is then found by calculating the free energy with respect to molecular hydrogen including zero-point energies and entropy terms (reprinted from Ref 83 with permission).

Stanski, electrodeposition, 1301,1303 Standard hydrogen electrode, 1108 see also hydrogen electrode potential, definition, 840, 1060, 1061 Steady state, 1147, 1212 current, 1248... 

Thus, a more appropriate question to ask is Is it possible to measure the absolute potential of the hydrogen reaction, /H+(abs) Actually it is possible. Remembering the definition of a standard hydrogen electrode potential (see Section 6.3.4), this was defined as the potential obtained when a metal comes in contact with a solution containing H+ under thermodynamically reversible conditions at unit activity, and H2 at 1 atm, at 298 K. As to the identity of the metal base, it can in principle be any metal at which it is possible to observe the reaction H2 H+ + e taking place at equilibrium. In practice, the metals used as substrates can only be noble metals because most other metals enter into equilibria with their own species in solution. Usually platinum is the metal chosen. 

Many half-reactions of interest to biochemists involve protons. As in the definition of AG °, biochemists define the standard state for oxidation-reduction reactions as pH 7 and express reduction potential as E °, the standard reduction potential at pH 7. The standard reduction potentials given in Table 13-7 and used throughout this book are values for E ° and are therefore valid only for systems at neutral pH Each value represents the potential difference when the conjugate redox pair, at 1 m concentrations and pH 7, is connected with the standard (pH 0) hydrogen electrode. Notice in Table 13-7 that when the conjugate pair 2ET/H2 at pH 7 is connected with the standard hydrogen electrode (pH 0), electrons tend to flow from the pH 7 cell to the standard (pH 0) cell the measured E ° for the 2ET/H2 pair is -0.414 V... 

Any surface (typically a piece of metal) on which an electrochemical reaction takes place will produce an electrochemical potential when in contact with an electrolyte (typically water containing dissolved ions). The unit of the electrochemical potential is volt (TV = 1JC1 s 1 in SI units).The metal, or strictly speaking the metal-electrolyte interface, is called an electrode and the electrochemical reaction taking place is called the electrode reaction. The electrochemical potential of a metal in a solution, or the electrode potential, cannot be determined absolutely. It is referred to as a potential relative to a fixed and known electrode potential set up by a reference electrode in the same electrolyte. In other words, an electrode potential is the potential of an electrode measured against a reference electrode. The standard hydrogen electrode (SHE) is universally adopted as the primary standard reference electrode with which all other electrodes are compared. By definition, the SHE potential is OV, i.e. the zero-point on the electrochemical potential scale. Electrode potentials may be more positive or more negative than the SHE. 

Such a solution is e. g. one with an exact concentration of 1,18 N — HC1 the IN — HC1 solution does not give the standard electrode since the hydrogen ion activity in it is only Oh+ = 0,811. On defining tho standard hydrogen electrode the nature of the acid and its concentration need not be stated. This is justified by the fact, that for calculations of potentials we need the definition of such an electrode only, and its design is less important. 

Inserting an inert but highly conductive metal electrode into an aqueous solution allows electrons to transfer both from the electrode to the solution and vice versa. A potential difference (voltage) builds up, which can be determined in a current-less measurement. Per definition, this potential is measured relative to the standard hydrogen electrode with P(H2) = 100 kPa, pH = 0, temperature = 20°C and a potential of... 

According to this definition the standard potential of the hydrogen electrode is the arbitrary zero of potential [cf. equation (7a)] electrode potentials based on this zero are thus said to refer to the hydrogen scale. Such a potential is actually the e.m.f. of a cell obtained by combining the given electrode with a standard hydrogen electrode it has, consequently, a definite thermodynamic value. For example, the potential (E) on the hydrogen scale of the electrode M, M (aM+), which is reversible with respect to the 2-valent cations M, in a solution of activity aM is the E.M.F. of the cell... 

The definition of Eh, and thus Pe, is given by the Nemst equation, in which the Eh of a solution is related to concentrations of aqueous redox couples at chemical equilibrium and the voltage of a standard hydrogen electrode ( ). For example, when concentrations of aqueous Fe and Fe " " are at equilibrium. Eh is defined as... 

According to the definition of electrochemical potential given in Eq. (13), it does not make sense to talk about absolute potential values because only differences in potential can be measured. Values of potentials are reported and tabulated with respect to a reference electrode. The potential of the reference electrode, by definition, is zero (there is no potential difference between two electrodes of the same type). The primary reference electrode by convention is the standard hydrogen electrode (SHE) Pt/H2 [14]... 

Because the silver electrode is on the right, the measured potential is, by definition, the standard electrode potential for the silver half-reaction, or the silver couple. Note that the silver electrode is positive with respect to the standard hydrogen electrode. Therefore, the standard electrode potential is given a positive sign, and we write... 

In the above equation, if a = 1, then E = eP. The standard potential of an electrode eP is the potential of an electrode in contact with a solution of its ions of unit activity. The standard potentials are always expressed against the standard hydrogen electrode (SHE), the potential of which is zero by definition. The standard potentials are a function of temperature they are usually tabulated for 25° C. Standard electrode potential is also called normal electrode potential. 

As already discussed, the standard hydrogen electrode (SHE) is the chosen reference half-cell upon which tables of standard electrode potentials are based. The potential of this system is zero by definition at all temperatures. Although this reference electrode was often used in early work in electrochemistry, it is almost never seen in chemical laboratories at the present time. It is simply too awkward to use because of the requirement for H2 gas at 1 bar pressure and safety considerations. 

Other abbreviations may also be used in tables, such as SHE for the standard hydrogen electrode or SCE for the saturated calomel electrode. The abbreviation NHE has been widely used for the normal hydrogen electrode , which is by definition identical to the SHE. It should nevertheless be noted that NHE customarily refers to a standard state pressure of 1 atm, whereas SHE always refers to a standard state pressure of 0.1 MPa (1 bar) in this review. 

The activity of electrons in equilibrium with the standard hydrogen electrode is arbitrarily defined to be unity and — log (a ) is defined as pE in a manner analogous to the definition of pH as — logP ]. Hostettler (1984) discusses the difficulty of associating pE with an electron activity. By substitution, an expression similar to the Nemst equation is obtained for pE. The term pE is — AG/ (2.303z T) ... 

The tendency for a specific substance to lose or gain electrons is called its redox or reduction potential. The redox potential of a conjugate redox pair is measured in an electrochemical cell against a reference standard, usually a standard hydrogen electrode. The redox potential of the standard hydrogen electrode is 0.0 V at 1 atm, by definition. Substances with a more negative... 

To determine the standard electrode potential of an element M we set up a cell as illustrated in Fig. 7,6. The element is placed in a solution of its ions at unit activity (standard state, based on the unit-molality definition) and coupled to a standard hydrogen electrode.f The potential of element M with respect to the platinum of the hydrogen electrode is called the standard electrode potential of M. (If the element M is positive with respect to the hydrogen electrode then the standard electrode potential of M is positive and vice versa.) If the metal in the cell is zinc we find... 

The overall reaction in a cell is made up of the reactions occurring at the electrodes. It is reasonable to assume that the overall emf of the cell is also made up of contributions from each electrode. To put this on a quantitative and comparative basis it is necessary to couple each electrode to one standard electrode. The Pt(s)IH2(g) 1 atmlHCl(aq) (unit activity) electrode is such a standard and by definition is taken to have zero electrode potential. Every other electrode potential is relative to this standard hydrogen electrode which Is always written on the left of the cell. 

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