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Standard hydrogen electrode

Allesandro Volta (1745-1827) constructed the first chemical battery. He appears on the Italian 10,000 Lire note. Standard hydrogen electrode.

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Standard hydrogen electrode

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  1. Allesandro Volta (1745-1827) constructed the first chemical battery. He appears on the Italian 10,000 Lire note.

  2. Standard hydrogen electrode The standard hydrogen electrode (abbreviated SHE), also called normal hydrogen electrode (NHE), is a redox electrode which forms the basis of the thermodynamic scale of oxidation-reduction potentials.

  3. The scheme of the standard hydrogen electrode: • platinized platinum electrode • hydrogen blow • solution of the acid with activity of H+ = 1 mol kg-1 • hydroseal for prevention of the oxygen interference • reservoir through which the second half-element of the galvanic cell should be attached. This creates an ionically conductive path to the working electrode of interest.

  4. Eo = +0.80 V    Ag+1(aq)  + e-1  --->  Ag(s)  reduction

  5. Eo = -0.76 V    Zn+2(aq)  + 2e-1  ---> Zn(s)  oxidation

  6. Ecell = +1.56 V        2 Ag+1(aq) + Zn(s) ---- > 2Ag(s)  + Zn+2(aq)

  7. Hydrogen's standard electrode potential (E0) is declared to be zero at all temperatures. 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

  8. 2 H+ + 2e- --> H2 E = Eo - RT/2F ln P(H2) / a(H+)2 = Eo -(0.0591/2)log P(H2) / a(H+)2

  9. Example:

  10. The Saturated calomel electrode (SCE) is a reference electrode based on the reaction between elemental mercury and mercury(I) chloride. The aqueous phase in contact with the mercury and the mercury(I) chloride (Hg2Cl2, "calomel") is a saturated solution of potassium chloride in water. The electrode is normally linked via a porous frit to the solution in which the other electrode is immersed. This porous frit is a salt bridge. In cell notation the electrode is written as: Cl- (4M)|Hg2Cl2(s)|Hg(l)|Pt

  11. Hg2Cl2 + 2e = 2Hg + 2Cl- E0 = 241mV VS NHE(25°C)

  12. The Nernst equation is • The mercury concentration can be calculated from the solubility product • The concentration of Cl- is fixed by the solubility of potassium chloride, • The activity of a solid is 1. Two electrons are transferred per Hg2 dication. The Nernst equation thus becomes

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