Nernst Equation ph

Nernst Equation ph    


The Nernst equation expresses the quantitative relationship between the standard redox potential of a redox couple given potential observed and the ratio of concentrations between the electron donor and acceptor. The Nernst equation is presented as:

          RT [acceptor]Eh = E ° + --------- --- ln (1)
          nF [donor]

where E ° = standard redox potential at pH = 7.0,
     T = 298 ° K and all concentrations are at 1.0 M.
     Eh = observed electrode potential.
     R = gas constant, 8.31 J / ° mol.
     T = absolute temperature in ° K.
     n = number of e-transferred.
     F = Faraday constant, 23.062 cal / V or 96.406 J / V.If T = 298 ° K, the term 2303 (RT / nF) has a value of 0.059 mV / decade when n = 1 and 0.03 when n = 2.The equilibrium potential of an ion through an interface is described by the equation:

       C2 RTE ° = ---- ln ---- (2)
       ZF C1

where RT / SF = 25.3 mV, if T = 293.15 ° K and Z = 1.
     C1 and C2 are the concentrations at each side of the interface.Since the Nernst equation is universally applicable for steady state potentials generated by a concentration difference across any type of interfaces (such as the electrochemical cells, artificial and biological membranes or selective electrode), it is possible to demonstrate validity testing predictions derived from this equation. This can be done easily by using a cation-selective electrode.For an ion selective electrode for a univalent as H +, K + or Na +, equation (1) can be written as follows:E ° = A + 0.059 log C1 (3)where C1 is the concentration external to the electrode, A is a constant which includes terms of internal concentration of the electrode, the potential exists asymmetric through its membrane, which is characteristic for each electrode and the reference electrical potential.As can be seen, equation (2) has the form of the equation of a line of slope m = - 59 when plotted in mV electrical potential against-log C1 or pC1.Acidic and basic solutions have measurable electrical potentials through electrodes, the most used is glass, which consists of a Ag-AgCl half-cell immersed in HCl to a known concentration. The half cell is a calomel electrode consisting of Mg-MgCl in a solution of KCl at a known concentration. The very thin glass membranes are permeable to H + ions selectively. The potential across this membrane is a concentration due to potential differences in the concentration of H + and theoretically is governed by the Nernst equation.The glass electrode is applicable to virtually all kinds of substances, including those containing oxidizing or reducing strong. You can enter as cheese-solid substances and obtain satisfactory values ​​of pH. It is also ideal for measuring pH of biological fluids.

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