PHYSICAL CHEMISTRY IN BRIEF

CHAP. 11: ELECTROCHEMISTRY [CONTENTS] 407

Example

An example is the so-called glass electrode. This half-cell is comprised of a silver chloride electrode

dipped into an electrolyte containing a HCl solution of the molalitym 1. This electrolyte is enclosed

in a vessel made of special glass permeable only for hydrogen ions. Outside the vessel there is

another HCl solution of the molalitym 2. The hydrogen ions are exchanged between these two

solutions.

Nernst equation:

Φ = Φ◦−

RT

F

ln

m 2 γ±, 2

m 1 γ±, 1

= const−

RT

F

ln (m 2 γ±, 2 ).

In this caseΦ◦does not have the physical meaning of the standard potential, but it is a constant

dependent on the material of the half-cell.

11.8.11 Classification of galvanic cells

If electric energy is generated in a galvanic cells in consequence of chemical reactions, we speak
about achemical cell. If, on the other hand, it is generated in consequence of the equalization
of differences in the concentrations of substances present in different parts of the cell, we speak
about aconcentration cell.
Concentration cells are divided into two categories:

• Electrolyte concentration cells
  The anodic and cathodic compartments are formed by the same electrolyte of different
  concentrations. Both electrodes are identical. Electrolyte concentration cells are further
  divided into
  
  
  • cells with transference,
  
  
  • cells without transference.
  
  
  
  


• Electrode concentration cells
  The concentration differences occur at the (otherwise identical) electrodes. These are
  dipped into a shared electrolyte. Electrode concentration cells are further divided into
  
  
  • gas concentration cells comprised of two identical gas electrodes with different partial
    pressures of the gas