Reactions included in equilibrium:

The reactions included in this equilibrium are:

AgI (s) ↔ Ag ⁺+ I ^-                                                                                 ... (2.31)

Ag⁺+ e ↔ Ag (s) ; E^o  Ag  =  0.799 V                                                  ... (2.32)

Combining these two equations

AgI (s) + e ↔ Ag (s) + I^-

E^o_agi = - 0.151 V                                                                                    ... (2.33)

Applying Nernst equation to that given half reaction, we get the following relationship among the electrode potential and the anion activity.

E = - 0.151  - 0.0591 log [I¯  ]                                                             ... (2.34)

This categories of electrode, where silver is serving as an indicator electrode for iodide as called an electrode of the second order because it measures the concentration of an ion that is not directly involved in the electrode process.

The potential is directly dependent on the participant ion in the electrode process if it is used for determination of silver ion then it is an electrode of the first order.