Electrode potentials metal coated with sparingly soluble salt

Electrodes classified in the second group of electrode systems are those in which the metal electrode is coated with a layer of a sparingly soluble salt of the electroactive species and the metal ion of the metal electrode, such that the potentiometric response is indicative of the concentration of the inactive anion species. Thus the silver/silver-chloride electrode system, which is representative of this class of electrodes, gives a potential response that is directly related to the logarithm of the chloride ion activity (see also Chapter 1, section 1.5), even though it is not the electroactive species ... 

A metal electrode of the second kind consists of a metal coated with one of its sparingly soluble salts (or immersed in a saturated solution of its sparingly soluble salt). This electrode responds to the anion of the salt. For example, a silver wire coated with AgCl will respond to changes in chloride activity because the silver ion activity is controlled by the solubility of AgCl. The electrode reaction is AgCl(s) -I- e Ag(s) -I- Cl , with a potential = 0.222 V. The Nernst equation expression for the electrode potential at 25°C is = 0.222 - 0.05916 log[Cr]. 

These comprise metal electrodes coated with a thin layer of sparingly soluble salt of this metal. The potential difference depends on the... 

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