Silver chloride electrode

A silver chloride electrode is a type of reference electrode, commonly used in electrochemical measurements. For environmental reasons it has widely replaced the saturated calomel electrode. For example, it is usually the internal reference electrode in pH meters and it is often used as reference in reduction potential measurements. As an example of the latter, the silver chloride electrode is the most commonly used reference electrode for testing cathodic protection corrosion control systems in sea water environments.

The electrode functions as a reversible redox electrode and the equilibrium is between the solid (s) silver metal (Ag(s)) and its solid salt—silver chloride (AgCl(s), also called silver(I) chloride) in a chloride solution of a given concentration.

In electrochemical cell notation, the silver chloride electrode is written as, e.g., for an electrolyte solution of KCl 3 M:

${\displaystyle {\ce {{Ag(s)}\ |\ {AgCl(s)}\ |\ KCl(aq)\ (3M)}}}$

The corresponding half-reaction can be presented as follows:

${\displaystyle {\ce {AgCl(s) + e^- <=> Ag(s) + Cl^- (aq)}}}$

Which is a summary of these two reactions:

${\displaystyle {\ce {Ag^+ (aq) + e^- <=> Ag(s)}}}$

${\displaystyle {\ce {AgCl(s) <=> Ag^+ (aq) + Cl^- (aq)}}}$

AgCl does not form by direct combination of Ag⁺ and Cl^-, rather through the transformation of soluble species AgCl_{n + 1}^–n (0 ≤ n ≤ 3) first formed from the combination of the Ag⁺ and Cl^- into the solid AgCl phase.^[1]

This reaction is a reversible reaction and is characterized by fast electrode kinetics, meaning that a sufficiently high current can be passed through the electrode with 100% efficiency of the redox reaction (anodic oxidation and dissolution of the Ag metal along with cathodic reduction and deposition of the Ag⁺
ions as Ag metal onto the surface of the Ag wire). The reaction has been proven to obey these equations in solutions of pH values between 0 and 13.5.

The Nernst equation below shows the dependence of the potential of the silver-silver(I) chloride electrode on the activity or effective concentration of chloride-ions:

${\displaystyle E=E^{0}-{\frac {RT}{F}}\ln a_{{\ce {Cl-}}}}$

The standard electrode potential E⁰ against standard hydrogen electrode (SHE) is 0.230 V ± 10 mV.^{[citation needed]} The potential is however very sensitive to traces of bromide ions which make it more negative. The more exact standard potential given by an IUPAC review paper is +0.22249 V, with a standard deviation of 0.13 mV at 25 °C.^[2]