Silver-silver chloride electrode

The silver-silver chloride electrode , like the calomel electrode , is an electrode of the second kind .

construction

Schematic representation of a silver-silver chloride electrode.

Silver-silver chloride electrode.

With this type of electrode, a silver wire coated with silver chloride is usually immersed in a solution containing chloride ions (usually 3 M or saturated potassium chloride solution). The electrically conductive connection to the electrolyte solution is made via a diaphragm , a ground-glass stopper or a melted-down magnesia stickmanufactured. This sintered magnesium oxide material is somewhat porous. These pores and the tiny crevices and channels between the glass and the magnesia fill with the electrolyte. This ensures a minimal ion flow, which is sufficient to set the redox equilibrium on the electrode surface. The input resistance of a voltmeter or the reference electrode input of a potentiostat is very high resistance, so that no currents worth mentioning arise here. A special electrode that is used in construction and anchors is the CMS electrode .

Working principle

The step that determines the potential is the oxidation of silver with the subsequent formation of solid silver chloride. The reaction equation of this half-element is:

${\displaystyle \mathrm {Ag\;+\;Cl^{-}\rightarrow \;AgCl\;+\;e^{-}} }$

The corresponding formulation of the Nernst equation is as follows:

${\displaystyle E=E^{0}+{RT \over F}\cdot \ln \;a{\mathrm {(Ag^{+})} }}$

Since silver chloride is a compound that is sparingly soluble in water, the activity of the silver ions is determined by the solubility product of the silver chloride from the activity of the chloride ions:

${\displaystyle K_{L}=a_{eq}\mathrm {(Ag^{+})} \cdot a_{eq}\mathrm {(Cl^{-})} }$

If the Ag ⁺ ion concentration becomes smaller than the saturation concentration according to the solubility product, then solid AgCl dissolves and vice versa. This dependence of the Ag ⁺ concentration on the Cl ^- concentration allows a constant potential to be set which only depends on the chloride ion concentration.

${\displaystyle E=E^{0}+{RT \over F}\cdot \ln \;{K_{L} \over a{\mathrm {(Cl^{-})} }}}$

The logarithmic term can now be further simplified according to:

${\displaystyle E=E^{0}+{RT \over F}\cdot \ln \;{K_{L}}-{RT \over F}\cdot \ln \;a{\mathrm {(Cl^{-})} }}$

The term containing the solubility product is constant and is therefore tabulated together with E ⁰ for an electrode of the second type.

${\displaystyle E=E'^{0}-{RT \over F}\cdot \ln \;a{\mathrm {(Cl^{-})} }}$

With values ​​of E ⁰ = 0.800 V, E ' ⁰ = 0.222 V and K _L = 2 * 10 ⁻¹⁰ mol ² / l ² one obtains meaningful results. Instead of activity, concentration or molality is often used.

application areas

Due to its constant potential, the Ag / AgCl electrode is used as a reference electrode in many electrochemical analysis processes. With it, for example, the potential of an ion-selective measuring electrode in an analyte solution can be determined using a high-resistance voltmeter (see potentiometry ) in order to determine the ion activity.

In current research, Ag / AgCl chloride sensors are used and instrumented to measure the free chloride ions in the pore water of reinforced concrete components, for example. Such sensors fall into the category of ion-sensitive sensors and consist of a silver wire coated with silver chloride ( CMS electrode ). The aim is to create a continuous recording of measured values ​​to evaluate the depth of the chloride penetration front from the outside into the concrete. Depassivation of the reinforcement and thus corrosion of the reinforcement should be detected early in order to be able to counteract this with suitable measures.

literature

• Gerd Wedler : Textbook of physical chemistry. 5th, completely revised and updated edition. Wiley-VCH, Weinheim 2004, ISBN 3-527-31066-5 .
• Erwin Riedel, Christoph Janiak: Inorganic Chemistry. 5th edition. Walter de Gruyter, Berlin et al. 2002, ISBN 3-11-017439-1 .
• Yves Schiegg: Online monitoring to record the corrosion of the reinforcement of reinforced concrete structures. Zurich 2002 (dissertation, ETH Zurich, 2002), doi : 10.3929 / ethz-a-004319266 .