Silver chloride electrode

The silver -silver chloride electrode, as well as the calomel electrode, an electrode of the second kind

Construction

In this type of electrode usually dips containing silver wire coated with silver chloride in a chloride ion solution (usually 3 M or saturated potassium chloride solution). The electrically conductive connection to the electrolyte solution is prepared via a diaphragm, a glass stopper, or a fused Magnesiastift. This material made ​​from sintered magnesia is somewhat porous. These pores and tiny crevices and channels between the glass and magnesia filled with the electrolyte. Thus, a minimal flow of ions is assured that sufficient to adjust the redox equilibrium of the electrode surface. The meter voltmeter or the reference electrode input of a potentiostat is a very high impedance, so here are no significant flows materialize. A special electrode which finds its application in the construction anchors, the CMS electrode.

Principle of operation

The potential-determining step is the oxidation of silver, followed by formation of the solid silver chloride. The redox reaction of this half- element is:

The corresponding formulation of the Nernst equation is as follows:

Since silver chloride is a substantially water insoluble compound, the activity of the silver ions on the solubility of silver chloride is determined by the activity of chloride ions:

The Ag ion concentration is lower than the saturation concentration for the solubility, as are solid AgCl is dissolved and vice versa. Due to this dependence of the Ag concentration of the Cl - concentration can be adjusted to a constant potential, which depends only on the concentration of chloride ions.

The logarithmic term can now be further simplified as:

Here, the term containing the solubility product constant and therefore is tabulated together with E0 for an electrode of the second kind.

With values ​​of E0 = 0.800 V, 0.222 V and E'0 = KL = 2 * 10-10 to get meaningful results. Instead of the activity, the concentration or the molality is often used.

Areas of application

Through their constant potential, the Ag / AgCl electrode as a reference electrode in many electrochemical analysis methods. With it, for example via a high-impedance voltmeter, the potential of an ion-selective measuring electrode are determined in an analyte solution (see potentiometry), so as to determine the ion activity.

In current research Ag / AgCl chloride sensors are used for example to measure the free chloride ions in the pore water of concrete structures and instrumented. Such sensors fall into the category of the ion-sensitive sensors and consist of a silver chloride coated silver wire (CMS - electrode). There are several research projects that have the objective to evaluate the durability of the sensors at the same time reliability of the results. The target is in the creation of a continuous recording of measured values ​​for the evaluation of the depth of Chlorideindringfront from the outside into the concrete. This repair measures can be planned targeted. It should thus be avoided that it is for depassivation of the reinforcement and hence the same in corrosion. This problem has been discussed by research at the ETH Zurich and the TU Braunschweig, but these sensors are currently the state of research is and let no ratings from the practice.