page_title: Silver chloride electrode
section_title: Biological electrode systems
hierarchical_section_title: Silver chloride electrode / Applications / Biological electrode systems
caption_attribution_description: English: Tab electrode (silver/silver chloride sensor) used in electrocardiography (ECG)
context_page_description: 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 redox electrode and the equilibrium is between the silver metal and its salt—silver chloride.
  The corresponding half-reactions can be presented as follows:
  or can be written together:
  which can be simplified:
  This reaction 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. The reaction has been proven to obey these equations in solutions of pH values between 0 and 13.5.
context_section_description: Silver chloride electrodes are also used by many applications of biological electrode systems such as biomonitoring sensors as part of electrocardiography (ECG) and electroencephalography (EEG), and in transcutaneous electrical nerve stimulation (TENS) to deliver current. Historically, the electrodes were fabricated from solid materials such as silver, brass coated with silver, tin and nickel. In today's applications, most biomonitoring electrodes are silver/silver chloride sensors which are fabricated by coating a thin layer of silver on plastic substrates and the outer layer of silver is converted to silver chloride.
  The principle of silver/silver chloride sensors operation is the conversion of ion current at the surface of human tissues to electron current to be delivered through the lead wire to the instrument to read. An important part of the operation is electrolyte gel which is applied between the electrode and tissues. The gel contains free chloride ions such that the charge can be carried through the electrolyte, therefore the electrolyte can be considered as conductive for ion current as the human tissues. When the ion current exists, the silver atoms in the electrode oxidize and discharge cations to the electrolyte and the electrons carry charge through the lead wire. At the same time, the chloride ions which are anions in the electrolyte travel toward the electrode and they are reduced as they bond with silver of the electrode resulting in silver chloride and free electrons to deliver to the lead wire. The reaction allows current to pass from electrolyte to electrode and the electron current passes through the lead wire for the instrument to read.
  When there is an uneven distribution of cations and anions, there will be a small voltage called half-cell potential associated with the current. In the DC system that is used by the ECG and EEG instruments, the difference between the half-cell potential and the zero potential is shown as DC offset which is an undesirable characteristic. Silver/silver chloride is a popular choice of biological electrodes due to its low half-cell potential of approximately 220 mV and low impedance.
caption: Tab electrode using silver/silver chloride sensing for electrocardiography (ECG)[6]
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