Electrode

An electrode [ elɛktro de ː ] ( from Ancient Greek ηλεκτρόν electron, " amber ", i.ü.S. " electric," and ὁδός Hodos, " way") is an electronic conductor, which in combination with a counter electrode with a medium located between two electrodes interacts. Electrodes consist of electrical conductors, usually a metal or graphite. They are used to connect non-electron -conducting regions with cables and find this example application in electrochemical elements, tool and possibly material dispenser in electric welding, as terminals and electron-optical elements in electron tubes. About the electrical function of electrode material can also be deposited or consumed, or it may include physical processes taking place in the electrode as the anode of an X-ray tube.

Electrodes in a gas or vacuum or insulator

Depending on the nature of the medium surrounding the electrode, there are different forms of the interactions:

If the medium is an insulator, an electric field is established between the electrodes. This configuration is called condenser. See, however, silent electric discharge.

The medium is a vacuum or a gas, an electric field between the electrodes is built up as in the case of the insulator. However, electrons can move to another from one electrode (cathode ) when the discharge from the cathode is possible, for example by field emission or thermal emission or the Edison Richardson effect ( → electron tube X-ray tube, thermionic cathode ).

The medium is a gas, the atoms or molecules of the gas is partially ionized, so that a plasma is formed. In plasma, the ions move in addition to the electrons in the electric field (→ gas discharge tube).

The electrodes of the spark plug, the welding electrodes in electric welding, and the electrode inside the nozzle for plasma cutting fall into this category. The welding electrodes generate an electric arc welding with the material to be welded. In the heat of the arc melts the two, and the electrode serves as a filler material so that the materials are joined.

Electrochemical electrodes

Here the medium is adjacent to the electrode, a liquid or solid ion conductor, the electrolyte. By oxidation and reduction reactions, or by an external voltage, an electrochemical potential is built up on the electrode.

It differs according to the nature of the dependence of potential on the concentration of the electrolyte, four types of electrodes:

It includes two electrodes immersed in electrolyte solutions of different concentrations or are of different materials, a power circuit to each other, a galvanic element is obtained. Between the electrodes, a voltage is measured, resulting from the potential difference, and which is called the electro-motive force (EMF) or " reversible cell voltage ". Such an arrangement can give power (battery). By connecting an external voltage to run other chemical reactions at the electrodes from (electrolysis ). The electrodes can be made of metals or semiconductors, for example, graphite, glassy carbon, and may be liquid (mercury) or solid.

An electrode used for corrosion protection is the sacrificial anode.

In the fuel cell, wherein the gas sensors, and in some batteries, the gas diffusion electrode is used

Polarity of electrochemical electrodes

For the electrochemical electrodes applies:

• The electrode at which oxidation takes place, is the anode. Electrons flow from the anode through a conductor. Anions in solution flow to the anode.
• The electrode at which reduction takes place, is the cathode. Electrons flow through a conductor to the cathode. Cations in solution flow to the cathode.

Which of the two electrodes which is positive and negative, depending on the electrochemical device from:

• If the chemical reaction is enforced by an induced current flow from an external voltage (electrolysis, electroplating ), the oxidation is caused by the electron withdrawing at the positively charged anode: The anode in this case is the positive ( ).
• When the electric voltage is generated by the chemical processes by themselves, such as in galvanic cells ( batteries or fuel cells ), the anode is negatively charged, as in the voluntary running oxidation electrons are free. The anode is then the negative (- ).

As with electrolysis, compared to a galvanic cell, the polarity of the electrodes is reversed, the assignment of the anode and cathode is often confusing. However, it can be oriented to the direction of flow of electrons. To give an example, the cell schematic ago: The prefix ana - is upward, the prefix kata - down means. In general, the anode on the left in the drawings is shown, the right cathode.

Electrodes of the first kind

Electrodes are electrodes of the first type, the potential depends directly on the concentration of the surrounding electrolyte solution. These are, for example, all metal, immerse in a solution of their metal ion (electrolyte solution). At the phase boundary leads to the formation of an equilibrium between the solution pressure of the metal and the osmotic pressure of the electrolyte solution.

• The solution pressure of the metal is thus concluded that each metal is trying to break away from its lattice cations. By the excess of electrons in the metal, the metal charges negatively. Due to the Coulomb attraction, the cations remain in relative proximity to the electrode. It is formed from an electrochemical double layer. The ability of a metal to release cations from its grid, was listed for each metal in the electrochemical series. The lower it is, the more noble it is, and the higher is thus its ability to release cations.
• The opposite tendency is by the osmotic pressure of the electrolyte solution materialize or more simply electrolyte solutions would be diluted. They achieve this by pushing the dissolved metal ions in the lattice of the electrode and install there. They do this particularly well when many metal ions are present in solution. It comes as the formation of an electrochemical double layer with the opposite sign. The opposite trend is supported by the electrostatic attraction of the dissolved salt by the ion remaining in the dissolution process of the metal electrons in the metal.

In equilibrium, therefore, keep solution pressure, and osmotic pressure and electrical pressure balance. On what page (solution vs. pressure. Osmotic pressure), the equilibrium, ie depends both on the position of the metal in the electrochemical series and on the other hand the concentration of the electrolyte solution.

Electrodes of the second kind

Electrodes are electrodes of the second type, the potential depends only indirectly on the concentration of the surrounding electrolyte solution. However, the deviation from the electrode of the first kind is only a constant voltage difference. Electrodes of the second kind are used as the reference electrode.

The concentration of independence of the potential is achieved by the special construction of the electrode. More specifically, the potential is held constant by the specific composition of the electrolyte solution. The electrolyte solution consists firstly of a saturated solution of a sparingly soluble salt, which as the electrode is made of the same metal as the cation and on the other from a highly soluble and just concentrated alkali salt containing the same anion as the sparingly soluble salt.

The potential depends on the concentration of the cation of the sparingly soluble salt. This concentration in turn is coupled about the solubility to the concentration of the anion. Is the concentration of the anion is held constant, and consequently the potential remains constant. This anion can be kept by their concentration is chosen very large, nearly constant. By subtracting these values ​​from the voltage reading is obtained the actual potential, or the emf of a solution.

Important reference electrodes are silver -silver chloride electrode and the calomel electrode. They are used for example in potentiometry.

Ion-selective electrodes

Measured at the ion selective electrode potential is a function of the concentration or activity of a specific ion species. Such an electrode consists in principle of an electrode which does not participate in the electrochemical reaction, for example, a graphite electrode and an associated electrode phase, which consists in the simplest case of a sparingly soluble salt with the ionic species, communicating with the corresponding ion in the solution in equilibrium. In practice, ion-selective electrodes are constructed similarly as a glass electrode, the membrane, for example, ion- specific of the sparingly soluble salt (crystal membrane ) or of potash or Natrongläsern. Liquid membranes consist of an inert support on which are mounted ionophores dissolved in organic solvents. The selectivity of the electrode is largely determined by the solubility and ionic conductivity. This should also be noted that many other ions present simultaneously in the solution can form a disruptive factor.

Examples:

• In a cadmium- selective electrode is the fixed electrode or the membrane phase of cadmium sulfide.
• With a silver chloride - silver sulfide electrode having a silver chloride - silver sulfide solid solution membrane can be both the concentrations of silver ions and the determination of chloride and sulfide ions.

Meanwhile, a large number of ions are selectively determined. In analytical practice, inter alia, the fluoride electrode for the determination of fluoride ions in water with an accuracy of 0.01 mg / l and in a modified principle of the ammonia electrode ( a gas-permeable membrane electrode) used. A special development are electrochemical biosensors, eg Enzyme electrodes.

Microelectrodes

The advantages of microelectrodes are the very small total current (only minor perturbation of to the system under investigation by reaction products, only small voltage drop in the solution), no influence ( moderate ) currents on the measurement result, non-repudiation of very small concentrations, negligibility capacitive effects are (very high scan rates are possible), and feasibility of very high current densities. These are offset by disadvantages over like a small total power despite the high current density and an extremely large ratio of sample volume to the electrode surface (even if only present traces of surface-active substances, so they easily cover the entire electrode surface).

Historical

The terms electrode, electrolyte, anode and cathode formed at the suggestion of Michael Faraday (1791-1867) and were made ​​public by him. Faraday, who had not learned Greek, was advised by William Whewell (1794-1866), the rector of Trinity College, University of Cambridge.