ELECTRODES

From Big Medical Encyclopedia

ELECTRODES — the structural element of the electronic, ionic or electrotechnical device or installation representing the conductor by means of to-rogo the site of an electric chain falling on handling medium connects to other part of this chain formed by wires. AA. call various as on designs, and to destination devices, including used in the devices used in medicine, e.g. in electron-beam tubes, photoelectronic multipliers, devices for electrotreatments (see), electrodiagnoses (see), electrophoresis (see), etc. Electrodes widely apply in medicobiological researches (see. Microelectrode method of a research ) for measurement bioelectric potential (see), studying conductivity of biological systems (see), conductivity of nervous tissue (see. Nervous impulse ) etc. In biochemical researches, sanitary and hygienic and laboratory and clinical practice E. found broad application for measurement of the size pH of the environment (indicator electrodes) and concentration (activity) of ions of sodium, potassium, calcium, chlorine, etc. in solutions (see. Ion-selective electrodes ). On the basis of ion-selective E. mo gutnut to be made electrode systems for measurement of partial pressure of nek-ry gases, e.g. carbon dioxide gas, ammonia, hydrogen sulfide, and also so-called fermental E., allowing to measure amount of substrate. By means of such E. define, e.g., quantity urea (see) or glucose (see).

Majority E., applied in biol. and medical practice, are the galvanic, i.e. metal or other electronic conductors which are in contact with the ionic conductor — electrolyte (see. Electrolytes ). The major characteristic such E. the electrolytic potential which is established on border an electrode — electrolyte thanks to space division of positive and negative charges near the interphase boundary is. Such division is caused by uneven distribution of electrons in a surface layer of metal and formation of the oriented layer of dipole molecules (in water solutions of electrolytes it is water molecules), and also emergence of a double electric layer (see. Electrokinetic phenomena ). The potential which was established thus is called equilibrium electrolytic potential. Electrolytic potential electronegative E., called by the cathode, has a negative sign, and electropositive E., called by the anode — a positive sign.

Among galvanic E. including and for medicobiological researches, so-called reversible electrodes, i.e. galvanic E have the greatest value., on to-rykh in this solution a certain potential with the size depending only by nature and concentration of the oxidized and recovered components of system according to their thermodynamic properties is established. Reversible E. divide into two main types: indicator E., potential to-rykh depends on ion concentration in solution, and AA. the comparisons having a certain well reproduced potential and applied to measurement of sizes of potentials of others E. As E. comparisons most often use chlorsilver and calomel electrodes.

Reversible E. divide on E. first, second and third sort. Treat electrodes of the first sort metal E., their salts shipped in solution, gas E. and amalgamny couples of Zn/Zn can serve so-called to E. Primerami E. in solution of its salt from metal 2+ , Ag/Ag + , Hg/Hg 2+ etc. Representing such E. scheme MOM Z + , where M Z + — a metal cation of M in solution, and Z + — its electrovalency, electrode reaction can be written down the equation: M = M Z + + Z +e .

In this case the redox system consists of ions of M Z + in solution (the oxidized form of system) and the metal M (got into condition system). Electrolytic potential of such system linearly increases with increase in value of a logarithm of ion activity of metal in solution. Tenfold increase in ion activity in solution at 20 ° does electrolytic potential to more positive on 58 mV in case of single shot ions and on 29 mV — in case of two-charging ions. Gas E., relating also to E. the first sort, are called so because at them except the substances which are in solution gases, and metal E take part in electrode reaction. is inactive (inert), i.e. does not participate in reaction. The most usable gas E. hydrogen E. K E. of the first sort is belong also amalgamny E., in to-rykh active metal (e.g., sodium) it is applied not in pure form, and in the form of solution in mercury (see). Potential amalgamny E. potential usual metal E changes with change of ion concentration of metal in solution, as well as., but unlike the last it depends also on concentration of metal in amalgam, becoming more negative with its increase.

K E. the second sort belong E. from the active metal shipped in the saturated solution of its almost insoluble salt containing also the electrolyte having the general anion with this salt. Examples such E. can serve chlorsilver E., to-ry represents metal silver E. in solution of hydrogen chloride, being in balance with solid silver chloride, and also calomel, or mercury and calomel, AA. (calomel — Hg mercury chloride 2 Cl 2 ). Potentials such E. depend on activity of the anion which is a part of a deposit of almost insoluble salt the same as potential E. the first sort — from activity of a cation. Therefore E. the first sort call E., reversible on a cation, and AA. the second sort — reversible on anion. Electrodes of the second sort are also metal oxide E., to-rykh can be an example antimonial E., representing metal antimony (see), covered with a film of oxide and shipped in water solution antimonial to - you are HSb(OH) 6 .

K E. the third sort belong E., of to-rykh (with smaller solubility) the general cation with metal E has the containing rainfall of two almost insoluble electrolytes, one. (as in E. the second sort), and another has the general anion with the first electrolyte. Use E. this sort it is very limited.

Though all considered electrode systems are oxidation-reduction (see. Redoxreactions ), oxidation-reduction E. usually call also electrode systems, in to-rykh metal E. (usually platinum) it is not active and all substances participating in electrode reaction are in solution. The simplest example of such electrode system is platinum E., shipped in the solution containing ions of two - and trivalent iron, in Krom reversible electrode test proceeds: Fe 2+ ↔ Fe 3+ + e. Redox potential (see) such system depends only on the relation of ion concentrations of two - and trivalent iron, but not on their absolute value.

Unlike considered higher than E., potentials to-rykh are defined by oxidation-reduction processes, essentially represent other type ion-exchange E. Ionoobmenny E. — it is the complex electrochemical system consisting from E. the first or second sort, solution of electrolyte and a membrane from ion-exchange substance. Specific properties of such system decide by interaction of the ion-exchanger membrane on solution of electrolyte. Example such E. serves glass electrode (see), i.e. an electrode, a membrane to-rogo it is made of the special grades of glass having the increased ability to absorb certain ions from solution. They behave like reversible E., e.g. on hydrogen, sodium, potassium or other ions.

AA., including and the electrodes used in biology and medicine are various by the sizes, a form and material, from to-rogo they are made. At electrocardiography (see), electroencephalography (see) and other methods of electrodiagnosis usually use rather big on the area lamellar superficial E. from noncorrosive metals. At electromyography (see) apply needle E. from chemical inert materials. In fiziol. experiments on cells, nerve and muscle fibers use microelectrodes, the metal end to-rykh has diameter of an order of units of a micrometer less, or the glass capillaries filled with salt solution, so-called salt bridges. During the studying of conductivity biol. systems and konduktometrichesky measurements (see. Conductometry ) for reduction of influence of electrode polarization (see) apply platinum E., covered with the platinum black possessing spongy structure that considerably increases the area of their surface.



Bibliography: Vladimirov Yu. A., etc. Biophysics, M., 1983; Glinka N. L. General chemistry, L., 1978; Pervis R. D. Microelectrode methods of intracellular recording and an ionophoresis, the lane with English, M., 1983, bibliogr.; Ferris C. D. Introduction to bioelectrodes, N. Y. — L., 1974.

G. A. Kurella, G. E. Fedorov.

Яндекс.Метрика