Tashkent Medical Academy Department of Bioorganic Chemistry and Biological Item: bioinorganic chemistry and fizkolloidnaya Theme of the lecture: "The electrode and the oxidation-reduction potentials" (for teachers and students of one course of treatment, medical - teachers, health-care and dental faculties) Lecture number 6 Author: Professor Kasymova S.S. Tashkent - 2013

Electrode potentials and the mechanism of their formation When immersed in an aqueous solution of the metal of his own salt polar water molecules oriented around metal cations on the surface, tear them and facilitate the transition to the solution. Transition cations in solution violates the electrical neutrality of the metal and the solution. Metal surface acquires an excess negative charge, since the electrons in the metal remains. However, the solution gets an excess positive charge due to passed into solution cations.

Addition to the features of dissolution of cations, there is still the possibility of the return of their deposition on the surface of the metal. Cations lose their solvation shell, and are part of the crystal lattice of the metal on the surface. After a while the balance will come when the rate of dissolution (oxidation) and the deposition (reduction) of the metal will be equal. Cations accumulated at the metal surface, can not get away from it into the solution. This is prevented by the electrostatic attraction of cations negatively charged surface of the metal containing an excessive amount of electrons. Due to the attraction of the ions formed electric double layer.

V oxidation = V recovery Me0 ↔ MeZ+ + Ze At the moment of equilibrium: V oxidation = V recovery металл раствор

Between the two charged layers exist equilibrium potential difference, or just the equilibrium potential. Such electrodes, when the metal dropped into a solution containing its own salt, called metal electrodes.   Metal electrodes can be schematically represented as: Me | MeZ+. For example: , Cu | Cu2+ или Cu | CuSO4

Some noble (inert) metals, such as: Pt, Pd, Au, etc Some noble (inert) metals, such as: Pt, Pd, Au, etc. have such a strong chemical lattice that being immersed in the solution can not give their ions in solution. However, if the electrode immersed in the solution, which contains the oxidized and reduced forms of the same substance, then at the metal - solution also raises the equilibrium potential. Inert electrode in this solution is a "source" of the electrons, because at least there are free electrons of the metal. H p: Pt lowered into a solution which contains ions Fe2 and Fe3. Fe3 ions can attach electrons, taking them from Pt, and become ions Fe2. The reverse process. Fe2 ions donate electrons to platinum and become ions Fe3.

V oxidation = V recovery Fe2+ Fe3+ металл раствор Fe2+ ↔ Fe3+ + e- At the moment of quilibrium: V oxidation = V recovery

At the metal - solution of an electric double layer is formed and there is the equilibrium potential. These electrodes, when the inert metal dropped into a solution containing a redox system, called redox electrodes, and the equilibrium potential - oxidation-reduction potential.

Redox electrodes can be represented as follows.

where E 0 - standard potential, which depends only on the nature of the electrode system; R - gas constant, equal to 8.31 J / grad.mol; T - absolute temperature, K; F - the Faraday = 96500 C / mol; z - number of electrons involved in the oxidation-reduction reaction.

in the electrode reaction aokis. and avosst in the electrode reaction aokis. and avosst. - Activity respectively oxidized and reduced forms of the matter. At t = 25 ° C the value of F = 0,059 V.

wherein Eox / red - the redox potential of the electrode; wherein Eox / red - the redox potential of the electrode;? E0ox/red - standard redox potential of the Pt electrode is immersed in a solution in which both forms are active (i.e. a solution of ions with an activity equal to 1);? z - numerically equal to the number of charges loaned;? F - Faraday constant. This equation is often called the Peters equation for a system of Fe3 + / Fe2 + is:

Nernst equation to calculate the electrode potentials

Normal hydrogen electrode The equilibrium potential for boundary inert metal solution also appears when immersed in a solution of an inert metal, through which the purged gas (H2, O2, Cl2, etc) Such electrodes are called gas electrodes. In its simplest form - it is U - shaped tube with one end closed, into which the platinum wire is welded to it platinized platinum plate.

Electrodes. Hydrogen electrode

H2⇄2H+ + 2e

Calomel and silver chloride electrodes. calomel electrode Hg22+ + 2e ⇄ 2Hg0 Обратимость относительно катионов Hg22+ Hg2Cl2+2e ⇄ 2Hg + 2C- Обратимость относительно анионов Cl-

Silver chloride electrode. Ag, AgCl | KCl

Reversible electrodes of the first and second kind Electrodes of the first kind are reversible relative to the cation or anion. где или .

The electrodes of the second kind in relation to both reversible cation and anion.

Quinhydrone electrode (x).

The glass electrode.

Diffusion and membrane potentials and their role in the genesis Bioelectric potentials Henderson equation: wherein U and V - respectively the mobility of the cation and anion.

For example, to determine the value of the diffusion potential at the 0.01 / 0.1 M solution HCl, if the mobility of the ions H + and Cl-are respectively 316.4 and 68.8. Average activity coefficients (γ ±) 0,01 M and 0.1 M HCl solutions are respectively 0.905 and 0.796. Temperature = 25 ° C. According to equation (6) to 25 ° C, we have:

Donnan potential for ion exchangers in the case of electrolyte (KCl, CuSO4, etc.) expressed by the equation:

Potentiometric methods. Potentiometry in biological systems