Department of Bioorganic and Biological Chemistry. Bioinorganic chemistry.

I COURSE

LECTURER: Professor A.D.DZHURAEV

LECTURE 7. THE ELECTROD AND REDOX POTENTIIALS.

The LECTURE PURPOSE:

Give an idea of the students about the electrodes, redox potentials and mechanisms of their occurrence. Know that determining the redox potentials for the various biochemical systems can predict the direction of the reaction. Know that in the medical practice, a lot of methods based on the measurement of potentials, and determine the nature of disorders of the heart, brain, muscle redox potentials and in general, the laws of equilibrium processes taking place at the electrodes are of great importance in studying the life of the human.

The LECTURE PURPOSE: • To introduce the students with electrode and redox

potentials , and mechanism of their rise. Determined redox potentials for different biochemical systems is possible to predict the direction of reactions. Many processes in organism it is impossible to explain without redox potentials. The total bioelectric potentials of different systems and organs reflect their functional state. In medical practices used many methods, founded on measurement of potentials, and defining nature of the breach of the work of heart, brain of the muscles and others. In this connection future physician must to know, how appear the potentials and how their to measure. In this connection is considered also method of potentiometric titration. Potentiometric determination of pH is the most exact, broadly applicable method for determination of active acidity of the medium, including blood and other biological liquids.

DEALT of questions kinds of potentialsThe emergence of electrochemical potentialMeasurement and calculation of the electrochemical potentialThe emergence of the redox (red-ox) potentialUnlike red-ox potential of electrochemical potentialGalvanic cells and electrodesThe value of red-ox potential in medicine

Kinds of potentials1. Diffuse potential - arises at the interface of the two solutions of different concentrations2. Contact potential - there is a border between the two types of metal3. Oxidation-reduction potential - occurs at the boundary inert metal and a solution containing the oxidized and reduced forms of the same substance4. Electrochemical potential - there is at the metal and water or a salt solution of the metal5. Membrane potential - occurs at the boundary of cell membranes

Electrochemical potential

Electrochemical potential– occurs at the interface between metal and water or a solution of the metal: Cu + Zn -

CuSO4

++++++

ZnSO4

++++++

Nernst equation to calculate the electrochemical potential

RT E = E0 + --------- ln CMe

+z

zFE – electrochemical potentialE0 – normal electrode potentialR – universal gas constantT – absolute temperatureF – Faraday number, z – valence metalCMe

+z – the concentration of metal ions

Galvanic element Elements consisting of two electrodes and

converts chemical energy into electrical energy is called a galvanic element

Zn / ZnSO4 / KCI / CuSO4 / Cu

+ + + + + +

ZnSO4

- - - - - -

CuSO4

Сu+Zn-Zn + CuSO4 → Cu + ZnSO4 Zn0 + Cu+2 → Cu0 + Zn+2

Zn0 - 2e → Zn+2 Cu+2 + 2e → Cu0

2

2

ln

aCuaZn

nFRT

E = E0 -

The electrodesThe electrodes of the first kind : Си / Сu2+, Ag / Ag+ and others

Electrodes of the second kind - calomelthe electrode.

The electrodes

Relative magnitude of the potential:Reference Electrodes - normal hydrogen electrodecalomel and silver chloride electrodes.Electrode measurements - glass and quinhydroneelectrodes. Quinhydrone electrode - platinum wire loweredin a solution containing 1 mole of quinone – C6 H4O2 and 1 molhydroquinone С6Н4 (ОН)2. C6 H4O2 - С6Н4(ОН)2 ↔ C6 H4O2 + С6Н4(ОН)2

е = e0 + nehydroquino

quinonelnСС

nFRT

Scheme of the structure the glass electrode

Ag/AgCl

glass membrane

0,1 M HCl

glass electrode

Cu

Concentration element

Е = 0,058 lg С1 / С2

Е = O,O58 lg 0,1/0,01 = 0,058 lg 10= 0,058 в * 1000 = 58 мв.

Diffuse potential

Diffuse potential – occurs at the interface between the two solutions of the same material with different concentrations:

The membrane potential

The membrane potential- Occurs from the difference of concentration of ions on either side of a biological membrane.On the border of the outer and inner side of a biological membrane potential difference of 75 mV. Changing this value changes the order of passage of ions through the cell.

The redox potential

The redox potential- occurs at the boundary of an inert metal and a solution containing the oxidized and reduced forms of the same substance : + Pt

[Fe3+]>[Fe2+]

Fe3+ Fe2+

PtFeCl3FeCl2; PtSnCl4SnCl2

Pt K3[Fe(CN)6] K4[Fe(CN)6]

Redox systems

Pt K3[Fe(CN)6] AgCl; Ag K4[Fe(CN)6] KCl (Saturation)

Ag AgCl;HCl glass. definable AgCl; Ag [H+]=const membrane-solution KCl [H+]=x (Saturation)

Scheme of galvanic element

Peters equation RT oxidation. form r = r

0 - ------- l n --------------------- nF Restore. form

r – the redox potentialr

0 – standard redox potentialR – universal gas constant;T - absolute temperature; F – Faraday number, n - number of electronsoxides. form - the concentration of the oxidized form the substance recovery. form - the concentration of the reduced form the substance

Peters equation 0,058 oxidation. form

r = r0 - ------- l g ---------------------

n Restore. form

For difficult redox systems: 0,058 oxidation. form

[H+]r = r

0 - ------- l g ------------------------ n Restore. form