The concept of membrane and diffusion potentials.

Go to Main page When creating any electrode pair, a “salt bridge” is always used. The use of a “salt bridge” solves several problems that arise for researchers of electrochemical processes. One of these tasks is to increase the accuracy of determinations by eliminating or significantly reducing the diffusion potential. Diffusion potential + in galvanic cells occurs when solutions of different concentrations come into contact. The electrolyte from a solution with a higher concentration diffuses (transfers) into a less concentrated solution. If the absolute speeds of movement of the cations and anions of the diffusing electrolyte are different, then the less concentrated solution acquires the potential of the charge sign of the “faster ions,” and the more concentrated solution acquires the potential of the opposite sign. To eliminate the diffusion potential, it is necessary to minimize the difference in the rates of movement of cations and anions of the diffusing electrolyte. For this purpose, a saturated KCl solution was chosen, because absolute movement speeds K ¯ and Cl

almost identical and have one of the highest values. The emergence of a diffusion potential is also characteristic of biological systems

. For example, when a cell is damaged, when the semi-permeability of its membrane is disrupted, electrolyte begins to diffuse into or out of the cell. This creates a diffusion potential, which is called “damage potential” here. Its value can reach 30 - 40 mV, the “damage potential” is stable for approximately one hour.

The value of the diffusion potential increases significantly if solutions of electrolytes of different concentrations are separated by a membrane that allows only cations or anions to pass through. The selectivity of such membranes is due to their own charge. Membrane potentials are very stable and can persist for several months.

Potentiometry

Types of electrodes

For analytical and technical purposes, many different electrodes have been developed to form electrode pairs (elements).

There are two main types of classification of electrodes. By :

1. chemical composition - these are electrodes whose electrode reaction is reversible only with respect to the cation or anion. For example, the electrodes that form the Jacobi-Daniel element are copper and zinc (see above).

2. Electrodes 2 types - these are electrodes whose electrode reaction is reversible for two types of ions: cations and anions.

3. Redox Electrodes (Red – Ox) . The term “Red – Ox – electrode” refers to an electrode where all the elements of the half-reaction (both oxidized and reduced forms) are in solution. Metal electrodes immersed in the solution do not participate in the reaction, but serve only as a carrier of electrons.

By purpose:

1. Reference electrodes .

Reference electrodes are electrodes whose potential is precisely known, is stable over time and does not depend on the concentration of ions in the solution. Such electrodes include: standard hydrogen electrode, calomel electrode and silver chloride electrode. Let's look at each electrode in more detail.

Standard hydrogen electrode.

This electrode is a closed vessel into which a platinum plate is inserted. The vessel is filled with a solution of hydrochloric acid, the activity of hydrogen ions in which is 1 mol/l. Hydrogen gas is passed into a vessel under a pressure of 1 atmosphere. Hydrogen bubbles are adsorbed on a platinum plate, where they are dissociated into atomic hydrogen and oxidized.

Characteristics of standard hydrogen electrode:

1.Electrode diagram: Pt(H 2) / H +

2. Electrode reaction: ½ Н 2 – ē ↔ Н +

As is easy to see, this reaction is reversible only for the cation (H +), therefore the standard hydrogen electrode is a type 1 electrode.

3.Calculation of electrode potential.

The Nernst equation takes the form:

eH 2 /H+ = e ° N 2 /N + RT ln a n +

nF (P n 2) 1/2

Because and n+ = 1 mol/l, р n+ = 1 atm, then ln a n+ = 0, That's why

(Rn 2) 1/2

eH 2 /H+ = e ° N 2 /H+

Thus, with a n + = 1 mol/l and p(n 2) = 1 atm, the potential of the hydrogen electrode equal to zero and is called the "standard hydrogen potential".

Another example – calomel electrode(see picture)

It contains a paste including calomel (Hg 2 Cl 2), mercury and potassium chloride. The paste is based on pure mercury and filled with a solution of potassium chloride. A platinum plate is immersed inside this system.

Electrode characteristics:

1. Electrode diagram: Hg 2 Cl 2, Hg(Pt) / Cl¯

2. Two parallel reactions occur in this electrode:

Hg 2 Cl 2 ↔2Hg + +2Cl¯

2 Hg + + 2ē →2Hg

Hg 2 Cl 2 + 2ē → 2Hg +2Cl¯ - total reaction.

From the above equations it is clear that the calomel electrode is a type 2 electrode.

3. The electrode potential is determined using the Nernst equation, which after appropriate transformations takes the form:

e = e o - RT ln a Cl¯

Another important example is silver chloride electrode(see pic).

Here, the silver wire is coated with a layer of poorly soluble salt AgCl and immersed in a saturated solution of potassium chloride.

Electrode characteristics:

1. Electrode diagram: Ag, AgCl / Cl¯

2. Electrode reactions: AgCl ↔ Ag + + Cl¯

Ag + + ē → Ag

AgCl + ē ↔ Ag + Cl¯ -total reaction.

As can be seen from this reaction, the resulting metal settles on the wire, and Cl¯ ions go into solution. The metal electrode acquires a positive charge, the potential of which depends on the concentration (activity) of Cl¯ ions.

3. The electrode potential is determined using the Nernst equation, which, after appropriate transformations, already takes known species:

e = e o - RT ln a Cl¯

In silver chloride and calomel electrodes, the concentration of Cl¯ ions is maintained constant and therefore their electrode potentials are known and constant over time.

2. Definition electrodes - these are electrodes whose potential depends on the concentration of any ions in the solution, therefore the concentration of these ions can be determined by the value of the electrode potential.

The most commonly used indicator electrodes are: hydrogen, glass and quinhydrone electrodes.

Hydrogen electrode is designed similarly to a standard hydrogen electrode, but if an acidic solution with an activity of H + ions greater than one is placed in the container of the hydrogen electrode, then a positive potential appears on the electrode, proportional to the activity (i.e. concentration) of protons. When the proton concentration decreases, on the contrary, the electrode will be negatively charged. Therefore, by determining the potential of such an electrode, it is possible to calculate the pH of the solution in which it is immersed.

Electrode characteristics.

1. Electrode diagram: Pt(H 2) / H +

2. Electrode reaction: ½ Н 2 – ē ↔ Н +

3. e H 2 /H+ = e o H 2 /H + + 0.059 lg a n+

n

Because n =1, and e o N 2 / H+= 0, then the Nernst equation takes the form:

e H2/H+ = 0.059 lg a n+ = - 0.059 pH pH = - e

0,059

Glass electrode is a silver plate coated with an insoluble silver salt, enclosed in a glass shell made of special glass, ending in a thin-walled conductive ball. The internal medium of the electrode is a solution of hydrochloric acid. The electrode potential depends on the concentration of H + and is determined by the Nernst equation, which has the form:

e st = e o st + 0.059 lg a n+

Quinhydrone electrode consists of a platinum plate immersed in a solution of quinhydrone - an equal molar mixture of quinone C 6 H 4 O 2 and hydroquinone C 6 H 4 (OH) 2, between which a dynamic equilibrium is quickly established:

Since protons are involved in this reaction, the electrode potential depends on pH.

Electrode characteristics:

1. Electrode diagram: Pt / H +, C 6 H 4 O 2, C 6 H 4 O 2-

2. Electrode reaction:

C 6 H 4 (OH) 2 - 2ē ↔ C 6 H 4 O 2 + 2H + -

redox process.

3. The electrode potential is determined using the Nernst equation, which after appropriate transformations takes the form:

e x. g. = e o x. g. + 0.059 lg a H +

The quinhydrone electrode is used only to determine the pH of those solutions where this indicator is not more than 8. This is due to the fact that in an alkaline environment hydroquinone behaves like an acid and the value of the electrode potential ceases to depend on the concentration of protons.

Because in a quinhydrone electrode A plate of noble metal is immersed in a solution containing both the oxidized and reduced forms of one substance, then it can be considered as a typical “red – ox” system.

The components of the redox system can be both organic and inorganic substances, For example:

Fe 3+ / Fe 2+ (Pt).

However, for organic substances, "red - ox" - electrodes are especially important because are the only way to form an electrode and determine its potential.

The magnitude of the electrode potentials arising at metal plates V red-ox – systems, can be calculated not only by the Nernst equation, but also by the Peters equation:

2 * 10 -4 C ox

e red-ox = e 0 red-ox + * T * lg ;(IN)

T– temperature, 0 K.

C ox And C red– concentrations of the oxidized and reduced forms of the substance, respectively.

e 0 red - ox is the standard redox potential that occurs in the system when the ratio of the concentrations of the oxidized and reduced forms of the compound is equal to 1.

Diffusion potential is the potential difference that occurs at the interface between two unequal electrolyte solutions. It is caused by the diffusion of ions across the interface and causes inhibition of faster diffusing ions and acceleration of slower diffusing ions, whether cations or anions. Thus, soon the equilibrium potential at the interface is established and reaches a constant value, which depends on the number of ion transfers, the magnitude of their charge and the concentration of the electrolyte.

E.m.f. concentration chain (see)

expressed by the equation

is the sum of two electrode potentials and the diffusion potential. The algebraic sum of two electrode potentials is theoretically equal to

hence,

Let's assume that then

or in general for an electrode reversible with respect to the cation,

and for an electrode reversible with respect to the anion,

For electrodes reversible with respect to the cation, when if then the value is positive and is added to the sum of the electrode potentials; if then the value is negative and e. d.s. element in this case is less than the sum of the electrode potentials. Attempts have been made to eliminate the diffusion potential by introducing a salt bridge containing a concentrated solution and other salts for which. In this case, since the solution is concentrated, diffusion is determined by the electrolyte of the salt bridge itself, and instead of the diffusion potential of the cell, we have two diffusion potentials acting in opposite directions and having a value close to zero. In this way, it is possible to reduce diffusion potentials, but it is almost impossible to completely eliminate them.

External cellular membrane– plasmalemma – is basically a lipid layer, which is a dielectric. Since there is a conducting medium on both sides of the membrane, this entire system, from an electrical engineering point of view, is capacitor. Thus, alternating current through living tissue can pass both through active resistances and through electrical capacitances formed by numerous membranes. Accordingly, the resistance to passage alternating current through living tissue will exert two components: active R - resistance to the movement of charges through the solution, and reactive X - resistance to the current of electrical capacitance on membrane structures. Reactive resistance has a polarization nature, and its value is related to the value of electrical capacitance by the formula:

where C is the electrical capacitance, w is the circular frequency, f is the current frequency.

These two elements can be connected in series or in parallel.

Equivalent electrical diagram living tissue– this is a connection of elements of an electrical circuit, each of which corresponds to a specific element of the structure of the tissue being studied.

If we take into account the basic structures of the tissue, we get the following diagram:

Figure 2 - Equivalent electrical circuit of living tissue

R c - resistance of the cytoplasm, R mf - intercellular resistance, Cm - electrical capacitance of the membrane.

Concept of impedance.

Impedance– the total complex resistance of the active and reactive components of the electrical circuit. Its value is related to both components by the formula:

where Z is impedance, R is active resistance, X is reactance.

The magnitude of the impedance when connecting reactive and active resistance in series is expressed by the formula:

The magnitude of the impedance when connecting reactive and active resistance in parallel is written as:

If we analyze how the value of impedance changes with changes in R and C, we will come to the conclusion that with both series and parallel connection of these elements, as the active resistance R increases, the impedance increases, and as C increases, it decreases, and vice versa.

The impedance of living tissue is a labile quantity that depends, firstly, on the properties of the tissue being measured, namely:

1) on the structure of the tissue (small or large cells, dense or loose intercellular spaces, degree of lignification of cell membranes);

2) tissue water content;

4) the state of the membranes.

Secondly, the impedance is affected by the measurement conditions:

1) temperature;

2) frequency of the current being tested;

3) electrical circuit diagram.

When membranes are destroyed by various extreme factors, a decrease in the resistance of the plasmalemma, as well as the apoplast, will be observed due to the release of cellular electrolytes into the intercellular space.

The direct current will flow mainly through the intercellular spaces and its magnitude will depend on the resistance of the intercellular space.

Figure 3 - Change in capacitance (C) and resistance (R) of tissue when changing the frequency of alternating current (f)

The preferential path of alternating current depends on the frequency of the applied voltage: as the frequency increases, an increasing proportion of the current will flow through the cells (through the membranes), and the complex resistance will decrease. This phenomenon - a decrease in impedance with increasing frequency of the testing current - is called electrical conductivity dispersion.

The slope of the dispersion is characterized by the polarization coefficient. The dispersion of the electrical conductivity of living tissues is the result of polarization at low frequencies, as with DC. Electrical conductivity is related to polarization - as frequency increases, polarization phenomena have less effect. Dispersion of electrical conductivity, as well as the ability to polarize, is inherent only in living tissues.

If you look at how the polarization coefficient changes as tissue dies, then in the first hours it decreases quite significantly, then its decline slows down.

The liver of mammals has a polarization coefficient of 9-10, the liver of a frog 2-3: the higher the metabolic rate, the higher the polarization coefficient.

Practical significance.

1. Determination of frost resistance.

2. Determination of water availability.

3. Determination of a person’s psycho-emotional state (Tonus device)

4. Component of a lie detector - polygraph.

Membrane diffusion potential

Diffusion potential– electric potential arising as a result of microscopic separation of charges due to differences in the speed of movement of various ions. And different speeds of movement through the membrane are associated with different selective permeability.

For its occurrence, contact of electrolytes with different concentrations and different mobility of anions and cations is necessary. For example, hydrogen and chlorine ions (Fig. 1). The interface is equally permeable to both ions. The transition of H + and Cl - ions will occur towards lower concentrations. The mobility of H + when moving through the membrane is much higher than Cl - , due to this, a large concentration of ions will be created with right side from the electrolyte interface, a potential difference will arise.

The resulting potential (membrane polarization) inhibits further ion transport, so that eventually the total current through the membrane will stop.

In plant cells, the main ion flows are the flows of K +, Na +, Cl -; they are found in significant quantities inside and outside the cell.

Taking into account the concentrations of these three ions and their permeability coefficients, it is possible to calculate the value membrane potential, due to the uneven distribution of these ions. This equation is called the Goldmann equation, or constant field equation:

Where φ M - potential difference, V;

R - gas constant, T - temperature; F - Faraday number;

P - ion permeability;

0 - ion concentration outside the cell;

I is the ion concentration inside the cell;

Diffusion potentials arise at the interface between two solutions. Moreover, these can be either solutions of different substances or solutions of the same substance, only in the latter case they must differ from each other in their concentrations.

When two solutions come into contact, particles (ions) of dissolved substances interpenetrate into them due to the process of diffusion.

The reason for the emergence of a diffusion potential in this case is the unequal mobility of the ions of dissolved substances. If the electrolyte ions have different diffusion rates, then the faster ions gradually appear ahead of the less mobile ones. It is as if two waves of differently charged particles are formed.

If solutions of the same substance are mixed, but with different concentrations, then the more dilute solution acquires a charge that coincides in sign with the charge of more mobile ions, and the less diluted solution acquires a charge that coincides in sign with the charge of less mobile ions (Fig. 90).

Rice. 90. The emergence of a diffusion potential due to different ion speeds: I– “fast” ions, negatively charged;
II– “slow” ions, positively charged

A so-called diffusion potential arises at the solution interface. It averages the speed of movement of ions (slows down the “faster” ones and accelerates the “slower” ones).

Gradually, with the completion of the diffusion process, this potential decreases to zero (usually within 1-2 hours).

Diffusion potentials can also arise in biological objects when cell membranes are damaged. In this case, their permeability is disrupted and electrolytes can diffuse from the cell into the tissue fluid or vice versa, depending on the difference in concentration on both sides of the membrane.

As a result of the diffusion of electrolytes, a so-called damage potential arises, which can reach values ​​of the order of 30-40 mV. Moreover, damaged tissue is most often charged negatively in relation to undamaged tissue.

The diffusion potential arises in galvanic cells at the interface between two solutions. Therefore, when accurately calculating the emf. galvanic circuits must necessarily introduce a correction for its value. To eliminate the influence of diffusion potential, electrodes in galvanic cells are often connected to each other by a “salt bridge”, which is a saturated KCl solution.

Potassium and chlorine ions have almost identical mobilities, so their use makes it possible to significantly reduce the influence of the diffusion potential on the emf value.

The diffusion potential can greatly increase if solutions of electrolytes of different compositions or different concentrations are separated by a membrane that is permeable only to ions of a certain charge sign or type. Such potentials will be much more persistent and can persist for a longer time - they are called differently membrane potentials. Membrane potentials arise when ions are unevenly distributed on both sides of the membrane, depending on its selective permeability, or as a result of the exchange of ions between the membrane itself and the solution.

The principle of operation of the so-called ion-selective or membrane electrode.

The basis of such an electrode is a semi-permeable membrane obtained in a certain way, which has selective ionic conductivity. A feature of the membrane potential is that electrons do not participate in the corresponding electrode reaction. Here an exchange of ions takes place between the membrane and the solution.

Solid membrane electrodes contain a thin membrane on either side of which there are different solutions containing the same detectable ions, but at different concentrations. WITH inside the membrane is washed by a standard solution with a precisely known concentration of the ions being determined, and on the outside - by the analyzed solution with an unknown concentration of the ions being determined.

Due to the different concentrations of solutions on both sides of the membrane, ions are exchanged with the inner and external sides membranes in different ways. This leads to the formation of different electric charge and as a result of this, a membrane potential difference arises.

DIFFUSION POTENTIAL,

potential difference at the boundary of two contacting solutions of electrolytes. It is due to the fact that the rates of transfer of cations and anions across the boundary, caused by the difference in their electrochemical properties. potentials in solutions 1 and 2 are different. The presence of a D. point can cause an error in measuring the electrode potential, so efforts are made to calculate or eliminate the D. point. Accurate calculation is impossible due to the uncertainty of the coefficient. ion activity, as well as the lack of information about the distribution of ion concentrations in the boundary zone between adjacent solutions. If solutions of the same z are in contact, z - charging electrolyte (z - number of cations equal to the number of anions) decomp. concentrations and we can assume that the transfer numbers of anions and cations, respectively. t + and t_ do not depend on their activity, but the coefficient. The activities of anions and cations are equal to each other in both solutions, then D. p.

Where a 1 and a 2 - average activities of ions in solutions 1 and 2, T - abs. t-ra, R - , F - Faraday's constant. There are other approximate formulas for determining D. p. Reduce D. p. to a small value in the plural. cases, it is possible by separating solutions 1 and 2 with a “salt bridge” from the concentrate. solutions, cations and cut have approximately equal transfer numbers (KCl, NH 4 NO 3, etc.). Lit.: Fetter K., Electrochemical kinetics, trans. from German, M., 1967, p. 70-76; Rotinyan A. L., Tikhonov K. I., Shoshina I. A., Theoretical. L., 1981, p. 131-35. A. D. Davydov.

Chemical encyclopedia. - M.: Soviet Encyclopedia. Ed. I. L. Knunyants. 1988 .

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