The concept of membrane and diffusion potentials. Concentration chains
Membrane electrical potentials exist in virtually all cells of the body. Some cells, such as nerve and muscle cells, are capable of generating rapidly varying electrochemical impulses that are used to transmit signals along the membranes of these cells. In other cell types, such as glandular, macrophage, and ciliated cells, local changes in membrane potentials also activate many cellular functions. This chapter discusses membrane potentials generated by resting and active nerve and muscle cells.
Diffusion potential, due to the difference in ionic concentrations on both sides of the membrane. The concentration of potassium ions inside the nerve fiber is high, but outside it is very low. Let us assume that in this case the membrane is permeable to potassium ions, but impermeable to other ions. Because of the large concentration gradient, there is a strong tendency for large numbers of potassium ions to diffuse out of the cell across the membrane. During the process of diffusion, they carry positive electrical charges outward, as a result, the membrane is charged positively on the outside and negatively on the inside, since the negative anions remaining inside do not diffuse out of the cell along with potassium ions.
Within approximately 1 ms the difference potentials between the inner and outer sides of the membrane, called the diffusion potential, becomes large enough to block further diffusion of potassium ions outward, despite their high concentration gradient. In mammalian nerve fibers, the potential difference required for this is about 94 mV, with a negative charge inside the fiber. These ions also have a positive charge, but this time the membrane is highly permeable to sodium ions and impermeable to other ions. The diffusion of positively charged sodium ions into the fiber creates a membrane potential of opposite polarity to the membrane potential in the figure - with a negative charge on the outside and a positive charge on the inside.
As in the first case, membrane potential during a fraction of a millisecond becomes sufficient to stop the diffusion of sodium ions into the fiber. In this case, for mammalian nerve fibers the potential is approximately 61 mV with a positive charge within the fiber.
Thus, the difference ion concentrations through a selectively permeable membrane under appropriate conditions can create a membrane potential. In the following sections of this chapter we will show that the rapid changes in membrane potentials observed during the transmission of nerve and muscle impulses result from rapid changes in diffusion potentials.
Communication diffusion potential with concentration differences. Nernst potential. The level of membrane diffusion potential that completely stops the overall diffusion of a particular ion across the membrane is called the Nernst potential for that ion. The magnitude of the Nernst potential is determined by the ratio of the concentrations of a specific ion on both sides of the membrane. The larger this ratio, the greater the tendency of the ion to diffuse in one direction and, therefore, the higher the Nernst potential needed to prevent overall diffusion. Using the following Nernst equation, you can calculate the Nernst potential for any monovalent ions at normal body temperature (37°C):
EMF (mV) = ± 61 log (Concentration inside/Concentration outside), where EMF is electromotive force (potential difference).
When using this formulas The potential of the extracellular fluid outside the membrane is usually taken to be zero, and the Nernst potential represents the potential inside the membrane. In addition, the sign of the potential is positive (+) if the ion diffusing from inside to outside is negative, and negative (-) if the ion is positive. Therefore, if the concentration of positive potassium ions inside is 10 times greater than outside, the tenth logarithm of 10 is 1, so the potential inside, according to Nernst's equation, must be -61 mV.
The voltage of an electrochemical system with a liquid interface between two electrolytes is determined by the difference in electrode potentials accurate to the diffusion potential.
Rice. 6.12. Eliminating diffusion potential using electrolytic bridges
Generally speaking, diffusion potentials at the interface of two electrolytes can be quite significant and, in any case, often make the measurement results uncertain. Below are the values of diffusion potentials for some systems (the electrolyte concentration in kmol/m 3 is indicated in parentheses):
In this regard, the diffusion potential must either be eliminated or accurately measured. Elimination of the diffusion potential is achieved by including an additional electrolyte with similar cation and anion mobilities into the electrochemical system. When making measurements in aqueous solutions, saturated solutions of potassium chloride, potassium nitrate or ammonium are used as such electrolytes.
An additional electrolyte is included between the main electrolytes using electrolytic bridges (Fig. 6.12) filled with the main electrolytes. Then the diffusion potential between the main electrolytes, for example in the case shown in Fig. 6.12, - between solutions of sulfuric acid and copper sulfate, is replaced by diffusion potentials at the boundaries of sulfuric acid - potassium chloride and potassium chloride - copper sulfate. At the same time, at the boundaries with potassium chloride, electricity is mainly transferred by K + and C1 – ions, which are much more numerous than the ions of the main electrolyte. Since the mobilities of K + and C1 – ions in potassium chloride are almost equal to each other, the diffusion potential will be small. If the concentrations of the main electrolytes are low, then with the help of additional electrolytes the diffusion potential is usually reduced to values not exceeding 1 - 2 mV. Thus, in the experiments of Abbeg and Cumming, it was established that the diffusion potential at the boundary of 1 kmol/m 3 LiCl - 0.1 kmol/m 3 LiCl is equal to 16.9 mV. If additional electrolytes are included between lithium chloride solutions, then the diffusion potential decreases to the following values:
Additional electrolyte Diffusion potential of the system, mV
NH 4 NO 3 (1 kmol/m 3) 5.0
NH 4 NO 3 (5 kmol/m 3) –0.2
NH 4 NO 3 (10 kmol/m 3) –0.7
KNO 3 (saturated) 2.8
KCl (saturated) 1.5
Elimination of diffusion potentials by including an additional electrolyte with equal ion transfer numbers gives good results when measuring diffusion potentials in non-concentrated solutions with slightly different mobilities of the anion and cation. When measuring voltages of systems containing solutions of acids or alkalis
Table 6.3. Diffusion potentials at the interface of KOH – KCl and NaOH – KCl (according to V. G. Lokshtanov)
with very different rates of movement of the cation and anion, special care must be taken. For example, at the HC1 - KS1 (saturated) boundary, the diffusion potential does not exceed 1 mV only if the concentration of the HC1 solution is below 0.1 kmol/m 3 . Otherwise, the diffusion potential increases rapidly. A similar phenomenon is observed for alkalis (Table 6.3). Thus, the diffusion potential, for example in a system
(–) (Pt)H 2 | KOH | KOH | H 2 (Pt) (+)
4.2 kmol/m 3 20.4 kmol/m 3
is 99 mV, and in this case it cannot be significantly reduced using a salt bridge.
To reduce diffusion potentials to negligible values, Nernst proposed adding a large excess of some indifferent electrolyte to the contacting solutions. Then the diffusion of basic electrolytes will no longer lead to the emergence of a significant activity gradient at the interface, and, consequently, the diffusion potential. Unfortunately, the addition of an indifferent electrolyte changes the activity of the ions participating in the potential-determining reaction and leads to distorted results. Therefore, this method can only be used in those
cases where the addition of an indifferent electrolyte cannot affect the change in activity or this change can be taken into account. For example, when measuring system voltage Zn | ZnSO 4 | CuSO 4 | Cu, in which the concentration of sulfates is not lower than 1.0 kmol/m 3, the addition of magnesium sulfate to reduce the diffusion potential is quite acceptable, since the average ionic activity coefficients of zinc and copper sulfates will practically not change.
If, when measuring the voltage of an electrochemical system, diffusion potentials are not eliminated or must be measured, then first of all care should be taken to create a stable contact boundary between the two solutions. A continuously renewed boundary is created by slow directed movement of solutions parallel to each other. In this way, it is possible to achieve stability of the diffusion potential and its reproducibility with an accuracy of 0.1 mV.
The diffusion potential is determined by the Cohen and Tombrock method from voltage measurements of two electrochemical systems, the electrodes of one of them being reversible to the salt cation, and the other to the anion. Let's say we need to determine the diffusion potential at the ZnSO 4 (a 1)/ZnSO 4 (a 2) interface. To do this, we measure the voltages of the following electrochemical systems (assume that a 1< < а 2):
1. (–) Zn | ZnSO 4 | ZnSO 4 | Zn(+)
2. (–) Hg | Hg 2 SO 4 (solid), ZnSO 4 | ZnSO 4, Hg 2 SO 4 (solid) | Hg(+)
System 1 voltage
systems 2
Considering that φ d 21 = – φ d 12, and subtracting the second equation from the first, we obtain:
When measurements are carried out at not very high concentrations, at which one can still assume that = and = or that : = : the last two terms of the last equation cancel and
The diffusion potential in system 1 can also be determined in a slightly different way, if instead of system 2 we use a dual electrochemical system:
3. (–) Zn | ZnSO 4, Hg 2 SO 4 (solid) | Hg - Hg | Hg 2 SO 4 (solid), ZnSO 4 | Zn(+)
System voltage
Therefore, the voltage difference between systems 1 and 3 will be expressed by the equation:
If, as before, the ratio of the activities of zinc ions is replaced by the ratio of the average ionic activities of the zinc salt, we obtain:
Since the last term of this equation can usually be calculated accurately, the value of the diffusion potential can be determined from the measurements of E p1 and E p 3.
The diffusion potential at the boundary of two different solutions is determined in a similar way. For example, if they want to determine the diffusion potential at the boundary of solutions of zinc sulfate and copper chloride, they create two electrochemical systems:
4. (–) Zn | ZnSO 4 | CuCl2 | Cu(+)
5. (–) Hg | Hg 2 Cl 2 (solid), CuCl 2 | ZnSO 4, Hg 2 SO 4 (solid) | Hg(+)
System voltage 4
systems 5
Hence
Naturally, the greater the number of terms included in the equation for the diffusion potential, the less likely the determination is to be highly accurate.
Related information.
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 that are 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.
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 .
See what "DIFFUSION POTENTIAL" is in other dictionaries:
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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. On the 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 differently with the inner and outer sides of the membrane. This leads to the fact that different electrical charges are formed on different sides of the membrane and, as a result, a membrane potential difference arises.
