Diffusion potential. Diffuse potential, mechanism of origin and biological significance. Substance current output
Diffusion potential is the potential difference that occurs at the interface between two dissimilar electrolyte solutions. It is caused by the diffusion of ions across the interface and causes deceleration of more rapidly diffusing ions and acceleration of more slowly diffusing ions, be they cations or anions. Thus, the potential at the interface soon becomes equilibrium and reaches a constant value, which depends on the number of ion transport, the magnitude of their charge and the concentration of the electrolyte.
E. d. With. concentration chain (see)
expressed by the equation
is the sum of two electrode potentials and a diffusion potential The algebraic sum of two electrode potentials is theoretically equal to
hence,
Suppose that, then
or, in the general case, for an electrode reversible with respect to a cation,
and for an electrode reversible with respect to the anion,
For electrodes reversible with respect to the cation, when if that value is positive and is added to the sum of the electrode potentials; if then the value is negative and e. etc. with. 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 due to 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 the diffusion potentials, but it is practically impossible to completely eliminate them.
Diffusion potentials arise at the interface between two solutions. Moreover, it can be both solutions of different substances, and solutions of the same substance, only in the latter case they must necessarily differ from each other in their concentrations.
When two solutions come into contact, particles (ions) of dissolved substances interpenetrate in them due to the diffusion process.
The reason for the appearance of the diffusion potential in this case is the unequal mobility of ions of dissolved substances. If electrolyte ions have different diffusion rates, then faster ions gradually appear ahead of less mobile ones. As if two waves of differently charged particles are formed.
If solutions of the same substance are mixed, but with different concentrations, then a more dilute solution acquires a charge that coincides in sign with the charge of more mobile ions, and a less dilute one - 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 velocities: I- "fast" ions, negatively charged;
II- "slow" ions, positively charged
The so-called diffusion potential arises at the interface between solutions. It averages the speed of ion movement (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 the cell membranes are damaged. In this case, their permeability is disturbed 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, the damaged tissue is most often charged negatively in relation to the undamaged one.
Diffusion potential arises in galvanic cells at the interface between two solutions. Therefore, with accurate calculations of the emf galvanic circuits must be corrected for its value. To eliminate the influence of the diffusion potential, electrodes in galvanic cells are often connected to each other with a "salt bridge", which is a saturated solution of KCl.
Potassium and chlorine ions have almost the same mobility; therefore, their use makes it possible to significantly reduce the effect of the diffusion potential on the emf value.
The diffusion potential can greatly increase if electrolyte solutions of different compositions or different concentrations are separated by a membrane that is permeable only for 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 ion exchange 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 a 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.
Membrane electrodes with a solid membrane contain a thin membrane, on both sides of which there are different solutions containing the same detectable ions, but with different concentrations. From the inside, the membrane is washed standard solution with a precisely known concentration of the ions to be determined, from the outside - the analyzed solution with an unknown concentration of the ions to be determined.
Due to the different concentration of solutions on both sides of the membrane, ions are exchanged with the inner and outer sides of the membrane in a different way. This leads to the fact that a different electric charge is formed on different sides of the membrane, and as a result of this, a membrane potential difference arises.
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. Elimination of diffusion potential with electrolytic bridges
Generally speaking, the diffusion potentials at the interface between 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 brackets):
Therefore, the diffusion potential must be either eliminated or accurately measured. Elimination of the diffusion potential is achieved by including an additional electrolyte with close values of the cation and anion mobilities into the electrochemical system. In measurements in aqueous solutions, saturated solutions of potassium chloride, potassium nitrate or ammonium are used as such an electrolyte.
Additional electrolyte is connected between the main electrolytes using electrolytic bridges (Fig. 6.12) filled with basic 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 carried by ions K + and C1 -, which are much more than ions of the main electrolyte. Since the mobilities of K + and C1 - ions in potassium chloride are practically equal to each other, the diffusion potential will also 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. So, 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 16.9 mV. If additional electrolytes are included between the 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 (sat.) 2.8
KCl (sat.) 1.5
Elimination of diffusion potentials by incorporating an additional electrolyte with equal ion transfer numbers gives good results when measuring diffusion potentials in unconcentrated solutions with slightly different anion and cation mobilities. When measuring the stresses of systems containing solutions of acids or alkalis
Table 6.3. Diffusion potentials at the KOH - KCl and NaOH - KCl interface (according to V.G. Lokshtanov)
with very different speeds of movement of cation and anion, special care should be taken. For example, at the HC1 - KC1 (saturation) 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). So, the diffusion potential, for example, in the 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 is impossible to achieve a significant reduction using the salt bridge.
To reduce the diffusion potentials to negligible values, Nernst suggested adding a large excess of some electrolyte indifferent for the given system to the contacting solutions. Then the diffusion of the main 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 distortion of the results. Therefore, this method can only be used in those
in 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 the 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, because 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 interface between the two solutions. A continuously renewing border is created by a slow directional movement of solutions parallel to each other. Thus, it is possible to achieve the stability of the diffusion potential and its reproducibility with an accuracy of 0.1 mV.
The diffusion potential is determined by the method of Cohen and Thombrock from measurements of the voltages of two electrochemical systems, and the electrodes of one of them are reversible to the salt cation, and the other to the anion. Let's say you 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 (tv.), ZnSO 4 | ZnSO 4, Hg 2 SO 4 (tv.) | Hg (+)
System voltage 1
system 2
Considering that φ d 21 = - φ d 12, and subtracting the second equation from the first, we get:
When measurements are carried out at not very high concentrations, at which it can still be assumed that = and = or that: =: the last two terms of the last equation cancel out and
The diffusion potential in system 1 can also be determined in a slightly different way, if instead of system 2 we use a double electrochemical system:
3. (-) Zn | ZnSO 4, Hg 2 SO 4 (tv.) | Hg - Hg | Hg 2 SO 4 (tv.), ZnSO 4 | Zn (+)
System voltage Z
Therefore, the difference between the voltages of 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 is usually amenable to accurate calculation, from measurements of E p1 and E p 3, you can determine the value of the diffusion potential.
The diffusion potential at the interface of two different solutions is determined in a similar way. For example, if one wants to determine the diffusion potential at the interface between zinc sulfate and copper chloride solutions, two electrochemical systems are made up:
4. (-) Zn | ZnSO 4 | CuCl 2 | Cu (+)
5. (-) Hg | Hg 2 Cl 2 (tv.), CuCl 2 | ZnSO 4, Hg 2 SO 4 (tv.) | Hg (+)
System voltage 4
system 5
Hence
Naturally, the larger the number of terms included in the equation for the diffusion potential, the lower the probability of a high determination accuracy.
Similar information.
The practically measured exact value of the EMF usually differs from the theoretically calculated one by the Nernst equation by some small value, which is associated with the potential differences arising at the point of contact of different metals (“contact potential”) and different solutions (“diffusion potential”).
Contact potential(more precisely, the contact potential difference) is associated with a different value of the electron work function for each metal. At each given temperature, it is constant for a given combination of metal conductors of a galvanic cell and enters the EMF of the cell as a constant term.
Diffusion potential occurs at the interface between solutions of different electrolytes or the same electrolytes with different concentrations. Its occurrence is explained by the different rate of diffusion of ions from one solution to another. Diffusion of ions is caused by different values of the chemical potential of ions in each of the half-elements. Moreover, its speed changes over time due to a continuous change in concentration, and therefore m ... Therefore, the diffusion potential has, as a rule, an indefinite value, since it is influenced by many factors, including temperature.
In normal practical work, the value of the contact potential is minimized by using wiring with conductors made of the same material (usually copper), and the diffusion potential by using special devices called electrolytic(saline)bridges or electrolytic keys. They are tubes of various configurations (sometimes equipped with taps) filled with concentrated solutions of neutral salts. For these salts, the mobility of the cation and the anion should be approximately equal to each other (For example, KCl, NH 4 NO 3, etc.). In the simplest case, the electrolytic bridge can be made from a strip of filter paper or an asbestos flagellum moistened with a KCl solution. When using electrolytes based on non-aqueous solvents, rubidium chloride is usually used as the neutral salt.
The minimum values of the contact and diffuse potentials achieved as a result of the measures taken are usually neglected. However, for electrochemical measurements requiring high accuracy, contact and diffusion potentials should be considered.
The fact that this galvanic cell has an electrolytic bridge is indicated by a double vertical line in its formula, which stands at the point of contact between two electrolytes. If there is no electrolytic bridge, then a single line is put in the formula.
