As already indicated, concentration chains have a large practical significance, since with their help it is possible to determine such important quantities as the activity coefficient and activity of ions, the solubility of slightly soluble salts, transfer numbers, etc. Such chains are practically easy to implement and the relationships connecting the EMF of the concentration chain with the activities of the ions are also simpler than for other chains. Let us recall that an electrochemical circuit containing the boundary of two solutions is called a transfer circuit and its diagram is depicted as follows:

Me 1 ½ solution (I) solution (II) ½ Me 2 ½ Me 1,

where the dotted vertical line indicates the existence of a diffusion potential between two solutions, which is galvani - the potential between points located in different directions chemical composition phases, and therefore cannot be accurately measured. The magnitude of the diffusion potential is included in the amount for calculating the EMF of the circuit:

The small value of the EMF of a concentration chain and the need for its accurate measurement make it especially important to either completely eliminate or accurately calculate the diffusion potential that arises at the boundary of two solutions in such a chain. Consider the concentration chain

Me½Me z+ ½Me z+ ½Me

Let us write the Nernst equation for each of the electrodes of this circuit:

for left

for the right

Let us assume that the activity of metal ions at the right electrode is greater than that at the left, i.e.

Then it is obvious that j 2 is more positive than j 1 and the emf of the concentration circuit (E k) (without diffusion potential) is equal to the potential difference j 2 – j 1.

Hence,

, (7.84)

then at T = 25 0 C , (7.85)

where and are the molal concentrations of Me z + ions; g 1 and g 2 are the activity coefficients of Me z + ions, respectively, at the left (1) and right (2) electrodes.

a) Determination of average ionic activity coefficients of electrolytes in solutions

To most accurately determine the activity coefficient, it is necessary to measure the EMF of the concentration chain without transfer, i.e. when there is no diffusion potential.

Consider an element consisting of a silver chloride electrode immersed in a solution of HCl (molality C m) and a hydrogen electrode:

(–) Pt, H 2 ½HCl½AgCl, Ag (+)

Processes occurring on the electrodes:

(–) H 2 ® 2H + + 2

(+) 2AgCl + 2 ® 2Ag + 2Cl –

current-generating reaction H 2 + 2AgCl ® 2H + + 2Ag + 2Cl –

Nernst equation

for hydrogen electrode: ( = 1 atm)

for silver chloride:

It is known that

= (7.86)

Considering that the average ionic activity for HCl is

And ,

where C m is the molal concentration of the electrolyte;

g ± – average ionic activity coefficient of the electrolyte,

we get (7.87)

To calculate g ± from EMF measurement data, it is necessary to know the standard potential of the silver chloride electrode, which in this case will also be the standard EMF value (E 0), since The standard potential of a hydrogen electrode is 0.

After transforming equation (7.6.10) we get

(7.88)

Equation (7.6.88) contains two unknown quantities j 0 and g ±.

According to the Debye–Hückel theory for dilute solutions of 1-1 electrolytes

lng ± = –A ,

where A is the coefficient of Debye’s limit law and, according to reference data for this case, A = 0.51.

Therefore, the last equation (7.88) can be rewritten as follows:

(7.89)

To determine, build a dependence graph from and extrapolate to C m = 0 (Fig. 7.19).

Rice. 7.19. Graph for determining E 0 when calculating g ± HCl solution

The segment cut off from the ordinate axis will be the value j 0 of the silver chloride electrode. Knowing , you can use the experimental values ​​of E and the known molality for a solution of HCl (C m), using equation (7.6.88), to find g ±:

(7.90)

b) Determination of the solubility product

Knowledge of standard potentials makes it easy to calculate the solubility product of a sparingly soluble salt or oxide.

For example, consider AgCl: PR = L AgCl = a Ag + . a Cl –

Let us express L AgCl in terms of standard potentials, according to the electrode reaction

AgCl – AgCl+,

running on a type II electrode

Cl – / AgCl, Ag

And the reactions Ag + + Ag,

running on the I-type electrode with a current-generating reaction

Cl – + Ag + ®AgCl

; ,

because j 1 = j 2 (electrode is the same) after transformation:

(7.91)

= PR

The values ​​of standard potentials are taken from the reference book, then it is easy to calculate the PR.

c) Diffusion potential of the concentration chain. Definition of carry numbers

Consider a conventional concentration chain using a salt bridge to eliminate the diffusion potential

(–) Ag½AgNO 3 ½AgNO 3 ½Ag (+)

The emf of such a circuit without taking into account the diffusion potential is equal to:

(7.92)

Consider the same circuit without a salt bridge:

(–) Ag½AgNO 3 AgNO 3 ½Ag (+)

EMF of the concentration circuit taking into account the diffusion potential:

E KD = E K + j D (7.93)

Let 1 faraday of electricity pass through the solution. Each type of ion transfers a portion of this amount of electricity equal to its transport number (t + or t –). The amount of electricity that cations and anions will transfer will be equal to t +. F and t – . F accordingly. At the border of contact of two AgNO 3 solutions of different activities, a diffusion potential (j D) arises. Cations and anions, overcoming (j D), perform electrical work.

Per 1 mole:

DG = –W el = – zFj D = – Fj d (7.94)

In the absence of diffusion potential, ions perform only chemical work when crossing the solution boundary. In this case, the isobaric potential of the system changes:

Similarly for the second solution:

(7.98)

Then according to equation (7.6.18)

(7.99)

Let us transform expression (7.99), taking into account expression (7.94):

(7.100)

(7.101)

Transport numbers (t + and t –) can be expressed in terms of ionic conductivities:

;

Then (7.102)

If l – > l +, then j d > 0 (diffusion potential helps the movement of ions).

If l + > l – , then j d< 0 (диффузионный потенциал препятствует движению ионов, уменьшает ЭДС). Если l + = l – , то j д = 0.

If we substitute the value jd from equation (7.101) into equation (7.99), we obtain

E KD = E K + E K (t – – t +), (7.103)

after conversion:

E KD = E K + (1 + t – – t +) (7.104)

It is known that t + + t – = 1; then t + = 1 – t – and the expression

(7.105)

If we express ECD in terms of conductivity, we get:

E KD = (7.106)

By measuring ECD experimentally, it is possible to determine the transport numbers of ions, their mobility and ionic conductivity. This method is much simpler and more convenient than the Hittorf method.

Thus, using the experimental determination of various physicochemical quantities, it is possible to carry out quantitative calculations to determine the EMF of the system.

Using concentration chains, it is possible to determine the solubility of poorly soluble salts in electrolyte solutions, activity coefficient and diffusion potential.

Electrochemical kinetics

If electrochemical thermodynamics studies equilibria at the electrode-solution boundary, then measuring the rates of processes at this boundary and elucidating the laws to which they obey is the object of studying the kinetics of electrode processes or electrochemical kinetics.

Electrolysis

Faraday's laws

Since the passage electric current through electrochemical systems is associated with a chemical transformation, then there must be a certain relationship between the amount of electricity and the amount of reacted substances. This dependence was discovered by Faraday (1833-1834) and was reflected in the first quantitative laws of electrochemistry, called Faraday's laws.

Electrolysis – the occurrence of chemical transformations in an electrochemical system when an electric current from an external source is passed through it. By electrolysis it is possible to carry out processes whose spontaneous occurrence is impossible according to the laws of thermodynamics. For example, the decomposition of HCl (1M) into elements is accompanied by an increase in the Gibbs energy of 131.26 kJ/mol. However, under the influence of electric current this process can easily be carried out.

Faraday's first law.

The amount of substance reacted on the electrodes is proportional to the strength of the current passing through the system and the time of its passage.

Mathematically expressed:

Dm = keI t = keq, (7.107)

where Dm is the amount of reacted substance;

kе – some proportionality coefficient;

q – amount of electricity equal to the product of force

current I for time t.

If q = It = 1, then Dm = k e, i.e. the coefficient k e represents the amount of substance that reacts when a unit amount of electricity flows. The proportionality coefficient k e is called electrochemical equivalent . Since different quantities can be chosen as a unit of the amount of electricity (1 C = 1A. s; 1F = 26.8 A. h = 96500 K), then for the same reaction one should distinguish between electrochemical equivalents related to these three units : A. with k e, A. h k e and F k e.

Faraday's second law.

During the electrochemical decomposition of various electrolytes with the same amount of electricity, the content of the electrochemical reaction products obtained on the electrodes is proportional to their chemical equivalents.

According to Faraday's second law, at a constant amount of electricity passed, the masses of reacted substances are related to each other as their chemical equivalents A.

. (7.108)

If we choose the faraday as the unit of electricity, then

Dm 1 = F k e 1; Dm 2 = F k e 2 and Dm 3 = F k e 3, (7.109)

(7.110)

The last equation allows us to combine both Faraday's laws in the form of one general law, according to which an amount of electricity equal to one Faraday (1F or 96500 C, or 26.8 Ah) always electrochemically changes one gram equivalent of any substance, regardless of its nature .

Faraday's laws apply not only to aqueous and non-aqueous salt solutions at ordinary temperatures, but are also valid in the case of high-temperature electrolysis of molten salts.

Substance output by current

Faraday's laws are the most general and precise quantitative laws of electrochemistry. However, in most cases, a smaller amount of a given substance undergoes electrochemical change than calculated on the basis of Faraday's laws. So, for example, if you pass a current through an acidified solution of zinc sulfate, then when 1F of electricity passes, not 1 g-eq of zinc is usually released, but about 0.6 g-eq. If solutions of chlorides are subjected to electrolysis, then as a result of passing 1F electricity, not one, but a little more than 0.8 g-equiv of chlorine gas is formed. Such deviations from Faraday's laws are associated with the occurrence of side electrochemical processes. In the first of the examples discussed, two reactions actually occur at the cathode:

zinc precipitation reaction

Zn 2+ + 2 = Zn

and the reaction to form hydrogen gas

2Н + + 2 = Н 2

The results obtained during the release of chlorine will also not contradict Faraday’s laws, if we take into account that part of the current is spent on the formation of oxygen and, in addition, the chlorine released at the anode can partially go back into solution due to secondary chemical reactions, for example according to the equation

Cl 2 + H 2 O = HCl + HСlO

To take into account the influence of parallel, side and secondary reactions, the concept was introduced current output P . Current output is the portion of the amount of electricity flowing that accounts for a given electrode reaction.

R = (7.111)

or as a percentage

R = . 100 %, (7.112)

where q i is the amount of electricity spent on this reaction;

Sq i is the total amount of electricity passed.

So, in the first example, the current efficiency of zinc is 60%, and that of hydrogen is 40%. Often the expression for current efficiency is written in a different form:

R = . 100 %, (7.113)

where q p and q p are the amount of electricity, respectively calculated according to Faraday’s law and actually used for the electrochemical transformation of a given amount of substance.

You can also define the current output as the ratio of the amount of changed substance Dm p to that which would have to react if all the current were spent only on this reaction Dm p:

R = . 100 %. (7.114)

If only one of several possible processes is desired, then it is necessary that its current output be as high as possible. There are systems in which all the current is spent on just one electrochemical reaction. Such electrochemical systems are used to measure the amount of electricity passed and are called coulometers, or coulometers.

Diffusion potential

In electrochemical circuits, potential jumps occur at the interfaces between unequal electrolyte solutions. For two solutions with the same solvent, such a potential jump is called diffusion potential. At the point of contact between two solutions of the spacecraft electrolyte, which differ from each other in concentration, diffusion of ions occurs from solution 1, which is more concentrated, into solution 2, which is more dilute. Typically, the diffusion rates of cations and anions are different. Let us assume that the rate of diffusion of cations is greater than the rate of diffusion of anions. Over a certain period of time, more cations than anions will pass from the first solution to the second. As a result, solution 2 will receive an excess of positive charges, and solution 1 will receive an excess of negative charges. Since solutions acquire electrical charges, the rate of diffusion of cations decreases, that of anions increases, and over time these rates become the same. IN stationary state the electrolyte diffuses as a single unit. In this case, each solution has a charge, and the potential difference established between the solutions corresponds to the diffusion potential. Calculation of the diffusion potential is generally difficult. Taking into account some assumptions, Planck and Henderson derived formulas for calculating the central value. So, for example, when two solutions of the same electrolyte with different activities come into contact (b1b2)

where and are the maximum molar electrical conductivities of ions. The value of the CD is small and in most cases does not exceed several tens of millivolts.

EMF of an electrochemical circuit taking into account the diffusion potential

……………………………….(29)

Equation (29) is used to calculate (or) from the measurement results of E if (or) and are known. Since determining the diffusion potential is associated with significant experimental difficulties, it is convenient to eliminate the EMF when measuring it using a salt bridge. The latter contains a concentrated electrolyte solution, the molar electrical conductivities of the ions are approximately the same (KCl, KNO3). A salt bridge, which contains, for example, KS1, is placed between electrochemical solutions, and instead of one liquid boundary, two appear in the system. Since the concentration of ions in the KC1 solution is significantly higher than in the solutions it connects, almost only K+ and C1- ions diffuse through the liquid boundaries, at which very small and opposite-sign diffusion potentials arise. Their sum can be neglected.

Structure of the electrical double layer

The transition of charged particles across the solution-metal boundary is accompanied by the appearance of an electric double layer (DEL) and a potential jump at this boundary. An electrical double layer is created electric charges, located on the metal, and ions of opposite charge (counterions) oriented in the solution near the surface of the electrode.

In the formation of the ion plate d.e.s. Both electrostatic forces take part, under the influence of which the counterions approach the electrode surface, and the forces of thermal (molecular) motion, as a result of which the d.e.f. acquires a blurry, diffuse structure. In addition, in the creation of a double electrical layer at the metal-solution interface, the effect of specific adsorption of surface-active ions and molecules that may be contained in the electrolyte plays a significant role.

The structure of the electric double layer in the absence of specific adsorption. Under the building of the D.E.S. understand the charge distribution in its ionic plate. To put it simply, the ion plate can be divided into two parts: 1) dense, or Helmholtz, formed by ions that come almost close to the metal; 2) diffuse, created by ions located at distances from the metal exceeding the radius of the solvated ion (Fig. 1). The thickness of the dense part is about 10-8 cm, the diffuse part is 10-7-10-3 cm. According to the law of electrical neutrality

……………………………..(30)

where, is the charge density on the metal side, on the solution side, in the dense diffusion part of the emp. respectively.

Fig.1. The structure of the electrical double layer at the solution-metal interface: ab - dense part; bv - diffuse part

The potential distribution in the ionic plate of the electric double layer, reflecting its structure, is presented in Fig. 2. The magnitude of the potential jump μ at the solution-metal interface corresponds to the sum of the magnitudes of the potential drop in the dense part of the emp and in the diffuse part. Structure of the D.E.S. is determined by the total concentration of the solution. As it increases, the diffusion of counteractive substances from the surface of the metal into the mass of the solution is weakened, as a result of which the size of the diffuse part is reduced. This leads to a change in -potential. In concentrated solutions, the diffuse part is practically absent, and the double electric layer is similar to a flat capacitor, which corresponds to the model of Helmholtz, who first proposed the theory of the structure of electrical power.

Fig.1. Potential distribution in the ion plate at different solution concentrations: ab - dense part; bv - diffuse part; ts is the potential difference between the solution and the metal; w, w1 - potential drop in the dense and diffuse parts of the emp.

The structure of the electric double layer under conditions of specific adsorption. Adsorption - the concentration of a substance from the volume of phases at the interface between them - can be caused by both electrostatic forces and forces of intermolecular interaction and chemical ones. Adsorption caused by forces of non-electrostatic origin is usually called specific. Substances that can be adsorbed at the interface are called surface-active agents (surfactants). These include most anions, some cations, and many molecular compounds. The specific adsorption of the surfactant contained in the electrolyte affects the structure of the double layer and the value of the -potential (Fig. 3). Curve 1 corresponds to the potential distribution in the electric double layer in the absence of a surfactant in solution. If the solution contains substances that produce surface-active cations upon dissociation, then due to specific adsorption by the metal surface, the cations will enter the dense part of the double layer, increasing its positive charge (curve 2). Under conditions that enhance adsorption (for example, an increase in adsorbate concentration), the dense part may contain an excess amount of positive charges compared to the negative charge of the metal (curve 3). From the potential distribution curves in the double layer it is clear that the -potential changes during the adsorption of cations and may have a sign opposite to that of the electrode potential.

Fig.3.

The effect of specific adsorption is also observed on an uncharged metal surface, i.e. under conditions where there is no exchange of ions between the metal and the solution. The adsorbed ions and corresponding counterions form an electrical double layer located in close proximity to the metal on the solution side. Adsorbed polar molecules (surfactant, solvent) oriented near the metal surface also create an electrical double layer. The potential jump corresponding to the electric double layer with an uncharged metal surface is called the zero charge potential (ZPC).

The potential of zero charge is determined by the nature of the metal and the composition of the electrolyte. When adsorption of cations p.n.s. becomes more positive, anions - more negative. Zero charge potential is an important electrochemical characteristic of electrodes. At potentials close to p.s.e., some properties of metals reach limiting values: surfactant adsorption is high, hardness is maximum, wettability by electrolyte solutions is minimal, etc.

The results of research in the field of the theory of the electric double layer made it possible to more broadly consider the issue of the nature of the potential jump at the solution-metal interface. This jump is due to the following reasons: the transition of charged particles across the interface (), specific adsorption of ions () and polar molecules (). The galvanic potential at the solution-metal interface can be considered as the sum of three potentials:

……………………………..(31)

Under conditions under which the exchange of charged particles between the solution and the metal, as well as the adsorption of ions, does not occur, there still remains a potential jump caused by the adsorption of solvent molecules - . Galvani potential can be equal to zero only when and compensate each other.

At present, there are no direct experimental and computational methods for determining the magnitude of individual potential jumps at the solution-metal interface. Therefore, the question of the conditions under which the potential jump becomes zero (the so-called absolute zero potential) remains open for now. However, to solve most electrochemical problems, knowledge of individual potential jumps is not necessary. It is enough to use the values ​​of electrode potentials expressed in a conventional scale, for example, a hydrogen scale.

The structure of the electrical double layer does not affect the thermodynamic properties of equilibrium electrode systems. But when electrochemical reactions occur under nonequilibrium conditions, the ions are influenced electric field double layer, which leads to a change in the speed of the electrode process.

At the boundary of two unequal solutions, a potential difference always arises, which is called the diffusion potential. The emergence of such a potential is associated with the unequal mobility of cations and anions in solution. The magnitude of diffusion potentials usually does not exceed several tens of millivolts, and they are usually not taken into account. However, with accurate measurements, special measures are taken to reduce them as much as possible. The reasons for the occurrence of the diffusion potential were shown using the example of two adjacent solutions of copper sulfate of different concentrations. Cu2+ and SO42- ions will diffuse across the interface from more concentrated solution in less concentrated. The rates of movement of Cu2+ and SO42- ions are not the same: the mobility of SO42- ions is greater than the mobility of Cu2+. As a result, an excess of negative SO42- ions appears at the solution interfaces on the side of the solution with a lower concentration, and an excess of Cu2+ appears on the more concentrated side. A potential difference arises. The presence of excess negative charge at the interface will inhibit the movement of SO42- and accelerate the movement of Cu2+. At a certain potential, the rates of SO42- and Cu2+ will become the same; a stationary value of the diffusion potential will be established. The theory of diffusion potential was developed by M. Planck (1890), and subsequently by A. Henderson (1907). The calculation formulas they obtained are complex. But the solution is simplified if the diffusion potential arises at the boundary of two solutions with different concentrations C1 and C2 of the same electrolyte. In this case, the diffusion potential is equal. Diffusion potentials arise during nonequilibrium diffusion processes, therefore they are irreversible. Their magnitude depends on the nature of the boundary of two contacting solutions, on the size and their configuration. Accurate measurements use techniques that minimize the magnitude of the diffusion potential. For this purpose, an intermediate solution with the lowest possible mobility values ​​of U and V (for example, KCl and KNO3) is included between solutions in half-cells.

Diffusion potentials play an important role in biology. Their occurrence is not associated with metal electrodes. It is interfacial and diffusion potentials that generate biocurrents. For example, in electric stingrays and eels, a potential difference of up to 450 V is created. Biopotentials are sensitive to physiological changes in cells and organs. This is the basis for the use of electrocardiography and electroencephalography methods (measurement of the biocurrents of the heart and brain).

55. Interfluid phase potential, mechanism of occurrence and biological significance.

A potential difference also arises at the boundary of contact of immiscible liquids. Positive and negative ions in these solvents are distributed unevenly, and their distribution coefficients do not coincide. Therefore, a potential jump occurs at the interface between liquids, which prevents the unequal distribution of cations and anions in both solvents. In the total (total) volume of each phase, the number of cations and anions is almost the same. It will differ only at the phase interface. This is the interfluid potential. Diffusion and interfluid potentials play an important role in biology. Their occurrence is not associated with metal electrodes. It is interfacial and diffusion potentials that generate biocurrents. For example, in electric stingrays and eels, a potential difference of up to 450 V is created. Biopotentials are sensitive to physiological changes in cells and organs. This is the basis for the use of electrocardiography and electroencephalography methods (measurement of the biocurrents of the heart and brain).

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 numbers transfer (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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