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Equilibriums kompleksoobrazovanija in systems «metal - a ligand» and their research on the basis of catalytic kinetic currents

In the analysis sledovyh quantities of ions of metals methods voltamperometricheskogo the analysis are popular enough at the expense of low limits of detection, accuracy, reproducibility, and also cost and speed of the analysis.

Electrolytic accumulation of ions of metals in the form of electroactive complexes with the ligand-catalyst adsorbed on a working electrode became one of ways of sensitisation in these methods.

Many azo compounds, including eriochrome black T [21; 22; 23; 24], display surface-active properties that is displayed in their adsorption from a solution on a solid phase surface including on an electrode surface [39; 59; 71; 81], and also selectively co-ordinate ions of metals [84; 85], that does by their ideal ligands for specific adsorption.

The first description of this process is yielded in works Brdichka [94] in connection with supervision of influence of specific adsorption of reagents and products of electrode reaction on reversible poljarograficheskie waves.

Catalytic [95] in voltamperometricheskom the analysis name
Kinetic currents of restoration simple and complex ions of the metals, displayed on voltamperogrammah in the form of a prewave and proceeding with participation of the ligand-catalyst adsorbed on an electrode.

The catalytic effect of a ligand for voltamperometricheskih definitions of ions of nickel (II), cobalt (II) has very much great value as gidratirovannye and the hydrolyzed ions of nickel (II) and cobalt (II) are reduced on the river k.e. With a high overstrain that result ins to very extended wave unsuitable for voltamperometricheskogo of the analysis, or the wave because of a high overstrain is overlaped by discharge of background electrolyte [96].

At restoration of ions of nickel (II) and cobalt (II) catalytic effect of a ligand result ins to wave displacement in area of positive potentials and to increase in its steepness, and also in occurrence of the prewaves which are responsible kompleksoobrazovanie of an ion of metal with a ligand-catalyst. Such changes speak electrode processes in which participate both simple ions of metal, and the complexes which are available in volume of a solution.

In systems «metal - a ligand» formation of several complexes so, in equilibrium composition of system «metal - a ligand» it is necessary to consider all complex particles, and also constants of their stability [97] is possible.

As a result of the fundamental scientific researches devoted to studying of restoration of complexes of metals, the ligands complicated by specific adsorption and complexes for a case of parallel reversible electrode reactions [95; 96], the general law has been revealed

The superficial catalytic processes basing on passage of parallel superficial catalytic reactions which has made a basis of the concept of series-parallel superficial reactions kompleksoobrazovanija [98].

In the concept of series-parallel superficial reactions kompleksoobrazovanija all chemical reactions localised on are considered

36 surfaces of an electrode, in prielektrodnom bed and in solution volume, as shown in the schema 10 [95; 96].

Ligand-induced adsorption of complexes name process, when simple and the complex ions of metal which are in volume of a solution close to external plane Gelmgoltsa, and the ligand - adsorbed on an electrode katalizatorv result of passage on an electrode parallel

Superficial bimolecular reactions step kompleksoobrazovanija form electroactive комплексы:Из schemas (10)

It is visible, that all particles as in solution volume

Surfaces elektrodanahodjatsja in balance, and ligand -

katalizatorpolnostju it is reclaimed in a catalytic cycle.

Proceeding from the concept of series-parallel superficial reactions kompleksoobrazovanija in the field of a limiting current, speed of catalytic process is defined by total diffusion rate of particles and total speed of their interaction with adsorbed on an electrode

Ligand [96].

In a number of works [45; 69] for a finding of equilibrium constants use voltamperometricheskie methods on the basis of shift of potential of a half wave (∆E12).odnako accuracy and reproducibility of definition of value Е1/2 are frequently insufficient because of effect of adsorption of a ligand and its complexes [95].

Alternative and more optimum methods of a finding of equilibrium constants are the methods based on measurement of limiting catalytic current Ikl.

In references [95; 96] classification of methods of a finding of equilibrium constants by the limiting catalytic current, based on the mechanism of catalytic electrode process which evolves three types of catalytic processes is offered:

1. Catalytic reactions of electroreduction of ions of metal at catalysis by a ligand [63; 81; 99-101];

2. Catalytic reactions of electroreduction of the oxidizer (ligand), catalyzed by metal ions [95; 102];

3. Catalytic reactions of electroreduction of a complex with hydrogen allocation [81; 103; 104; 105].

S.Tribolja [106] was one of the first who has developed an effective method of a finding of equilibrium constants in systems «metal - a ligand», based on measurement of limiting catalytic currents at catalysis by a ligand.

In spite of the fact that recently there was variety of methods of a finding of equilibrium constants on a limiting catalytic current at catalysis by a ligand, it is possible to evolve a little the most simple and universal, but at the same time successfully solving problems of studying of balance kompleksoobrazovanija.

In S.Tribolja's work and employees [106] the method of a finding of the equilibrium constants, based on decrease in a limiting catalytic current has been developed at introduction in system «metal - a ligand» catalyticly inactive ions
Other metal (M '), connecting a ligand-catalyst in catalyticly inactive complex (M'L). The basic condition is that concentration of volume complexes (ML, ML2, ML3и etc.) in system «metal - a ligand» in comparison with concentration of a free ligand could be neglected that low concentration of a ligand in comparison with ion density of metal are reached at the expense of a choice of such catalytic systems where forms unstable complexes, and also use enough.

On decrease limiting catalytic tokapri SM = const find

Share of the ligand-catalyst which has entered into complex M'L.

Constant of stability of complex M'L count by means of the equation (11):

Equilibrium concentration of a free ligand [L] define under the curve of a catalytic current for lack of ions of inactive metal.

Calculation of a constant of stability of complexes βycnвыполняется for the same limiting catalytic current, therefore value of equilibrium concentration of a free ligand in the presence of inactive metal remains to the same, as for lack of inactive metal.

In N.K.Strizhova and J.I.Turjana's works [95; 96] the method of a finding of the equilibrium constants, based on braking of catalytic process with increase in ion density of metal at the expense of linkage of a part of a ligand-catalyst in a volume complex [ML], and, as consequence, decrease in active concentration of ligand-catalyst Lw adsorbed on an electrode has been developed. Thus it is not necessary to find out the mechanism and kinetics of catalytic process.

This method describes dependence of function of the limiting catalytic

39

Stability constants kompleksovv to system «metal - a ligand» can be calculated proceeding from linear zavisimostikak

The relation of a tangent of angle of lean of this prjamojk to a piece cut

Straight line on an axis of ordinates. The basic working condition of this method

Performance of a constancy of activity coefficients and fast establishment of adsorption equilibrium is.

In works [95; 107] N.K.Strizhovym is offered a perspective method of a finding of values of constants of stability of complexes in systems «metal - a ligand». This method does not depend on a parity of sizes Cmи Cl, number of complexes in volume of a solution, number of parallel stages in the schema (10) and a kind of the adsorptive isotherm for a ligand-catalyst.

At the heart of this method of a finding of values of constants of stability βycnв to system «metal - the ligand» lays the theoretical kinetic model (13) describing dependence of the sum of limiting catalytic currents from concentration of a ligand-catalyst:

Where [L] - equilibrium concentration of the ligand, which value it is possible

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Value of equilibrium concentration of a ligand [L] is identical as to solution Cli (Cmi), so for solution Cl2 (См2).

Constants of stability of complexes βycnв to system «metal - a ligand» find under the formula (15):

Having found equilibrium concentration of a ligand [L], it is possible to calculate not only constants of stability of complexes of composition 1:1, but also constants of stability more vysokokoordinirovannyh the complexes which are present at a solution, and also to estimate prospects of their existence in investigated conditions.

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A source: Abramova Natalia Stepanovna. ELECTROCATALYTIC REACTIONS of COMPLEXES of NICKEL (II) And COBALT (II) With ERIOCHROME BLACK T ON the MERCURY DRIPPING ELECTRODE. The dissertation on competition of a scientific degree of a Cand.Chem.Sci. Krasnodar - 2017. 2017

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