DETERMINATION OF POLYLIGAND COMPLEXES OF COBALT (II) WITH CITRATE AND PYROPHOSPHATE IONS
In the work it is shown by the spectrophotometry method that depending on the concentration ratio of ligands [PPi4–]/[Cit3–] in the pyrophosphate-citrate electrolyte, cobalt (II) ions form not only citrate [Co(Cit)2]4– and pyrophosphate [Co(PPi)2]6–, but also polyligand complexes [Co(PPi)m(Cit)n]+2–...
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| Дата: | 2022 |
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V.I.Vernadsky Institute of General and Inorganic Chemistry
2022
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Репозитарії
Ukrainian Chemistry Journal| _version_ | 1871465822870831104 |
|---|---|
| author | Nikitenko, Vasyl Kublanovsky, Valeriy Yapontseva, Yuliya |
| author_facet | Nikitenko, Vasyl Kublanovsky, Valeriy Yapontseva, Yuliya |
| author_institution_txt_mv | [
{
"author": "Vasyl Nikitenko",
"institution": "Vernadsky Institute of General & Inorganic Chemistry of National Academy of Sciences of Ukraine, Academic Palladin Avenue, 32\/34, Kyiv, 03142"
},
{
"author": "Valeriy Kublanovsky",
"institution": "Vernadsky Institute of General & Inorganic Chemistry of National Academy of Sciences of Ukraine, Academic Palladin Avenue, 32\/34, Kyiv, 03142"
},
{
"author": "Yuliya Yapontseva",
"institution": "V.I. Vernadsky Institute of General and Inorganic Chemistry of NAS of Ukraine, Palladin av., 32-34, 03680, Kiyev, Ukraine"
}
] |
| author_sort | Nikitenko, Vasyl |
| baseUrl_str | https://ucj.org.ua/index.php/journal/oai |
| collection | OJS |
| datestamp_date | 2026-07-22T08:23:49Z |
| description | In the work it is shown by the spectrophotometry method that depending on the concentration ratio of ligands [PPi4–]/[Cit3–] in the pyrophosphate-citrate electrolyte, cobalt (II) ions form not only citrate [Co(Cit)2]4– and pyrophosphate [Co(PPi)2]6–, but also polyligand complexes [Co(PPi)m(Cit)n]+2–(4m+3n). The composition of polyligand complexes [Co(PPi)Cit]5– was determined, and the equilibrium constant of the reaction of their formation and the constant of their stability were calculated (pβ=8.47). The dependence of the degree of formation of citrate, polyligand, and pyrophosphate complexes of cobalt (II) in the pyrophosphate-citrate electrolyte on the logarithm of the ratio of equilibrium concentrations of ligands is calculated. |
| doi_str_mv | 10.33609/2708-129X.88.04.2022.113-122 |
| first_indexed | 2025-09-24T17:43:44Z |
| format | Article |
| fulltext |
113
UDC 541.49. doi: 10.33609/2708-129X.88.04.2022.113-122
DETERMINATION OF POLYLIGAND COMPLEXES OF COBALT (II)
WITH CITRATE AND PYROPHOSPHATE IONS
V. М. Nikitenko, Yu. S. Yapontseva*, V. S. Kublanovsky
V. I. Vernadsky Institute of General and Inorganic Chemistry of the NAS of Ukraine,
Acad. Palladina av. 32/34, 03142 Kyiv, Ukraine
*е-mail: juliya_yap@ukr.net
In the work it is shown by the spectrophotometry method that depending on the concen-
tration ratio of ligands [PPi4–]/[Cit3–] in the pyrophosphate-citrate electrolyte, cobalt (II) ions
form not only citrate [Co(Cit)2]
4– and pyrophosphate [Co(PPi)2]
6–, but also polyligand com-
plexes [Co(PPi)m(Cit)n]+2–(4m+3n). The composition of polyligand complexes [Co(PPi)Cit]5– was
determined, and the equilibrium constant of the reaction of their formation and the constant
of their stability were calculated (pβ = 8.47). The dependence of the degree of formation of
citrate, polyligand, and pyrophosphate complexes of cobalt (II) in the pyrophosphate-citrate
electrolyte on the logarithm of the ratio of equilibrium concentrations of ligands is calculated.
Keywords: cobalt (II), polyligand complex, composition, stability constant, spectro
photometry.
INTRODUCTION. The structure of metal
and alloy coatings deposited from complex
electrolytes, the uniformity of their distribution
on the sample surface are determined by the
magnitude of the overvoltage of the cathodic
process – a preliminary chemical reaction and
the magnitude of activation energy of electro-
chemically active complexes (EAC). Overvolt-
age is the most important characteristic of the
electrode process, as it affects the morphology,
structure and functional properties of metal
and alloy coatings. In the development of tech-
nological processes for coating from complex
electrolytes not only the composition and sta-
bility of coordination compounds, but also the
geometric structure and electronic configura-
tion of the EAC are important. These factors
ultimately determine the rate and mechanism
of electrode processes.
Monoligand and polyligand (pyrophos-
phate, citrate and pyrophosphate-citrate) elec-
trolytes have been widely used for the deposi-
tion of coatings of binary and ternary alloys of
refractory metals (Mo, W and Re) with iron
subgroup metals (Fe, Co, Ni), in particular co-
balt. [1].
A pyrophosphate-citrate electrolyte was
chosen as the study object because it allows
the deposition of high-quality coatings of mo-
lybdenum, tungsten, and rhenium alloys with
114 ISSN 2708-129X. Укр. хім. журн., 2022
DETERMINATION OF POLYLIGAND COMPLEXES OF COBALT (II) WITH CITRATE AND PYROPHOSPHATE IONSPHISICAL CHEMISTRY
cobalt with a higher current efficiency than
monoligand electrolytes [1–9]. These alloys
have valuable physicochemical and operation-
al properties [2], which make them indispen-
sable in practical use.
Determining the ionic composition in the
bulk electrolyte for polyligand systems such
as electrolyte containing cobalt (II) ions and
two ligands simultaneously – citrate (Cit3–)
and pyrophosphate (PPi4–) is an important and
challenging task. Information on the compo-
sition of complex compounds in the bulk of
pyrophosphate-citrate electrolyte, kinetics and
mechanism of deposition of alloys will allow to
control the relevant processes of their produc-
tion, and hence the structure and functional
properties of the resulting coatings.
The aim of the work aim is to establish the
composition and determine the stability con-
stant of polyligand complexes of cobalt (II)
with Cit3– and PPi4– ions by spectrophotomet-
ric method and to study the effect of the ionic
composition in the electrolyte on the ratio of
ligand equilibrium concentrations.
Cobalt (II) ions form with Cit3- and PPi4-
ions, depending on the solution pH and the
equilibrium concentration of ligands, pro-
tonated and unprotonated pyrophosphate
[CoHPPi]–, [Co(HPPi)2]
4–, [Co(HPPi)(PPi)]5–,
[CoPPi]2–, [Co(PPi)2]
6– and citrate [CoH2Cit]+,
[CoHCit], [CoCit]–, [Co(Cit)2]
4– complexes
[10, 11]. Polymeric, polynuclear complex-
es of cobalt (II) with citrate of the composi-
tions [Co4(Cit)4 [Co(H2O)5]2
4–, K3[Co2(Cit)2
(H2O)4]·6H2O were synthesized and isolated in
solid the state [12, 13] and others, which have
magnetic properties [13] and biocompatibility
[14, 15]. The thermodynamics of protonated
cobalt (II) citrate complexes formation have
also been studied [16]. There are no data on
the possibility of formation of polyligand com-
plexes of cobalt (II) with Cit3– and PPi4– ions
in the pyrophosphate-citrate electrolyte, their
composition and stability constant are not
available in the literature.
Spectrophotometry is one of the most pre-
tentious methods for determining the com-
position and stability constants of colored
complex compounds. General characteristics
of methods for determining the composition
and stability constants of metal complex-
es from spectrophotometric measurements,
their advantages, scope, limitations and dis-
advantages are detailed in the monograph of
A.K. Babko [17].
The spectrophotometric study of the forma-
tion of polyligand complexes of cobalt (II) with
Cit3- and PPi4- ions involves significant difficul-
ties, because along with them in the system
under study, depending on the pH of the solu-
tion, there may also exist monoligand proto-
nated and unprotonated citrate and pyrophos
phate complexes of Co (II). The distribution of
complexes in the bulk electrolyte in this case
strongly depends on the equilibrium concen-
tration of metal ions, the ratio of the equilibri-
um concentrations of free ligands and the pH
of the solution.
Since Cit3– and PPi4– ions are anions of
weak acids, their equilibrium concentrations
in the test solution depend on the pH of the
solution and can be determined by the pH-po-
tentiometric method. Therefore, when con-
ducting spectrophotometric studies it is nec-
essary to maintain the optimal and constant
concentration of hydrogen ions (pH = const)
to form complex ions of constant composition
at a certain ratio of the concentrations of the
main components of the investigated solution
СCo
2+:CPPi
4– and СCo
2+:CCit
3– = const.
115https://ucj.org.ua
V. М. Nikitenko, Yu. S. Yapontseva, V. S. Kublanovsky UCJ № 4 / Vol. 88
The values of the equilibrium concentra-
tions of free ligands [PPi]4– and [Cit]3– were de-
termined from the spectrophotometric studies,
considering their total concentrations and the
pH according to the equation (1) [18, 19]:
3
method. Therefore, when conducting spectrophotometric studies it is necessary to maintain the
optimal and constant concentration of hydrogen ions (pH=const) to form complex ions of constant
composition at a certain ratio of the concentrations of the main components of the investigated
solution СCo
2+:CPPi
4– and СCo
2+:CCit
3– = const.
The values of the equilibrium concentrations of free ligands [PPi]4– and [Cit]3– were
determined from the spectrophotometric studies, considering their total concentrations and the pH
according to the equation (1) [18, 19]:
4
1
2*
][1
)][(
][
2
i
i
i
i
CoLHm
HK
CoCnC
L m , (1)
where CHmL, [L]m– are the total and equilibrium concentrations of ligands (citrate and
pyrophosphate) in the test solution (mol·l–1) respectively; CCo
2+ and [Co]2+ are the total and
equilibrium concentrations of cobalt (II) ions in the test solution (mol·l–1) respectively; n* is the
average coordination number of pyrophosphate and citrate complexes formed in the test solution; Ki
is the general stability constants of citrate and pyrophosphate ions (pK1 = 5.68; pK2 = 10.03; pK3 =
12.90 і pK1 = 8.30; pK2 = 14.30; pK3 = 17.00; pK4 = 19.50) [20].
The equilibrium concentrations of free ligands [Cit]3– and [PPi]4–, determined by
spectrophotometric studies conditions based on their total concentrations, mol·l–1: CCo
2+ = 0.01;
CPPi
4– = 0.05; CCit
3– = 0.05 at the solution pH 9.0 according to equation (1) [18, 19] are 3.0·10–2 and
2.5·10–2 (n* = 2); 4.0·10–2 and 3.3·10–2 (n* =1), respectively. The ratio of free ligand equilibrium
concentrations [PPi]4:[Cit]3– in a pyrophosphate-citrate electrolyte at a pH of 9.0 practically does
not change, that is [Cit]3–/[PPi]4–= 1.2 at 1 n* 2.
The relation between the absorption and composition of the studied solution in the ideal case
described by the Lambert-Bouguer-Beer law [21, 22] is:
D = l ·.∑ i Ci , (2)
where D is the absorption of the solution; l is the thickness of the absorbing layer (cuvette length,
cm); i is the molar absorption coefficient of the i-th particle at a given wavelength and
temperature; Ci is the molar concentration of the absorbing particle.
The main requirement for the spectrophotometric method is that the absorption of the
solution D, property of the measured system must be, according to the Lambert-Beer law, a strictly
linear function of the molar concentration of absorbing complexes Ci. In addition, the colored
complex should have high stability (stability constant), constant composition (in a wide range of
ratios of the concentrations of the main components and pH of the solution), as well as high color
intensity per 1 g mole of the substance (molar absorption coefficient i) [23 , 24].
, (1)
where CHmL, [L]m– are the total and equilibri-
um concentrations of ligands (citrate and py-
rophosphate) in the test solution (mol·l–1) re-
spectively; CCo
2+ and [Co]2+ are the total and
equilibrium concentrations of cobalt (II) ions
in the test solution (mol·l–1) respectively; n* is
the average coordination number of pyrophos-
phate and citrate complexes formed in the test
solution; Ki is the general stability constants
of citrate and pyrophosphate ions (pK1 = 5.68;
pK2 = 10.03; pK3 = 12.90 і pK1 = 8.30; pK2 =
14.30; pK3 = 17.00; pK4 = 19.50) [20].
The equilibrium concentrations of free li
gands [Cit]3– and [PPi]4–, determined by spec-
trophotometric studies conditions based on
their total concentrations, mol·l–1: CCo
2+ = 0.01;
CPPi
4– = 0.05; CCit
3– = 0.05 at the solution pH 9.0
according to equation (1) [18, 19] are 3.0·10–2
and 2.5·10–2 (n* = 2); 4.0·10–2 and 3.3·10–2
(n* = 1),respectively. The ratio of free ligand
equilibrium concentrations [PPi]4 : [Cit]3–
in a pyrophosphate-citrate electrolyte at a
pH of 9.0 practically does not change, that is
[Cit]3– / [PPi]4– = 1.2 at 1 ≤ n* ≤ 2.
The relation between the absorption and
composition of the studied solution in the ide-
al case described by the Lambert – Bouguer –
Beer law [21, 22] is:
D = l ·.∑ εi Ci , (2)
where D is the absorption of the solution; l is
the thickness of the absorbing layer (cuvette
length, cm); εi is the molar absorption coeffi-
cient of the i-th particle at a given wavelength λ
and temperature; Ci is the molar concentration
of the absorbing particle.
The main requirement for the spectropho-
tometric method is that the absorption of the
solution D, property of the measured system
must be, according to the Lambert – Beer law,
a strictly linear function of the molar concen-
tration of absorbing complexes Ci. In addition,
the colored complex should have high stability
(stability constant), constant composition (in
a wide range of ratios of the concentrations of
the main components and pH of the solution),
as well as high color intensity per 1 g mole of
the substance (molar absorption coefficient εi)
[23, 24].
EXPERIMENT AND DISCUSSION OF
THE RESULTS. Composition of the studied
solutions, mol·1-1: Co2+ = 0.01; PPi4– = 0.05;
Cit3– = 0.05; Na2SO4 = 0.30; pH 9.0. Analytical
grade reagents were used to prepare electro-
lytes. The pH of the studied solutions was cor-
rected on an electronic pH meter (pH-150MI)
using NaOH and Na2SO4.
The absorption spectra of the studied solu-
tions of cobalt (II) complexes were recorded on
a UV-VIS Spectrophotometer UV mini 1240
(Shimadzu) in a 1 cm cuvette.
The possibility of the formation of polyli-
gand complexes of cobalt (II) from Cit3- and
PPi4- ions was studied by the method of isomo-
lar series [17, 21, 25] at a constant ionic strength
of the solution (0.3 mol·1-1 Na2SO4 ) using ini-
tial solutions with a constant ratio of the con-
centrations of ions СCo
2+ : CCit
3– = 1 : 5 and
СCo
2+ : CPPi
4– = 1 : 5. The use of this solution
is due to the fact that at pH 9.0, in contrast to
the citrate solution with СCo
2+ : CCit
3– = 1 : 2 it is
116 ISSN 2708-129X. Укр. хім. журн., 2022
DETERMINATION OF POLYLIGAND COMPLEXES OF COBALT (II) WITH CITRATE AND PYROPHOSPHATE IONSPHISICAL CHEMISTRY
impossible to experimentally prepare an initial
pyrophosphate solution with a constant con-
centration ratio of СCo
2+ : СPPi
4– = 1 : 2 because
under these conditions, partial hydrolysis of the
cobalt(II) salt and, as a result, clouding of the
solution take place, since the solubility product
of cobalt(II) hydroxide SP(Co(OH)2) = 1,8·10–18
[26]; the instability constant K of [Co(PPі)2]
6–
pyrophosphate complexes is 1.8·10–9 [10]; the
total concentration of cobalt (II) ions in the
test solution is 0.01 mol·1-1.
To establish the composition and determine
the stability constant of the polyligand com-
plexes of cobalt (II) formed in the system un-
der study, the absorption of the solution D, as a
function of its composition was determined by
the method of isomolar series (Ostromyslensky
Zhoba method) [17, 21, 25]. A series of solu-
tions was prepared by mixing initial solutions
with a constant concentration ratio of СCo
2+ :
CCit
3– = 1 : 5 (х ml) and СCo
2+ : CPPі
4– = 1 : 5
(10 – х) ml at a constant total concentration of
cobalt (II) ions (СCo
2+ = 0.01 mol·1-1) and wave-
length λ. In this case, the total concentration
of metal ions in the studied solutions remains
constant, and the ratio of the total concentra-
tions of ligands CPPI
4– : (CPPі
4– + CCit
3–) decreases
from 1 to 0. The ratio of the total concentra-
tions of ligands CCit
3- : (CPPі
4– + CCit
3–) increas-
es from 0 to 1. The ratios of the equilibrium
concentrations of free ligands [PPі]4–:([PPі]4– +
[Cit]3–) and [Cit]3–:([Cit]3– + [PPі]4–) change in
a similar way.
The absorption spectra of the studied solu-
tions are shown in Fig. 1. The presence of two
isosbestic points on the absorption curves at
the wavelengths λ = 592 and 632 nm allows us
to assume that there are three complex com-
pounds in the solution under study, namely:
citrate [Co(Cit)2]
4–, pyrophosphate [Co(PPі)2]
6–
and polyligand complexes of cobalt (II). At the
isosbestic point (λ = 592 nm), as can be seen
from Fig. 1, two complex compounds are in
equilibrium: pyrophosphate [Co(PPі)2]
6– (cur
ve 1) and polyligand [Co(PPi)m(Cit)n]+2–(4m+3n)
(curves 2–6) cobalt(II) complexes. Citrate
complexes [Co(Cit)2]
4– do not take part in
the equilibrium, since curve 7 does not pass
through this isosbestic point (λ = 592 nm).
Fig. 1. Absorption spectra in pyrophosphate-cit-
rate electrolyte obtained by the isomolar series
method at pH 9.0, a concentration of cobalt (II)
ions of 0.01 mol·l−1 with the ratio of concentrations
CPPі
4– : CCit
3– = 10 : 0 (1); 9 : 1 (2); 7 : 3 (3); 5 : 5 (4);
3 : 7 (5); 1 : 9 (6); 0 : 10 (7).
Since [Co(Cit)2]
4– citrate complexes do not
take part in the formation of polyligand com-
plexes [Co(PPi)m(Cit)n]+2–(4m+3n), the equilib-
rium between the latter and pyrophosphate
complexes [Co(PPі)2]
6– is described by the
equation:
[Co(PPі)2]
6– + [Cit]3– ↔ [Co(PPі)Cit]5– +
+ [PPі]4– . (3)
Therefore, the equilibrium constant Kr of
the reaction for the formation of polyligand
complexes of cobalt (II) is:
Kr = β / β'2 , (4)
117https://ucj.org.ua
V. М. Nikitenko, Yu. S. Yapontseva, V. S. Kublanovsky UCJ № 4 / Vol. 88
where β and β'2 are the stability constants of
polyligand and pyrophosphate complexes
[Co(PPі)2]
6–, respectively.
It should be noted that the interpretation of
spectrophotometric data for the systems under
study, in which three or more complex com-
pounds are formed, can be ambiguous [26],
since the error in determining the stability
constants of the corresponding complexes in-
creases. In this case, the determination of the
stability constants of the complexes formed
in the system under study from spectropho-
tometric data must be carried out at a wave-
length corresponding to the isosbestic point
in absorption spectra, at which the extinction
coefficients of all absorbing species are equal
(ε1 = ε2 = … = εn = ε*) [26].
The results of the analysis of spectrophoto-
metric data are shown in Fig. 2 at the wave-
lengths λ = 500 and 540 nm in the coordinates:
D = f [ CPPі
4–] / [CPPі
4– + CCit
3–) (curves 1, 2)
and D = f (CCit
3– / (CCit
3– + CPPі
4–) (curves 3, 4)
[21, 25], where D is the absorption of the test
solution, CPPі
4– and CCit
3– are the total concen-
trations of ligands. The linear plots indicate
the formation of stable polyligand complexes
of cobalt (II) with Cit3- and PPi4- ions in the
system under study. The abscissa of the point
of maximum absorption хmax does not depend
on the wavelength = 500 (curves 1, 3) and
540 nm (curves 2, 4) and is given by:
Хmax = CPPі
4– /(CPPі
4– + CCit
3–) =
=n /(n + m) = 0.5. (5)
This is consistent with the data [21] that
when a polyligand (mixed) complex with the
composition MLX is formed by the reaction:
ML2 + MX2 ↔ 2 MLX, (6)
the partial mole fraction of each of the com-
plexes ML2 and MX2 is 0.50 since α1 + α2.= 1.
Fig. 2. Absorption of isomolar series in a pyro
phosphate-citrate electrolyte at pH 9.0 and wave-
lengths λ = 500 nm (curves 1, 3) and 540 nm
(curves 2, 4) and the concentration of cobalt ions
in the investigated solutions СCo
2+ = 0.01 mol·l−1.
The obtained data (Fig. 2) make it possible
to determine the composition of the cobalt (II)
polyligand complexes formed under the ex-
perimental conditions. The maximum coordi-
nation numbers m and n for the polydentate
ligands PPi4- and Cit3- of cobalt (II) polyligand
complex formed in the system under study, as
follows from Fig. 2 and equation (5), are equal
to m = n = 1. Thus, polyligand complexes [Co
(PPi)Cit]5– are formed in the pyrophosphate–
citrate electrolyte at pH 9.0.
The composition of the cobalt (II) polyli-
gand complexes formed in the system under
study under given conditions and their stabi
lity constant were also determined by the
equilibrium shift method [17]. We denote
the maximum absorption of the solution un-
der study at the corresponding wavelength
λ = 500 nm by Do, and the optical density of
the solution at certain ratios of equilibrium
concentrations of ligands [PPI]4– / [Cit]3– until
almost complete binding of the central ion into
a colored polyligand complex Dx. Since the
118 ISSN 2708-129X. Укр. хім. журн., 2022
DETERMINATION OF POLYLIGAND COMPLEXES OF COBALT (II) WITH CITRATE AND PYROPHOSPHATE IONSPHISICAL CHEMISTRY
optical density of the solution by Dx is proportio
nal to the concentration of the colored complex
[Co(PPi)m(Cit)n]+2–(4m+3n), the ratio is [Co(PPi)m
(Cit)n]+2–(4m+3n) / [Co(PPi)2]
6– = Dx / (Do – Dx).
So, from the plots of lg Dx / (Do – Dx) = f (lg
[PPi]4–/ [Cit]3–) (Fig. 3a, curve 1) and lg Dx /
(Do – Dx) = f (lg [Cit]3–/ [PPi]4–) (Fig. 3a, cur
ve 2) it is possible to establish the composition
of the cobalt (II) polyligand complexes that are
formed in the studying system at pH 9.0 and
their stability constant.
The plots of lg Dx/(Do – Dx) = f (lg [PPi]4–/
[Cit]3–) (Fig. 3a, curve 1) and lg Dx/(Do – Dx) =
f (lg [Cit]3–/[PPi]4–) (Fig. 3a, curve 2) are rec-
tilinear; the abscissa of the intersection point
of them lg ([PPi]4–/ [Cit]3–) = 0. This indicates
that the molar ratio of the equilibrium con-
centrations of polydentate ligands [PPi]4–/
[Cit]3– in the polyligand complex [Co(PPi)m
(Cit)n]+2–(4m+3n) is 1, i.e. m=n=1 . The ratio [Cit]3–/
[PPi]4–, as can be seen from Fig. 3a, curve 2 is 1.
Thus, polyligand complexes [Co(PPi)Cit]5– are
formed in the system under study at pH 9.0.
The slope of the straight line lg Dx/(Do–Dx) =
f(lg [PPi]4–/[Cit]3–) (Fig. 3a, curve 1) is –0.4886
and is equal to the logarithm of the equilibri-
um constant Kr of the reaction of formation
of polyligand complexes [Co(PPi)Cit]5–. The
value of the equilibrium constant Kr calculated
from equation (3) is 0.3246.
Fig. 3. Dependences lg Dx / (Do – Dx) obtained by the method of equilibrium shift in the studied system
at pH 9.0, the concentration of ions СCo
2+ – 0.01 mol·l−1, the wavelength λ = 500 nm on lg([PPi]4–: [Cit]3–)
(Fig. 3а, (1)); on lg([Cit]3–: [PPi]4–) (Fig. 3а, (2)); on lg[PPi]4– (Fig. 3b, (1)) and on lg[Cit]3– (Fig. 3b, (2)).
The value of the stability constant β of
[Co(PPi)Cit]5– polyligand complexes calculat-
ed by the equilibrium shift method [17] using
experimental data (Fig. 3a) according to equa-
tion (4) is 1.80·108 (pβ = 8.26).
The composition of cobalt (II) polyligand
complexes, and not only the ratio of coordina-
tion numbers m/n, was determined from spec-
trophotometric data (Fig. 1) by the equilibrium
shift method [17]; plots of lg Dx / (Do – Dx) =
f (lg [PPi]4–) (Fig. 3b, curve 1) and lg Dx /
(Do – Dx) = f (lg [Cit]3–) (Fig. 3b, curve 2) ob-
tained from the spectrophotometric data (Fig. 1)
by the equilibrium shift method in the system
under study at pH 9.0, the total concentration
of СCo
2+ ions – 0.01 mol·l–1, and the wavelength
λ = 500 nm (Fig. 3b) are shown in Fig. 3b.
The slope of the straight line lg Dx/(Do–Dx)
119https://ucj.org.ua
V. М. Nikitenko, Yu. S. Yapontseva, V. S. Kublanovsky UCJ № 4 / Vol. 88
= f (lg [PPi]4–) (Fig. 3b, curve 1) at [Cit]3– → 0
is equal to the coordination number m of the
polyligand complex for [PPi]4– ions and is 1.07:
. (7)
The value of the coordination number n,
calculated from the slope of the straight line
lg Dx / (Do – Dx) = f (lg [Cit]3–) at [PPi]4– → 0,
is 0.96. The obtained values of the coordina-
tion numbers m and n (m=n=1) indicate the
formation of [Co(PPi)Cit]5– polyligand com-
plexes in the pyrophosphate–citrate electrolyte
at pH 9.0.
The composition of [Co(PPi)Cit]5– polyli-
gand complexes in a pyrophosphate–citrate
electrolyte was confirmed by the Newman
and Hume method [27]. This method is one
of the most reliable methods for determining
the composition of polyligand complexes and
their formation constants [25]. The equilibri-
um constant Kr of reaction (3) for the forma-
tion of [Co(PPi)Cit]5– polyligand complexes in
the system under study at pH 9.0 calculated by
the Newman and Hume method (Fig. 4a) ac-
cording to the equation:
D = CCo ·εCit-PPi – 1/Kr[(D – CCo·εPPi)·
( [PPi]4–/ [Cit]3–), (8)
where εCit-PPi and εPPi are the molar absorp-
tion coefficients of the polyligand and pyro
phosphate complexes of cobalt (II), respective-
ly, is 0.5755.
As can be seen from Fig. 4a, the experimental
points lie on a straight line, which confirms the
correctness of the determination from equa-
tion (8) of the numbers of coordination groups
of ligands (m=n=1) in the polyligand complex
[Co(PPi)m(Cit)n]+2–(4m+3n). Thus, polyligand co-
balt (II) complexes [Co(PPi)Cit]5– are formed
in the system under study at pH 9.0.
Fig. 4. Graphical determination of the equilibrium constant Kr of the polyligand cobalt (II) complexes
formation reaction in pyrophosphate-citrate electrolyte at pH 9.0, wavelength λ = 500 nm by the method:
(a) – Newman and Hume; (b) – Watters.
The equilibrium constant Kr of formation of
[Co(PPi)Cit]5– polyligand complexes at pH 9 by
reaction (3) was also determined by the Watters
method [28] (Fig. 4b) according to the equation:
([Cit]3–/[PPi]4–)/(ε*– εp) = 1/[(εзм – εпір)·Kr] +
+ 1/(εзм – εпір)·( ([Cit]3–/[PPi]4–), (9)
120 ISSN 2708-129X. Укр. хім. журн., 2022
DETERMINATION OF POLYLIGAND COMPLEXES OF COBALT (II) WITH CITRATE AND PYROPHOSPHATE IONSPHISICAL CHEMISTRY
where εCit-PPi and εPPi are the molar absorption
coefficients of a polyligand and a pyrophos-
phate complex of cobalt (II), respectively; ε* is
the average coefficient of molar absorption of
all forms of cobalt (II) ions.
The value of the equilibrium constant Kr
of the reaction of formation of polyligand
complexes [Co(PPi)Cit]5– formation reaction,
calculated from the slope of the linear plot of
([Cit]3–/[PPi]4–)/(ε*– εp) = f ([Cit]3–/[PPi]4–),
which is 1/(εCit-PPi – εPPi) = 0.0794 (Fig. 4b),
and the y-intercept (1/(εзм – εпір)Kr = 0.160) is
0.4963. The value of the equilibrium constant
Kr of formation of polyligand complexes ob-
tained by the Watters method [28] agrees with
the Kr value obtained by the Newman and
Hume method [27] (0.5755).
The stability constants of [Co(PPi)Cit]5–
polyligand complexes calculated by the Wat-
ters [28], Newman and Hume [27] methods
using the equilibrium constants Kr of the re-
action of their formation of 0.4963 and 0.5755
respectively, in accordance with Eq. (4) are
2.76·108 (pβ = 8.44) and 3.20·108 (pβ = 8.51),
respectively. The obtained average value of the
stability constant of [Co(PPi)Cit]5– polyligand
complexes (β = 2.98·108 (pβ = 8.47)) agrees
with the value calculated from experimental
data (Fig. 3a) by the equilibrium shift method
(pβ = 8.26).
Taking into account the stability constants
of citrate [Co(Cit)2]
4– (pβ2 = 5.30), pyrophos
phate [Co(PPi)2]
6– (pβ'2 = 8.74) and polyli-
gand [Co(PPi)Cit]5– (pβ = 8.47) complexes
of cobalt (II), the dependence of the degree
of their formation in a pyrophosphate-citrate
electrolyte on the ratio of equilibrium con-
centrations of ligands lg ([PPi]4–/ [Cit]3–) was
calculated (Fig. 5). As can be seen from fig. 5,
citrate complexes [Co(Cit)2]
4– exist in the
range of the ratios of equilibrium concentra-
tions of ligands lg ([PPi]4–/ [Cit]3–) from –1
to +1, polyligand complexes [Co(PPi)Cit]5– in
the range from –1 to +2, and [Co(PPi)2]
6– pyro
phosphate complexes in the range >0.
Fig. 5. Dependence of the degree of formation
of citrate (1), polyligand (2) and pyrophosphate (3)
complexes of cobalt (II) in pyrophosphate-citrate
electrolyte on the logarithm of the ratio of equi-
librium concentrations of ligands: 1 – [Co(Cit)2]
4–;
2 – [Co(PPi)Cit]5–; 3 – [Co(PPi)2]
6–.
CONCLUSIONS.
It has been established that in a pyrophos
phate-citrate electrolyte at pH 9.0, cobalt
(II) ions form with Cit3– and PPi4– ions not
only citrate [Co(Cit)2]
4– and pyrophosphate
[Co(PPi)2]
6–, but also polyligand complexes
[Co(PPi)m(Cit)n]+2–(4m+3n). The composition of
[Co(PPi)Cit]5– polyligand complexes was de-
termined, the equilibrium constant Kr of the
reaction of their formation and their stability
constant (pβ=8.47) were calculated. In terms of
stability, [Co(PPi)Cit]5– (pβ = 8.47) polyligand
complexes approach pyrophosphate complex-
es [Co(PPi)2]
6– (pβ'2 = 8.74). The dependence
of the degree of formation of citrate, polyli-
gand, and pyrophosphate complexes of cobalt
121https://ucj.org.ua
V. М. Nikitenko, Yu. S. Yapontseva, V. S. Kublanovsky UCJ № 4 / Vol. 88
(II) in a pyrophosphate-citrate electrolyte on
the logarithm of the ratio of equilibrium con-
centrations of ligands was calculated.
ACKNOWLEDGMENT. This work
was carried out in the framework of
the target research program of the
NAS of Ukraine «Promising basic research
and innovative development of nanomate-
rials and nanotechnologies for the needs of
industry, health and agriculture», project
№ 28/22-N, 2022.
ВИЗНАЧЕННЯ ПОЛІЛІГАНДНИХ КОМПЛЕКСІВ
КОБАЛЬТУ (II) З ЦИТРАТНИМИ ТА ПІРОФОС-
ФАТНИМИ ІОНАМИ
В. М. Нікітенко, Ю. С. Японцева*,
В. С. Кублановський
Інститут загальної та неорганічної хімії
ім. В. І. Вернадського НАН України,
просп. Академіка Палладіна, 32/34, Київ
03142, Україна
*е-mail: juliya_yap@ukr.net
Спектрофотометричним методом вста-
новлено, що у пірофосфатно-цитратному
електроліті іони кобальту (II) утворюють
залежно від співвідношення концентрацій
лігандів [PPi4–] / [Cit3–] не тільки цитрат-
ні [Co(Cit)2]
4–, пірофосфатні [Co(PPi)2]
6–,
а й полілігандні комплекси [Co(PPi)m
(Cit)n]+2–(4m+3n). Визначено склад полілі-
гандних комплексів [Co(PPi)Cit]5–, обчис-
лено константу рівноваги реакції їхнього
утворення та константу їхньої стійкості
(pβ = 8.47). Розраховано залежність ступе-
ня утворення цитратних, полілігандних та
пірофосфатних комплексів кобальту(II) в
пірофосфатно-цитратному електроліті від
логарифму співвідношення рівноважних
концентрацій лігандів.
Ключові слова: кобальт (II), поліліганд-
ні комплекси, склад, константа стійкості,
спектрофотометрія.
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Стаття надійшла 18.04.2022.
|
| id | oai:ojs2.1444248.nisspano.web.hosting-test.net:article-436 |
| institution | Ukrainian Chemistry Journal |
| keywords_txt_mv | keywords |
| language | English |
| last_indexed | 2026-07-23T01:08:09Z |
| publishDate | 2022 |
| publisher | V.I.Vernadsky Institute of General and Inorganic Chemistry |
| record_format | ojs |
| resource_txt_mv | ucjorgua/5d/e1207668daa90e0fdee735d59bbe3f5d.pdf |
| spelling | oai:ojs2.1444248.nisspano.web.hosting-test.net:article-4362026-07-22T08:23:49Z DETERMINATION OF POLYLIGAND COMPLEXES OF COBALT (II) WITH CITRATE AND PYROPHOSPHATE IONS Nikitenko, Vasyl Kublanovsky, Valeriy Yapontseva, Yuliya cobalt (II), polyligand complex, composition, stability constant, spectrophotometry. In the work it is shown by the spectrophotometry method that depending on the concentration ratio of ligands [PPi4–]/[Cit3–] in the pyrophosphate-citrate electrolyte, cobalt (II) ions form not only citrate [Co(Cit)2]4– and pyrophosphate [Co(PPi)2]6–, but also polyligand complexes [Co(PPi)m(Cit)n]+2–(4m+3n). The composition of polyligand complexes [Co(PPi)Cit]5– was determined, and the equilibrium constant of the reaction of their formation and the constant of their stability were calculated (pβ=8.47). The dependence of the degree of formation of citrate, polyligand, and pyrophosphate complexes of cobalt (II) in the pyrophosphate-citrate electrolyte on the logarithm of the ratio of equilibrium concentrations of ligands is calculated. V.I.Vernadsky Institute of General and Inorganic Chemistry 2022-05-25 Article Article Physical chemistry Физическая xимия Фізична xімія application/pdf https://ucj.org.ua/index.php/journal/article/view/436 10.33609/2708-129X.88.04.2022.113-122 Ukrainian Chemistry Journal; Vol. 88 No. 4 (2022): Ukrainian Chemistry Journal; 113-122 Украинский химический журнал; ##issue.vol## 88 ##issue.no## 4 (2022): Ukrainian Chemistry Journal; 113-122 Український хімічний журнал; Том 88 № 4 (2022): Український хімічний журнал; 113-122 2708-129X 2708-1281 en https://ucj.org.ua/index.php/journal/article/view/436/224 Copyright (c) 2022 Vasyl Nikitenko, Valeriy Kublanovsky, Yuliya Yapontseva https://creativecommons.org/licenses/by-nc/4.0 |
| spellingShingle | Nikitenko, Vasyl Kublanovsky, Valeriy Yapontseva, Yuliya DETERMINATION OF POLYLIGAND COMPLEXES OF COBALT (II) WITH CITRATE AND PYROPHOSPHATE IONS |
| title | DETERMINATION OF POLYLIGAND COMPLEXES OF COBALT (II) WITH CITRATE AND PYROPHOSPHATE IONS |
| title_full | DETERMINATION OF POLYLIGAND COMPLEXES OF COBALT (II) WITH CITRATE AND PYROPHOSPHATE IONS |
| title_fullStr | DETERMINATION OF POLYLIGAND COMPLEXES OF COBALT (II) WITH CITRATE AND PYROPHOSPHATE IONS |
| title_full_unstemmed | DETERMINATION OF POLYLIGAND COMPLEXES OF COBALT (II) WITH CITRATE AND PYROPHOSPHATE IONS |
| title_short | DETERMINATION OF POLYLIGAND COMPLEXES OF COBALT (II) WITH CITRATE AND PYROPHOSPHATE IONS |
| title_sort | determination of polyligand complexes of cobalt (ii) with citrate and pyrophosphate ions |
| topic_facet | cobalt (II) polyligand complex composition stability constant spectrophotometry. |
| url | https://ucj.org.ua/index.php/journal/article/view/436 |
| work_keys_str_mv | AT nikitenkovasyl determinationofpolyligandcomplexesofcobaltiiwithcitrateandpyrophosphateions AT kublanovskyvaleriy determinationofpolyligandcomplexesofcobaltiiwithcitrateandpyrophosphateions AT yapontsevayuliya determinationofpolyligandcomplexesofcobaltiiwithcitrateandpyrophosphateions |