DETERMINATION OF THE COMPOSITION AND STABILITY CONSTANT OF COBALT (II) POLYLIGAND CITRATE-MOLYBDATE COMPLEXES
The complex formation of cobalt (II) with citrate (cit3–) and molybdate (MoO42–) ions in 0.3 mol·l-1 Na2SO4 solution at 20°C was studied by spectrophotometric method. It has been established that in a citrate-molybdate electrolyte at pH 9.0, cobalt (II) ions form, depending on the ratio of the equil...
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Ukrainian Chemistry Journal| _version_ | 1871465920353796096 |
|---|---|
| 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 | 2026-07-22T08:23:51Z |
| description | The complex formation of cobalt (II) with citrate (cit3–) and molybdate (MoO42–) ions in 0.3 mol·l-1 Na2SO4 solution at 20°C was studied by spectrophotometric method. It has been established that in a citrate-molybdate electrolyte at pH 9.0, cobalt (II) ions form, depending on the ratio of the equilibrium concentrations of ligands: molybdate [Co(MoO4)2]2–, citrate [Co(cit)2]4– and polyligand complexes [Co(cit)m(MoO4)n]+2-(3m+2n). The composition of [Co(cit)(MoO4)]3– polyligand complexes was determined, the equilibrium constant Ke of the reaction of their formation and their stability constant (lgβMLX = 5.86) The dependence of the degree of formation of molybdate, citrate and polyligand complexes of cobalt (II) in citrate-molybdate electrolyte on the logarithm of the ratio of equilibrium concentrations of ligands was calculated. |
| doi_str_mv | 10.33609/2708-129X.89.03.2023.15-24 |
| first_indexed | 2025-09-24T17:43:50Z |
| format | Article |
| fulltext |
15
УДК 541.49 doi: 10.33609/2708-129X.89.03.2023.15-24
DETERMINATION OF THE COMPOSITION AND STABILITY
CONSTANT OF COBALT (II) POLYLIGAND
CITRATE-MOLYBDATE COMPLEXES.
V.М. Nikitenko, Yu.S. Yapontseva*, V.S. Kublanovsky
V.I. Vernadsky Institute of General and Inorganic Chemistry of the NAS of Ukraine,
32/34 Aсad. Palladina ave., 03142 Kyiv, Ukraine
*е-mail: juliya_yap@ukr.net
The complex formation of cobalt (II) with citrate (cit3–) and molybdate (MoO4
2–) ions in
0.3 mol·l-1 Na2SO4 solution at 20°C was studied by spectrophotometric method. It has been
established that in a citrate-molybdate electrolyte at pH 9.0, cobalt (II) ions form, depending
on the ratio of the equilibrium concentrations of ligands: molybdate [Co(MoO4)2]
2–, citrate
[Co(cit)2]
4– and polyligand complexes [Co(cit)m(MoO4)n]+2-(3m+2n). The composition of [Co(cit)
(MoO4)]3– polyligand complexes was determined, the equilibrium constant Ke of the reaction
of their formation and their stability constant (lgβMLX = 5.86) The dependence of the degree
of formation of molybdate, citrate and polyligand complexes of cobalt (II) in citrate-molyb
date electrolyte on the logarithm of the ratio of equilibrium concentrations of ligands was
calculated.
Keywords: cobalt (II), molybdate, citrate and polyligand complexes, composition, equilib-
rium constant, stability constant, spectrophotometry.
INTRODUCTION. In modern electroplat-
ing, polyligand electrolytes are widely used for
applying functional coatings with metals and
alloys. This is due to the fact that polyligand
electrolytes are non-toxic, stable, and easily
disposed of. In addition, acid-type polyden-
tate ligands form a wide range of stable com-
plex compounds with almost all metal ions and
have a pronounced ability to be compatible
with weak-field ligands in one coordination
sphere to form polyligand electrochemically
inactive complexes (EIAC). The advantage of
polyligand electrolytes is also a high buffer ca-
pacity and, as a consequence, a slight alkalini
zation of the near-cathode layer during their
long-term operation.
When choosing ligands for polyligand elec-
trolytes, one should be guided by the basic ide-
as of modern ligand field theory, the nature of
chemical bonds, the requirements of their non-
toxicity and indifference to electrode processes
occurring during the reduction of metal com-
plexes. Properly selected ligands for complex
electrolytes lead to the formation of EIAC in
16 ISSN 2708-129X. Укр. хім. журн., 2023
DETERMINATION OF THE COMPOSITION AND STABILITY CONSTANT OF COBALT (II)
POLYLIGAND CITRATE-MOLYBDATE COMPLEXES.PHISICAL CHEMISTRY
the polyligand electrolyte, which makes it pos-
sible to purposefully influence the inhibition
of the electrode process, the composition of
electrochemically active complexes (EAC), the
mechanism of the process, and consequent-
ly, the morphology, structure and functional
properties of the resulting metal coatings and
alloys [1, 2].
The effect on the inhibition of electrode re-
actions occurring in a polyligand electrolyte
and, consequently, on the morphology, struc-
ture, and functional properties of coatings ob-
tained by binary and ternary alloys of cobalt,
nickel or iron with refractory metals (Mo, W,
and Re) is impossible without reliable data on
the composition of electrolytes, the stability
constants of monoligand and polyligand com-
plexes of these metal ions with citrate and mo-
lybdate ions, the composition of electrochemi
cally active complexes (EAC), the mechanism
of electrode processes and the kinetic regulari-
ties of limiting stages.
Information about the composition of com-
plex compounds, including polyligand ones,
in the bull of a citrate-molybdate electrolyte,
the areas of their existence and predominance
make it possible to consciously influence the
choice of the optimal composition of the elec-
trolyte, the kinetics and mechanism of depo-
sition of binary and ternary alloys of molyb-
denum with metals of the iron subgroup and,
consequently, the structure and functional
properties of the resulting coatings.
The object of study is the processes of com-
plex formation that occur in a citrate electro-
lyte for the deposition of binary and ternary
alloys of cobalt with refractory metals (Mo,
Wo, and Re), which are of not only scientific
but also practical interest [3–5], since these al-
loys have valuable physicochemical and opera
tional properties, which makes them indispen-
sable for practical use.
Previously [6], we found that in a pyrophos
phate–citrate electrolyte at pH9.0, not only
monoligand citrate [Co(cit)2]
4– and pyrophos
phate [Co(ppi)2]
6– complexes, but also
polyligand complexes of cobalt (II) with cit-
rate-(cit3–) and pyrophosphate– (ppi4–) ions
[Co(ppi)m(cit)n]+2–(4m+3n), their composition
[Co(ppi)cit]5–, the equilibrium constant Ke of
the reaction of their formation and the cons
tant of their stability were determined (lgβMLX =
8.47).
The purpose of this work is to determine the
composition and stability constant of polyli-
gand complexes of cobalt (II) with citrate (cit3–)
and molybdate – (MoO4
2–) ions by the spectro-
photometric method; to study the effect of the
ratio of ligands equilibrium concentrations on
the ionic composition of the electrolyte.
EXPERIMENT AND DISCUSSION OF THE
RESULTS. Cobalt(II) forms with citrate (cit3–)
ions, depending on the pH of the solution and
the equilibrium concentration of the ligand,
protonated and non-protonated complex
[CoH2cit]+, [CoHcit], [Cocit]–, [Co(cit)2]
4– cit-
rate compounds [7, 8]. Polymeric, polynuclear
complex compounds of cobalt (II) with citrate
of various compositions [9, 10], which have
magnetic properties [10] and biological com-
patibility [11, 12], have been synthesized and
isolated in the solid state. The authors of [13]
also studied the thermodynamic parameters
(∆Go, ∆H, ∆S) of the formation of protonated
citrate complexes [CoHcit]− at pH 6, a tempe
rature of 25°C in a 100 mM·l-1 NaClO4 solu-
tion. With molybdate (MoO4
2–) ions, cobalt
(II) also forms protonated and non-proto-
nated complex compounds [Co(HMoO4)6]
4–,
[CoMoO4], [Co(MoO4)2]
2– [7, 8].
17https://ucj.org.ua
V. М. Nikitenko, Yu. S. Yapontseva, V. S. Kublanovsky UCJ № 03 / Vol. 89
In the absence of cobalt(II) ions in the test
solution, Mo(VI) can form mononuclear and
polynuclear protonated hydroxide complex-
es with citrate [14]. The composition of the
mononuclear and polynuclear complex com-
pounds isolated from the solution corresponds
to the formulas: K4[MoO3(cit)]·2H2O and
K4[(MoO2)2O(Hcit)2]·4H2O, respectively.
The data on the possibility of formation of
polyligand complexes of cobalt(II) with citrate
(cit3-) and molybdate (MoO4
2-) ions in a mo-
lybdate-citrate electrolyte, their composition
and stability constant are practically absent in
the literature.
Under the conditions of spectrophotometric
studies at component concentrations, mol·l-1:
CCo
2+ 0.01; Ccit
3– 0.01; CMoO4
2– 0–0.10; Na2SO4
0.30; pH 9.0, as is seen from absorption spectra
of cobalt(II) complexes in a citrate-molybdate
electrolyte (Fig. 1), only polyligand complexes
of cobalt(II) [Cocit(MoO4)]3– are formed in the
system under study according to the reaction:
[Co(cit)2]
4– + [MoO4]
2– ↔ [Cocit(MoO4)]
3– + cit3–.
(1)
This is confirmed by the results of further
studies presented in this paper.
Thus, the study of polyligand complexes of
cobalt (II) with citrate (cit3-) and molybdate
(MoO4
2-) ions is certainly of scientific interest,
since polyligand electrolytes based on them
are promising for use in electroplating for ob-
taining electrocatalytic coatings of binary and
ternary alloys of cobalt, nickel or iron with re-
fractory metals (Mo, W and Re).
The processes of formation of polyligand
complexes of cobalt (II) from citrate- (cit3-)
and molybdate- (MoO4
2-) ions in a molyb-
date-citrate electrolyte, determination of their
composition and stability constants were stu
died by spectrophotometric method. Spectro-
photometry is one of the most precise methods
for determining the composition and stability
constants of colored complex compounds.
Methods for determining the composition,
stability constants of colored metal complexes
from spectrophotometric measurements, their
scope, advantages, limitations and disadvan-
tages are discussed in detail in the monograph
by A.K. Babko [6].
The spectrophotometric study of the forma-
tion of polyligand complexes of cobalt (II) with
citrate– (cit3–) and molybdate– (MoO4
2–) ions
causes significant difficulties, since along with
them, depending on the pH of the solution,
monoligand protonated and nonprotonated
citrate and molybdate cobalt (II) complexes.
The distribution of cobalt (II) complexes in
the bulk of the electrolyte in this case strongly
depends on the equilibrium concentration of
metal ions, the ratio of the equilibrium con-
centrations of free ligands, and the pH of the
solution.
The equilibrium concentrations of citrate–
(cit3–) and molybdate– (MoO4
2–) ions in the test
solution strongly depend on the pH of the solu-
tion, since they are anions of weak acids, citrate
H3cit and molybdate H2MoO4. Therefore, when
conducting spectrophotometric studies, it is
necessary to maintain an optimal and constant
concentration of hydrogen ions (pH=const) in
order to form complex ions of constant com-
position at a certain ratio of concentrations
of the main components of the test solution
(СCo
2+ /Ccit
3– = const) and a variable concentra-
tion molybdate ions (CMoO4
2– ≠ const).
The values of the equilibrium concentra-
tions of free ligands [cit]3– and [MoO4]
2– were
determined from spectrophotometric studies,
taking into account their total concentrations
18 ISSN 2708-129X. Укр. хім. журн., 2023
DETERMINATION OF THE COMPOSITION AND STABILITY CONSTANT OF COBALT (II)
POLYLIGAND CITRATE-MOLYBDATE COMPLEXES.PHISICAL CHEMISTRY
and the pH of the solution according to the
equation (2) [15]:
4
4
maintain an optimal and constant concentration of hydrogen ions (pH=const) in order to form
complex ions of constant composition at a certain ratio of concentrations of the main components of
the test solution (СCo
2+/Ccit
3–=const) and a variable concentration molybdate ions (CMoO4
2– const).
The values of the equilibrium concentrations of free ligands [cit]3– and [MoO4]2– were determined
from spectrophotometric studies, taking into account their total concentrations and the pH of the
solution according to the equation (2) [15]:
mi
1i
m
iLH
m ]H[K1/C]L[
m
, (2)
where CHmL, [L]m– are the total and equilibrium concentrations of ligands (citrate and molybdate) in
the test solution, mol·l–1 respectively; Ki are the general stability constants of citrate and molybdate
(pK1 = 5.68; pK2 = 10.03; pK3 = 12.90 and pK1 = 6.00; pK2 = 7.02 respectively) [8].
Composition of the studied solutions, mol·l-1: CCo
2+ – 0.01; Ccit
3– – 0.01; CMoO4
2– – 0 (1); 0.002
(2); 0.01 (3); 0.02 (4); 0.05 (5); 0.10 (6); Na2SO4 – 0.30; pH 9.0. Analytical grade reagents were used
to prepare electrolytes. The studied solutions pH was corrected on an electronic pH meter (pH-150
MI) with NaOH and H2SO4 solutions.
The absorption spectra of solutions of the cobalt (II) complexes under study were recorded on a
UV-VIS Spectrophotometer UV mini 1240 (Shimadzu) in a 1 cm cuvette at a temperature of 20°C.
The composition and stability constant of polyligand complexes of cobalt (II) with citrate- (cit3-)
and molybdate- (MoO4
2-) ions formed in the system under study at pH 9.0 were determined from the
data of spectrophotometric measurements by the method of isomolar series. In this case, the ratio of
the total concentrations of cobalt (II) ions and citrate ions remained constant (СCo
2+ /Ccit
3–=1:1),
while the concentration of molybdate ions CMoO4
2– varied from 0 to 0.10 mol l–1.
The relationship between absorbance and concentration of the studied solution is ideally
described by the Lambert-Beer law [16, 17]:
D = l .∑ i Ci , (3)
where D is the absorption of the solution; l is the thickness of the absorbing layer (cell 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.
It should be emphasized that, according to the Lambert – Beer law [16, 17], the property of the
measured system must be a strictly linear function of the molar concentration Ci of absorbing
complexes.
The ratio of the coordination numbers m and n of the polydentate ligands cit3– and MoO4
2– during
the formation of polyligand complexes [Co(cit)m(MoO4)n]+2-(3m+2n) of constant composition (m/n =
const) was determined by the Harvey and Manning slope ratio method [6, 16] from the dependence
of the absorption of solution D on the total concentration of molybdate ions CMoO4
2– at a constant
(2)
where CHmL, [L]m– are the total and equilibrium
concentrations of ligands (citrate and molyb-
date) in the test solution, mol·l–1 respectively;
Ki are the general stability constants of cit-
rate and molybdate (pK1 = 5.68; pK2 = 10.03;
pK3 = 12.90 and pK1 = 6.00; pK2 = 7.02 respec-
tively) [8].
Composition of the studied solutions,
mol·l-1: CCo
2+ – 0.01; Ccit
3– – 0.01; CMoO4
2– – 0 (1);
0.002 (2); 0.01 (3); 0.02 (4); 0.05 (5); 0.10 (6);
Na2SO4 – 0.30; pH 9.0. Analytical grade rea-
gents were used to prepare electrolytes. The
studied solutions pH was corrected on an elec-
tronic pH meter (pH-150 MI) with NaOH and
H2SO4 solutions.
The absorption spectra of solutions of the
cobalt (II) complexes under study were record-
ed on a UV-VIS Spectrophotometer UV mini
1240 (Shimadzu) in a 1 cm cuvette at a tempe
rature of 20 °C.
The composition and stability constant of
polyligand complexes of cobalt (II) with cit-
rate- (cit3-) and molybdate- (MoO4
2-) ions
formed in the system under study at pH 9.0
were determined from the data of spectropho-
tometric measurements by the method of iso-
molar series. In this case, the ratio of the total
concentrations of cobalt (II) ions and citrate
ions remained constant (СCo
2+ /Ccit
3– = 1:1),
while the concentration of molybdate ions
CMoO4
2– varied from 0 to 0.10 mol l–1.
The relationship between absorbance and
concentration of the studied solution is ideally
described by the Lambert-Beer law [16, 17]:
D = l .∑ εi Ci , (3)
where D is the absorption of the solution;
l is the thickness of the absorbing layer (cell
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 concentra-
tion of the absorbing particle.
It should be emphasized that, according to
the Lambert – Beer law [16, 17], the property
of the measured system must be a strictly li
near function of the molar concentration Ci of
absorbing complexes.
The ratio of the coordination numbers m
and n of the polydentate ligands cit3– and
MoO4
2– during the formation of polyligand
complexes [Co(cit)m(MoO4)n]+2-(3m+2n) of con-
stant composition (m/n = const) was deter-
mined by the Harvey and Manning slope ratio
method [6, 16] from the dependence of the
absorption of solution D on the total concen-
tration of molybdate ions CMoO4
2– at a constant
ratio of the total concentrations of cobalt(II)
and citrate ions CCo
2+ / Ccit
3–
= 1:1, solution pH,
and wavelength λ.
Analysis of the data of spectrophotometric
studies (Fig. 1) at constant total concentrations
of cobalt(II) ions, citrate Ccit
3-, solution pH and
wavelength λ in the coordinates D = f (CMoO4
2–)
[6, 16] (Fig. 2) , where D – absorption of the
test solution; CMoO4
2– the total concentration of
molybdate ions made it possible to determine
the composition (ratio of the coordination
numbers of polydentate ligands m/n) in the
polyligand complex [Co(cit)m(MoO4)n]+2-(3m+2n)
formed in the system under study at pH 9.0.
Curves D = f (CMoO4
2−) obtained at wavelengths
λ = 529 (3), maximum absorption in the sys-
tem under study, 550 (2) and 560 (1) nm, in-
dicate the formation of stable polyligand com-
plexes of cobalt (II).
19https://ucj.org.ua
V. М. Nikitenko, Yu. S. Yapontseva, V. S. Kublanovsky UCJ № 03 / Vol. 89
Fig. 1. Absorption spectra of cobalt (II) in cit-
rate-molybdate electrolyte at pH 9.0, component
concentrations, mol·l−1: CCo
2+ − 0,01; Ccit
3– − 0,01;
CMoO4
2- − 0 (1); 0,002 (2); 0,01 (3); 0,02 (4); 0,05 (5);
0,10 (6).
The horizontal section observed on the
absorption curve D = f (CMoO4
2–) at a 10-fold
excess of molybdate ions (Fig. 2) indicates
the almost complete binding of cobalt (II)
ions into the colored polyligand complex
[Co(cit)m(MoO4)n]+2-(3m+2n) or [Cocit(MoO4)]3−
[6, 16], since citrate− (cit3–) and molybdate−
(MoO4
2–) ions are polydentate ligands, and al-
lows us to calculate it molar absorption coeffi-
cient. The abscissa of the intersection points of
two straight lines constructed from the slopes
of the curvilinear sections of the experimental
curves D = f (CMoO4
2–) (Fig. 2) is equal to (1). This
indicates that the molar ratio of the coordina-
tion numbers of polydentate ligands m/n in the
polyligand complex [Co(cit)m(MoO4)n]+2-(3m+2n),
formed in the system under study at pH 9.0, is
equal to 1. Therefore, in an alkaline medium
(pH 9.0) with a 10-fold excess of molybdate
ions, polyligand complexes of the composition
[Co(cit)(MoO4)]3−, since the maximum coor-
dination number is m=n=1.
The composition of polyligand complex-
es [Co(cit)m(MoO4)n]+2-(3m+2n) formed in the
system under study, and not only the ratio of
coordination numbers m/n, was determined
by the Bent and French method [6, 16], and
their constant stability by the equilibrium shift
method proposed by Bodlander and Abegg [6].
Fig. 2. Curves of formation of polyligand com-
plexes of cobalt (II) in 0.30 mol·l–1 Na2SO4 solution
at pH 9.0 and at wavelengths λ = 529(3), 550(2),
and 560(1) nm. Component concentration, mol·l−1:
CCo
2+ − 0,01; Ccit
3– − 0,01.
The results of the analysis of spectropho-
tometric data (Fig. 1) at wavelengths λ = 500,
529, 550 and 560 nm in coordinates: D =
f [Ccit
3– / (Ccit
3– + CMoO4
2–)] (cr. 2, 4, 6, 8) and
D = f [CMoO4
2– / (CMoO4
2– + Ccit
3–)] (cr. 1, 3, 5, 7)
[6, 16], where D – absorption of the test solu-
tion; CMoO4
2– and CCit
3– are the total concentra-
tions of ligands, mol·l-1, are shown in fig. 3. The
obtained linear dependences indicate the for-
mation of stable polyligand complexes of cobalt
(II) with citrate-(cit3–) and molybdate–(MoO4
2–)
ions of composition [Co(cit)m(MoO4)n]
+2-(3m+2n).
The abscissa of the point of maximum absorp-
tion хmax in the system under study, as can be
5
5
ratio of the total concentrations of cobalt(II) and citrate ions CCo
2+/Ccit
3–
= 1:1, solution pH, and
wavelength .
Analysis of the data of spectrophotometric studies (Fig. 1) at constant total concentrations of
cobalt(II) ions, citrate Ccit
3-, solution pH and wavelength in the coordinates D=f(CMoO4
2–) [6, 16]
(Fig. 2) , where D – absorption of the test solution; CMoO4
2– the total concentration of molybdate ions
made it possible to determine the composition (ratio of the coordination numbers of polydentate
ligands m/n) in the polyligand complex [Co(cit)m(MoO4)n]+2-(3m+2n) formed in the system under study
at pH 9.0. Curves D = f (CMoO4
2) obtained at wavelengths = 529 (3), maximum absorption in the
system under study, 550 (2) and 560 (1) nm, indicate the formation of stable polyligand complexes of
cobalt (II).
Fig. 1. Absorption spectra of cobalt (II) in citrate-molybdate electrolyte at pH 9.0, component
concentrations, mol·l−1: CCo
2+ 0,01; Ccit
3– 0,01; CMoO4
2- 0 (1); 0,002 (2); 0,01(3); 0,02 (4); 0,05
(5); 0,10 (6).
The horizontal section observed on the absorption curve D=f(CMoO4
2–) at a 10-fold excess of
molybdate ions (Fig. 2) indicates the almost complete binding of cobalt (II) ions into the colored
polyligand complex [Co(cit)m(MoO4)n]+2-(3m+2n) or [Cocit(MoO4)]3 [6, 16], since citrate− (cit3–) and
molybdate− (MoO4
2–) ions are polydentate ligands, and allows us to calculate it molar absorption
coefficient. The abscissa of the intersection points of two straight lines constructed from the slopes of
the curvilinear sections of the experimental curves D=f(CMoO4
2–) (Fig. 2) is equal to (1). This
indicates that the molar ratio of the coordination numbers of polydentate ligands m/n in the
polyligand complex [Co(cit)m(MoO4)n]+2-(3m+2n), formed in the system under study at pH 9.0, is equal
to 1. Therefore, in an alkaline medium (pH 9.0) with a 10-fold excess of molybdate ions, polyligand
complexes of the composition [Co(cit)(MoO4)]3, since the maximum coordination number is mn1.
The composition of polyligand complexes [Co(cit)m(MoO4)n]+2-(3m+2n) formed in the system under
study, and not only the ratio of coordination numbers m/n, was determined by the Bent and French
6
6
method [6, 16], and their constant stability by the equilibrium shift method proposed by Bodlander
and Abegg [6].
Fig. 2. Curves of formation of polyligand complexes of cobalt (II) in 0.30 mol·l–1 Na2SO4
solution at pH 9.0 and at wavelengths = 529(3), 550(2), and 560(1) nm. Component concentration,
mol·l−1: CCo
2+ 0,01; Ccit
3– 0,01.
The results of the analysis of spectrophotometric data (Fig. 1) at wavelengths = 500, 529, 550
and 560 nm in coordinates: D=f[Ccit
3–/(Ccit
3–+CMoO4
2–)] (cr. 2, 4, 6, 8) and D=f[CMoO4
2–
/(CMoO4
2–+Ccit
3–)] (cr. 1, 3, 5, 7) [6, 16], where D – absorption of the test solution; CMoO4
2– and
CCit
3– are the total concentrations of ligands, mol·l-1, are shown in fig. 3. The obtained linear
dependences indicate the formation of stable polyligand complexes of cobalt (II) with citrate-(cit3–)
and molybdate–(MoO4
2–) ions of composition [Co(cit)m(MoO4)n]+2-(3m+2n). The abscissa of the point
of maximum absorption хmax in the system under study, as can be seen from Fig. 3 does not depend
on the wavelength and is equal to:
хmax = Ccit
3–/(Ccit
3–+CMoO4
2–) = n/ (n + m) = 0.5. (4)
Hence, the molar ratio of the maximum coordination numbers of polydentate ligands m/n in the
[Co(cit)(MoO4)]3- polyligand complex formed in the system under study at pH 9.0 is 1.
20 ISSN 2708-129X. Укр. хім. журн., 2023
DETERMINATION OF THE COMPOSITION AND STABILITY CONSTANT OF COBALT (II)
POLYLIGAND CITRATE-MOLYBDATE COMPLEXES.PHISICAL CHEMISTRY
seen from Fig. 3 does not depend on the wave-
length λ and is equal to:
хmax = Ccit
3– / (Ccit
3– + CMoO4
2–) = n / (n + m) = 0.5.
(4)
Hence, the molar ratio of the maximum
coordination numbers of polydentate ligands
m/n in the [Co(cit)(MoO4)]3- polyligand com-
plex formed in the system under study at pH
9.0 is 1.
Fig. 3. Dependence of cobalt (II) complexes ab-
sorption in a citrate-molybdate electrolyte at pH 9.0
and wavelengths λ = 500 (5, 6), 529 (1, 2), 550 (3,
4), and 560 (7, 8) nm. The composition of the stu
died solutions, mol·l−1: CCo
2+ – 0.01; Ccit
3− – 0.01;
CMoO4
2− – 0 (1); 0.002 (2); 0.01 (3); 0.02 (4); 0.05 (5);
0.10 (6); 0.010 (7).
The composition of the [Co(cit)m(MoO4)
n]+2-(3m+2n) polyligand complexes formed in the
system under study at pH 9.0, their stability
constant βMLX was determined from spectro-
photometric data by the method proposed by
A.K. Babko [6]. The maximum optical den-
sity of the test solution corresponding to the
horizontal section on the absorption curve D
= f (CMoO4
2–) shown in Fig. 2, denoted by Do,
and the optical density of the solution at a cer-
tain equilibrium concentration of molybdate
ions [MoO4]
2– until the complete binding of
the ligand into the colored polyligand com-
plex [Co(cit)m(MoO4)n]+2-(3m+2n) via Dx. Since
the optical density of the solution Dx is pro-
portional to the concentration of the colored
cobalt (II) polyligand complex, then the ratio
[Co(cit)m(MoO4)n]+2-(3m+2n)/ [Cocit]− = [Dx /
(Do − Dx)]. So, from the dependence lg[Dx /
(Do − Dx) = f [lg([cit]3– / [MoO4]
2–)], shown in
fig. 4a, it is possible to determine the composi-
tion of the polyligand complexes formed in the
system under study at pH 9.0, the equilibrium
constant Ke of their formation according to the
reaction:
[Co(cit)2]
4– + [MoO4]
2– ↔ [Co(cit)(MoO4)]
3– + cit3–.
(5)
According to equation (4), the equilibri-
um constant Ke of the reaction of formation
of polyligand complexes [Co(cit)(MoO4)]3− is
equal to:
Ke = βMLX / β2 , (6)
where βMLX and β2 are the stability constants of
polyligand [Co(cit)(MoO4)]3− and citrate com-
plexes [Co(cit)2]
4– according.
Dependences lg[Dx / (Do − Dx)] = f [lg([cit]3–/
[MoO4]
2–)] (cur. 1, 2) and lg[Dx / (Do – Dx)] = f
[lg([MoO4]
2–/[cit]3–)] (cur. 3, 4), as can be seen
from Fig. 4 are rectilinear, the abscissa of the
intersection point of which is equal to 0. This
indicates that the molar ratio of the equilibri-
um concentrations of lidentate ligands [cit]3–/
[MoO4]
2– in the polyligand complex [Co(cit)
m(MoO4)n]+2-(3m+2n) is equal to 1, that is, m=n=1.
Thus, polyligand complexes [Co(cit)(MoO4)]3-
are formed in the system under study at pH 9.0.
Tangent of the slope of the straight line lg [Dx /
(Do – Dx)] = f [lg([MoO4]
2–/[cit]3–)] (Fig. 4,
6
6
method [6, 16], and their constant stability by the equilibrium shift method proposed by Bodlander
and Abegg [6].
Fig. 2. Curves of formation of polyligand complexes of cobalt (II) in 0.30 mol·l–1 Na2SO4
solution at pH 9.0 and at wavelengths = 529(3), 550(2), and 560(1) nm. Component concentration,
mol·l−1: CCo
2+ 0,01; Ccit
3– 0,01.
The results of the analysis of spectrophotometric data (Fig. 1) at wavelengths = 500, 529, 550
and 560 nm in coordinates: D=f[Ccit
3–/(Ccit
3–+CMoO4
2–)] (cr. 2, 4, 6, 8) and D=f[CMoO4
2–
/(CMoO4
2–+Ccit
3–)] (cr. 1, 3, 5, 7) [6, 16], where D – absorption of the test solution; CMoO4
2– and
CCit
3– are the total concentrations of ligands, mol·l-1, are shown in fig. 3. The obtained linear
dependences indicate the formation of stable polyligand complexes of cobalt (II) with citrate-(cit3–)
and molybdate–(MoO4
2–) ions of composition [Co(cit)m(MoO4)n]+2-(3m+2n). The abscissa of the point
of maximum absorption хmax in the system under study, as can be seen from Fig. 3 does not depend
on the wavelength and is equal to:
хmax = Ccit
3–/(Ccit
3–+CMoO4
2–) = n/ (n + m) = 0.5. (4)
Hence, the molar ratio of the maximum coordination numbers of polydentate ligands m/n in the
[Co(cit)(MoO4)]3- polyligand complex formed in the system under study at pH 9.0 is 1.
21https://ucj.org.ua
V. М. Nikitenko, Yu. S. Yapontseva, V. S. Kublanovsky UCJ № 03 / Vol. 89
cur. 3, 4) is 0.5618 and is equal to the logarithm
of the equilibrium constant Ke of the formation
of polyligand complexes [Co(cit)(MoO4)]3−.
The value of the equilibrium constant Ke calcu-
lated by equation (4) is 3.6459. Hence, the sta-
bility constant of [Co(cit)(MoO4)]3− polyligand
complexes according to equation (5) is 7.29·105
(lgβMLX = 5.86).
Fig. 4. Dependence of the function lg[Dx /
(Do − Dx)] on the logarithm of the ratio of equilib-
rium concentrations of ligands lg([cit]3–/[MoO4]
2–)
(cur. 1, 2) and lg([MoO4]
2–/[cit]3–) (cur. 3, 4), ob-
tained by the method of equilibrium shift in the
studyed system at pH 9.0, СCo
2+ ion concentration
0.01 mol·l–1, wavelength λ = 529(1. 3) and 550
(2. 4) nm.
The composition of polyligand cobalt (II)
complexes, and not only the ratio of the coor-
dination numbers of polydentate ligands m/n,
was determined from spectrophotometric
data (Fig. 1) also by the equilibrium shift me
thod [6]. The dependence lg[Dx /(Do – Dx)] =
f (lg[MoO4]
2–) was obtained in the system un-
der study at pH 9.0, total concentration of СCo
2+
ions – 0.01 mol·l–1, and wavelengths λ =500,
529 and 550 nm, is shown in fig. 5.
Fig. 5. Dependence of function lg [Dx /
(Do − Dx)] on the logarithm of the equilibrium
concentration of molybdate ions in citrate-mo-
lybdate electrolyte at pH 9.0, concentration of
components, mol·l-1: CCo
2+ − 0.01; Ccit
3– − 0.01
and wavelengths 500 (1), 529 (2) and 560
(3) nm.
The tangent of the slope of the straight line
lg[Dx / (Do – Dx)] = f (lg[MoO4]
2–) (Fig. 5,
cur. 3) is 0.5556 and is equal to the logarithm
of the equilibrium constant Ke of the formation
of polyligand complexes [Co(cit)(MoO4)]3−.
The value of the equilibrium constant Ke of the
reaction of their formation calculated by equa-
tion (4) is 3.5942. Hence, the stability constant
of [Co(cit)(MoO4)]3− polyligand complexes,
according to equation (5), is equal to 7.19·105
(lgβ MLX = 5.86).
The value of the stability constant of polyli-
gand complexes [Co(cit)(MoO4)]3− calculat-
ed from spectrophotometric data (lgβMLX =
5.86) is consistent with the value of this con-
stant calculated from pH-potentiometric data
(lgβMLX = 6.25) [7] which testifies to their au-
thenticity. The reasons for the discrepancies
in the values of the stability constants of the
7
7
Fig. 3. Dependence of cobalt (II) complexes absorption in a citrate-molybdate electrolyte at pH
9.0 and wavelengths = 500 (5, 6), 529 (1, 2), 550 (3, 4), and 560 (7, 8) nm. The composition of the
studied solutions, mol·l−1:CCo
2+ – 0.01;Ccit
3– 0.01; CMoO4
2 – 0 (1); 0.002 (2); 0.01 (3); 0.02 (4);
0.05 (5); 0.10 (6); 0.010 (7).
The composition of the [Co(cit)m(MoO4)n]+2-(3m+2n) polyligand complexes formed in the system
under study at pH 9.0, their stability constant MLX was determined from spectrophotometric data by
the method proposed by A.K. Babko [6]. The maximum optical density of the test solution
corresponding to the horizontal section on the absorption curve D = f (CMoO4
2–) shown in Fig. 2,
denoted by Do, and the optical density of the solution at a certain equilibrium concentration of
molybdate ions [MoO4]2– until the complete binding of the ligand into the colored polyligand
complex [Co(cit)m(MoO4)n]+2-(3m+2n) via Dx. Since the optical density of the solution Dx is
proportional to the concentration of the colored cobalt (II) polyligand complex, then the ratio
[Co(cit)m(MoO4)n]+2-(3m+2n)/[Cocit]=[Dx/(DoDx)]. So, from the dependence
lg[Dx/(DoDx)=f[lg([cit]3–/[MoO4]2–)], shown in fig. 4a, it is possible to determine the
composition of the polyligand complexes formed in the system under study at pH 9.0, the equilibrium
constant Ke of their formation according to the reaction:
[Co(cit)2]– + [MoO4]2– [Co(cit)(MoO4)]3– + cit3–. (5)
According to equation (4), the equilibrium constant Ke of the reaction of formation of polyligand
complexes [Co(cit)(MoO4)]3 is equal to:
Ke = MLX / 2 , (6)
where MLX and 2 are the stability constants of polyligand [Co(cit)(MoO4)]3 and citrate complexes
[Co(cit)2]4– according.
Fig. 4. Dependence of the function lg[Dx/(DoDx)] on the logarithm of the ratio of equilibrium
concentrations of ligands lg([cit]3–/[MoO4]2–) (cur. 1, 2) and lg([MoO4]2–/[cit]3–) (cur. 3, 4), obtained
8
8
by the method of equilibrium shift in the studyed system at pH 9.0, СCo
2+ ion concentration 0.01
mol·l–1, wavelength = 529(1. 3 ) and 550 (2. 4) nm.
Dependences lg[Dx/(DoDx)]=f[lg([cit]3–/[MoO4]2–)] (cur.1,2) and lg[Dx/(Do–
Dx)]=f[lg([MoO4]2–/[cit]3–)] (cur. 3, 4), as can be seen from Fig. 4 are rectilinear, the abscissa of the
intersection point of which is equal to 0. This indicates that the molar ratio of the equilibrium
concentrations of lidentate ligands [cit]3–/[MoO4]2– in the polyligand complex [Co(cit)m(MoO4)n]+2-
(3m+2n) is equal to 1, that is, m=n=1. Thus, polyligand complexes [Co(cit)(MoO4)]3- are formed in the
system under study at pH 9.0. Tangent of the slope of the straight line lg[Dx/(Do–
Dx)]=f[lg([MoO4]2–/[cit]3–)] (Fig. 4, cur. 3, 4) is 0.5618 and is equal to the logarithm of the
equilibrium constant Ke of the formation of polyligand complexes [Co(cit)(MoO4)]3. The value of
the equilibrium constant Ke calculated by equation (4) is 3.6459. Hence, the stability constant of
[Co(cit)(MoO4)]3 polyligand complexes according to equation (5) is 7.29·105 (lgMLX = 5.86).
The composition of polyligand cobalt (II) complexes, and not only the ratio of the coordination
numbers of polydentate ligands m/n, was determined from spectrophotometric data (Fig. 1) also by
the equilibrium shift method [6]. The dependence lg[Dx/(Do–Dx)]=f(lg[MoO4]2–) was obtained in
the system under study at pH 9.0, total concentration of СCo
2+ ions – 0.01 mol·l–1, and wavelengths
=500, 529 and 550 nm, is shown in fig. 5.
Fig. 5. Dependence of function lg [Dx / (Do Dx)] on the logarithm of the equilibrium
concentration of molybdate ions in citrate-molybdate electrolyte at pH 9.0, concentration of
components, mol·l-1: CCo
2+ 0.01; Ccit
3– 0.01 and wavelengths 500 (1), 529 (2) and 560 (3) nm.
The tangent of the slope of the straight line lg[Dx/(Do–Dx)]=f(lg[MoO4]2–) (Fig. 5, cur. 3) is
0.5556 and is equal to the logarithm of the equilibrium constant Ke of the formation of polyligand
complexes [Co(cit)(MoO4)]3. The value of the equilibrium constant Ke of the reaction of their
22 ISSN 2708-129X. Укр. хім. журн., 2023
DETERMINATION OF THE COMPOSITION AND STABILITY CONSTANT OF COBALT (II)
POLYLIGAND CITRATE-MOLYBDATE COMPLEXES.PHISICAL CHEMISTRY
polyligand complexes [Co(cit)(MoO4)]3− are
due not only to the methods of their determi-
nation, but also to different values of the ionic
strength and temperature of the studied solu-
tions. Thus, taking into account the above, it
should be noted that the question of the com-
position and stability constant of polyligand
citrate-molybdate complexes of cobalt (II)
should be considered resolved.
Fig. 6. Dependence of the degree of formation
of molybdate (1), citrate (2), polyligand (3), cobalt
(II) complexes in molybdate-citrate electrolyte on
the logarithm of the ratio of equilibrium concentra-
tions of ligands: 1− [Co(MoO4)2]
2–; 2 – [Co(cit)2]
4–;
3 – [Co(cit)(MoO4)]3–.
Taking into account the stability constants
of molybdate [Co(MoO4)2]
2– (lgβ2 = 3.09),
citrate [Co(cit)2]
4– (lgβ2 = 5.30) and polyli-
gand [Co(cit)(MoO4)]3–, (lgβMLX = 5.86) of
cobalt (II) complexes, the dependence of the
degree of their formation in a molybdate-cit-
rate electrolyte on the logarithm of the ratio
of the equilibrium concentrations of ligands
lg ([cit]3–/ [MoO4]
2–). In a molybdate-citrate
electrolyte, as can be seen from Fig. 6, molyb-
date complexes [Co(MoO4)2]
2– exist depending
on the ratio of equilibrium concentrations of
ligands lg([cit]3–/ [MoO4]
2–) in the range from
0 to +1, citrate complexes [Co(cit)2]
4– – in the
range from 0 to +2, and polyligand complexes
[Co(cit)(MoO4)]3– – in the region >0).
CONCLUSIONS. It has been established
that, in a citrate-molybdate electrolyte at pH
9.0, cobalt (II) ions form with citrate- (cit3–)
and molybdate- (MoO4
2-) ions, depending on
the ratio of the equilibrium concentrations
of the ligands, non-protonated molybdate
[Co(MoO4)2]
2–, citrate [Co(cit)2]
4– and polyli-
gand complexes [Co(cit)m(MoO4)n]+2-(3m+2n)).
The composition of [Co(cit)(MoO4)]3– polyli-
gand complexes was determined, the equilibri-
um constant Ke of the reaction of their forma-
tion and their stability constant (lgβMLX = 5.86)
were calculated. Polygand complexes [Co(cit)
(MoO4)]3– (lgβMLX = 5.86) are more stable than
molybdate [Co(MoO4)2]
2– (lgβ2 = 3.09) and
citrate [Co(cit)2]
4– (lgβ2 = 5.30) cobalt (II)
complexes.
The dependence of the degree of forma-
tion of molybdate [Co(MoO4)2]
2–, citrate
[Co(cit)2]
4– and polyligand [Co(cit)(MoO4)]3–
complexes of cobalt (II) in a citrate-molybdate
electrolyte on the logarithm relative to equilib-
rium concentrations ligands.
ACKNOWLEGEMENTS
This work was performed with the finan-
cial support of the National Academy
of Sciences of Ukraine within the state
budget theme «Finishing processing of
materials in order to give them unique
functional properties» 0123U100650.
23https://ucj.org.ua
V. М. Nikitenko, Yu. S. Yapontseva, V. S. Kublanovsky UCJ № 03 / Vol. 89
ВИЗНАЧЕННЯ СКЛАДУ ТА КОНСТАНТИ
СТІЙКОСТІ ПОЛІЛІГАНДНИХ ЦИТРАТНО-
МОЛІБДАТНИХ КОМПЛЕКСІВ КОБАЛЬТУ (II)
В. М. Нікітенко, Ю. С. Японцева*,
В. С. Кублановський
Інститут загальної та неорганічної хімії
імені В. І. Вернадського НАН України,
просп. Академіка Палладіна, 32/34, Київ
03142, Україна
*е-mail: juliya_yap@ukr.net
Спектрофотометричним методом вивче
но комплексоутворення кобальту (II) з цит
рат- (cit3–) і молібдат- (MoO4
2–) іонами в
0,3 моль·л−1 розчині Na2SO4 за 20 оС. Уста-
новлено, що в цитратно-молібдатному елек-
троліті при pH 9.0 іони кобальту (II) утво-
рюють залежно від співвідношення рівно-
важних концентрацій лігандів молібдатні
[Co(MoO4)2]
2–, цитратні [Co(cit)2]
4– та полілі-
гандні комплекси [Co(cit)m(MoO4)n]
+2-(3m+2n).
Визначено склад полілігандних комплексів
[Co(cit)(MoO4)]3–, обчислено константу рів-
новаги Kr реакції їхнього утворення та кон-
станту їхньої стійкості (lgβMLX = 5.86). Роз-
раховано залежність ступеня утворення
молібдатних, цитратних та полілігандних
комплексів кобальту (II) в цитратно-моліб-
датному електроліті від логарифму відно-
шення рівноважних концентрацій лігандів.
Ключові слова: кобальт (II), молібдатні,
цитратні та полілігандні комплекси, склад,
константа рівноваги, константа стійкості,
спектрофотометрія.
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Стаття надійшла 29.03.2023.
|
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| institution | Ukrainian Chemistry Journal |
| keywords_txt_mv | keywords |
| language | English |
| last_indexed | 2026-07-23T01:09:42Z |
| publishDate | 2023 |
| publisher | V.I.Vernadsky Institute of General and Inorganic Chemistry |
| record_format | ojs |
| resource_txt_mv | ucjorgua/44/43ac9bd05a4b86ee9b78b80cf5d6e144.pdf |
| spelling | oai:ojs2.1444248.nisspano.web.hosting-test.net:article-5372026-07-22T08:23:51Z DETERMINATION OF THE COMPOSITION AND STABILITY CONSTANT OF COBALT (II) POLYLIGAND CITRATE-MOLYBDATE COMPLEXES Nikitenko, Vasyl Kublanovsky, Valeriy Yapontseva, Yuliya cobalt (II), molybdate, citrate and polyligand complexes, composition, equilibrium constant, stability constant, spectrophotometry. The complex formation of cobalt (II) with citrate (cit3–) and molybdate (MoO42–) ions in 0.3 mol·l-1 Na2SO4 solution at 20°C was studied by spectrophotometric method. It has been established that in a citrate-molybdate electrolyte at pH 9.0, cobalt (II) ions form, depending on the ratio of the equilibrium concentrations of ligands: molybdate [Co(MoO4)2]2–, citrate [Co(cit)2]4– and polyligand complexes [Co(cit)m(MoO4)n]+2-(3m+2n). The composition of [Co(cit)(MoO4)]3– polyligand complexes was determined, the equilibrium constant Ke of the reaction of their formation and their stability constant (lgβMLX = 5.86) The dependence of the degree of formation of molybdate, citrate and polyligand complexes of cobalt (II) in citrate-molybdate electrolyte on the logarithm of the ratio of equilibrium concentrations of ligands was calculated. V.I.Vernadsky Institute of General and Inorganic Chemistry 2023-04-28 Article Article Physical chemistry Физическая xимия Фізична xімія application/pdf https://ucj.org.ua/index.php/journal/article/view/537 10.33609/2708-129X.89.03.2023.15-24 Ukrainian Chemistry Journal; Vol. 89 No. 3 (2023): Ukrainian Chemistry Journal; 15-24 Украинский химический журнал; ##issue.vol## 89 ##issue.no## 3 (2023): Ukrainian Chemistry Journal; 15-24 Український хімічний журнал; Том 89 № 3 (2023): Український хімічний журнал; 15-24 2708-129X 2708-1281 en https://ucj.org.ua/index.php/journal/article/view/537/272 Copyright (c) 2023 Vasyl Nikitenko, Valeriy Kublanovsky, Yuliya Yapontseva https://creativecommons.org/licenses/by-nc/4.0 |
| spellingShingle | Nikitenko, Vasyl Kublanovsky, Valeriy Yapontseva, Yuliya DETERMINATION OF THE COMPOSITION AND STABILITY CONSTANT OF COBALT (II) POLYLIGAND CITRATE-MOLYBDATE COMPLEXES |
| title | DETERMINATION OF THE COMPOSITION AND STABILITY CONSTANT OF COBALT (II) POLYLIGAND CITRATE-MOLYBDATE COMPLEXES |
| title_full | DETERMINATION OF THE COMPOSITION AND STABILITY CONSTANT OF COBALT (II) POLYLIGAND CITRATE-MOLYBDATE COMPLEXES |
| title_fullStr | DETERMINATION OF THE COMPOSITION AND STABILITY CONSTANT OF COBALT (II) POLYLIGAND CITRATE-MOLYBDATE COMPLEXES |
| title_full_unstemmed | DETERMINATION OF THE COMPOSITION AND STABILITY CONSTANT OF COBALT (II) POLYLIGAND CITRATE-MOLYBDATE COMPLEXES |
| title_short | DETERMINATION OF THE COMPOSITION AND STABILITY CONSTANT OF COBALT (II) POLYLIGAND CITRATE-MOLYBDATE COMPLEXES |
| title_sort | determination of the composition and stability constant of cobalt (ii) polyligand citrate-molybdate complexes |
| topic_facet | cobalt (II) molybdate citrate and polyligand complexes composition equilibrium constant stability constant spectrophotometry. |
| url | https://ucj.org.ua/index.php/journal/article/view/537 |
| work_keys_str_mv | AT nikitenkovasyl determinationofthecompositionandstabilityconstantofcobaltiipolyligandcitratemolybdatecomplexes AT kublanovskyvaleriy determinationofthecompositionandstabilityconstantofcobaltiipolyligandcitratemolybdatecomplexes AT yapontsevayuliya determinationofthecompositionandstabilityconstantofcobaltiipolyligandcitratemolybdatecomplexes |