Effect of 4(2)-Octylaminopyridine Distribution in a Water (HCl)/Chloroform System on the Extraction of Metal Ions from Chloride Solutions

Article abstract of DOI:10.1134/S0036024419050066

Abstract: Distributions of 4(2)-octylaminopyridines (4(2)-OAPs) as extractants in an H₂O(HCl)/CHCl₃ system were studied by UV and IR spectroscopy and potentiometry. The ionization constant of monoprotonated 4-OAP, the logarithmic distribution constant of 4-OAP and its chloride, and the logarithmic extraction constant of hydrochloric acid were calculated. These values were compared with those for 2-OAP. The isotherm (20 ± 2°C) of HCl extraction with 0.1 M 4-OAP in chloroform is given. The difference in the extraction chemistry of hydrochloric acid with 2- and 4-OAPs and specific application features of these reagents for extraction of metals from chloride solutions are discussed.

Full text of DOI:10.1134/S0036024419050066

ISSN 0036-0244, Russian Journal of Physical Chemistry A, 2019, Vol. 93, No. 5, pp. 828–834. © Pleiades Publishing, Ltd., 2019.
Russian Text © N.A. Borshch, L.S. Ageeva, A.Yu. Frolova, 2019, published in Zhurnal Fizicheskoi Khimii, 2019, Vol. 93, No. 5, pp. 661–667.
PHYSICAL CHEMISTRY
OF SOLUTIONS
Effect of 4(2)-Octylaminopyridine Distribution in a Water
(
HCl)/Chloroform System on the Extraction of Metal Ions
from Chloride Solutions
a,
a
a
N. A. Borshch *, L. S. Ageeva , and A. Yu. Frolova
a
Southwest State University, Kursk, 305040 Russia
*е-mail: nborsch@mail.ru
Received June 27, 2018; revised July 10, 2018; accepted September 11, 2018
Abstract—Distributions of 4(2)-octylaminopyridines (4(2)-OAPs) as extractants in an H O(HCl)/CHCl
2
3
system were studied by UV and IR spectroscopy and potentiometry. The ionization constant of monoproton-
ated 4-OAP, the logarithmic distribution constant of 4-OAP and its chloride, and the logarithmic extraction
constant of hydrochloric acid were calculated. These values were compared with those for 2-OAP. The iso-
therm (20 ± 2°C) of HCl extraction with 0.1 M 4-OAP in chloroform is given. The difference in the extraction
chemistry of hydrochloric acid with 2- and 4-OAPs and specific application features of these reagents for
extraction of metals from chloride solutions are discussed.
Keywords: 4(2)-octylaminopyridine, monoprotonated 4-octylaminopyridine, ionization constant, extraction
of hydrochloric acid, extraction constant, distribution constant, chloroform–water system
DOI: 10.1134/S0036024419050066
INTRODUCTION
-Octylaminopyridine (2-OAP), synthesized for
system was investigated as a first stage of its extraction
studies. The data obtained for 4-OAP are discussed
along with similar data for 2-OAP obtained earlier by
the authors in [2, 3].
2
the first time by the authors in [1, 2] and proposed as
an extractant for recovery and separation of platinum
metals (2-ОАP) in [3–5], is now widely used for the
extraction of both platinum metals and other rare and
scattered metals, e.g., Pd(II), Rh(III), Ir(III), and
Ru(III) [6–9], Th(IV), Tl(III), Ga(III), In(III),
La(III), Sm(III), Zr(IV), Mo(VI), Cr(VI), V(V),
Bi(III), Pb(II), and Hg(II) [10–22], respectively.
EXPERIMENTAL
Reagents and Equipment
The reagents we used were 4-aminopyridine (98%,
Acros Organics), sodium amide (99%, Acros Organ-
ics), octyl iodide (98%, Acros Organics), dioxane
Ionic associates with metal in the anionic part,
neutral coordination–solvation compounds, and
compounds containing an amine in the outer or inner
coordination sphere of a metal ion can be extracted,
depending on the conditions [3, 5].
(
analytical grade), chloroform (reagent grade), etha-
nol (rectified), hexane (reagent grade), isopropanol
chemically pure), acetic acid (chemically pure),
hydrochloric acid (chemically pure), perchloric acid
(
In contrast, 4-octylaminopyridine (4-OAP) has (chemically pure), potassium hydroxide (analytical
virtually never been studied as an extractant for metals, grade), and bidistilled water obtained via distillation
though it was shown in [23] that iridium (III, IV) on a laboratory distiller.
could be extracted quantitatively with 4-OAP in chlo-
The liquids were sampled using graduated glass-
roform from hydrochloric acid solutions with very
ware, and all substances were weighed on a Nevskie
high partition coefficients, as was predicted in [1]. In
VSL-200/0.1A analytical scale.
addition, a procedure was proposed in [24] for the
Potentiometric measurements were performed at
extractive separation of small amounts of platinum
metals (Pd, Pt, Ph, and Ir) from complex-composi-
tion hydrochloric acid solutions of copper–nickel ori-
gin with solutions of 4-OAP in chloroform, followed
by determination via inductively coupled plasma
2
0 ± 2°C using a Multitest KSL-101 potentiometer,
glass hydrogen and silver chloride reference elec-
trodes, and a magnetic stirrer.
IR spectra were recorded on an IS 10 Nicolet FT-IR
atomic emission spectroscopy. In this work, the distri- spectrometer equipped with a SMART accessory
bution of 4-OAP in a H O(HCl)/CHCl two-phase (a germanium crystal).
2
3
828

EFFECT OF 4(2)-OCTYLAMINOPYRIDINE DISTRIBUTION
829
EXPERIMENTAL
phase than that of 4-AOP chloride. The total concen-
tration of HCl was determined for the first sample.
The main portion of HCl in the second sample was
Synthesis of 4-OAP and Its Chloride
4
-OAP was prepared using the Chichibabin reac- neutralized to pH 3–3.5, and the residual HCl and
tion of 4-aminopyridine (4-AP) and sodium amide in 4-OAP chloride were titrated. The concentration of
dry dioxane, followed by alkylation with octyl iodide 4-OAP chloride was calculated using the resulting
[
25]. 4-OAP was isolated from the reaction mixture via potentiometric curve, and the content of unbound
extraction with hexane and recrystallized first from acid was calculated from the difference between the
hexane and then from acetone (99.9%, m.p. 64 ± total content of HCl and the content of 4-OAP chlo-
0
.2°C). The molecular and structural formulas were ride.
1
confirmed via elemental analysis plus IR and H
NMR spectroscopy. OAP chloride was prepared by
shaking a 0.1 M solution of 4-OAP (100 mL) with 1 M
HCl (100 mL). The organic phase was separated and
filtered through a paper filter. All chloroform was
evaporated under an air stream. The residue was dried
at 60°C. The yield was 95%. According to the data
from potentiometric titration, the content of the main
substance was 99.8%.
Titration was performed in a 100-mL glass beaker
containing a 0.02 M solution (30 mL) of 4-OAP (or
OAP chloride) in ethanol or a sample for determining
the components of HCl extraction adjusted to 30 mL
using the solvent. The beaker was placed on a mag-
netic stirrer, glass and silver chloride electrodes were
immersed, and titration was performed using an alkali
or acid solution, depending on the component to be
determined. During titration, the ∆E value (mV) was
measured with an accuracy of ±1 mV, depending on
the volume of the added titrant. The titration curve
Spectrophotometric Measurements
was plotted in the ∆Е = f(V ) coordinates using the
HCl
A test portion (5–10 mg) was placed on the crystal measurement data. The equivalence point was found
of the SMART accessory, and the IR spectrum was to be the midpoint on the ∆E jump interval.
recorded. UV-spectra were recorded, and the absor-
bances of the solutions were measured using an
UV-1800 spectrophotometer (Shimadzu) and quartz
RESULTS AND DISCUSSION
Pyridine and its derivatives are strong organic
cells (l = 1 cm).
Experiments on separating 4-OAP into chloroform bases, so the extraction of metals from HCl solutions
and water were performed in a separatory funnel at can proceed by both anion-exchange and coordina-
2
5 ± 2°C; the phase contact time was 15 min. Buffer tion–solvation mechanisms. To gain an insight into
solutions were prepared on the basis of 0.01 M H РO
the mechanism of extraction, it is important to know
the constants that characterize the acid–base proper-
ties of OAP, the distribution constants of neutral mol-
ecules and the chloride, and the constants of hydro-
chloric acid extraction. These constants are part of the
constant of metal ion extraction.
The protolytic properties of 4(2)-OAP (Аm) as a
bifunctional molecule are described by three constants
of acid–base equilibrium:
3
4
to prevent acid from being drawn into the organic
phase. The concentration of OAP in CHCl , which had
3
been in preliminary contact (15 min) with a buffer solu-
tion having the same composition as in separation
−
5
experiments, was held constant at a level of 4.3 × 10 M.
Hydrochloric acid was extracted with a 0.1 M solu-
tion of OAP in CHCl in tubes equipped with a
3
ground-glass stopper at equal volumes of organic and
aqueous phases and a contact time of 15 min. It was
preliminarily found that equilibrium was reached
within 5 min. The equilibrium concentrations of com-
ponents were calculated using data from potentiomet-
ric titration.
+
2
+
+
АmН2 ↔ АmН + Н ,
+
+
+2
2
K
= [АmН ][Н ]/[АmН
],
1
+
+
АmН ↔ Аm + Н ,
+
+
Potentiometric titration was used to determine the
content of the main substance in 4-OAP and its chlo-
ride, along with the equilibrium concentrations of
components when extracting HCl with 4-OAP. To
determine the concentrations of HCl and free 4-OAP
in the organic phase, a sample dissolved in isopropa-
nol and acetic acid was titrated with solutions of KOH
K = [Аm][Н ]/[АmН ],
2
−
+
Аm ↔ Аm + Н ,
−
+
K = [Аm ][Н ]/[Аm].
3
To understand specific features of metal ion
extraction from acidic and slightly acidic solutions of
HCl, the IR spectra for 4-OAP and its chloride, plus
the UV spectra for free amine and its protonated form
in water, were preliminarily studied.
and HClO in ethanol, respectively. The concentra-
4
tion of hydrochloric acid and 4-OAP chloride in the
aqueous phase was calculated from one potentiomet-
ric curve which displayed two distinct potential jumps.
The IR spectrum for 4-OAP obtained from 4-AP
It was determined using different samples at a consid- [25] displays the stretching and bending vibration
erably higher concentration of HCl in the aqueous bands of octyl –CH
and –CH – groups at 2956,
2
3
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8
30
BORSHCH et al.
−1
2
924, 2850 (ν , ν ), and 1467, 1370, 725 (δ , δ ) cm ,
A
as
s
as
s
which are missing from the IR spectrum of 4-AP. At 0.3
the same time, the pattern of N-H stretching vibration
bands changes, corresponding to a transition from a
primary to secondary amine. For example, two
absorption bands at 3433 and 3300 cm−1 (ν , ν ) dis-
as
s
0.2
−
1
appear, and a new band appears at 3230 cm instead.
All remaining absorption bands of the pyridine ring
are present, with slight shifts to lower frequencies.
2
1
As with 2-OAP [2], the vibration frequencies of the
0
.1
aromatic ring increase when protons are added (ν
cch
,
c–c
δ ). A new broad band of ν
vibrations (an ammo-
NH+
nium band) appears in the higher-frequency region of
−1
2
600–3000 cm , and the ν band of amino group
NH
2
00
250
300 λ, nm
broadens and shifts toward low frequencies. All this
testifies to the addition of protons to the heterocyclic
nitrogen, and the formation of hydrogen bonds
between the counter-ions (chloride) and both the
ammonium and amino group protons.
Fig. 1. UV spectra for solutions of 4-OAP (1.25 × 10− M,
l = 1 cm): (1) 0.01 M HCl and (2) 0.01 M NaOH.
5
The UV spectra for the neutral and protonated
forms of 4-OAP (Fig. 1) display two absorption bands
with maxima at 254.5, 213.2 nm and 274, 209.6 nm,
respectively (π–π* electron transitions). Adding pro-
tons to 4-OAP results in a strong bathochromic shift of
the low-energy π–π* transition (∆ = 19.5 nm) and a
hypsochromic shift of the high-energy π–π* transition
the absorbance of the protonated form (pH 2), deter-
mined at the same wavelength.
A specified pH was maintained using buffer solu-
tions prepared by mixing 0.05 M Na B O ·10H O with
2
4
7
2
0
.1 M HCl or 0.1 M NaOH. To move to the thermo-
T
(
∆ = 3.6 nm).
dynamic value of ionization constant (рK
of the ionic strength of a solution produced by a buffer
solution was considered using the formula
2
), the effect
Analysis of the UV spectra and comparing them to
the literature data shows that alkylation of the amino
T
C
group of aminopyridine (242.3 nm, pK = 9.18 [26])
2
рK = рK − 0.505 μ/(1 +1.6 μ).
2 2
results in a bathochromic shift of the low-energy π–π*
transition (∆ = 12.2 nm) and an increase in the basic-
ity of the heterocyclic nitrogen, due to the induction
effect.
The average ionic strength produced by the buffer
solution was μ = 0.165.
Table 1 gives the data from which the average K
2
values for 4-OAP were calculated. The average con-
To describe the extraction of HCl, it is sufficient to
C
T
2
know the constant of ionization of the monoproton- stants were рK
= 9.89 ± 0.07 and рK
= 9.77 ± 0.07
2
ated form K , since protons are in this case added to (P = 0.95, n = 5).
2
the nitrogen of heterocycles. The addition of a second
proton can occur only in extremely acidic media. We
Constant of Distribution between Chloroform
can also ignore the dissociation of 4(2)-OAP (K ),
3
and Water (K )
D
since aminopyridines like pyrazoles and amidines are
known to be very weak NH acids, and they form
sodium salts only when treated with sodium amide.
The constant of the distribution of long-chain
amines is generally high. It is therefore difficult to
The wavelength for determining K was selected
2
from the UV spectra: the maximum difference Table 1. Data from determining the constant of ionization
−4
between the absorption of the neutral and protonated of protonated 4-octylaminopyridine: 20 ± 2°C, 1 × 10 M.
Cell length, 1 cm; λ = 240 nm; A = 0.902, A = 0.218
forms of 4-OAP was observed in the range of 240–
n
p
2
70 nm. The K value was determined using via spec-
2
log(A
C
pH
A
−(A − A ) −(A − A)
trophotometric technique in [27], according to which
n
p
pK2
A )/(A − A)
−
n
p
C
2
the values of the concentration constant (рK ) for a
8
.37
0.234
0.264
0.304
0.476
0.523
0.668
0.638
0.598
0.426
0.379
0.016
0.046
0.086
0.258
0.305
1.62
9.97
9.84
9.84
9.92
9.89
series of solutions with identical concentrations of
8.70
1.14
0.84
0.22
0.09
4
-OAP and different pH values were calculated using
C
9.00
.70
9.80
the formula рK = pH + log(A – A )/(A – A), where
2
n
p
9
A is the absorbance of a solution with known pH; A is
n
the absorbance of the neutral form (pH 12); and A is
p
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EFFECT OF 4(2)-OCTYLAMINOPYRIDINE DISTRIBUTION
831
Table 2. Data from determining the coefficient of separa- can be of the same order as its concentration. The data
tion of 4-OAP in a water–chloroform system, depending on
from determining the distribution constants of 4-AOP
are given in Table 2. The formulas used in our calcula-
tions were
the pH of the aqueous phase (λ = 300 nm; A = 1.86;
0
C
= 5 × 10−5 M)
OAP
No.
pH
A
r
D
−log D
( )
Dr/(1 + Dr) = A/A0; D = A/r A0 – A ,
1
5.2
0.061
0.077
0.143
0.192
0.235
0.338
0.061
0.077
0.143
0.192
0.235
0.338
0.25
0.25
0.25
0.25
0.25
0.25
0.25
0.25
0.25
0.25
0.25
0.25
0.136
0.173
0.333
0.460
0.578
0.888
0.136
0.173
0.333
0.460
0.578
0.888
0.866
0.762
0.476
0.337
0.238
0.052
0.866
0.762
0.476
0.337
0.238
0.052
where r is the organic to aqueous phase volume ratio.
Figure 2 shows the logD = f(pH) dependence. Its
2
3
4
5
6
1
2
3
4
5
6
5.58
5.97
6.40
6.75
7.18
5.2
5.58
5.97
6.40
6.75
7.18
extrapolation to pH = 0 allows one to determine logK
D
–
pK = –3.04, from which logK = 9.77 – 3.04 =
a D
6.73.
The extraction of hydrochloric acid with a 0.1 M
solution of 4-OAP in chloroform was studied in the
range of 0.05–12 M HCl. The extraction isotherm
(25 ± 2°C) is shown in Fig. 3. As is seen, the quantita-
tive extraction of HCl with 0.1 M 4-OAP proceeds at a
concentration of 0.2–3 M HCl in the aqueous phase.
At a concentration of 6–10 M HCl in the aqueous
phase, the hyperstoichiometric extraction of HCl pro-
ceeds when a second HCl molecule is added through
hydrogen bonding. At a hydrochloric acid concentra-
tion of 12 M in the aqueous phase, a third phase is pro-
duced after extraction, where the concentration of
HCl is 2.57 M.
determine this constant in direct experiments, i.e.,
through spectrophotometric measurements of the
equilibrium concentrations of amine in its aqueous
and organic phases. We must in this case allow for the
activity factors and the association of amine in the
organic phase. With aminopyridines, association is
especially high because of hydrogen bonding. To
determine the distribution constants, we therefore
used the equation
In light of the above data, the equation of hydro-
chloric acid extraction via neutralization can be writ-
ten as
+
w
–
w
+
–
Am + H + Cl ↔ [AmH Cl ]
0
0
with the extraction constant
log D = pH + log K − pK ,
D
a
+
–
[
AmH Cl ] γ°
+
–
T
ex
C
0
AmH Cl
whose applicability was demonstrated in determining
the ionization constants of several heterocyclic amines
K
= K γ =
ex
+
–
w
H
w
Cl
[
Am] [H ] [Cl ] γ
+
γ
–
γ°
Am
0
w
w
[
27]. Coefficients D of distribution in a two-phase sys-
tem were determined via spectrophotometry. At low and the distribution constant of OAP chloride,
concentrations, the association of 4-AOP in the
+
–
organic phase can be ignored, since chloroform can
form strong hydrogen bonds with electron donors. In
addition, the activity of amines in the organic phase
[
AmH Cl ] γ°
+
–
T
D
–
C
D
–
0
AmH Cl
K (Cl ) = K (Cl )γ =
.
+
+
–
w
w
[AmH ]
[Cl ]
γ
–
γ
w
w
Cl AmH
−logD
1.0
0.8
0.6
0.4
0.2
0
0
.20
.16
.12
y = −0.4179x + 3.0376
0
0
2
R = 0.9816
0.08
0.04
0
2
4
6
8
10
5
6
7 pH
HCl in the aqueous phase, M
Fig. 2. Logarithmic coefficient of the distribution of
-OAP between chloroform and water, as a function of pH.
Fig. 3. Isotherm of HCl extraction (20 ± 2°C) with 0.1 M
4-OAP in chloroform.
4
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8
32
BORSHCH et al.
Table 3. Data from determining the constant of extraction of HCl and the constant of distribution of 4-OAP chloride
Equilibrium concentrations of the components in water and chloroform, M
C
−
C
ex
K (Cl )
K
AmH+]
+
[Cl–]
+
–
D
[
[H ]
[Am]0
[AmH Cl ]
0
w
w
w
0
0
.0350
.0390
0.0017
0.0017
0.0017
0.0033
0.0620
0.0367
0.0407
0.0423
0.0466
0.0893
0.0487
0.0273
0.0080
0.0016
0.0007
0.0273
0.0452
0.0663
0.0663
0.0858
1.33
1.45
1.56
1.51
1.54
3.95
4.37
5.06
5.40
4.33
0
0
.0406
.0433
0
.0273
Mean values (n = 5, P = 0.95)
1.48 ± 0.12
4.62 ± 0.40
T
C
T
−
C
−
tions show, this is mainly due to its pK being two units
a
Here, K , K and K (Cl ), K (Cl ) are constants of
ex
ex
D
D
–
higher: 9.77 versus 7.35. Conversely, logK (Cl ) is
thermodynamics and concentration, respectively;
D
γ denotes the total and individual activity factors of the higher for 2-OAP by more than unity: 2.49 versus 1.54,
corresponding equilibrium forms. The constants of while the distribution constant of free amine is consid-
concentration were calculated using these equations erably lower: 5.94 versus 6.73.
and the equilibrium concentrations of the components
The observed similarity and difference between the
extraction behaviors of 2- and 4-OAP in the water
(HCl)–chloroform system can be explained by con-
sidering specific features of the structure and electron
density distribution in the free amine and its cation
from the example of simple and illustrative concept of
resonance structures (Figs. 4 and 5).
(
Table 3):
C
ex
C
–
K
= 4.62 ± 0.4, K (Cl ) = 1.48 ± 0.12
D
(
P = 0.95, n = 5).
Allowing for the activity factors of components in
the aqueous phase (the activity factors of amine and its
salt in the organic phase can be ignored [28]), we
M
The contribution from the pyridine imine structure
obtained the corrected values of these constants (K )
which were slightly different from the constants of in free 2-OAP, as calculated experimentally from the
concentration and apparently close to the thermody- ESCA data on nitrogen atoms [1, 3], is 9% (the degree
namic values:
of delocalization is 18%), resulting in considerably
higher basicity of heterocyclic nitrogen, compared to
alkylpyridines [2]. With 4-OAP, this contribution can
M
M
D
–
Kex = 4.68 ± 0.4, K (Cl ) = 1.54 ± 0.12
(
P = 0.95, n = 5).
be even greater, since its pK is two orders of magni-
a
tude higher than that for 2-OAP. The protonated
amine is an electron-deficient structure, so we would
expect a deeper resonance than with a neutral mole-
cule. For example, the degree of charge delocalization
for 2-OAP, calculated with the experimental data from
ESCA spectroscopy and the chemical shifts of nitro-
gen protons in [1] is 56%. The dependence of the con-
stant of the distribution of amine chloride on the
depth of resonance is easily demonstrated. It can be
As with 2-OAP, the constant of HCl extraction
includes the constant of dissociation of the protonated
form of the amine, along with the constants of the dis-
tribution of neutral molecules of the amine and its salt.
The following equations are valid [2]:
–
K (Cl )
D
Kex
=
,
K K (Am)
a
D
–
log Kex = log K (Cl ) – log K – log K (Am),
D
a
D
–
log Kex = log K (Cl ) + pK – log K (Am),
D
a
D
with which the extraction constant of HCl can be cal-
culated from the experimental values of constants con-
tained in it to verify the correctness of an experiment.
In our case, 4.68 ≈ 1.54 + 9.77 − 6.73 = 4.58. As is
seen, the experimental and calculated extraction con-
stants coincide within the experimental error.
C H
C H
8 17
8
17
N
N
H
N
N
H
C H
8
C H
8 17
17
N
H
N
H
N
H
N
In comparing the data from studying the behavior
of 4-OAP extraction in the water (HCl)–chloroform
two-phase system and the analogous data for 2-OAP
H
Cl
Cl
[
2], it should be noted that logK is considerably
ex
Fig. 4. Limiting (resonance) structures of the neutral and
protonated molecules of 2-OAP.
higher for 4-OAP: 4.68 versus 4.13. As the above equa-
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EFFECT OF 4(2)-OCTYLAMINOPYRIDINE DISTRIBUTION
833
Such anions are also characterized by lower energies of
hydration. Consequently, in the case of 4(2)-OAP one
should expect high two-phase stability constants for
anions with high polarizability. The ratio of two-phase
stability constants is in turn the constant of anion
exchange in a system with cationic extractant; i.e.,
C H
N H
C8H17 N H
8
17
N
N
K
= K (A )/K (A ). Upon the extraction of metal
D 1 D 2
C H
8
N H
C8H17 N H Cl
ex(1, 2)
17
ions with 4(2)-OAP as ionic associates, the highest
constants of exchange should therefore be expected for
metal-containing anions with high polarizability.
Compared to other nitrogen-containing extractants
used as anion exchangers, 4(2)-OAP is more selective
toward anions with high polarizability. In our opinion,
this feature is more pronounced for 2-OAP, due to the
expectedly greater depth of resonance in the organic
cation. In contrast, 4-OAP is preferred as an extract-
ant when extracting metal ions in the form of neutral
N
H Cl
N
H
Fig. 5. Limiting (resonance) structures of the neutral and
protonated molecule of 4-OAP.
presented as the two-phase stability constant of the coordination–solvation compounds, since it has
associate K (Cl ) = K (Am)β , where
–
higher electron density on the electron-donating
nitrogen atom.
D
D
ass
+
−
+
−
βass = [AmH Cl ] /[AmH ] [Cl ]
w
−
w
+
w
and
+
−
K = [AmH Cl ] /[AmH Cl ] .
CONCLUSIONS
D
0
w
The constant of the stability of the associate in the
The abovementioned features of the extraction of
aqueous phase, determined using the equation ∆G = 4-OAP in a water (HCl)–chloroform system relative
–
RTln β , is due mainly to the difference between the to 2-OAP were confirmed by comparing the data on
ass
the extraction of iridium(III) from hydrochloric acid
solutions using these extractants [4, 23]. The
extraction recovery of iridium has always been a prob-
lem, due to the abnormally high kinetic inactivity of its
complexes. Both extractants nevertheless allow us to
quantitatively recover iridium from acidic solutions
with very high distribution coefficients. In the case of
energy of interaction of an ion pair and the energy of
hydration of a cation and anion; with a large organic
cation, the hydration of an inorganic anion makes the
main contribution to the energy of hydration. With
4(2)-OAP, the energy of ion pair interaction increases
due to charge delocalization. The two-phase stability
constants of 4-OAP chloride and 2-OAP chloride are
therefore one and two orders of magnitude higher,
respectively, than for p-isooctylaniline chloride [30],
in which the degree of conjugation between the nitro-
gen electron pair and the aromatic ring is less than in
4-OAP, however, recovery can be done from more
acidic solutions in the range of 0.1–3 M HCl at con-
siderably lower concentrations of the extractant. The
authors’ assumption [1] that 4-OAP could be a more
efficient extractant for recovering and concentrating
iridium from solutions of complex composition was
confirmed experimentally. Further studies of 4(2)-
OAP as extractants are required for isolating and sep-
arating platinum metals and other rare and scattered
metals.
2
(4)-OAP. The gain in the energy of delocalization for
2
-OAP compared to the one for p-isooctylaniline is as
high as 5.8 kcal/mol [1].
Stabilization due to charge delocalization is likely
the main reason why the hyperstoichiometric
extraction of hydrochloric acid with 4-OAP starts after
reaching an HCl concentration of 6 mol/L and attains
the maximum value for the extracted Am·2HCl asso-
ciate at an HCl concentration of 10 mol/L. The addi-
tion of the second HCl molecule results in a loss in the
energy of delocalization. With 2-OAP, this leads to an
additional loss caused by the cleaving of the energeti-
cally favorable cycle based on hydrogen bonding [1, 2].
The hyperstoichiometric extraction of HCl with
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Translated by K. Utegenov
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A
Vol. 93
No. 5
2019