OXIDATION OF Co(II) BY OZONE AND REACTIONS OF Co(III)
2359
optical density measured on an Agilent 8453 UV–Vis In studying the interaction between trivalent cobalt
spectrophotometer; the length of the liquid cell’s opti- and chloride ions, an equal volume of HCl or NaCl
cal path was 1 cm.
was rapidly added with a syringe to the solution of
Co(III) in sulfuric acid in the cell of the spectropho-
tometer, and the absorption spectra were registered
over time.
In finding molar absorption coefficients, the con-
centration of trivalent cobalt was determined by back
titration with hydrogen peroxide. Up to 20 mL of a
0
.1 M solution of Н О was added to 10 mL of a solu-
2 2
tion of sulfuric acid (C(H SO ) = 6–8.2 M) with a
2
4
RESULTS AND DISCUSSION
Со(III) concentration of no more than 0.1 M. The
reaction that occurred was 2Со(III) + Н О →
Solutions of trivalent cobalt in sulfuric acid have
two absorption maxima in the visible spectrum: 406
and 613 nm in 5 M H SO and 409–410 and 617–
2
2
2+
+
2
Со + О + 2Н . The amount of residual hydrogen
2
2
4
peroxide was determined via titration with a 0.1 M
6
18 nm in 8 M H SO (see Fig. 1). The maxima shift
2 4
solution of KМnО . The equivalence point was deter-
4
slightly toward long wavelengths when the concentra-
tion of acid is increased, but the value of the molar
absorption coefficient at the maximum remains
almost unchanged. The following values were found
for the molar absorption coefficients of Co(III) in
aqueous solutions of sulfuric acid with concentrations
mined from the emergence of a stable color of per-
manganate that was clearly visible against the pale red-
dish-pink color of bivalent cobalt. In our experiments,
almost no catalytic decomposition of hydrogen perox-
ide occurred under the action of Co ions. The con-
centration of Со(III) was calculated according to the
formula
2
+
–1
–1
of 1.8–8.2 M: ε4
= 52.2 L mol cm and
05–409 nm
ε611‒617 nm = 44.2 L mol– cm . At 405–409 nm, the
dependence of optical density А on the concentration
of trivalent cobalt is linear at least up to [Co(III)] =
0.021 M (A = 1.05), and up to [Co(III)] = 0.071 M
1
–1
[Co(III)], M = [V(H O )С(Н О )
2
2
2
2
5
2
−
V(KMnO )С(KMnO )] /V(Co(III)),
4
4
(
А = 3.15) at 611–617 nm. The literature contains val-
ues of the absorption coefficients of Co(III) in solu-
tions of perchloric acid: ε = 39.6 ± 0.2 [10], ε
where V(H O ) = 20 mL is the volume of the added
2
2
=
400
solution of peroxide; С(Н О ) is the concentration of
402
2
2
peroxide, M; V(KMnO ) is the volume of the solution 40.0 ± 0.5 [11], ε602 = 34.5 ± 0.1 [12], ε605 = 35.3 [13],
4
–1
–1
of permanganate consumed for the titration, mL; and ε = 35.0 ± 0.4 L mol cm [11]. The spectrum
605
С(KMnO ) is the concentration of permanganate, M;
4
of Co(III) in 9M H SO was presented in [14] (the
2 4
and V(Co(III)) = 10 mL is the volume of the portion
of the trivalent cobalt solution. The precise concentra-
tion of the initial solution of Н О was determined via
same spectrum was reproduced in [15]); from it, ε
≈
409
–1
–1
5
6 and ε ≈ 50 L mol cm can be obtained. Our
615
2
2
results on the absorption coefficients of Co(III) are
permanganometric titration in the presence of sulfuric
therefore in agreement with the literature data.
acid; the titer of KMnO solution was determined
4
In solutions of sulfuric acid, trivalent cobalt ions
decompose according to the stoichiometric equation
using oxalic acid as the primary standard [9]. The
–
1
–1
molar absorption coefficient of Co(III), L mol cm
at a particular wavelength λ, was calculated according
2+
1
+
2
Co(III) + H О → 2Co + / O + 2H
(1)
2
2
2
to the formula
2+
and are thus always present along with Со ions. In
the visible region, the absorption maximum of
2
+
2+
ε(Co(III)) = (A − ε(Co ) ([Co ]
0
λ
λ
λ
2+
Со (aq.) is found at 511 nm (see Fig. 1). The molar
−
[Co(III)]))/[Co(III)],
absorption coefficients of bivalent cobalt determined
in this work were ε = 0.35, ε = 4.83, and ε
=
=
where A is the optical density of the solution after ozo-
405
511
613
–1
–1
0
.34 L mol cm . According to literature data, ε
509
–1 –1 2+
nation; [Co(III)] is the concentration of trivalent
2
+
4.84 L mol cm [13] for Со in aqueous solutions
cobalt in this solution, M; [Co ] is the concentration
0
of HClO at λ
= 509 nm; in 2 М H SO , ε
≈
511
of bivalent cobalt in the initial solution, M; and
4
max
2
4
–
1
–1
2+
6.6 L mol cm [14, 15].
ε(Co ) is the molar absorption coefficient of bivalent
–1
–1
cobalt, L mol cm .
The kinetics of decomposition of trivalent cobalt
was studied in aqueous solutions of 0.1–8.2 М sulfuric
acid; here, the initial concentration of Со(III) was 3 ×
In studying the kinetics of the spontaneous decom-
position of trivalent cobalt by means of spectropho-
tometry, the drop in its concentration in solutions of
sulfuric acid was determined as a function of time. The
order of the reaction (n) was determined using an inte-
gral method, so the integral or semi-integral value of n
–
3
–2
1
0 to 7 × 10 M. The reaction is of the second order;
i.e., the drop in the concentration of trivalent cobalt is
described by the differential equation –d[Co(III)]/dt =
2
k [Co(III)] . The rate of decomposition falls dramati-
1
(
1–n)
cally as the concentration of sulfuric acid rises. The
at which the kinetic curves in the ([Co(III)]
, time)
half-life period of Co(III) in 0.1 М H SO is thus
coordinates ((ln [Co(III)], time) for n = 1) are most
2
4
3
linear was sought.
6 min (at [Co(III)] SO , it
0 2 4
= 3 × 10– М); in 6 М H
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A Vol. 90 No. 12 2016