Polarographic Behavior of Manganese(II) in the Presence of Oxalate Ions in Perchlorate and Sulfate Solutions
ABSTRACT The dc polarographic method has been applied to study coordination equilibria between Mn(II) and oxalate ions in perchlorate
and sulfate solutions. The stoichiometries of complexes formed in solution and those reduced at a dropping mercury electrode
were established. The stability constants of the Mn(II) oxalate and sulfate complexes, as well as their diffusion coefficients,
were determined at a constant ionic strength 0.5mol⋅L−1 and25 °C. The stabilities of these Mn(II) complexes were compared with the corresponding complexes of other divalent metal
ions. The polarographic method was able to identify complexes that have not been established by other methods and to determine
their stability constants with high accuracy.
KeywordsPolarography–Manganese(II) complexes–Oxalate complexes–Stability constants–Electrode reduction mechanism
- Bulletin of The Chemical Society of Japan - BULL CHEM SOC JPN. 01/1961; 34(7):1040-1045.
- [show abstract] [hide abstract]
ABSTRACT: The extraction of 32 metals (Be, Mg, Ca, Sr, Ba, Sc, La, Ti, Zr, Th, V, Nb, Cr, Mo, W, U, Mn, Fe, Co, Ni, Pd, Cu, Ag, Zn, Cd, Hg, Al, Ga, In, Tl, Pb and Bi) with oxine solution has been studied. The effects of pH, oxine concentration and water-soluble complexing agents (oxalic acid, tartaric acid, hydrocyanic acid, nitrilotriacetic acid, ethylenediaminetetraacetic acid and 1,2-diaminocyclohexanetetraacetic acid) have been investigated. From these results the extraction constants and stability constants ot the metal complexes with the various complexing agents investigated have been calculated.RésuméL'auteur a effectué une étude sur l'extraction de 32 métaux, au moyen d'oxine. L'influence de divers facteurs (pH, concentration, agents complexants) a été examiée. Les constantes d'extraction et de stabilité ont été déterminées.ZusammenfassungBeschreibung einer Untersuchung über das Verhalten von 32 Metallen bei der Extraktion mit Oxinlösung unter Berücksichtigung des Einflusses von pH und anderer Faktoren. Extraktions und Stabilitätskonstanten mit verschiedenen Komplexbildnern werden angegeben.Analytica Chimica Acta. 01/1963; 28:132-149.
- Inorganic Chemistry - INORG CHEM. 04/2002; 17(3).
J Solution Chem (2011) 40: 247–260
Polarographic Behavior of Manganese(II) in the Presence
of Oxalate Ions in Perchlorate and Sulfate Solutions
Jadwiga Urba´ nska
Received: 3 November 2009 / Accepted: 7 July 2010 / Published online: 20 January 2011
© The Author(s) 2011. This article is published with open access at Springerlink.com
Abstract The dc polarographic method has been applied to study coordination equilibria
between Mn(II) and oxalate ions in perchlorate and sulfate solutions. The stoichiometries of
complexes formedinsolutionandthosereducedatadroppingmercuryelectrode wereestab-
lished. The stability constants of the Mn(II) oxalate and sulfate complexes, as well as their
diffusion coefficients, were determined at a constant ionic strength 0.5 mol·L−1and 25°C.
The stabilities of these Mn(II) complexes were compared with the corresponding complexes
of other divalent metal ions. The polarographic method was able to identify complexes that
have not been established by other methods and to determine their stability constants with
Keywords Polarography · Manganese(II) complexes · Oxalate complexes · Stability
constants · Electrode reduction mechanism
Manganese is among the metals most frequently used in the industry, which results in el-
evated levels of this metal in the environment: in water, soil and in biological materials.
Manganese is involved in the production of steel (as a hardening agent), and in Mn alloys,
dry-cell batteries, and in the chemical industry for coloring glasses, ceramics and pigments,
as well as in agriculture as a fertilizer and fungicide (e.g., MANEB, ZINEB). In gasoline, it
is applied as anti-knocking agent and to improve octane ratings (MMT). Recently, its com-
pounds have been used as the contrasting agent in nuclear magnetic resonance tomography
(Mn-DPDP for liver and pancreas scans) [1, 2].
On the other hand, manganese is important in the activation of many enzymes involved
in metabolic processes of all organisms. It is required for protein and fat metabolism, for
healthy nerves and a healthy immune system, for blood sugar regulation, the production of
cellular energy and bone growth [1, 3, 4]. Manganese elevates the level of anti-oxidative
J. Urba´ nska (?)
Faculty of Chemistry, University of Wrocław, 14 F. Joliot-Curie, 50-383 Wrocław, Poland
248 J Solution Chem (2011) 40: 247–260
protection by decreasing the concentration of free radicals , assists in the utilization of
vitamins B1and E, and prevents clotting effects .
However, increased manganese levels are known to damage the central nervous system,
resulting in the motor coordination abnormalities and psychic disorder and, finally, can even
result in symptoms similar to Parkinson’s disease [1, 4, and references cited therein]. Man-
ganese toxicity is also a serious constraint to crop cultivation since manganese is taken up
by plants and can easily be passed into the food chain, again causing symptoms similar to
Parkinson’s disease .
The exact mechanism of toxicity of manganese is not fully understood. To understand
better the role of this element in biological reactions, it is necessary to investigate its inter-
action with various inorganic and organic ligands occurring in the environment, in order to
identify which species influence its bioavailability, mobility and toxicity.
A survey of the literature reveals that manganese speciation, in spite of its biological im-
portance, has been rarely investigated [6–9] in comparison to studies of other 3d-electron
elements. The polarographic method has seldom been used in manganese coordination equi-
libria studies [10–13] although, in some favorable cases, it can detect complexes that have
not been established by other methods and can be used to determine their stability constants
with high accuracy or, at least, comparable to the accuracy achieved by the potentiometric
method [14, 15].
Mn(II) is reduced on a mercury electrode at strongly negative potentials, and in acidic
media its reduction wave (or that of its complexes) overlaps with the hydrogen ion wave, and
thus it is extremely difficult to separate them. On the other hand, in alkaline media Mn(II)
precipitates. Secondly, the reduction wave of Mn(II) to Mn(0) in some media has a slope
higher than that for a fully reversible 2-electron process [10, 16], and it is uncertain which
equation should be used to determine the stability constants of Mn(II) complexes, that for
a reversible reduction or that for an irreversible one. Application of both equations yields
The purpose of the present work is study Mn(II) speciation that may help in recognizing
its beneficial or toxic role in living organisms. The main goal was to demonstrate a simple
method of analysis of Mn(II) waves that provides valuable information concerning the elec-
trode reduction mechanism and to determine stability constants of the complexes. As model
systems, the reduction of Mn(II) in the presence of oxalate ions in perchlorate and sulfate
solutions have been chosen. The discharge of Mn(II) in the presence of oxalate ions in per-
chlorate solutions was partly studied by Verdier and Piro [10, 11]. However, the conclusions
reported in that study need to be revised since their detailed analysis of the current-potential
curves for the reduction mechanism differed from that proposed in the present paper. The
results obtained are presented and discussed below.
Measurements were made on a Radelkis OH-105 polarograph using a dropping mercury
electrode. The mercury container was placed at a height of 50 cm, the mercury flow rate was
2.36 mg·s−1, and the resulting drop time (t1) was 3.8 s. A saturated calomel electrode (SCE)
with a large surface area was used as both anode and reference electrode. It was connected
to the examined solution with an electrolytic bridge, filled with saturated sodium perchlorate
(or sulphate) solution.
All stock solutions were prepared from analytical-reagent grade chemicals. The concen-
trations of manganese(II) in MnSO4solutions was fixed at 5 × 10−5mol·L−1while those
J Solution Chem (2011) 40: 247–260249
of oxalate (Na2C2O4) were varied over the range 0.001–0.1500 mol·L−1. A constant ionic
strength equal to 0.5 mol·L−1was maintained by adding the necessary amounts of sodium
perchlorate or sodium sulfate. The pH values of all investigated solutions were in the range
6.0–6.5. The pH values were checked by means of a Radelkis OP-211 digital pH meter.
All experiments were carried out under an argon atmosphere at 25°C. The current of
interest was recorded as the difference between the total current and the residual current
measured in the pure supporting electrolyte and ligand solution at the same potential.
3 Results and Discussion
The wave shifted towards more negative potentials as the oxalate ions concentration in-
creased and its limiting current decreased in both supporting electrolytes (sodium perchlo-
rate and sodium sulfate), see Fig. 2, indicating the formation of complexes. Logarithmic
analyses were performed for all of the waves to determine the half-wave potentials (E1/2)
and their slope coefficients (2.303RT/αnαF). Some of the curves, characteristic of this in-
vestigation, are shown in Fig. 3.
Plots of log10[i/(il− i)] versus E for the reduction of the free Mn(II) ion (aqua ion)
and of its oxalate complexes formed at concentrations of oxalate lower than 0.03 mol·L−1,
in both supporting electrolytes NaClO4and Na2SO4, were straight lines with a slope of
33 mV, which is a little higher than that for a fully reversible two-electron process. When
the concentration of oxalate ions exceeded 0.03 mol·L−1, the slope in the lower part of
the wave was still 33 mV, whereas it increased in the upper part. The half-wave potentials
E1/2for these waves were obtained by extrapolation of the lower sections of each wave
(decreased by 33 mV) to zero value of log10[i/(il−i)]. The values of E1/2obtained plotted
against log10coxare presented in Fig. 4.
Fig. 1 Selected polarographic waves for Mn(II)–oxalate complexes in 0.5 mol·L−1NaClO4
250J Solution Chem (2011) 40: 247–260
Fig. 2 Dependence of the limiting current on log10coxin NaClO4(1) and in Na2SO4(2) solutions
Fig. 3 Selected logarithmic wave analysis for Mn(II)–oxalate complexes in 0.5 mol·L−1NaClO4cox: 0 (1),
0.001 (2), 0.003 (3), 0.008 (4), 0.015 (5), 0.03 (6), 0.06 (7), 0.10 (8), and 0.15 (9) mol·L−1
This method of determination of the half-wave potential of the wave was previously
applied in the Ni(II)–oxalate system , and the stability constants of the complexes deter-
mined from the shift of this potential with respect to the oxalate ions concentration produced
values very close to those obtained in a different way by Crow .
J Solution Chem (2011) 40: 247–260251
Fig. 4 Changes in the half-wave potentials of Mn(II) with log10coxin NaClO4(1 and 3) and in Na2SO4
(2 and 3) solutions; for curves 1 and 2, the E1/2values were obtained from the wave with slope of 33 mV,
whereas the values for curve 3 were taken directly from the logarithmic curve
Agreement of the slope for reduction of the free Mn(II) ion in perchlorate solution with
that for reduction of Mn(II) oxalate complexes indicated a common character of the elec-
trode process mechanisms in both electrolytes, i.e. reduction of the Mn(II) aqua ion .
However, following the Brönsted theory, the perchlorate ion cannot form complexes with
Mn(II) but the sulfate ion has same tendency for complexation. The results for these sys-
tems are discussed separately below.
3.1 Reduction of Mn(II) in NaClO4Solutions
To determine the stoichiometry of Mn(II)–oxalate complexes formed in solution, two tests
wereperformed:plotting ?E1/2versus log10coxandplottinglog10[i/(il−i)] versus log10cox
at a fixed potential E. The slope of both plots gave the difference in the average ligand
numbers between the complexes predominating in the solution and those that were directly
reduced at the mercury electrode .
The limiting slope of the plot ?E1/2versus log10coxwas 2.32 for (E1/2)cextrapolated
from the lower part of the wave, and 2.51 for (E1/2)ctaken directly from the logarithmic
curve. In the sulfate solutions these values were 2.39 and 2.50 respectively.
The diagram of log10[i/(il− i)] versus log10coxplotted at the three potentials −1.53,
−1.56 and −1.59 V (Fig. 5), at oxalate concentration higher than 0.03 mol·L−1, gave a
difference in the average ligand numbers of 2.40. These results evidently confirm that, in
this system, only the free Mn(II) aqua ion undergoes direct reduction at the mercury elec-
trode, whereas in the solutions there exist three oxalate complexes: Mn(ox), Mn(ox)2−
then the difference of the average ligand numbers should be less than two. These results dif-
fer from those obtained by Verdier and Piro . They have stated that Mn(II) formed only
two oxalate complexes, i.e. Mn(ox) and Mn(ox)2−
ion, were reduced irreversibly at the mercury electrode.
3. When any other species besides of the free Mn(II) ion is reduced at the electrode,
2, and that both, together with Mn(II) aqua