25th International Meeting on Organic Geochemistry IMOG 2011

P-152
Stability of Cu(II) and Pb(II) salycilate complexes determined by
modified Schubert's method
Ivana Kostic 1 , Tatjana Andjelkovic 1 , Ruzica Nikolic 1 , Milovan Purenovic 1 , Aleksandar
Bojic 1 , Darko Andjelkovic 2 , Miljana Radovic 1
1 University of Nis, Faculty of Sciences and Mathematics, Nis, Serbia, 2 Water Works Association "Naissus",
Nis, Serbia (corresponding author:ivana.kostic83@gmail.com)
Understanding environmental behavior of some
heavy metals, such as Cu(II) and Pb(II) in systems
containing natural organic matter (NOM) requires
knowledge of metal-NOM complexes. Salicylic acid is
a commonly used model for NOM and has an
aromatic nucleus and phenolic and carboxylic
functional groups. [1]
One approach for determining an average stability
constant for a metal-ligand system is to use the wellknown
Schubert equation that can be used effectively
to determine binding constants for metal/ligand
complexes under environmentally relevant
concentrations of metal ions. [2]
The complexation of salicylic acid with two heavy
metal ions Cu(II) and Pb(II) was investigated at pH
4.0, under constant ionic strength of 0.1 and at
temperature 25°C. This investigation was done by
using modified Schubert‘s ion-exchange method. [3]
Stock solutions (1000 mg/l) of Pb(II) and Cu(II) were
prepared by dissolving an appropriate quantity of
these metal nitrate powder in 0.1M HCl. All the metal
solutions were adjusted to pH 4.0 by 0.1M HCl/0.1M
NaOH and ionic strenght was set by NaCl to value
0.01. The cation-exchange resin Dowex 50WX8 (100-
200 mesh), Na-form, analytical grade from Fluka was
used. Metal ions were determined by atomic
absorption spectrophotometry with a Varian Aanalyst
300 with an air-acetylene flame.
The linear range for copper was obtained from
isotherm curve to estimate the Do in order to choose
the appropriate concentrations for metal ion and
ligand to avoid the effect of metal loading. Clasical
Schubert‘s method is applicable for determing stability
constants logβmn of mononuclear complexes and is
shown by following equation:
Possible problems with the Schubert‘s method
occur when complex, MmLn, is not mononuclear (m ≠
1). The following equation presents equation of
modified Schubert‘s method and was used to
eliminate this source of errors and presents the
modified method of data treatment and analysis:
It is expected that salicylic acid form polynuclear
complexes due to the presence of two functional
groups, thus modified Schubert‘s method was
applied.
Metal-ligand ratios were obtained from slopes of
curves log MC vs. log CL according to modified
Schubert‘s method. The slopes of these plots give the
composition of these complexes and values of n near
1.5 indicate that stoichiometries of complexes are 2:3.
Table. Stability constant, log βmn, and metal-ligand
ratio for complexes Cu(II)-salicylate and Pb(II)salicylate,
at pH 4.0 and ionic strength of I = 0.01, by
modified Schubert‘s method
Pb
(ppm)
M:L logβmn
5 2:3 10.90
10 2:3 10.60
15 2:3 10.44
Cu
(ppm)
M:L logβmn
5 2:3 10.54
10 2:3 10.22
15 2:3 10.10
The modified Schubert‘s ion-exchange method
applied for conditional stability constants
determination of Cu-salicylic polynuclear complex
gave comparable results to the reference data. Obtain
results indicate that Pb(II) and Cu(II) form polynuclear
complexes with salicylic acid with metal : ligand ratio
2:3. Pb(II)-salicylate complex show higher stability
constant than Cu(II)-salicylate. It was found that log
βmn decreases with increase of metal concentration.
Reference
[1] E. Tipping, Cation binding by humic
substances, Cambridge University Press, Cambridge,
2002.
[2] Schnitzer M. and Skinner S.I.M. (1966) Soil
Science, 6, 361-363.
[3] H. Baker, F. Khalili, Annals of Environmental
Science, 2007, 1, 35-44.
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