25th International Meeting on Organic Geochemistry IMOG 2011

P-151
Cu(II) complexation with humic acid and humic-like ligands
studied by Schubert’s method
Ivana Kostic 1 , Tatjana Andjelkovic 1 , Ruzica Nikolic 1 , Milovan Purenovic 1 , Aleksandar
Bojic 1 , Darko Andjelkovic 2 , Jelena Mitrovic 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)
Humic substances are colloidal, heterogeneous,
polydisperse macromolecules. They are the major
organic constituents of soil, water sediments, being
main representatives of complex environmental
chemical systems. Due to the presence of hydroxyl,
phenyl and carboxyl-reactive groups, coordination
compounds of HA with metals are formed. This is
extremely important in affecting the retention and
mobility of metal contaminants in soil and water. In an
effort to understand and quantify humic-metal
interactions some ligand models (such as citric,
salicylic, benzoic and phthalic acids) have been used
in the study of complexation properties of humics. In
this paper, salicylic and benzoic acids are used as
humic-model ligands. [1]
Schubert‘s ion-exchange method for determing
conditional stability constants involves measuring the
distribution coefficients of a metal ion between a
cation-exchange resin and solution phase, in both the
presence and absence of the complexing agent. [2, 3]
Metal solutions were prepared by dissolving an
appropriate quantity of Cu(NO3)2 in HNO3. All the
metal solutions were adjusted to pH 4.0 by 0.1M HCl
and 0.1M NaOH with ionic strenght is set by NaCl to
value 0.01. The concentration of stock solutions of
humic (HA), benzoic (BA) and salicylic acid (SA) were
5, 7.5 and 10 moldm -3 . The concentration of stock
solutions of all three acids were 5, 7.5 and 10 meq/l
with ionic strenght of 0.01. The cation-exchange resin
used was Dowex 50WX8 (100-200 mesh), Na-form,
analytical grade from Fluka. Copper was determined
by atomic absorption spectrophotometry with a Varian
Aanalyst 300 with an air-acetylene flame.
The logarithm of the stability constant of the
complex (log βmn) was evaluated from the following
relationship:
� D0
�
log��
� 1 � log K � n log
D
��
� �
�L� where D0 is distribution constant of the Cu 2+ in
absence of ligand; D is distribution constant of Cu 2+ in
the presence of ligand; log βmn is stability constant of
Cu(II)complex; n is number of moles of ligand which
combine with one mole of Cu(II); [L] is concentration
of ligand in mole per litre.
Do was determined from the expression:
0
0
(100 0 ) resin
m α
α �V
D �
� �
where α0 is percent of total metal bound to
exchange resin (Dowex AG 50W-X8); 100-α0 is
percent of total metal remaining in solution; V is
volume of solution; mresin is weight of exchange resin.
D is measured in the same manner as D0 in presence
of different ligand concentrations. Metal-ligand ratios
were obtained from slopes of curves log (D0/D)-1 vs.
log CL for Cu-benzoate and Cu-humate complexes
according to Schubert‘s method, and log MC vs. log
CL for Cu-salicylate complexes according to modified
Schubert‘s method. The obtained results are shown in
Table.
Table. Stability constants of Cu(II)-complexes and
metal : ligand ratio at pH 4.0,
under constant ionic strenght of 0.01 and at 25˚C.
BA SA HA
M:L 1:1 2:3 1:1
log K 1.65 10.34 2.36
The Schubert‘s ion-exchange method is applicable
to determination of conditional stability constants of
mononuclear complexes such as Cu-benzoate and
Cu-humate. Salicylic acid forms polynuclear
complexes with Cu(II). Formation of polynuclear
salicylate complexes disables the application of
classical Schubert‘s method. Value of log βmn and
stoichiometry of Cu-salicylate was determinated by
modified Schubert‘s method.
The obtained log K values show the following
sequence: SA > HA > BA. Thus, the most stable
complex is formed between Cu 2+ and SA, due to Cu 2+
ions form polynuclear complexes with salicylic acid.
References
[1] E. Tipping, Cation binding by humic
substances, Cambridge University Press, Cambridge,
2002.
[2] H. Baker, F. Khalili, Annals of Environmental
Science, 2003, 497, 235-248.
[3] Schnitzer M. and Skinner S.I.M. (1966) Soil
Science, 6, 361-363.
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