Journal of Basic & Applied Sciences, 2015, 11, 583-589 583
ISSN: 1814-8085 / E-ISSN: 1927-5129/15 © 2015 Lifescience Global
Cu
2+
-Citrate Dimer Complexes in Aqueous Solutions
Yahia Z. Hamada
*
, Robin Cox and Hasan Hamada
Division of Natural and Mathematical Sciences, LeMoyne-Owen College, 807 Walker Ave., Memphis, TN
38126, USA
Abstract: The UV-Vis spectra, speciation diagrams, and potentiometric profiles for Cu
2+
-citrate complexes in aqueous
solutions are presented. As the pH increases from 2.29 to 5.15, the UV-Vis spectral profile of the Cu
2+
-citrate complexes
showed a blue shift from 820 nm to 760 nm. We have set the conditions to construct the speciation diagram as follow:
Cu
2+
:citric acid was in 1:1 ratio with concentration of 1.0 x 10
-4
mol.L
-1
, 0.1023 mol.L
-1
NaOH solution, and pKw = 13.781
± 0.006 taken from Sweeton, Mesmer, and Baes. The current report is the first potentiometric study that has taken into
accounts two Cu-Cit dimeric species to be refined simultaneously. These spectroscopic and potentiometric data are
discussed which augment what had been reported in the literature.
Keywords: Cu
2+
solutions, UV-Vis spectra, Cu
2+
-Citrate-dimer, Potentiometric titrations, Speciation diagrams.
1. INTRODUCTION
1.1. Copper
Copper (the most stable oxidation state Cu
2+
will be
used as the shorthand notation) is an essential trace
metal ion involved in many metalloproteins including:
ceruloplasmin [1-8], cytochrome oxidase [9-13],
superoxide dismutase [14-19], dopamine- -hydro-
xylase [20-22], ascorbate oxidase [23-27], lysyl oxidase
[28-30], and tyrosinase [31-35]. To build up these Cu
2+
containing metalloproteins/metalloenzymes, the biolo-
gical machinery has to transport Cu
2+
and the raw
materials (low molecular mass organic molecules
building-blocks) through a very complex transport/
storage mechanism system. A detailed American
Chemical Society (ACS) library search that was
conducted on September 26, 2013 showed, as
expected, very large number of publications when it
comes to the search key term searches “Copper or
Cu
2+
”. When the key search term “Copper” was spelled
out ~150 thousands (~150 K) papers appeared
anywhere in the paper. When this search term was
narrowed within the abstract ~ 10 K papers appeared.
When the search was narrowed within the title only ~ 3
K papers appeared. When the key search term “Cu
2+
”
was used; instead of copper; within the title 475 papers
were returned. When the term “Cu
2+
combined with
citrate” was included within the title no papers were
received. When the term “Cu
2+
combined with citrate
and combined with the term dimer” more than 500
papers were returned anywhere, but zero paper in both
title and abstract. Figure 1 shows these detailed library
searches.
*Address correspondence to this author at the Division of Natural and
Mathematical Sciences, LeMoyne-Owen College, 807 Walker Ave., Memphis,
TN 38126, USA; Tel: (901) 435-1392; Fax: (901) 435-1424;
E-mail: Yahia_hamada@loc.edu
1.2. Citrate and the Copper-Citrate System
Citrate is considered to be a pre-eminent low
molecular mass metal binder. Cu
2+
citrate complexes
(or more precisely Benedict’s solution) have been
studied for over 100 years [36]. The Cu
2+
citrate
reaction was revisited in 1953 and 1976 at which
various Cu
2+
citrate complexes or Cu
2+
citrate species
were discovered. The Cu
2+
citrate one-to-one complex
in solution was first identified by Warner and Weber
[37]. The Cu
2+
citrate dimer was first crystallized in
1976 by Schugar et al. [38]. Because of the essentiality
of copper in biological systems and the pre-eminence
of citrate as metal binder, we are testing the interaction
of citrate/citric acid with the essential metal ions Cu
2+
.
2. EXPERIMENTAL SECTION
2.1. Materials
Ligand and Copper aqueous solutions were
prepared using Fisher reagent grade citric acid,
C
6
H
8
O
7
, formula weight 192.12 g.mol
-1
, tri-trisodium
citrate salt monohydrate, C
6
H
5
Na
3
O
7
. H
2
O, formula
weight 258.08 g.mol
-1
, copper nitrate hemipentahy-
drate, Cu(NO
3
)
2
2.5H
2
O, formula weight 232.59
g.mol
-1
, or copper sulfate pentahydrate, Cu(SO
4
)
2
5H
2
O, formula weight 249.68 g.mol
-1
, using doubly
deionized water. Potentiometric titrations were conduc-
ted using the Orion pH electrode-meter combination
model 720A+ which measures pH’s to the accuracy of
0.001 in aqueous solutions at room temperature.
2.2. Preparation of the Potentiometric Titration
Solutions
Cu
2+
-Citrate or Cu
2+
-Citric acid potentiometric
titrations were conducted using standard NaOH
solution as the titrant. The NaOH solutions were
prepared from NaOH solid pellets in carbonate free