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VOL. 72 (3), 463-487, 2006 SPECTROPHOTOMETRIC CHARACTERISATION OF THE...

aCu2+ = [Cu2+ ]' aP2O74- = [P2O74- ]'
[Cu2+ ] [P2O74- ]

Therefore: K1 K2
k ' =1 k ' =2 2

a aCu2+ P2O74- a aCu2+ P2O74-

The equilibrium constants that describe the parasite reactions of
Cu(II) with OH– and of PPi with H+ in solution are well documented

in the literature. The value of aCu(II), aPPi, k1’ and k2’ can therefore be
expressed as a function of the experimental pH by means of the

equilibrium constants. The dependence of these variables on pH is

depicted in Fig. 2. Due to the presence of the parasite reactions, the

conditional constants k1’ and k2’ for the system Cu(II)/PPi never reach
the theoretical values for K1 and K2, 1e6.9 and 1e9.0 respectively
(Fig. 2).

2. The absorptivity shift of Cu(II) in the presence of PPi is
2. maximum at pH 9.0

The conditional constants of the Cu(II)/ PPi system have
maximum values in the pH range between 7 and 10 (see Fig. 2). In
the determination of Cu(II) by dipyrophosphatecuprate(II) titration,
the pH used is also in that interval (19). In light of this, the effects
of pH and of the nature of the buffer solution on the change in the
absorbance due to PPi were checked in that pH interval. The results
are shown in Fig. 3. A strong dependence of the shift in absorbance
on both the pH and the nature of the buffer was observed. These
variations might arise as a consequence of further interactions
between Cu(II) and side groups of the buffer molecule.

For a Cu(II):PPi ratio of 1:20 and a buffer concentration in the
sample of 50 mM, the largest change in A280 was observed for
AMPSO/NaOH at pH 9.4 (Fig. 3). These conditions were therefore
chosen for further experiments. The value of pH 9.4 corresponds to
the pKa of the AMPSO buffer and the pKa4 of pyrophosphoric acid.
The values for the parasite reaction coefficients and for the
conditional constants for the system at pH 9.4 are: aCu(II) = 4.06e3,
aPPi = 2.00, k1’ = 6.15e4 and k2’ = 6.17e2 (Fig. 2). The value 2.0 for

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