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

Given the values of the conditional constants, a diagram of species

for the system at pH 9.4 for [Cu(II)]tot = 0.5 mM as a function of
[PPi]tot can be built (Fig. 4). It can be observed that for values of
[PPi]tot lower than 0.50 mM the introduction of PPi into the system
leads to a linear increase in both [CuP2O72–] and [Cu(P2O7)26–]. In this
range the slope of the straight line representing the variation of the

species with two ligands is twice the slope of that representing the
variation of the species with one ligand. CuP2O72– predominates for
[PPi]tot values lower than 10.7 mM, and the species with two ligands
Cu(P2O7)26– predominates when [PPi]tot increases.

FIGURE 4. Diagram of species for the Cu(II)/PPi, system added as CuSO4 and
Na4P2O7 respectively. Calculations were made for [Cu(II)]tot = 5.0e-4 M, pH 9.4,
considering the formation of the CuP2O72– and Cu(P2O7)26– complexes, with
constants of formation log k1’ = 2.79 and log k2’ = 4.79 respectively.

3. Complexes of Cu(II) and PPi have a lower molar
3. absorptivity than Cu(II) in solution.

In a multi-component system such as the one studied Cu(II)/PPi
the Beer-Lambert law is additively applied to all the absorbent
species. Thus, assuming that there is no interaction between the
different components:

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