Page 132 - 72_03

M. FE DE LA TORRE Y COLS. AN. R. ACAD. NAC. FARM.

The presence of PPi decreases the absorptivity of Cu(II) in
solution at 280 nm. Since monitoring an absorbance increase is more
sensitive than monitoring a decrease, we chose a blank containing a
high concentration of PPi and followed the increase in absorbance
in the samples, which contain a lesser amount of PPi. Two blanks
with different PPi concentrations were chosen, thus making the
method suitable for use at different concentrations ranges of PPi:
the first one with a cation: ligand ratio of 1:20 (0.5 mM CuSO4, 10
mM Na4P2O7) and the second one with a ratio of 1:5 (0.5 mM CuSO4,
2.5 mM Na4P2O7). Absorbance was maximum in the absence of PPi,
decreased when the concentration of PPi in the sample increased,
and was zero when [PPi] was 10 mM or 2.5 mM respectively.

Cu(II) and PPi form complexes in solution, rendering the species
CuP2O72- and Cu(P2O7)26-. The corresponding equilibria are:

Cu2+ + P2O74– ??K1=?1e6.?7? CuP2O72–

Cu2+ + 2P2O74– ??K2=?1e9.?0? Cu(P2O7)26–

Cu(II) and PPi are also involved in different side reactions, which
must also be considered. Thus Cu(II) participates in reactions of
hydroxicomplexes formation and PPi in protonation reactions, which
are considered to be parasites of the main reactions. To deal with
such parasite reactions, the constants that describe the main
equilibria are redefined taking into account the simultaneous
involvement of the chemical species in the different equilibria. For
the Cu(II)/ PPi system the conditional constants are:

k1' = [CuP2O72- ] ]' k 2' = [Cu(P2O7 )26- ]
[Cu2+]' [P2O74- [Cu2+]' [P2O74- ]'2

where [Cu2+]’ represents the concentration of Cu(II) in all the forms
in solution, except the complexes with PPi, and [P2O74-]’ represents
the concentration of PPi in all species except the complexes with

Cu(II). The relationship between the conditional constants and the

equilibrium constants is given by the coefficient of parasite reactions,

defined as:

470