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

assumption is the fact described above, with nearly all the Cu(II) not

complexed with PPi being in the form of dihydroxycuprate. In the

first model, the molar absorptivity of the Cu-PPi complexes is

negligible, and absorbance is due exclusively to dihydroxycuprate. In
the second model, the molar absorptivity of Cu(P2O7)26– is negligible,
and CuP2O72– and dihydroxycuprate have the same molar absorptivity.
In the third model, the molar absorptivity of [CuP2O72–] and
[Cu(P2O7)26–] is the same, and different from that of dihydroxycuprate.
Finally, in the fourth model the molar absorptivities of [CuP2O72–],
[Cu(P2O7)26–] and dihydroxycuprate are different. The chemical
description of these models is given in Table 3.

TABLE 2. Empirical expressions representing the dependence of [Cu]’, [CuP2O72]
and [Cu(P2O7)26–] on [Cu]tot for the Cu2+/P2O74– system at pH 9.4 for different ratios

of cation: ligand, [Cu]tot :[PPi]tot , and with [Cu]tot in the 0 to 0.30 mM range.

Cu(II): PPi [Cu]’ = 0.9212 [Cu]tot
2:1 [CuP2O72–] = 1.660e – 3[Cu]tot + 285.0[Cu]tot2 + 1.580e5[Cu]tot3
[Cu(P2O7)26–] = –1.167e – 4 [Cu]tot + 1.665[Cu]tot2 – 5.986e3[Cu]tot3

Cu(II): PPi [Cu]’ = 0.8505 [Cu]tot
1:1 [CuP2O72–] = 4.908e – 3[Cu]tot + 546.2[Cu]tot2 – 3.579e5[Cu]tot3
[Cu(P2O7)26–] = –4.476e – 4 [Cu]tot + 6.915[Cu]tot2 + 2.080e4[Cu]tot3

Cu(II): PPi [Cu]'= 1.361e - 4 [Cu]tot
1:10 1.248e - 4 + [Cu]tot

[CuP2O72–] = 0.4311e[Cu]tot + 479.0[Cu]tot2
[Cu(P2O7)26–] = –0.02649[Cu]tot + 576.2[Cu]tot2

The results of the changes in absorbance as a function of the PPi
concentration, for the four experimental series with different ratios
Cu(II):PPi were fitted to the four models described above by non-
linear multiple regression, using the same boundary conditions
and initial limits. The best fit corresponded to the third model,
in which the molar absorbance of the two complexes of Cu(II) with
PPi was the same, and different from that of Cu(II) in solution.
These values are e =CuP2O72– eCu(P2O7)26– = 1373.8 M–1cm–1 ± 107.3 and

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