New aspects of the reaction of silver(I) cations with the ethylenediaminetetraacetate ion.
The highly neutralized ethylenediaminetetraacetate (EDTA) titrant (95-99% as Y(4-) anion) precipitates with Ag(+) cations to form the Ag(4)Y species, in aqueous medium, which is well characterized from conductometric titration, thermal analysis and potentiometric titration of the silver content of the solid. The precipitate dissolves in excess Y(4-) to form a complex, AgY(3-). Equilibrium studies at 25 degrees C and ionic strength 0.50 M (NaNO(3)) have shown from solubility and potentiometric measurements that the formation constant (95% confidence level) beta(1) = (1.93 +/- 0.07) x 10(5) M(-1) and the solubility products are K(S0) = [Ag +](4)[Y(4-)] = (9.0 +/- 0.4) x 10(-18) M(5) and K(S1) = [Ag +](3)[AgY(3-)] = (1.74 +/- 0.08) x 10(-12)M(4). The presence of Na(+), rather than ionic strength, markedly affects the equilibrium; the data at ionic strength 0.10 M are: beta(1) = (1.19 +/- 0.03) x 10(6) M(-1), K(S0) = (1.6 +/- 0.4) x 10(-19) M(5) and K(S1) = (1.9 +/- 0.5) x 10(-13) M(4); at ionic strength tending to zero; beta(1) = (1.82 +/- 0.05) x 10(7) M(-1), K(S0) = (2.6 +/- 0.8) x 10(-22) M(5) and K(S1) = (5 +/- 1) x 10(-15) M(4). The intrinsic solubility is 2.03 mM silver (I) in 0.50 M NaNO(3). Well-defined potentiometric titration curves can be taken in the range 1-2 mM with the Ag indicator electrode. Thermal analysis revealed from differential scanning calorimetry a sharp exothermic peak at 142 degrees C; thermal gravimetry/differential thermal gravimetry has shown mass loss due to silver formation and a brown residue, a water-soluble polymeric acid (decomposition range 135-157 degrees C), tending to pure silver at 600 degrees C, consistent with the original Ag(4)Y salt.
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Departamento de Química, Universidade Federal de São Carlos, Cx. Post. 676, CEP 13.565-905, São Carlos, Brazil.
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