Polarographic studies of copper(II) complexes with N,N'-bis(2-hydroxyethyl)ethylenediamine.-N,N'-diacetic acid

Abstract

The polarographic behaviour of Cu(II)-N,N′-bis(2-hydroxyethyl)ethylenediamine-N,N′-diacetic acid (H2bhedda=H2L) complexes has been investigated in detail. De polarograms of Cu(II)−H2 bhedda systems gave two well-defined waves in an excess of the metal ion in the pH range 3–9. The first wave (e.g. 0.034 V vs. SCE, pH=3.29) is due to the reversible reduction of Cu(II) for any pH studied. The electrode process for the second wave (−0.205 V vs. SCE pH=3.55) changed from an irreversible electron transfer process (pH<5) to a reversible one (pH≥5) with increasing pH. Both limiting currents for wave A and B were diffusion controlled. Logarithmic stability constants of Cu(II)−H2 bhedda complexes were calculated using the data from the polarography (log KCuHL=5.24) and the potentiometric titration (log KCuL=11.3). The composition of Cu(II)−H2 bhedda complexes was determined by the amperometric or potentiometric titration (CuL or CuHL+). Electrode reaction mechanisms for the second wave are elucidated as follows: 3.0<pH≤5.0, CuHL++H++2e+Hg→Cu(Hg)+H2L (I) 5.0<pH≤7.0, CuL+H++2e+HgCu(Hg)+HL− (II) pH>7.0, CuL+2e+HgCu(Hg)+L2− (III) The standard rate constant of mechanism (I) was also obtained (kj=2.04×10−4 cm s−1). The frequency effect on the “cut-in” height for the complex was examined with the frequency variable Heyrovský-Forejt type oscillopolarograph. The electrode reaction rate constants for various Cu(II)-polyaminepolycarboxylic acid systems were also studied by oscillopolarography.