Dissolution of copper in aqueous solution is generally much more rapid in the presence of NH3 and NH4+, when stable complex ions of copper are formed. It has been known in previous experiments that the dissolution rate i under low and high oxygen pressure can be given by i=ka[O2] and i=k[NH3]+kc[NH3], respectively. The rate also can be given by i=k[Cu(NH3)42+]0.5 at low values of the ratio [O2]/[Cu(NH3)42+]. These experimental facts are interpreted in terms of a new mechanism based on the multi-electrode kinetics. In the presence of oxygen, the dissolution of copper proceeds according to an electrochemical process involving separate anodic and cathodic steps as follows: anodic process [α:Cu+mNH4→Cu(NH3)m+e+, ω: Cu+mNH4+→Cu(NH3)m++H++me], cathodic process [β: Cu(NH3)m+n2++e→Cu(NH3)m++mNH3]. In the presence of oxygen, Cu(NH3)m+ produced by the anodic and the cathodic processes is oxidized into Cu(NH3)m+n2+ by oxygen according to a coupled reaction of the anodic step of β and the cathodic step of [γ: O2+2H2O+4e→4OH-]. The dissolution rate is controlled by the diffusion rate of oxygen under low oxygen pressure. Under high oxygen pressure, on the other hand, the rate is controlled by the diffusion rate of NH3 or NH4+. Schematic polarization curves of this tri-electrode system involving α,β and γ are presented.
Please do not copy without permission.