The influence of hydrolysis on the deposition and CO-DE-position of iron-group metals (Fe, Co, Ni) at the dropping mercury electrode

The deposition and co-deposition of iron(II), cobalt(II) and nickel(II) ions were studied polarographically in weakly acid (pH 3-5.5) solutions. Variation of pH in this region does not affect the waves. By adding oxygen to the solution, high pH values were generated at the electrode surface (O2 + 2 H2O + 4 e → 4 OH-) leading to hydrolysis of the discharging metal ions. Hydrolysis not only reduces the concentration of the discharging species due to Me2+ + 2 OH- → Me(OH)2 ↓ but also leads to adsorption of hydroxides at the interface. Chronopotentiometric measurements showed that about 3 · 10-10 moles/cm2 Fe(OH)2 are adsorbed at the electrode. Hydrolysis changes the rate of single metal discharge only slightly, but has a strong effect on the co-deposition of metals. The discharge of one metal can be suppressed by the presence of a second iron-group metal. Ferrous ions cause the strongest suppression followed by cobaltous ions. The same sequence (Fe2+ > Co2+ > Ni2+) is found in the suppression of Tl+ discharge. This sequence is interpreted on the basis of adsorption measurements. © 1964.