The simplest Ni system with no complexing agents. Ni²⁺ is the only dissolved species; Ni(OH)₂ and Ni(OH)₃ are the solid phases. As Ni concentration rises, the dissolved-Ni²⁺ region expands and Ni(OH)₂ precipitation pH shifts left.
S = 0.05916 V (which is RT·ln10/F at 25 °C). Solids have activity = 1. Below the precipitation pH, [Ni²⁺] = C_Ni. Above it, [Ni²⁺] is pinned by Ksp.
| Line | Half-reaction | E° (V) | Nernst expression |
|---|---|---|---|
| A | Ni²⁺ + 2e⁻ → Ni(s) | −0.2405 | E = −0.2405 + (S/2)·log[Ni²⁺] |
| B | Ni(OH)₂(s) + 2H⁺ + 2e⁻ → Ni(s) + 2H₂O | +0.1098 | E = 0.1098 − S·pH |
| C | Ni(OH)₃(s) + H⁺ + e⁻ → Ni(OH)₂(s) + H₂O | +1.308 | E = 1.308 − S·pH |
| D | Ni(OH)₃(s) + 3H⁺ + e⁻ → Ni²⁺ + 3H₂O | +2.2389 | E = 2.2389 − 3S·pH − S·log[Ni²⁺] |
| Equilibrium | Ksp / pH |
|---|---|
| Ni(OH)₂(s) ⇌ Ni²⁺ + 2 OH⁻ | Ksp = 6.96×10⁻¹⁷ (log Ksp = −16.16) |
| Precipitation pH at C_Ni | pH = 14 + ½·log(Ksp/C_Ni) |
Source: standard E° values from Bard, Standard Potentials in Aqueous Solution. At C_Ni = 0.1 M, precipitation pH = 6.42; at 1 M, 5.92; at 10 M, 5.42.