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Ni-Mo-NH₃-Citrate-H₂O Pourbaix diagram (25 °C)

Diagram colors by dominant Ni-containing species. Defaults are set to Mo-rich, citrate-deficient conditions so the NiMo₆O₂₄H₆⁴⁻ heteropolymolybdate region is visible. Slide citrate UP to watch NiMo₆ break apart into Ni-citrate complexes. Slide Mo DOWN to make NiMo₆ disappear entirely.

0.1 M
1 M
0.1 M
0.1 M
All Nernst equations used in this diagram

S = 0.05916 V (which is RT·ln10/F at 25 °C). Solids have activity = 1. Dissolved species activities come from the speciation solver. The effective pH-slope of each D-line kinks at every ligand pKa.

Floor (A-lines, → Ni metal)

LineHalf-reactionE° (V)Nernst expression
A0Ni²⁺ + 2e⁻ → Ni−0.2405E = −0.2405 + (S/2)·log[Ni²⁺]
Ac1NiH₂cit⁺ + 2e⁻ → Ni + H₂cit⁻−0.2849E = −0.2849 + (S/2)·log([NiH₂cit⁺]/[H₂cit⁻])
Ac2NiHCit + 2e⁻ → Ni + HCit²⁻−0.3458E = −0.3458 + (S/2)·log([NiHCit]/[HCit²⁻])
Ac3NiCit⁻ + 2e⁻ → Ni + Cit³⁻−0.4019E = −0.4019 + (S/2)·log([NiCit⁻]/[Cit³⁻])
Ac4NiCit₂⁴⁻ + 2e⁻ → Ni + 2Cit³⁻−0.4925E = −0.4925 + (S/2)·log([NiCit₂⁴⁻]/[Cit³⁻]²)
An1Ni(NH₃)²⁺ + 2e⁻ → Ni + NH₃−0.3236E = −0.3236 + (S/2)·(log[Ni(NH₃)²⁺] − log[NH₃])
An2Ni(NH₃)₂²⁺ + 2e⁻ → Ni + 2NH₃−0.3908E = −0.3908 + (S/2)·(log[Ni(NH₃)₂²⁺] − 2·log[NH₃])
An3Ni(NH₃)₃²⁺ + 2e⁻ → Ni + 3NH₃−0.4431E = −0.4431 + (S/2)·(log[Ni(NH₃)₃²⁺] − 3·log[NH₃])
An4Ni(NH₃)₄²⁺ + 2e⁻ → Ni + 4NH₃−0.4807E = −0.4807 + (S/2)·(log[Ni(NH₃)₄²⁺] − 4·log[NH₃])
An5Ni(NH₃)₅²⁺ + 2e⁻ → Ni + 5NH₃−0.5046E = −0.5046 + (S/2)·(log[Ni(NH₃)₅²⁺] − 5·log[NH₃])
An6Ni(NH₃)₆²⁺ + 2e⁻ → Ni + 6NH₃−0.5090E = −0.5090 + (S/2)·(log[Ni(NH₃)₆²⁺] − 6·log[NH₃])
AmNiMo₆O₂₄H₆⁴⁻ + 2e⁻ → Ni + 6 MoO₄²⁻ + 6 H⁺−1.5524E = −1.5524 + (S/2)·log[NiMo₆⁴⁻] − 3S·log[MoO₄²⁻] + 3S·pH
BNi(OH)₂ + 2H⁺ + 2e⁻ → Ni + 2H₂O+0.1098E = 0.1098 − S·pH

Roof (D-lines, → Ni(OH)₃)

LineHalf-reactionE° (V)Nernst expression
D0Ni(OH)₃ + 3H⁺ + e⁻ → Ni²⁺ + 3H₂O+2.2389E = 2.2389 − 3S·pH − S·log[Ni²⁺]
Dc1Ni(OH)₃ + 3H⁺ + H₂cit⁻ + e⁻ → NiH₂cit⁺ + 3H₂O+2.1976E = 2.1976 − 3S·pH + S·log[H₂cit⁻] − S·log[NiH₂cit⁺]
Dc2Ni(OH)₃ + 3H⁺ + HCit²⁻ + e⁻ → NiHCit + 3H₂O+2.4495E = 2.4495 − 3S·pH + S·log[HCit²⁻] − S·log[NiHCit]
Dc3Ni(OH)₃ + 3H⁺ + Cit³⁻ + e⁻ → NiCit⁻ + 3H₂O+2.5619E = 2.5619 − 3S·pH + S·log[Cit³⁻] − S·log[NiCit⁻]
Dc4Ni(OH)₃ + 3H⁺ + 2Cit³⁻ + e⁻ → NiCit₂⁴⁻ + 3H₂O+2.7429E = 2.7429 − 3S·pH + 2S·log[Cit³⁻] − S·log[NiCit₂⁴⁻]
Dn1Ni(OH)₃ + 3H⁺ + NH₃ + e⁻ → Ni(NH₃)²⁺ + 3H₂O+2.4051E = 2.4051 − 3S·pH + S·log[NH₃] − S·log[Ni(NH₃)²⁺]
Dn2Ni(OH)₃ + 3H⁺ + 2NH₃ + e⁻ → Ni(NH₃)₂²⁺ + 3H₂O+2.5394E = 2.5394 − 3S·pH + 2S·log[NH₃] − S·log[Ni(NH₃)₂²⁺]
Dn3Ni(OH)₃ + 3H⁺ + 3NH₃ + e⁻ → Ni(NH₃)₃²⁺ + 3H₂O+2.6442E = 2.6442 − 3S·pH + 3S·log[NH₃] − S·log[Ni(NH₃)₃²⁺]
Dn4Ni(OH)₃ + 3H⁺ + 4NH₃ + e⁻ → Ni(NH₃)₄²⁺ + 3H₂O+2.7194E = 2.7194 − 3S·pH + 4S·log[NH₃] − S·log[Ni(NH₃)₄²⁺]
Dn5Ni(OH)₃ + 3H⁺ + 5NH₃ + e⁻ → Ni(NH₃)₅²⁺ + 3H₂O+2.7673E = 2.7673 − 3S·pH + 5S·log[NH₃] − S·log[Ni(NH₃)₅²⁺]
Dn6Ni(OH)₃ + 3H⁺ + 6NH₃ + e⁻ → Ni(NH₃)₆²⁺ + 3H₂O+2.7762E = 2.7762 − 3S·pH + 6S·log[NH₃] − S·log[Ni(NH₃)₆²⁺]
DmNi(OH)₃ + 9H⁺ + 6 MoO₄²⁻ + e⁻ → NiMo₆O₂₄H₆⁴⁻ + 3H₂O+4.8626E = 4.8626 − 9S·pH + 6S·log[MoO₄²⁻] − S·log[NiMo₆⁴⁻]
CNi(OH)₃ + H⁺ + e⁻ → Ni(OH)₂ + H₂O+1.308E = 1.308 − S·pH

Ligand acid-base equilibria (pH-only, vertical lines on the diagram)

EquilibriumpKa
H₃cit ⇌ H₂cit⁻ + H⁺2.95
H₂cit⁻ ⇌ HCit²⁻ + H⁺4.38
HCit²⁻ ⇌ Cit³⁻ + H⁺5.82
NH₄⁺ ⇌ NH₃ + H⁺9.25

Ni-ligand formation constants used

Equilibriumlog K or log β
Ni²⁺ + H₂cit⁻ ⇌ NiH₂cit⁺1.5
Ni²⁺ + HCit²⁻ ⇌ NiHCit3.56
Ni²⁺ + Cit³⁻ ⇌ NiCit⁻5.46
Ni²⁺ + 2 Cit³⁻ ⇌ NiCit₂⁴⁻ (log β₂)8.52
Ni²⁺ + n NH₃ ⇌ Ni(NH₃)ₙ²⁺ (log βₙ, n = 1…6)2.81 / 5.08 / 6.85 / 8.12 / 8.93 / 9.08
Ni²⁺ + 6 MoO₄²⁻ + 6 H⁺ ⇌ NiMo₆O₂₄H₆⁴⁻ (heteropolymolybdate)44.35
Ni(OH)₂ ⇌ Ni²⁺ + 2 OH⁻ (Ksp)−16.16 (Ksp = 6.96×10⁻¹⁷)

Mo redox lines (dashed purple on the diagram)

These show where Mo metal would be thermodynamically stable. Both lines sit below the H₂ evolution line in alkaline solution, which is why pure Mo cannot be electrodeposited from aqueous baths — HER outcompetes the Mo reduction kinetically. Ni catalyses the pathway via the surface intermediates discussed in the literature.

LineHalf-reactionE° (V)Nernst expression
Mo-1MoO₄²⁻ + 4H⁺ + 2e⁻ → MoO₂(s) + 2H₂O+0.606E = 0.606 − 2S·pH + (S/2)·log[MoO₄²⁻]
Mo-2MoO₂(s) + 4H⁺ + 4e⁻ → Mo(s) + 2H₂O−0.152E = −0.152 − S·pH