Co-H₂O Pourbaix Diagram

Co-H₂O Pourbaix diagram (25 °C)

Thermodynamic data from Chivot, Mendoza, Mansour, Pauporté, Cassir, Corros. Sci. 50 (2008) 62–69 — CODATA-consistent. Slide [Co] to see how the Co²⁺ corrosion field, Co(OH)₂(s) and Co(OH)₃⁻ regions shift. Toggle CoOOH(s) to compare with/without heterogenite.

Total Co concentration

10 mM

Reference electrode

Water lines Include CoOOH(s) Paper mode

Reference

J. Chivot, L. Mendoza, C. Mansour, T. Pauporté, M. Cassir, "New insight in the behaviour of Co–H₂O system at 25–150 °C, based on revised Pourbaix diagrams", Corrosion Science 50 (2008) 62–69. doi:10.1016/j.corsci.2007.07.002

Thermodynamic data: Table 1 of the above (CODATA-consistent ΔG°_f values). Pourbaix construction follows the standard method with metastable-intermediate trimming at the Co₃O₄/Co(OH)₃(s)/Co²⁺ triple point at low pH.

Nernst equations & thermodynamic data

S = 0.05916 V (RT·ln10/F at 25 °C). All ΔG°_f values (kJ/mol) from Chivot et al. (2008) Table 1. Solids and H₂O have activity = 1.

ΔG°_f values used (kJ/mol, 25 °C, 1 bar)

Species	State	ΔG°_f
Co	s	0
Co²⁺	aq	−55.6
CoOH⁺	aq	−240.1
Co(OH)₂°	aq	−423.7
Co(OH)₃⁻	aq	−586.0
Co(OH)₄²⁻	aq	−739.2
Co(OH)₂	s	−460.2
Co³⁺	aq	+133.62
CoOH²⁺	aq	−96.30
Co(OH)₃	s	−596.33
Co₃O₄	s	−794.901
CoOOH	s	−386.18
H₂O	l	−237.14

Floor (A-lines, → Co metal)

Line	Half-reaction	E° (V)	Nernst expression
A0	Co²⁺ + 2e⁻ → Co	−0.2881	E = −0.2881 + (S/2)·log[Co²⁺]
A1	Co(OH)₃⁻ + 3H⁺ + 2e⁻ → Co + 3H₂O	+0.6499	E = 0.6499 − (3S/2)·pH + (S/2)·log[Co(OH)₃⁻]
A2	Co(OH)₄²⁻ + 4H⁺ + 2e⁻ → Co + 4H₂O	+1.0849	E = 1.0849 − 2S·pH + (S/2)·log[Co(OH)₄²⁻]
B	Co(OH)₂(s) + 2H⁺ + 2e⁻ → Co + 2H₂O	+0.0730	E = 0.0730 − S·pH

Roof (D-lines, → Co(III) phases)

Line	Half-reaction	E° (V)	Nernst expression
D0	Co₃O₄(s) + 8H⁺ + 2e⁻ → 3Co²⁺ + 4H₂O	+1.6607	E = 1.6607 − 4S·pH − (3S/2)·log[Co²⁺]
D1	Co₃O₄(s) + 2H₂O + 2H⁺ + 2e⁻ → 3Co(OH)₂(s)	+0.5774	E = 0.5774 − S·pH
D2	Co₃O₄(s) + 5H₂O + 2e⁻ → 3Co(OH)₃⁻ + H⁺	−1.1536	E = −1.1536 + (S/2)·pH − (3S/2)·log[Co(OH)₃⁻]
D3	Co₃O₄(s) + 8H₂O + 2e⁻ → 3Co(OH)₄²⁻ + 4H⁺	−2.456	E = −2.456 + 2S·pH − (3S/2)·log[Co(OH)₄²⁻]
C	3Co(OH)₃(s) + H⁺ + e⁻ → Co₃O₄(s) + 5H₂O	+1.9859	E = 1.9859 − S·pH
C′	3CoOOH(s) + H⁺ + e⁻ → Co₃O₄(s) + 2H₂O (if CoOOH on)	+1.1466	E = 1.1466 − S·pH

Chemical equilibria (vertical lines: solid ⇌ dissolved)

Equilibrium	log K	Saturation pH at [Co]=C
Co(OH)₂(s) + 2H⁺ ⇌ Co²⁺ + 2H₂O	+12.21	pH = (12.21 − log C) / 2
Co(OH)₂(s) + H₂O ⇌ Co(OH)₃⁻ + H⁺	−19.50	pH = log C + 19.50
Co(OH)₂(s) + 2H₂O ⇌ Co(OH)₄²⁻ + 2H⁺	−34.21	pH = (log C + 34.21) / 2
Co²⁺ + H₂O ⇌ CoOH⁺ + H⁺ (minor)	−9.22	—

Co-H₂O Pourbaix diagram (25 °C)

ΔG°f values used (kJ/mol, 25 °C, 1 bar)

Floor (A-lines, → Co metal)

Roof (D-lines, → Co(III) phases)

Chemical equilibria (vertical lines: solid ⇌ dissolved)

ΔG°_f values used (kJ/mol, 25 °C, 1 bar)