Eh-pH Diagrams

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Eh-pH Diagrams
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What Are These Things Again?
EhpH diagram aka Pourbaix diagram,
potential-pH diagram, electro-chemical phase
diagram Invented in 1930s by Marcel Pourbaix
(Belgian) Used in lots of places extractive
metallurgy, corrosion (their original purpose),
environmental engineering, geochemistry Closely
tied to aqueous thermodynamics
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The Basics
x-axis is pH usually 014, but sometimes as low
as 3, and sometimes up to 16 pH log H
change of 1.0 pH unit changes H by factor of
10 y-axis is electrode potential relative to SHE
(range varies) positive is oxidiz-ing condition,
negative is reducing Assumes constant
temperature, aH2O 1
Diagram is divided into predominance regions,
where one phase prevails Requires definition of
predominance in terms of chemical potential For
solids, activity 1 for gases, set a partial
pressure for solutions, set an activity
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The Basics
Two lines shown here are present on nearly all
Eh-pH diagrams Line (a) is for 2 H 2 e H2
(g) Usually presumes pH2 1 atm Since ?G 0,
applying Nernst equation, E 0 0.05915
pH Result E 0 at pH 0 (SHE), slope of
straight line 0.05915
When conditions are below line, reduction
reaction generates H2 (g) when conditions are
above line, H2 (g) oxidizes to H Line (b) is for
4 H O2 4 e 2 H2O E 1.23 0.05915
pH Above line, oxidizing conditions generate O2
below line, reduction reaction generates H2O Most
hydrometallurgical processes operate between the
lines
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Add A Metal
Eh-pH diagram shows CuH2O system Dotted lines
represent water stability region solid lines
represent equi-libria between copper species Two
aqueous species, Cu2 and CuO22- Oxidation state
of Cu as Cu0 is 0 Oxidation state of Cu as Cu2O
is 1 Oxidation state of Cu in Cu2, CuO, and
CuO22- is 2
Lower oxidation states are stable at bottom,
higher oxidation states at top Activity of solid
compounds 1 when predominant varies for
aqueous species (1 in this case, could be as low
as 106) Predominance activity determined by
purpose, value of metal
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More on Metal H2O Diagrams
Type of stable ion depends on pH For CuO 2 H
Cu2 H2O, low pH drives reaction to
right Simple ions like Cu2 are stable at low
pH For CuO H2O 2 H CuO22, high pH drives
reaction to right Oxyions like CuO22 are stable
at high pH
Solid oxides, hydroxides most stable in center of
diagram
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More on Metal H2O Diagrams
Three kinds of lines separate copper species in
this diagram First is vertical CuO 2 H
Cu2 H2O CuO H2O CuO22 2 H Reactions
involve exchange of H, but no electrons (no
oxidation/reduc-tion) independent of E Second
type of line is horizontal Cu2 2 e Cu
Reaction involves oxidation/reduction, but no H
independent of pH Third type of line is diagonal
Cu2O 2 H 2 e 2 Cu H2O Reaction
involves both oxidation/reduction and H
exchange, so line is a function of E and pH (No
curved lines in most diagrams.)
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Why Does This Matter? (Part I)
Diagram at bottom left is CuH2O system Presence
of stability region between lines for Cu and ions
shows that Cu can be produced hydrometallurgically
Diagram at bottom right is AuH2O system No
stability region for gold ions between lines
cant dissolve Au in aqueous solutions (for now)
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The Effect of Ion Activity
Diagram shows CoH2O system Tiny 0, 2, 4, 6
represent base-10 log of ion activity (Co2,
HCoO2) As required activity of ions decreases,
predominance area for ions grows (sideways and
vertically) Easier to produce ions if desired
concentration isnt as high Easier to reduce ions
to metal is con-centration of ions is higher
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The Effect of Temperature
Partial Eh-pH diagrams below show CuH2O system
at 25 (left) and 100C (Use log aCu(2) 0
lines for lowtemperature diagram) Notice slight
change in slope of diagonal lines Cu2 region
shrinks (unusual), Cu2O region is smaller, CuO
and Cu regions ?
Water stability region also moves Can use changes
in temperature to our advantage
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Eh-pH Diagrams for Anions
Diagram shows SH2O system at 25C H2S is
dissolved in solution, not gas Can do this for
other anions as well Matters because pure oxide
minerals are uncommon, and anions are used for
leaching, precipitation need the right one!
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Why This Matters (Part II)
Diagrams below show AuH2O and AuCNH2O diagrams
at 25C Diagram at left shows why we cant
dissolve gold diagram at right shows how we
can (This is why cyanide is used) Notice vertical
line at bottom for H CN HCN (g) impacts
other lines Also notice curvature of lines
reflects changing activity coefficients
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Add An Anion And Another Metal
(Hope youre taking notes!) Diagram shows
CuFeSH2O sys-tem at 25C Requires setting
activity for aqueous Cu, Fe, and S species CuFeS2
is chalcopyrite, main copper mineral Cu5FeS4 is
bornite FeS2 is pyrite FeS is pyrrhotite Notice
separate predominance regions for several
species impact of changing predominant S species
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Why This Matters (Part III)
Chalcopyrite contains copper (3/lb) and iron
(0.08/lb). How to separate? Could smelt,
oxidize iron to slag re-quires energy, flux,
slag disposal Why not leach? Where on this
diagram can I put Cu into solution and leave Fe
behind?
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Limitations of Eh-pH Diagrams
Doesnt include impact of kinetics Presumes
only one predominant species (sometimes
activities of ions are nearly equal) Depends
on accurate thermodynamic data (not always
available for complex compounds)
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For More Information

• University of Montana Geology Department
• http//www.umt.edu/geosciences/faculty/moore/G431/
  lectur7.htm
• University of Idaho Geology Department
• http//www.sci.uidaho.edu/geol464_564/Powerpoint/L
  ecture_9a_468_568nc.ppt