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Eh-pH Diagrams. What Are These Things Again? Eh–pH diagram: aka Pourbaix diagram, potential-pH diagram, electro- chemical phase diagram Invented in 1930’s.

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Presentation on theme: "Eh-pH Diagrams. What Are These Things Again? Eh–pH diagram: aka Pourbaix diagram, potential-pH diagram, electro- chemical phase diagram Invented in 1930’s."— Presentation transcript:

1 Eh-pH Diagrams

2 What Are These Things Again? Eh–pH diagram: aka Pourbaix diagram, potential-pH diagram, electro- chemical phase diagram Invented in 1930’s by Marcel Pourbaix (Belgian) Used in lots of places: extractive metallurgy, corrosion (their original purpose), environmental engineering, geochemistry Closely tied to aqueous thermodynamics

3 The Basics x-axis is pH; usually 0–14, 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, a H2O = 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

4 The Basics Two lines shown here are present on nearly all Eh-pH diagrams Line (a) is for 2 H e – = H 2 (g) Usually presumes p H2 = 1 atm Since ΔG° = 0, applying Nernst equation, E = 0 – pH Result: E = 0 at pH = 0 (SHE), slope of straight line = – When conditions are below line, reduction reaction generates H 2 (g); when conditions are above line, H 2 (g) oxidizes to H + Line (b) is for 4 H + + O e – = 2 H 2 O; E = 1.23 – pH Above line, oxidizing conditions generate O 2 ; below line, reduction reaction generates H 2 O Most hydrometallurgical processes operate between the lines

5 Add A Metal Eh-pH diagram shows Cu–H 2 O system Dotted lines represent water stability region; solid lines represent equi- libria between copper species Two aqueous species, Cu 2+ and CuO 2 2- Oxidation state of Cu as Cu 0 is 0 Oxidation state of Cu as Cu 2 O is +1 Oxidation state of Cu in Cu 2+, CuO, and CuO 2 2- 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 10 –6 ) Predominance activity determined by purpose, value of metal

6 More on Metal – H 2 O Diagrams Type of stable ion depends on pH For CuO + 2 H + = Cu 2+ + H 2 O, low pH drives reaction to right Simple ions like Cu 2+ are stable at low pH For CuO + H 2 O = 2 H + + CuO 2 2–, high pH drives reaction to right Oxyions like CuO 2 2– are stable at high pH Solid oxides, hydroxides most stable in center of diagram

7 More on Metal – H 2 O Diagrams Three kinds of lines separate copper species in this diagram First is vertical: CuO + 2 H + = Cu 2+ + H 2 O; CuO + H 2 O = CuO 2 2– + 2 H + Reactions involve exchange of H +, but no electrons (no oxidation/reduc- tion); independent of E Second type of line is horizontal: Cu e – = Cu Reaction involves oxidation/reduction, but no H + ; independent of pH Third type of line is diagonal: Cu 2 O + 2 H e – = 2 Cu + H 2 O Reaction involves both oxidation/reduction and H + exchange, so line is a function of E and pH (No curved lines in most diagrams.)

8 Why Does This Matter? (Part I) Diagram at bottom left is Cu–H 2 O system Presence of stability region between lines for Cu and ions shows that Cu can be produced hydrometallurgically Diagram at bottom right is Au–H 2 O system No stability region for gold ions between lines; can’t dissolve Au in aqueous solutions (for now)

9 The Effect of Ion Activity Diagram shows Co–H 2 O system Tiny 0, –2, –4, –6 represent base-10 log of ion activity (Co 2+, HCoO 2 – ) As required activity of ions decreases, predominance area for ions grows (sideways and vertically) Easier to “produce” ions if desired concentration isn’t as high Easier to reduce ions to metal is con- centration of ions is higher

10 The Effect of Temperature Partial Eh-pH diagrams below show Cu–H 2 O system at 25° (left) and 100°C (Use log a Cu(2+) = 0 lines for low–temperature diagram) Notice slight change in slope of diagonal lines Cu 2+ region shrinks (unusual), Cu 2 O region is smaller, CuO and Cu regions ↑ Water stability region also moves Can use changes in temperature to our advantage

11 Eh-pH Diagrams for Anions Diagram shows S–H 2 O system at 25°C H 2 S 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!

12 Why This Matters (Part II) Diagrams below show Au–H 2 O and Au–CN–H 2 O diagrams at 25°C Diagram at left shows why we can’t 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

13 Add An Anion And Another Metal (Hope you’re taking notes!) Diagram shows Cu–Fe–S–H 2 O sys- tem at 25°C Requires setting activity for aqueous Cu, Fe, and S species CuFeS 2 is chalcopyrite, main copper mineral Cu 5 FeS 4 is bornite FeS 2 is pyrite; FeS is pyrrhotite Notice separate predominance regions for several species; impact of changing predominant S species

14 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?

15 Limitations of Eh-pH Diagrams Doesn’t 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)

16 For More Information… University of Montana Geology Department University of Idaho Geology Department t


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