The Iron Mystery If: The Fe +2 /Fe +3 boundary is at a pe o and - - PDF document

CEE 680 Lecture #48 4/29/2020 1

Lecture #48 Redox Chemistry: Log C vs pe Diagrams

(Stumm & Morgan, Chapt.8 )

Benjamin; Chapter 9

David Reckhow CEE 680 #48 1

Updated: 29 April 2020

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The Iron Mystery

 If:

 The Fe+2/Fe+3 boundary is at a peo and pe(w) of 13.03  Oxygen saturated water should have a pe(w) of 13.6, but

Pankow says the effective pe(w) of surface water is more like 12.6  Will reduced iron spontaneously oxidize to ferric in

surface waters?

1.

Yes

2.

No

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Then why is it so hard to keep reduced iron (ferrous) from oxidizing to the ferric form?

CEE 680 Lecture #48 4/29/2020 2

Redox and pH effects

 Often oxidation of metals results in a more hydrolyzed

species

 Acidity of oxidized species is higher, resulting in release of

protons

 Speciation changes and affects the overall reaction

 A good example is the oxidation of ferrous iron to ferric

 Fe+3 + e‐ ↔ Fe+2  peo = 13.03  This is very close to the theoretical peo defined by saturated O2

in water (13.6), or the effective peo (e.g., 12.6)

 But this is deceptive, because Fe+3 isn’t the dominant species at

neutral pH

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Ferrous Hydroxides: α diagram

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pH

1 2 3 4 5 6 7 8 9 10 11 12 13 14



1e-10 1e-9 1e-8 1e-7 1e-6 1e-5 1e-4 1e-3 1e-2 1e-1 1e+0

0 1 2 3

Fe+2 FeOH+ Fe(OH)2

• Fe(OH)3
• pH 7.0

 Fe+2 dominates at

neutral pH

CEE 680 Lecture #48 4/29/2020 3

Ferric Hydroxides: α diagram

David Reckhow CEE 680 #48 5

pH

1 2 3 4 5 6 7 8 9 10 11 12 13 14



1e-10 1e-9 1e-8 1e-7 1e-6 1e-5 1e-4 1e-3 1e-2 1e-1 1e+0 1e+1

0 1 2 3

Fe+3 FeOH+2 Fe(OH)2

• Fe(OH)3
• 4

Fe(OH)4

• pH 7.0

 Fe+3 is a minor

species at neutral pH

Iron redox diagram

 Analogous to log C vs pH diagram

David Reckhow CEE 680 #47 6

Stumm & Morgan, 1996;

• Fig. 8.1, pg. 435

Similar to: Benjamin, 2002 Fig 9-3, pg.486

] [ ] [ } {

3 2     Fe

Fe e K

𝐿 10.

CEE 680 Lecture #48 4/29/2020 4

Iron: considering speciation

David Reckhow CEE 680 #48 7

13.03 4.70

HOCl and H2S example

 Neutral pH (~7.0)

 0.1 mM HOCl, 1 mM Cl‐  1 mM H2S and SO4

‐2

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CEE 680 Lecture #48 4/29/2020 5

Summary

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Hypochlorite

 5 x 10‐4M ClT

 Where:

ClT = [HOCl] + [OCl‐] + [Cl‐]

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O H Cl e H HOCl

2 2 1 2 1 2 1 2 1

   

  

1 . 25 5 . 5 . 5 .

10 } { } { } { } {

   

  e H HOCl Cl K K n peo log 1  pH HOCl Cl H HOCl Cl Ox d n p p

• 2

1 2 1 5 . 5 . 5 .

} { } { log 1 . 25 } { } { } { log 1 . 25 ] [ ] [Re log 1                               

  

 

CEE 680 Lecture #48 4/29/2020 6

Determining Equilibrium Concentrations

 Graphical solution analogous to acid/base

problems

 Create LogC vs pe diagram  Determine location on graph using electron balance

 Analogous to proton balance in acid/base problems

 Example: HOCl and NaHS

 Reduced species: Cl‐ which is 2e‐ poor  Oxidized species: SO4

‐2 which is 8e‐ rich

David Reckhow CEE 680 #48 11

HOCl & HS‐

 10‐4M ST  5 x 10‐4 ClT

David Reckhow CEE 680 #48 12

CEE 680 Lecture #48 4/29/2020 7

Electron Balance

 Oxidation  Reduction  e‐ Balance:

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8[SO4

• 2] = 2[Cl-]

HS- HOCl

O H Cl e H HOCl

2

2    

  

O H HS e H SO

2 2 4

4 8 9    

   

Constants

 Reference reaction

 Where {e‐}=1, if all

chemical species activities are also unity

David Reckhow CEE 680 #47 14

) (

2 2 1

g H e H  

 

. 1 } }{ { )} ( {

5 . 2

 

 

e H g H K

CEE 680 Lecture #48 4/29/2020 8

pE bounds for water I

 Oxygen and Hydrogen half cell reactions

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Stumm & Morgan, 1996;

• Fig. 8.2, pg. 437

 p pH P H e H H e H e H H K

H g g g

2 2 log } log{ 5 . } log{ } log{ } { } }{ { . 1 } }{ { } {

2

) ( 2 5 . ) ( 2 5 . ) ( 2

       

     

pE bounds for water II

David Reckhow CEE 680 #48 16

CEE 680 Lecture #48 4/29/2020 9

Electron Balance for HOCl & HS‐

 Oxidation  Reduction  e‐ Balance:

David Reckhow CEE 680 #48 17

4[O2(aq)] + 8[SO4

• 2] = 2[Cl-] + 2[H2(aq)]

HS- HOCl

O H Cl e H HOCl

2

2    

  

O H HS e H SO

2 2 4

4 8 9    

   

H2O

O H e H O

2 2

2 4 4   

  2

2 4 4 H e H  

 

HOCl and HS‐

David Reckhow CEE 680 #48 18

CEE 680 Lecture #48 4/29/2020 10

Close‐up of electron balance

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Nitrogen

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Nitrogen & Chlorine

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Redox Predominance for N

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CEE 680 Lecture #48 4/29/2020 12

To next lecture

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DAR