Question

7) (14 pt) Given the Eh/pH diagram below, a) what would you expect the dominant speciation of Fe to be at Eh = 0.2 mV and pH = 4? b) what would you expect the dominant speciation of Fe to be at Eh = 0.2 mV and pH = 8? c) Considering the precipitation of siderite (Fe2+ + HCO3 → FeCO3 + H+), why is siderite unstable at pH <-6? +20 +15 +10 +0.41 Hematite Fe,o, Eh (V) Fe pe O o HO Siderite Pyrite Fesy Y Feco, o 2 Magnetite ... Fe, ollte -10 2 4 6 10 12 14 8 pH

Answer 1

a) AT Eh= 0.2mV and pH= 4, point A ,Fe ²⁺ is the dominant species

b) At Eh= 0.2 mV and pH =8 , point B, Fe³⁺ as Fe₂O₃ (haematite) is the dominant species

120. - - Treat 02. H₂o. - - ++10 +08 040- 7+5 pe: Hagnght 2,8).to ope. Feroz - Sicerite --5 4 6 2 8 10 12 14.

c) Given,

Fe²⁺ + HCO₃^-$\rightarrow$ FeCO₃ +H⁺ The diagonal boundary shows the involvement of acid base equilibria.The Pourbaix diagram shows that the siderite is stable upto pH (6 to 10.5) and Eh ( 0 to 0.15 mV) So, if pH is decreased below 6, the H+ ion concentration gets increased considerably, and following common ion effect, the equilibrium shifts to left leading to the formation of Fe²⁺ and bicarbonate ion.