섹션 개요

    • Again, we note the half-reaction equation and Nernst equation for the dissolution of iron hydroxide.

       

      Dissolution of iron hydroxide ( half-reaction equation)：　Fe(OH)3   +   3 H+   +    e-   = Fe2+   +     3 H2O      

      Nernst equation

      　E = E0　－RT/(F)・Ln {  [Fe2+] /（ [H+]3 ・[Fe(OH)3]）

        　=  0.969 － 0.024387・Ln｛ [Fe2+] / [Fe(OH)3]｝ － 0.168459pH

       

      (As a physical chemistry promise, in a solution reaction of a solid (Fe(OH)3), the solid concentration = 1, given that there is a sufficient amount of that solid.)


      The condition for separating the Fe2+ and Fe(OH)3 abundance ratios [Fe2+] / [Fe(OH)3] = 1,

      ([Fe(OH)3] = 1 (mol/L), so when [Fe2+] = 1 (mol/L))

       E = 0.969 - 0.168459pH.

       

      Taking pH on the horizontal axis and E on the vertical axis, this relational equation is depicted in the figure below.

      Fig. 1

      　The area above the black bold line in the boundary condition (Fe(OH)3 ⇔ Fe2+) means that solid Fe(OH)3 is present, and Fe2+ has reached dissolution equilibrium at less than 1 mol/L.

      　So what would the line be for [Fe2+] < 1 mol/L?

      　The line at the dissolution equilibrium concentration [Fe2+] = 1 µmol/L is added to the figure below. You can see a large shift.

      Fig. 2