(yet more...) Equilibria: Solubility, Acids and Bases

Group Work.9

IRON: The Problem of Bioavailability

Iron is required by all life....fuzzy kittens (mammals), daisies (plants), yogurt cultures (bacteria). There is a problem, though, for all these life-forms to obtain iron from the environment. This group work exercise will lead you through some equation writing and calculations that involve iron. You will see how the ever-clever Mother Nature comes up with a solution to the problem of iron bioavailability.

1. Write the equilibrium expression for the dissolution of iron(III) hydroxide. Ksp = 10-39.

Fe(OH)3(s) + H2O ó Fe3+(aq) + 3OH-(aq)

Ksp = [Fe3+][OH-]3

2. Calculate the concentration of ferric ion, [Fe3+], in water having pH 7. (Hint: what will be the [OH-]?)

pOH = 14-7 = 7

[OH-] = 10-7 = 1x10-7 M

Use the equilibrium expression to calculate [Fe3+]:

[Fe3+] = 1x10-39/(1x10-7)3

[Fe3+] = 1x10-18 M

3. Calculate the concentration of ferric ion, [Fe3+], at pH 1.

pOH = 14-1 = 13

[OH-] = 10-13 = 1x10-13 M

Use the equilibrium expression to calculate [Fe3+]:

[Fe3+] = 1x10-39/(1x10-13)3

[Fe3+] = 1 M

4. Calculate the concentration of ferric ion, [Fe3+], at pH 12.

pOH = 14-12 = 2

[OH-] = 10-2 = 1x10-2 M

Use the equilibrium expression to calculate [Fe3+]:

[Fe3+] = 1x10-39/(1x10-2)3

[Fe3+] = 1x10-33 M

5. Do your answers make sense?

Yes. When the concentration of hydroxide is high (basic environment) the equilibrium is shifted toward the reactants producing a very low concentration of Fe3+. Similarly when the concentration of hyroxide is low (acidic environment) the equilibrium reaction is shifted to the right increasing the concentration of Fe3+. The effect is dramatic because of the cubic dependence on hydroxide concentration in the equilibrium expression.

6. Bacteria and plants use two methods to extract more iron from their environment (soil, rocks):

(a) secrete acidic molecules at their roots to lower the pH in the local soil region.

(b) synthesize large molecules called siderophores to "trap" Fe3+. (see Figure 1; note Keq value)

How does the secretion of acidic molecules help plants get iron ?

As shown in the calculation in question #1, a low pH (i.e. from acidic molecule secretion) allows a very high concentration of Fe 3+ ion to be soluble.

 

The surface of the earth–the earth's crust–has a high concentration of iron (as well as Si, O, K, Mg, Ca).

The "concentration of iron in the earth's crust is 100, 000 ppm (1 g Fe for 10 g earth).

In seawater and fresh water, the concentration is much lower: only 1 ppm (1 g per ton water).

7. Why is the concentration of Fe so much lower in these waters?

The calculation in #2 shows that Fe3+ concentration is very low in a neutral environment like seawater.

8. What is the importance of this [Fe] to the algea, fish, turtles, etc that live there?

It can be difficult to get Fe needed to live.

More iron in biochemistry:

Cut open the over cooked egg. Be attentive with all your senses during this process.

The high concentration of iron in egg yolks is what makes overcooked egg yolks green! The presence of sulfides in egg yolks is what makes spoiled eggs smell!

More specifically, during cooking, it is the protonation of the iron sulfide cube clusters (as depicted at right) in the proteins by acidic groups on the proteins that causes them to fall apart and eventually form hydrogen sulfide, H2S, which is the source of the disagreeable odor. Also, the "free" iron combines with the sulfide to form black FeS, which gives the yolk the greenish appearance

The simple iron sulfide, FeS, is highly insoluble: FeS(s) D Fe2+ + S2-

The available equilibrium constant for FeS dissolution in water is for the reaction as written:

FeS(s) + H2O <=> Fe2+ + HS- + OH- Ksp = 7.934 x 10-19

  1. Write the relevant equilibria for the hydrogen sulfide system (H2S), indicating the pKa's.

    2. For the H2S / HS- /S2- system, pKa1 = 6.96 and pKa2 = 19.0.

    H2S + H2O <=> HS- + H3O+ pKa1 = 6.96

    HS- + H2O <=> S2- + H3O+ pKa2 = 19.0
  2. Calculate the equilibrium constant for FeS dissolution under acidic conditions, i.e.

    3. FeS(s) + 2H3O+ <=> Fe2+ + H2S + 2H2O Kspa = ?

    Couple participating equilibrium reactions to produce the above reaction. (There is more than one way to do this problem)

    rxn 1: FeS(s) + H2O <=> Fe2+ + HS- + OH- -----Ksp = 7.934 x 10-19

    rxn 2: HS- + H3O+ <=> H2S + H2O ---------------K = 1/(10-6.96) = 9.12x106

    rxn 3: OH- + H3O+ <=> 2H2O ---------------------Kw = 1/Kw = 1x1014

    _____________________________________________

    net rxn: FeS(s) + 2H3O+ <=> Fe2+ + H2S + 2H2O ------ K(net) = KspKKw = (7.934 x 10-19)( 9.12x106)( 1x1014) = 723