Question

To test the extent to which ocean phytoplankton are limited by the trace metal iron (Fe), experiments have been conducted where patches of ocean surface water are fertilized with dissolved inorganic iron (Fe²⁺).

a) The measured apparent solubility product of siderite (FeCO3(s)) in a NaCl solution with an ionic strength similar to seawater is:

FeCO₃(s) ⟺ Fe²⁺ + CO₃^2- Ksp’= 1.26x10^-11 mol²/kg²

Using:

[CO₃²⁺]_seawater = 200x10^-6 mol/kg

[Fe²⁺]_slurry = 0.1 mol/kg

What is the minimum dilution factor with seawater that should be used for the iron slurry to avoid the precipitation of siderite? Are there other things the experimenter could do toprevent siderite precipitation?

b) The target concentration for total Fe(II) for the experiment is 10 nmol/kg. At that total iron concentration, what fraction of total iron is bound in ion-ion pairs and what would be the activity for free Fe²⁺ given the following:

Fe²⁺ + CO₃^2- <=> FeCO₃⁰ Keq= 5500 mol^-1/kg^-1 equilibrium coefficient for ion-ion pair (note, this is NOT an apparent equilibrium coefficient)

γFe²⁺ = 0.65

γCO₃^2- = 0.23

γFeCO₃⁰ = 0.95

c) What other factors might affect the activity of Fe²⁺?

Answer 1

a) dilution factor is the unit volume in which the material is dissolved. it is the ratio of final volume to initial volume.here the dilution factor is: 0.000200 / 0.1 = 0.002. siderite is formed when:

Fe2+ on reacting with CO2 produces Fe(OH)3 which is red brown will further reverse to form Fe2+. inorder to stop siderite formation the better way is to remove the forming Fe(OH)3.

b) the target concentration is given 10nmol/kg. to get a target concentration of 0.1mol/kg, dissolve 0.01 gram of solute to solvent and make up to 10 ml.

activity is the ratio of chemical activity to molar concentration. activity can be expressed in many ways like amount fraction, mass fraction, mobility and concentration. here we are finding activity with concentration. where activity is calculated as product of concentration of Fe2+ (0.1)and equilibrium constant (0.65). that is 0.065.

c) factors effecting Fe2+ are: chemical potential, concentration (infinite dilution), partial pressure.