PLS 671: Environmental Soil Chemistry

Kinetic Redox Sequence
Lab #9- 11/19/02

Objective:  To measure kinetic redox sequence of N, Mn, and Fe on the threee soils.

Background:  A thermodynamic picture of the relative redox sequence in soils predicts that O2 should be used as
                           an electron acceptor followed by NO3-, Mn(III,IV) oxides, Fe(III) oxides, SO42-, and CO2.  
                           Weather this sequence occurs in soil depends on how effectively the respective half-reactions are
                           catalyzed.  Today, we want to follow the concentrations of some of these redox-active species over
                           time to observe whether the kinetic and thermodynamic redox sequences coincide.

I.  Procedure:

Kinetic Redox Sequence:

1.) Weigh out 3 g of your air-dries soil into a 60 mL glass bottle in duplicate outside the glovebox.  To
      both bottles, add a teflon coated magnetic stir bar.  To one bottle labeled :starch", add 0.24 g of
      starch to serve as an easily oxidizable C source to catalyze the reactions.    

2.)  Then add 60 mL of deoxygenated, DI water in the glovebox.  Please ask for assistance in ensuring
       that no O2 is allowed in the chamber.  Seal the bottles with a rubber septum and aluminum cap using
       the crimper inside the glovebox to prevent O2 from entering the system.

3.)  Place on a shaker outside the glovebox.  As soon as yoy begin shaking, take this as time zero.
      Remove 2 full syringes worth of suspension (~2.4 mL of filtrate) as subsamples at increasing reaction
      times as shown below:
      60 min, 120 min, 48 h, 72 h, and 168 h (1 week).
      and filter through 0.2 µm filter paper as before.  When pulling the subsamples, be sure and stir the
      suspension uniformly on a stir plate to ensure a uniform solid:solution ratio.
      Also, be careful of the needle when inserting into the rubber septum.

4.)  Dissolved Fe(II), total Fe, total Mn, dissolved NO3-, and NH4+ will be measured.  Due to the
      sensitivity of the Fe(II) analysis, perform this assay first using the scheme outlined below.

Filtrate ~2.4 mL
Outside glovebox, pipette 1.5 mL of filtrate to previously
prepared mixture of 3 mL pH 4 Na-acetate buffer:
0.3 mL ferrozine.  The presence of Fe(II) is indicated by pink color (562 nm).  Total Fe and Mn will be measured on these samples as well.

Remainder of filtrate used for N analysis
II.  Data Analysis:

1.)  Do the thermodynamics and kinetic redox sequences coincide with one another?
       Is there a difference between treatments?