Environmental Engineering Reference
In-Depth Information
Laboratory Exercise 2: Documenting Redox Conditions in Soil
Mesocosms Using Pt Electrodes and Alpha-Alpha Dipyridyl Dye
Introduction and Background
When soil pores are filled with water and the soil becomes saturated, the movement
of oxygen into the soil is greatly inhibited by the slow diffusion of gasses through
liquids. If oxidizable carbon compounds are present and temperatures are sufficiently
warm, aerobic heterotrophic microorganisms will begin to oxidize the organic matter
using oxygen as the electron acceptor. Once the oxygen has been depleted, various
anaerobic microbes will begin to use alternate compounds as electron acceptors.
Some of the important and common compounds that serve as electron acceptors in
saturated soils and thus can be reduced, include nitrate NO
3
to nitrite NO
2
(or eventually to dinitrogen N
2
gas), solid phase manganese oxides MnO
2
to soluble
Mn
2+
, solid phase iron oxyhydroxides FeOOH to soluble Fe
2+
, and sulfate SO4
¼
to
sulfide S
¼
. When water is removed from the saturated soil through drainage or
evapotranspiration, the reduced species can become oxidized. This reoxidation is
often microbially mediated, but some reactions can occur chemically.
The redox potential, or Eh, can be measured using Pt electrodes with a reference
electrode and the Eh, together with the pH, can be used to determine how reducing
or oxidizing a soil is in order to predict whether particular compounds would be
expected to be reduced or oxidized. This is done by comparing the measured Eh and
pH values to a line calculated using the Ksp for particular compounds and phases
based upon thermodynamic data. There are a number of variables which can affect
the precise calculation of the line (such as the concentration of soluble
components). Diagrams showing the redox stability lines (fields) for many
compounds have been created and published.
Objectives
1. To make Eh measurements in a soil mesocosm
2. To interpret soil redox conditions by using Eh and pH measurements and redox
stability diagrams
3. To use, and interpret the use of, alpha-alpha dipyridyl dye
4. To compare the Eh-pH data collected with the alpha-alpha dipyridyl dye
reactions observed
Materials and Equipment Needed
Soil Mesocosm 7.5 cm diam by 40 cm high
1
Six, 40 cm Pt wire electrodes
1
A 7.5 cm schedule 40 PVC pipe 50 cm in length should be sharpened on one end (bevel out) so that
the pipe can be driven into the soil to a depth of 40 cm and then excavated to collect the mesocosm.
