Wetland Biogeochemistry Techniques - Wetland Techniques - page 424

One reference electrode (with salt bridge)
One, high impedance voltmeter
pH meter and buffers
Light's Solution
Bucket
Duct tape
alpha-alpha dipyridyl dye 2

Procedures

1. Selection and testing of electrodes . Electrodes must be tested to ensure they are
working properly. Each group of students will require 6 Pt electrodes.
Electrodes can be made at fairly low expense if done according to the proce-
dure described by Owens et al. ( 2005 ). To test the Pt electrodes, place a group
of electrodes into a beaker containing Light's solution (Light 1972), which
contains Fe(II) and Fe(III) in a sulfuric acid solution (be careful). This solution
is poised and stable. Electrodes should be tested in groups using the same
reference electrode, and all individual electrodes within the group should be
within 1-2 mV the mean. Typically, they should read around Eh 675 (raw

voltage reading of about 430 if using a calomel reference). However, it is most
important that the electrode readings group together and sometimes values for
the Light's solution can drift. Groups of calomel reference electrodes can be
similarly tested, although they typically vary a bit more.
2. Selection of mesocosms . Each group of (2-4) students will work with one
mesocosm. See footnote #1 regarding collection of the mesocosm. A piece of
geotextile fabric should be attached across the bottom of the mesocosm using
duct tape, in order to help keep the soil in the mesocosm (Fig. 7.4 ). Care should
be taken to give support to the soil within the mesocosm so that it does not
accidentally slide out.
The depth from the top of the mesocosm to the top of the soil should be
measured and then marked on the outside of the core. After this, mark the core
at the 2 depths at which the Pt electrodes will be later installed. These depths
will be at 10 and 25 cm below the top of the soil (NOT below the top of the PVC
cylinder) as shown in Fig. 7.4 .
3. Instrumentation of the mesocosms . An overview of the instrumentation is
shown in Fig. 7.5 which illustrates how the mesocosm will appear when all
the electrodes have been installed. Probably, the best order in which to install
the electrodes is the following: (1) the deep (25 cm) Pt electrodes, (2) the salt
bridge and calomel electrode; and (3) the shallow (10 cm) Pt electrodes.

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2 This should be made according to the procedure of Childs ( 1981 ).