Environmental Engineering Reference
In-Depth Information
(even millimeters). Nevertheless, if one can measure soil Eh and pH, graphs such as
Fig.
7.2
can be useful in helping to predict which electrochemical phases might be
stable and which phases might be expected to be reduced or oxidized. For such
reasons, soil and wetland scientists are often interested in documenting or measur-
ing the redox status or condition of a soil.
7.8.2 Techniques to Measure Eh/Assess Reducing
Conditions
7.8.2.1 Direct Assessment: Pt Electrodes
One approach to documenting soil redox status is to measure the voltage generated
between a Pt electrode and a reference electrode (such as a calomel or Ag-AgCl)
inserted into the soil and then correcting the measured voltage for the difference
between the reference electrode and the standard hydrogen electrode, which is
reported as Eh (Bohn
1971
; James and Bartlett
2000
; Patrick et al.
1996
). The
Pt electrode is placed into the soil at the depth where one intends to measure the Eh;
the reference electrode can be placed nearer the soil surface if that is more
convenient. If measurements are to made at soil depths of more than a few cm, it
may be useful or necessary to make a pilot hole with some instrument prior to
inserting the Pt electrode to the desired depth. Best results are obtained when the
soil is moist or wet. Otherwise, a salt bridge may be used to ensure good electrical
contact between the reference electrode and the soil (Veneman and Pickering
1983
). This may be particularly important if upper part of the soil is not wet at
the time of measurement.
It is of particular importance to make the voltage measurement using a device
with a high internal resistance (Rabenhorst et al.
2009
). This can either be done
by using a research grade voltmeter, or by using a low cost multimeter that has
beenmodifiedtoeffectivelyincreases the internal resistance to
>
200 Gohms
(Rabenhorst
2009
). When an unmodified multimeter (resistance approximately
10 Mohms) is used to measure the voltage, too large a current is permitted to flow
during the measurement which alters the electrochemical environment in the
vicinity of the Pt electrode causing substantialdriftandanunreliablemeasure-
ment. Also, because there is a great deal of small scale microsite variability in
soils in the field, multiple (often as many as five or more) replicate measurements
are made. If multiple measurements are to be made over the course of a field
season, electrodes are sometimes leftinplaceandthenremovedattheendof
the field season and checked to be sure they are still functioning properly.
Automateddataloggersmayalsobeprogrammedtocolectrepeated
measurements. In other situations where it would be impractical to leave the
electrodes installed in the field, they may be removed and reinstalled each time
measurements are made.
