|
Chemical Analyses: Water: Redox
Redox (oxidation and reduction) reactions dictate the behavior
of many chemical constituents found in potable, process and wastewaters
and are of equal importance in biological systems where redox conditions
control the mobility and reactivity of elements such as iron, sulfur,
nitrogen, carbon and many metallic elements (Standard
Methods, 1998). Redox reactions involve the transfer of electrons
from the reduced specie (electron donor) to the oxidized specie
(electron acceptor). After the oxidation-reduction reaction is complete,
the specie that was reduced becomes oxidized and the specie that
was oxidized becomes reduced by the transfer of electrons (Kolthoff
et al., 1969). Measurement of redox potential can be useful
in the context of acid mine drainage for several reasons:
- It can estimate the relative amounts of ferrous (reduced) and
ferric (oxidized) iron in acidic drainage water (Pourbaix,
1966).
- It can be a useful index as of the state of a system (i.e. aerobic
or anaerobic) and can indicate which chemical and biological processes
may be occurring.
- It can be useful in predicting which metals may solubilize or
precipitate under different redox conditions.
Water treatment technologies for treating acid mine drainage work
to precipitate metals either aerobically or anaerobically. Redox
potential can indicate, along with other factors such as pH, whether
a metal will be in the soluble or precipitate form. Keeping a water
treatment system at the optimal pH and redox potential for precipitation
of target metals will ensure successful water treatment. For more
information on the different water treatment systems used to treat
acid mine drainage, click
here.
Oxidation-reduction potential (ORP) is determined potentiometrically
using an inert indicator electrode, usually made from gold or platinum,
and a reference electrode (calomel or silver/silver chloride). The
voltage measured between the two electrodes is the ORP and is proportional
to the ratio of the concentration of reduced and oxidized constituents
contained in a sample. Combination electrodes are also available
for ORP analysis. It is important to keep in mind that redox potential
is only an indicator of processes occurring in solution since redox
measurements are difficult if not impossible to interpret in any
fundamental chemical terms. Redox potential can be useful as an
index or "black box measurement" rather than a fundamental indicator
of a specific chemical environment (MEND,
2001; Snoeyink
and Jenkins, 1980).
Problem | Compliance
| Health & Safety | Sampling
| Analytical | Data
Quality
Site Assessment | Prediction
| Construction | GIS
| Monitoring & Assessment
|
