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The COD is a measure of the chemically oxidisable content of a sample. The dissolved chemicals may be either organic or inorganic in their nature.
A settled grab sample is mixed with an oxidising agent (typically
potassium dichromate acidified with sulphuric acid and a mercuric
salt catalyst) and heated to 120°C for over a 2 hour period
using a reflux condenser. The residual amount of oxidising agent
correlates to the COD of the sample.
The recognised method takes two hours to perform and uses a discrete sample.
On-line versions are semi-continuous taking up to 30 minutes to perform.
Real time events of peak load may be missed.
The technique is not always suitable especially for control applications.
The reagents are classified as hazardous and spent reagent needs careful disposal.
Total Organic Carbon - TOC can
be used to report the dissolved organic compounds in less than 3
minutes. By determining the TOC or dissolved organic component of
a sample stream, a correlation may be set up with COD that is typically
a 1:3 relationship (the process must be predominantly organic in
its nature). Peak organic loads may therefore be determined in real-time.
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