A Colorimetric Method for
Determining Chemical Oxygen Demand
A. F. GAUDY, JR., Professor
and
M. RAMANATHAN, Graduate Research Assistant
Engineering Laboratories
Oklahoma State University
Stillwater, Oklahoma
INTRODUCTION
In June, 1963, at the Texas Industrial Waste Conference, the authors presented the results of their research on a colorimetric COD technique (1). During
the past year we have extended the work to include a wider variety of waste
waters. The present report includes some of the work as well as the results previously reported.
Although the chemical oxygen demand (COD) test may not, under certain
circumstances, be directly correlated with the established biochemical oxygen
demand (BOD) test, it has been increasingly used for estimating the pollutional
potential of waste waters and the efficiency of biological waste treatment systems.
Today, even in cases where major reliance is placed upon the BOD test, waste
characterization usually includes both COD and BOD data. In some cases greater
reliance is placed upon the results of the COD test. Its chief advantage over the
BOD determination is the speed with which results are obtained, thereby suggesting its use as a valuable parameter for controlling biological treatment plant
operation. Regardless of the speed and relative ease with which the determination is performed, it is nevertheless necessary to determine the normality of the
ferrous ammonium sulfate titrant daily, and to prepare and titrate a blank regardless of the number of analyses made. When the number of samples is large,
the titration procedure itself becomes laborious and time-consuming. These
considerations and the lack of qualified manpower at some treatment plants discourage greater use of the COD test in plant operation. These and other aspects
which militate against more adequate laboratory control, especially in the small
treatment plant, have been delineated by Forman and Annett(2).
Current investigations in our laboratory make use of the COD test as one of
the parameters for assessing system behavior. At times up to 75 COD determinations may be made in one day, and efforts have been made to devise a more
rapid analytical procedure. Many investigators have reported short term tests in
which the reflux time is reduced. While for many of the substrates used in our
present investigations a reflux time shorter than the standard two-hr period could
be used, it was felt that the general application of any time-saving procedure
would be lessened by deviating from this requirement of the standard test. Accordingly it was decided to attempt a colorimetric determination, thus obviating
the need for titration. It was reasoned that a colorimetric procedure would also
be of general value since it might provide treatment plant personnel with an
easier, less time-consuming method, thus encouraging more complete laboratory
control at the plant.
Colorimetric tests for organic matter which depend upon the oxidizing
capabilities of dichromate in acid solution have been applied for some time in
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