1070-4272/05/7801-0101+2005 Pleiades Publishing, Inc.
Russian Journal of Applied Chemistry, Vol. 78, No. 1, 2005, pp. 101!107. Translated from Zhurnal Prikladnoi Khimii, Vol. 78, No. 1,
2005, pp. 102!107.
Original Russian Text Copyright + 2005 by Pisarevsky, Polozova, Hockridge.
OF CHEMISTRY AND TECHNOLOGY
Chemical Oxygen Demand
A. M. Pisarevsky, I. P. Polozova, and P. M. Hockridge
Research Institute of Chemistry, St. Petersburg State University, St. Petersburg, Russia
Virginia Commonwealth University, Richmond, the United States
Received November 28, 2003
Abstract-New instrumental methods and tendencies of development of direct potentiometric determination
of the chemical oxygen demand in the framework of the standard technique are considered.
In view of the increasing contamination of water,
both the number of analyses made and that of studies
of the analytical capacity of methods for determining
the quality of water of various kinds are steadily grow-
ing . The main parameter in the case in question
is the oxygen concentration maintaining the normal
ecological situation in water reservoirs. However, it is
insufficient to determine only this parameter. The
main task consists in monitoring the concentrations of
substances (reducing agents of mostly organic nature)
that can disturb the oxygen balance of water reser-
voirs, which leads to environmental disasters .
This problem has been solved by developing two
integrated techniques for determining the parameters
[biological oxygen demand] (BOD) and [chemical
oxygen demand] (COD): in both cases, the total con-
centration of substances that can be oxidized, bio-
logically or chemically, to inorganic final products is
In the COD method, the results obtained are recal-
culated to the oxygen concentration (oxidizing agent,
Determination of BOD is a complicated analytical
procedure requiring a high skill of analysts.
main difficulties have been noted [2, 5]: the necessity
for introduction of a bacterial microflora and biogenic
elements into a sample and the long duration of analy-
sis. In practice, results obtained after incubation for
five days (BOD
) are most frequently used, but com-
plete biochemical oxidation by oxygen may require
20325 days. Even with substances chosen as reference
for determination of BOD, the deviation from the cal-
The parameter BOD corresponds to a greater extent to the
natural conditions of oxidation of organic substances in water
(according to the commonly accepted procedure, its determi-
nation resembles the process used in aerotanks in treatment of
wastewater with activated sludge).
culated values is +5 rel. %, and the reproducibility in
the case of real wastewater is +15320 rel. % .
The introduction of such a parameter as COD into
the analytical practice made it possible to obviate the
above difficulties. The standard procedure involves
boiling of a sample (2 h) in an 8 M H
with introduction of K
and mercury salts for binding of chlorides. The un-
is removed by titration with Mohr’s
salt. The standard procedure for determining COD
makes it possible to achieve in many cases the re-
quired extent of oxidation, e.g., in analyses of house-
hold sewage (903100% relative to the theoretically
calculated values). In some cases, however, it is in-
efficient, which is primarily determined by the com-
position of water being studied. The COD method is
inapplicable to water containing (upon dilution) more
than 2 g l
Some countries use their own conditions of COD
determination. For example, Japanese scientists be-
lieve that it sufficient to make analysis only for easily
oxidizable components, which represent an immediate
danger to the state of water reservoirs . In this ap-
is chosen as an oxidizing agent and
the conditions of analysis (temperature, acidity, and
duration) are considerably milder. It is noteworthy that
use of integral parameters is always rather difficult,
because not only general principles are to be standar-
dized, but also all details of the analytical procedure.
The method of direct potentiometric determination
of COD (without sample titration) and the necessary
apparatus were suggested in .
The aim of this study was to reveal the most im-
portant issues associated with application of direct po-
Carried out at the Research Institute of Chemistry, St. Peters-
burg State University; the manufacture of the corresponding
analyzers has been commenced in Russia.