Comparison of Winkler, Polarography
and the New Beckman DO Electrode
RICHARD D. HOAK, Senior Fellow
and
HENRY C. BRAMER, Fellow
Mellon Institute
Pittsburgh, Pennsylvania
Among the various ways of judging the quality of natural water, measurement of dissolved oxygen (DO) concentration is the most important. This
is so because desirable aquatic organisms live aerobically. DO resources not
only govern to a large degree the diversity of species that will live and propagate in a body of water but they also determine the amount of organic matter
that can be dissimilated inoffensively.
The long-established Winkler method provides a dependable technique
for measurement of DO in relatively pure water. Difficulties are encountered,
however, where determinations are made on samples from streams carrying
appreciable amounts of man-made wastes. Many substances affect the accuracy of the primary Winkler method, and several modifications have been
developed to counteract the effects of specific interferences. Polarographic
techniques are relatively free from the interferences that affect Winkler determinations but a standard method has not yet been adopted. Neither of
these methods is very practical for field work because of the manipulations
that are required and the limited number of samples that can be analyzed in
a stream survey. A direct-reading instrumental method would thus he very
useful for study of the DO regime of rivers. The Beckman platinum-silver
electrode appeared to be adaptable to this purpose.
The Beckman electrode is responsive to slight changes in DO concentration but it has a large temperature coefficient. The coefficient of the
electrode used in the work reported here was 2. 6 per cent per degree JFahren-
heit as measured in our laboratory. Although readings could be corrected by
application of this factor, the electrode would be much more practical if it
were to read directly. A compensating network, employing an appropriate
thermistor, was therefore developed which yielded good linearity over the
range 32 to 90F.
EXPERIMENTAL
DO concentrations in rivers are of major importance. The Alterberg-
fluoride modification of the Winkler method was therefore adopted as the
reference analysis in this study. The primary Winkler method was used in
laboratory determinations on distilled water.
Laboratory analyses were run on samples syphoned from a battery jar
equipped with an immersion heater and a stirrer. Oxygen was dissolved in
the water from a stream of air flowing through a porous stone; it was stripped
from the water with a stream of nitrogen gas.
A comparison was made between the electrode and the Winkler method
at constant temperature.    The electrode was suspended in distilled water.
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