Science of Water : Concepts and Applications

Environmental Biomonitoring, Sampling, and Testing 255
DO fi eld kits using the Winkler method are relatively inexpensive, especially compared to a
meter and probe. Field kits run between $35 and $200, and each kit comes with enough reagents to
run 50–100 DO tests. Replacement reagents are inexpensive, and you can buy them already measured
out for each test in plastic pillows.
You can also purchase the reagents in larger quantities in bottles, and measure them out with
a volumetric scoop. The pillows’ advantage is that they have a longer shelf life and are much less
prone to contamination or spillage. Buying larger quantities in bottles has the advantage of considerably
lower cost per test.
The major factor in the expense for the kits is the method of titration used—eyedropper, syringetype
titrator. Eyedropper and syringe-type titration is less precise than digital titration, because a
larger drop of titrant is allowed to pass through the dropper opening, and on a micro-scale, the drop
size (and thus volume of titrant) can vary from drop to drop. A digital titrator or a burette (a long
glass tube with a tapered tip like a pipette) permits much more precision and uniformity for the
titrant it allows to pass.
If a high degree of accuracy and precision in DO results are required, a digital titrator should be
used. A kit that uses an eyedropper-type or syringe-type titrator is suitable for most other purposes.
The lower cost of this type of DO fi eld kit might be attractive if several teams of samplers and testers
at multiple sites at the same time are relied on.
Meter and Probe
A DO meter is an electronic device that converts signals from a probe placed in the water into
units of DO in milligrams per liter. Most meters and probes also measure temperature. The probe
is fi lled with a salt solution and has a selectively permeable membrane that allows DO to pass from
the stream water into the salt solution. The DO that has diffused into the salt solution changes the
electric potential of the salt solution, and this change is sent by an electric cable to the meter, which
converts the signal to milligrams per liter on a scale that the user can read.
Methodology
If samples are to be collected for analysis in the laboratory, a special APHA sampler or the equivalent
must be used. This is the case because, if the sample is exposed or mixed with air during collection,
test results can change dramatically. Therefore, the sampling device must allow collection of a sample
that is not mixed with atmospheric air and allows for at least 3X bottle overfl ow (see Figure 8.12).
Again, because the DO level in a sample can change quickly, only grab samples should be used
for DO testing. Samples must be tested immediately (within 15 min) after collection.
√ Note: Samples collected for analysis using the modifi ed Winkler titration method may be preserved
for up to 8 h by adding 0.7 mL of concentrated sulfuric acid or by adding all the chemicals
required by the procedure. Samples collected from the aeration tank of the activated sludge process
must be preserved using a solution of copper sulfate–sulfamic acid to inhibit biological activity.
The disadvantage of using the DO oxygen meter method is that the meter can be used to determine
DO concentration directly (see Figure 8.13). In the fi eld, a direct reading can be obtained using
a probe (see Figure 8.14) or by collection of samples for testing in the laboratory using a laboratory
probe (see Figure 8.15).
√ Note: The fi eld probe can be used for laboratory work by placing a stirrer in the bottom of the
sample bottle, but the laboratory probe should never be used in any situation where the entire
probe might be submerged.
The probe used in the determination of DO consists of two electrodes, a membrane, and a membrane
fi lling solution. Oxygen passes through the membrane into the fi lling solution and causes a
change in the electrical current passing between the two electrodes. The change is measured and
displayed as the concentration of DO. To be accurate, the probe membrane must be in proper operating
condition, and the meter must be calibrated before use.