Environmental Engineering Reference
In-Depth Information
scoop. The advantage of the pillows is that they have a longer shelf life and
are much less prone to contamination or spillage, but buying larger quanti-
ties in bottles has the advantage of considerably lower cost per test.
The major factor in the expense of the kits is the method of titration used—
eyedropper, syringe-type titrator, or digital 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 microscale
the drop size (and thus the 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 in the amount of titrant it
allows to pass.
DID YoU KNoW?
Oxygen concentration in air is about 21%, but in water it is only slightly
soluble. Oxygen saturation ranges from 7 mg/L in hot water to 15 mg/L
in cold water; it is 9.2 mg/L at 20°C and atmospheric pressure at sea
level.
If a high degree of accuracy and precision in DO results is required, a
digital titrator should be used. A kit that uses an eye dropper or syringe type
of titrator is suitable for most other purposes. The lower cost of this type of
DO field kit might be attractive if several teams of samplers and testers at
multiple sites at the same time are necessary.
Meter and Probe
A dissolved oxygen 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 filled 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 electric cable to the meter, which converts the signal to
milligrams per liter on a scale that the volunteer can read. DO meters are
expensive compared to field kits that use the titration method. Meter and
probe combinations run between $500 and $1200, including a long cable to
connect the probe to the meter. The advantage of a meter and probe is that
DO and temperature can be quickly read at any point where the probe is
inserted into the stream; also, DO levels can be measured at a certain point
on a continuous basis. The results are read directly as milligrams per liter,
unlike the titration methods, where the final titration result might have to be
converted by an equation to milligrams per liter. DO meters are more fragile
