Oxygen (dissolved)
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Methods
The level of dissolved oxygen in natural waters is often
approved by ASTM as the reference method for ppb
a direct indication of quality, since aquatic plants pro-
D.O. determination, the Rhodazine D compound in
duce oxygen, while microorganisms generally consume
reduced form reacts with dissolved oxygen to form a
it as they feed on pollutants. At low temperatures the
bright pink reaction product. The method is not subject
solubility of oxygen is increased; during summer, satura-
to salinity or dissolved gas interferences. Oxidizing
tion levels can be as low as 4 ppm. Dissolved oxygen
agents, including benzoquinone, can cause high results.
(D.O.) is essential for the support of fish and other
Reducing agents such as hydrazine and sulfite do not
aquatic life and aids in the natural decomposition of
interfere. Results are expressed as ppm (mg/L) or ppb
organic matter. Waste treatment plants that employ
(μg/L) O
2
.
aerobic digestion must maintain a level of at least 2
Low-range dissolved oxygen test kits include a special
ppm dissolved oxygen.
sampling tube (diagram) for use with boiler feedwater.
At elevated temperatures, oxygen is highly corrosive
This device allows the user to break the tip of the
to metals, causing pitting in ferrous systems such as
ampoule in a flowing sample stream in order to preclude
high-pressure boilers and deep well oil recovery equip-
error from contamination by atmospheric oxygen.
ment. To prevent costly corrosion damage, the liquids in
contact with the metal surfaces must be treated, usually
by a combination of physical and chemical means.
Deaeration can reduce the dissolved oxygen concentra-
tion of boiler feedwater from several ppm to a few
ppb. Chemical reducing agents such as hydrazine,
DEHA, or sodium sulfite, may be used instead of or in
conjunction with deaeration.
The Indigo Carmine Method
References: ASTM D 888-87, Colorimetric Indigo
Carmine, Test Method A. Gilbert, T. W., Behymer, T. D.,
Castañeda, H. B., “Determination of Dissolved Oxygen
in Natural and Wastewaters, “ American Laboratory,
March 1982, pp. 119-134.
Test kits for environmental and drinking water applica-
tions (ppm range) employ the indigo carmine method.
The reduced form of indigo carmine reacts with D.O. to
form a blue product. The indigo carmine methodology
is not subject to interferences from temperature, salini-
ty, or dissolved gases such as sulfide, which plague users
of D.O. meters. Results are expressed as ppm (mg/L) O
2
.
The Rhodazine D
™
Method
References: Developed by CHEMetrics, Inc. ASTM
Power Plant Manual, 1
st.
ed. p. 169 (1984). ASTM D
5543-94 (2005), Low Level Dissolved Oxygen in Water,
Test Method A.
Test kits for boiler waters and applications requiring
trace levels of D.O. (ppb range) employ the Rhodazine
D methodology. Developed by CHEMetrics, Inc., and
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