Methode DO
Manganous sulfate reacts with potassium or sodium hydroxide to give a white precipitate
of manganous hydroxide. In the
presence of oxygen,
brown manganic basic oxide is formed.
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Addition of sulfuric acid dissolves the brown
manganic sulfate which reacts instantly with iodide to yield iodine.
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From the above stoichiometric equations we can
find that four moles of thiosulfate are titrated for each mole of molecular oxygen (O2 ).
Thus 1 ml of 0.025 M sodium thiosulfate is equivalent to 0.025 meq of oxygen.
This
value is commonly multiplied by 8 mg/meq to convert to mg O2. When 200 ml of the original sample is
titrated, then 1 ml of 0.025M Na2S2O3 =
1 mg dissolved oxygen/L. (200ml /1000ml = 0.2 mg O2 and 8mg/meq*0.025 = 0.2 mg O2)
A sample bottle (typical 200 ml glass
BOD bottle with pointed ground glass stopper) is
filled completely with water. Special precautions are required to prevent the entrainment of or solution of atmospheric oxygen or loss of DO. This is particularly important for samples at either end of the spectrum - anoxic samples tend to be highly sensitive to oxygen dissolution into
the sample during handling, while those supersaturated may degas. To fill
the
bottle, it can be lowered into the water at the surface -
ensuring that sample water enters the bottle without splashing, to avoid oxygen entrainment. Samples from depth should be collected with a remote sample such as a
Nancens sampler, which has a tube on the outlet.
The tube should be placed at the
bottom
of
the BOD while it is
filled, and the bottle allowed to overflow for 10s to ensure that representative sample water is obtained. For running water, two to three times bottle volume replacement is suggested to get a representative sample. Samples from turbulent streams may be collected via
a funnel connected to a rubber tube which is placed in
the bottom of the BOD bottle. This will ensure
non-turbulent sample collection and avoid splashing in the bottle that could entrain oxygen. Temperature of the sample should be measured as accurately as possible.
Nitrate
The cadmium reduction method is
a colorimetric method that involves contact of the nitrate in the sample with
cadmium particles, which cause nitrates to be converted to nitrites. The
nitrites then react with another reagent to form a red color whose intensity is
proportional to the original amount of nitrate. The red color is then measured
either by comparison to a color wheel with a scale in milligrams per liter that
increases with the increase in color hue, or by use of an electronic
spectrophotometer that measures the amount of light absorbed by the treated
sample at a 543-nanometer wavelength. The absorbance value is then converted to
the equivalent concentration of nitrate by using a standard curve. Methods for
making standard solutions and standard curves are presented at the end of this
section.
This curve should be created by
the program advisor before each sampling run. The curve is developed by making
a set of standard concentrations of nitrate, reacting them and developing the
corresponding color, and then plotting the absorbance value for each
concentration against concentration. A standard curve could also be generated
for the color wheel. Use of the color wheel is appropriate only if nitrate
concentrations are greater than 1 mg/L. For concentrations below 1 mg/L, a
spectrophotometer should be used. Matching the color of a treated sample at low
concentrations to a color wheel (or cubes) can be very subjective and can lead
to variable results. Color comparators can, however, be effectively used to
identify sites with high nitrates. This method requires that the samples being
treated are clear. If a sample is turbid, it should be filtered through a
0.45-micron filter. Be sure to test whether the filter is nitrate-free. If
copper, iron, or other metals are present in concentrations above several mg/L,
the reaction with the cadmium will be slowed down and the reaction time will
have to be increased. The reagents used for this method are often prepackaged
for different ranges, depending on the expected concentration of nitrate in the
stream. For example, the Hach Company provides reagents for the following
ranges: low (0 to 0.40 mg/L), medium (0 to 4.5 mg/L), and high (0 to 30 mg/L).
You should determine the appropriate range for the stream being monitored.
Orthophospate
The
procedure reported by Dick and Tabatabai (1977) was followed.
However, the total assay volume was reduced to 1 mL
from 25 mL in the original report. The volumes of samples and reagents were
reduced accordingly. Specifically, to 0.32-mL of samples and buffer/water, 0.40
mL of reagent A, 0.08 mL of reagent B, and 0.20 mL of reagent C were added
sequentially. The solutions were stirred thoroughly with a cuvette stirring rod
after the addition of each reagent. The absorbance of the molybdenum blue was
recorded over time or read at 850 nm and/or 700 nm after 30min. Alternatively,
0.10 mL of 20% SDS was added prior to reagent A. In such case, the assay volume
totaled 1.1 mL. Total P in the water extracts of a swine manure was determined
in the same way after persulfate digestion of
1/8 diluted extracts (Rowland and Haygarth 1997).
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