Hach DR/700 Procedures Manual page 437

PHOSPHORUS REACTIVE,
STATISTICAL EVALUATION
A single operator repetitively tested samples of two laboratory prepared
solutions, using one DR/700, matched sample cells and two representative
lots of testing reagents. Testing 15.0 mg/L PO
the standard deviation was ±0.10 mg/L PO
Testing zero concentration samples, the limit of detection was 0.23 mg/L
3-
PO . The limit of detection was calculated as three times the standard
4
deviation when testing zero concentration samples (Adapted from
Analytical Chemistry, 1980, 52, 2242-2249).
INTERFERENCES
Samples with large amounts of turbidity may give inconsistent results.
Some of the suspended particles may dissolve because of the acid used in
the test. Also, results will vary because of the variable desorption of
orthophosphate from the particles. For highly turbid or colored samples,
add 1 mL of 10 N Sulfuric Acid Standard Solution to another 25-mL
sample. Use this in place of the sample as the blank to zero the instrument
in Step 10. Use a pipet and pipet filler when measuring the sulfuric acid
standard.
For best results, the temperature of the sample should be 21 ±3°C(70 ±5°F).
Sulfide interferes by forming a blue color directly with the molybdate
reagent. Remove sulfide interference by the following pretreatment:
a) Measure 350 mL of sample into a clean 500-mL erlenmeyer flask.
b) Add the contents of one Sulfide Inhibitor Reagent Powder Pillow. Swirl
to mix.
c) Filter the sample through a folded filter paper and use this solution in
Steps 4 and 8.
Nitrites bleach the blue color. Remove nitrite interference by adding 0.05 g
of sulfamic acid to the sample. Swirl to mix. Continue with Step 5.
The following may interfere if present in concentrations exceeding those
listed below:
Chloride
Calcium
Magnesium
continued
3-
concentration samples,
4
3-
.
4
150,000 mg/L as Cl
10,000 mg/L as CaCO
40,000 mg/L as CaCO
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3
3