Hach DR/700 Procedures Manual page 379

CHLORINE, FREE,
ACCURACY CHECK
Standard Additions Method
a) Snap the top off a Chlorine Voluette Ampule Standard Solution.
b) Use the TenSette Pipet to add 0.1, 0.2 and 0.3 mL of standard to three
25-mL samples. Swirl gently to mix. (For AccuVac ampuls, use
50-mL beakers.)
c) Analyze each sample as described above. Each 0.1 mL of standard
will cause an incremental increase in chlorine, the exact value of which
depends on the concentration in the Voluette. Check the certificate
enclosed with the Voluettes for this value.
d) If these increases do not occur, see Standard Additions (Section I) for
more information.
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 reagent. Testing 1.00 mg/L Cl
solutions, the standard deviation was ±0.007 mg/L Cl
Testing zero concentration samples, the limit of detection was 0.005 mg/L
Cl . The limit of detection was calculated as three times the standard
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deviation when testing zero concentration samples (adapted from
Analytical Chemistry, 1980, 52, 2242-2249).
Using two representative lots of AccuVacs, the standard deviation was
±0.009 mg/L Cl . and the limit of detection was 0.010 mg/L Cl
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INTERFERENCES
Samples containing more than 250 mg/L alkalinity or 150 mg/L acidity
as CaCO may not develop the full amount of color, or it may instantly
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fade. Neutralize these samples to a pH of 6 to 7 with 1 N sulfuric acid,
or 1 N sodium hydroxide. Determine the amount required on a separate
25-mL sample; then add the same amount to the sample to be tested.
Samples containing monochloramine will cause a gradual drift to higher
chlorine readings. When read within one minute of reagent addition,
3.0 mg/L monochloramine will cause an increase of less than 0.1 mg/L
in the free chlorine reading.
continued
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concentration
2
.
2
.
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