Hach DR/700 Procedures Manual page 402

IODINE,
continued
ACCURACY CHECK
Standard Additions Method
a) Snap the top off the Chlorine Voluette Ampule Standard Solution,
50 to 75 mg/L Cl
b) Use the TenSette Pipet to add 0.1, 0.2 and 0.3 mL of standard to three
25-mL water 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
should cause an incremental increase in iodine, the exact value of which
depends on the chlorine concentration in the Voluette. Check the
certificate enclosed with the Voluettes for the incremental chlorine
value. Multiply by 3.6 to obtain the value for iodine.
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 testing reagents. Testing 1.00 mg/L Cl
to 3.58 mg/L I ) concentration samples, the standard deviation was
2
±0.005 mg/L Cl
2
Testing zero concentration samples, the limit of detection was 0.010 mg/L
Cl (equivalent to 0.04 mg/L I
2
three times the standard deviation when tasting zero concentration
samples (Adapted from Analytical Chemistry, 1980, 52, 2242-2249).
Using two representative lots of AccuVacs, the standard deviation was
±0.005 mg/L Cl
2
was 0.015 mg/L Cl
INTERFERENCES
Samples containing more than 300 mg/L alkalinity or 150 mg/L acidity
as CaCO may not develop the full amount of color, or it may instantly
3
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. Add the same amount to the sample to be tested.
Correct for volume additions (See Section I).
.
2
(equivalent to 0.02 mg/L I
). The limit of detection was calculated as
2
(equivalent to 0.02 mg/L I
(equivalent to 0.06 mg/L I
2
).
2
) and the limit of detection
2
)
2
52-76
(equivalent
2