Hach DR/850, Procedures Manual, Page: 111 / 460

111
Sampling and Storage
Analyze samples for chlorine immediately after collection. Free
and combined chlorine are strong oxidizing agents and react
rapidly with various compounds. Many factors such as sunlight,
pH, temperature, and sample composition will influence
decomposition of chlorine in water.
•
Avoid plastic containers which may have a large chlorine
demand.
•
Pretreat glass sample containers to remove chlorine demand
by soaking in a dilute bleach solution (1 mL of commercial
bleach to 1 liter of deionized water) for at least one hour.
Rinse thoroughly with deionized or distilled water. If sample
containers are rinsed thoroughly with deionized or distilled
water after use, only occasional pre-treatment is necessary.
•
Use separate, dedicated sample cells for free and total
chlorine determinations. If trace iodide from the total
chlorine reagent is carried over to the free chlorine test,
monochloramine could interfere.
•
A common error in testing for chlorine is failure to obtain a
representative sample. If sampling from a tap, let the water
flow for at least five minutes to ensure a representative
sample. Let the sample container overflow with sample
several times. Cap the container so there is no air above the
sample.
•
If sampling with a sample cell, rinse the cell several times
with the sample, then carefully fill to the 5-mL mark. Proceed
with the chlorine test immediately.
Accuracy Check
1. Fill three mixing cylinders (Cat. No. 20886-38) with 5-mL
of sample.
2. Snap the neck of a HR Chlorine Ampule Standard,
50–75 mg/L Cl
2
. Using the TenSette
®
Pipet, add 0.1 mL, 0.2
mL, and 0.3 mL of standard, respectively, to each cylinder
and mix thoroughly.
3. Analyze each standard addition sample as described in the
procedure. Record each result.
CHLORINE, TOTAL, Ultra-High Range, continued