135
Sampling and Storage
Analyze samples for chlorine immediately after collection. Free
chlorine is a strong oxidizing agent and is unstable in natural
waters. It reacts rapidly with various inorganic compounds and
more slowly oxidizes organic compounds. Many factors,
including reactant concentrations, sunlight, pH, temperature, and
salinity influence decomposition of free chlorine in water.
Avoid plastic containers since these may have a large chlorine
demand. Pretreat glass sample containers to remove any
chlorine demand by soaking in a dilute bleach solution (1 mL
commercial bleach to l liter of deionized water) for at least 1
hour. Rinse thoroughly with deionized or distilled water. If
sample containers are rinsed thoroughly with deionized or
distilled water after use, only occasional pretreatment is
necessary.
A common error in testing for chlorine is obtaining an
unrepresentative sample. If sampling from a tap, let the water
flow for at least 5 minutes to ensure a representative sample. Let
the container overflow with the sample several times, then cap
the sample containers so there is no headspace (air) above the
sample. Perform the analysis immediately.
Accuracy Check
Standard Additions Method
a) Snap the top off a HR Chlorine PourRite
™
Ampule
Standard Solution.
b) Use a TenSette
®
Pipet to add 0.1 mL of the standard to the
reacted sample (this is the spiked sample). Swirl to mix.
c) Analyze the spiked sample, beginning at Step 8 of the
procedure.
d) Calculate the concentration of mg/L chlorine added to the
sample:
e) The spiked sample result (step c) should reflect the
analyzed sample result + the calculated mg/L Cl
2
added
(step d).
f) If this increase does not occur, see Standard Additions,
Section 1 for more information.
mg/L chlorine added =
0.1 vol. standard added()Label value mg L Cl
2
⁄()×
10.1 sample( standard volume)+
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CHLORINE, FREE continued
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