Hach DR/850 Procedures Manual page 143

CHLORINE, TOTAL,
Sampling and Storage
Accuracy Check
Standard Additions Method
continued
Analyze samples for chlorine immediately after collection. Free
and combined chlorine are strong oxidizing agents and are
unstable in natural waters. They react rapidly with various
inorganic compounds and more slowly oxidizes organic
compounds. Many factors, including reactant concentrations,
sunlight, pH, temperature and salinity influence decomposition of
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 pre-treatment 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.
a) Snap the top off a High Range Chlorine PourRite
Ampule Standard Solution.
b) Use a TenSette
mL of 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:
mg/L chlorine added
=
e) The spiked sample result (step c) should reflect the
analyzed sample result + the calculated mg/L Cl
(step d).
f) If this increase does not occur, see Standard Additions,
Section 1 for more information.
143
®
Pipet to add 0.1 mL of the standard to 10
0.1 (vol. standard added) Label value (mg/L Cl
------------------------------------------------------------------------------------------------------------------------ -
(
10.1 sample + standard volume
™
×
)
2
)
added
2