Chlorine, Free
Chlorine, Free
ChlorineFre_TNT_Other_DPD_Eng_Ody.fm Page 3 of 4
Sample Collection, Storage and Preservation
Analyze samples for chlorine immediately after collection. Free chlorine is a
strong oxidizing agent and it 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
pre-treatment is necessary.
A common error in testing for chlorine is not 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 chlorine analysis immediately.
Accuracy Check
Standard Additions Method (Sample Spike)
1. After reading test results, leave the sample cell (unspiked sample) in the
instrument. Verify that the units displayed are in mg/L.
2. Touch
Options. Touch Standard Additions. A keypad will appear. Enter the
average chlorine concentration shown on the certificate enclosed with the
standard solution. Touch
OK. A summary of the Standard Additions
procedure will appear. Touch
OK.
Monochloramine
For conventional free chlorine disinfection (beyond the breakpoint), typical monochloramine
concentrations are very low. If monochloramine is present in the sample, its interference in the
free chlorine test depends on the sample temperature, relative amount of monochloramine to
free chlorine, and the time required to do the analysis.Typical interference levels of
monochloramine in the free chlorine test are listed below (as mg/L Cl
2
).
Ozone, O
3
Interferes at all levels
Peroxides May interfere
Extreme sample pH or Highly
buffered samples
Adjust to pH 6–7 using acid (Sulfuric Acid, 1.000 N, Cat. No. 1270-32) or base (Sodium
Hydroxide, 1.00 N, Cat. No. 1045-32).
* Samples treated with sodium arsenite for interferences will be hazardous waste as regulated by Federal RCRA for arsenic
(D004). See Section 4 of the for further information on proper disposal of these materials.
(continued)
Interfering Substance Interference Levels and Treatments
NH
2
Cl
(as Cl
2
)
Sample Temp. °C (°F)
5 (40) 10 (50) 20 (68) 30 (83)
1.2 mg/L +0.15 0.19 0.30 0.29
2.5 mg/L +0.35 0.38 0.55 0.61
3.5 mg/L +0.38 0.56 0.69 0.73
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