Chlorine Monitoring and Dechlorination Techniques Handbook

Chlorine Monitoring and Dechlorinating Techniques Handbook
Be sure that the reagent expiry date is clearly marked and that staff check the expiry date
before using each reagent pouch.
2.2.3 Spectrophotometers
There are a number of field spectrophotometers that can be used for measurement of residual
chlorine levels. One model is a pocket-sized instrument that tests for free and total residual
chlorine over the operating range of 0 to 3.9 ppm. The meter reads in 0.01 ppm increments
with an accuracy of 0.05 ppm. The meter itself has dimensions of 4.5 x 12 x 8.9 cm. The
reagent used in these types of instruments is typically DPD, as described above.
The cost of the pocket device is approximately $300. The GVWD staff have had some
difficulties with using this equipment in the field due to weather conditions and prefer the
colour comparator kit which can be kept in the service truck.
2.2.4 Laboratory Analysis
Laboratory analysis for total residual chlorine in water is not recommended due to the very
short storage time that a sample can remain stable, and the relatively long turnaround time
for results. Chlorine dissipates from a water sample very quickly as a result of exposure to
light, increased temperature, and agitation. According to the US EPA standard laboratory
techniques for residual chlorine analysis Method 4500-Cl, samples to be analysed for
residual chlorine should not be stored at all, but analysed immediately.
The US EPA Standard Method 4500-Cl outlines different methods to detect residual
chlorine, including:
the Iodometric Method;
the Amperometric Titration Method;
the Low-Level Amperometric Titration Method
the DPD Colorimetric Method;
the Syringaldazine (FACTS) Method; and
the Iodometric Electrode Technique.
The advantage of these methods is that the detection limits are as low as 0.010 mg/L (ppm
or 10 g/L (ppb), depending on the sophistication of the equipment.
It is recommended that sampling for total residual chlorine be
completed in the field using a field testing kit to obtain the
best results. In special cases, lower detection limit methods
could be used by setting titration equipment in the field.
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