YSI 9300 Photometer - Manual (Page 45)

PHOTOMETER TEST INSTRUCTIONS
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Chlorine and chlorine release compounds are widely used for the disinfection of
water. When dissolved in water chlorine forms hypochlorous acid and hypochlorite
ions. Chlorine remaining in the water in this form is known as the free chlorine
residual.
Chlorine does however react with ammonia and nitrogen-based species to form
chloramines. These compounds are poor disinfectants and can also impart a
characteristic taste or odour to the water. It is important therefore in certain
applications to be able to distinguish between chlorine residual present as free
chlorine and as chloramines.
The YSI DPD Chlorine/Chloramines method provides a simple means of measuring
free chlorine (HOCl/HOCl
- ), monochloramine (NH
2
Cl) and dichloramine (NHCl
2
).
Method
The YSI Chlorine/Chloramines test uses the DPD method. This method is
internationally recognised as the standard method of testing for chlorine and other
residuals. In the YSI method the reagents are provided in tablet form for maximum
convenience and simplicity of use.
Free chlorine reacts with diethyl-p-phenylene diamine (DPD) in buffered solution to
produce a pink coloration. The intensity of the color is proportional to the free
chlorine concentration. Addition of a trace amount of potassium iodide induces
further reaction with any monochloramine present. The increase in color intensity is
therefore proportional to the monochloramine concentration. Subsequent addition
of excess potassium iodide causes dichloramine to react in a similar manner. The
increase in color intensity is now proportional to the dichloramine concentration.
In this way it is possible to differentiate between free chlorine, monochloramine
and dichloramine residuals present in the sample. The color intensities at each
stage of the test are measured using a YSI Photometer.
Photometer Method
AUTOMATIC
WAVELENGTH
SELECTION
0 – 5.0 mg/l
PHOT.7.1.AUTO