Hach DR/850 Procedures Manual page 105

CHLORINE, FREE, Ultra-high Range, continued
Interfering Substance
Monochloramine
Ozone
Peroxides
Extreme sample pH or highly
buffered samples
Summary of Method
Instrument Setup
Interference Levels and Treatments
For conventional free chlorine disinfection (beyond the breakpoint), monochlora-
mine concentrations are very low. If monochloramine is present in the sample, its
interference in the free chlorine test varies with the sample temperature, the rela-
tive amount of monochloramine to free chlorine, and the time required to do the
analysis. Approximate interference levels of monochloramine in the free chlorine
test are listed below (as mg/L Cl
NH Cl
2
(as Cl )
5 (40)
2
1.2
2.5
3.5
Interferes at all levels
May interfere
Adjust the sample pH to 6–7 with Sulfuric Acid or Sodium Hydroxide
The range of analysis using the DPD method for free chlorine can
be extended by adding more indicator in proportion to sample
volume. Thus, a larger fill powder pillow of DPD Free Chlorine
Reagent is added to a 5-mL sample portion.
Chlorine in the sample as hypochlorous acid or hypochlorite ion
(free chlorine or free available chlorine) reacts immediately with
DPD (N,N-diethyl-p-phenylenediamine) indicator to form a pink
color which is proportional in intensity to the chlorine
concentration.
The following procedure will add this method as a new Hach
program to a DR/800 instrument.
1. Turn on the instrument by pressing the
2. Press the
SETUP
3. Press the
DOWN
USER.
4. Press the
ENTER
5. Enter "8138", followed by
105
).
2
Sample Temperature °C (°F)
10 (50)
0.2 0.2
0.4 0.5
0.5 0.6
key.
arrow key until the prompt line shows
key.
.
ENTER
20 (68) 30(83)
0.3 0.3
0.6 0.6
0.7 0.8
key.
ON