Hach DR/850, Procedures Manual, Page: 105 / 460

105
Summary of Method
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.
Instrument Setup
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
ON
key.
2. Press the
SETUP
key.
3. Press the
DOWN
arrow key until the prompt line shows
USER.
4. Press the
ENTER
key.
5. Enter “8138”, followed by
ENTER
.
Monochloramine 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
2
).
Ozone Interferes at all levels
Peroxides May interfere
Extreme sample pH or highly
buffered samples Adjust the sample pH to 6–7 with Sulfuric Acid or Sodium Hydroxide
Interfering Substance Interference Levels and Treatments
NH
2
Cl
Sample Temperature °C (°F)
(as Cl
2
)
5 (40) 10 (50) 20 (68) 30(83)
1.2 0.2 0.2 0.3 0.3
2.5 0.4 0.5 0.6 0.6
3.5 0.5 0.6 0.7 0.8
CHLORINE, FREE, Ultra-high Range, continued