Hach DR/850 - Page 153

153
CHLORINE DIOXIDE, continued
Pollution Prevention and Waste Management
Samples treated with sodium arsenite for manganese or
chromium interferences will be hazardous wastes as regulated by
Federal RCRA for arsenic (D004).
Summary of Method
Chlorine dioxide reacts with DPD (N,N-diethyl-p-phenylenedi-
amine) Indicator Reagent (to the extent of one-fifth of its total
available chlorine content corresponding to reduction of chlorine
dioxide to chlorite) to form a pink color. The color intensity is
proportional to the ClO
2
in the sample. Chlorine interference is
eliminated by adding glycine, which converts free chlorine to
chloroaminoacetic acid, but has no effect on chlorine dioxide at
the test pH.
Iodine, I
2
Interferes at all levels.
Manganese, oxidized
(Mn
4+
, Mn
7+
)
or
Chromium, oxidized
(Cr
6+
)
Oxidized manganese interferes at all levels. Oxidized chromium interferes at levels
greater than 2 mg/L. To remove the interferences:
1. Adjust sample pH to 6–7.
2. Add 3 drops potassium iodide (30 g/L) to a 25-mL sample.
3. Mix and wait one minute.
4. Add 3 drops sodium arsenite (5 g/L) and mix.
5. Analyze 10 mL of the treated sample as described in the procedure.
6. Subtract the result of this test from the original analysis to obtain the correct
chlorine dioxide concentration.
Metals Various metals may interfere by combining with the glycine needed to remove the
chlorine interference. Metal interference is limited except when chlorine is present.
In the presence of 0.6 mg/L Cl
2
, both copper (>10 mg/L) and nickel (>50 mg/L)
interfere. Other metals may also interfere, depending on their ability to prevent
glycine from reacting with any Cl
2
in the sample. It may be necessary to add more
glycine to overcome this interference.
Monochloramine Causes a gradual drift to higher readings. When read within 1 minute after reagent
addition, 3 mg/L monochloramine causes less than a 0.1 mg/L ClO
2
increase in the
reading.
Ozone Interferes at levels greater than 1.5 mg/L.
Peroxides May interfere.
Extreme sample pH Adjust to pH 6–7. See Section 1, pH Interferences, in the DR/800 Procedures
Manual.
Highly buffered samples Adjust to pH 6–7. See Section 1, pH Interferences, in the DR/800 Procedures
Manual.
Interfering Substance Interference Levels and Treatments