LaMotte DPD-FAS TITRIMETRIC OUT FIT DT - Manual (Page 5)

D. NITROGEN TRICHLORIDE
The absence of color in Step A indicates the absence of nitrogen
trichloride, since this compound exists in water only in the presence of
free-chlorine. Nitrogen trichloride, readily identified by its distinctive
odor, is estimated by the following procedure.
1.
Use the 1.0 g spoon (0697) to add one small *Potassium Iodide
Crystal (6809) to a clean 250 mL Erlenmeyer flask (0433).
2. Use the 100 mL graduated cylinder ((0419) to add 100 mL of
sample water to the flask (0433). Swirl until crystals are dissolved.
3. Use the 0.5 g spoon (0698) to add 0.5 g of *DPD #1 Powder (6807)
to the flask (0433). Swirl until powder is dissolved. If Nitrogen
Trichloride is present, solution will turn pink.
4. Titrate rapidly with the diluted Ferrous Ammonium Sulfate Solution
in the buret until the pink color disappears. Record buret reading as
Reading D.
5. Obtain a new sample. Run the test for Free Chlorine (Part A).
Record result as Reading A.
NOTE:
If Free Chlorine has already been determined, use the result
(Reading A) in the equation below.
2 x (Reading D – Reading A), mL = (ppm) Nitrogen Trichloride
CALCULATIONS: (Sections I and II)
For a 100 mL sample, 1.00 mL diluted FAS solution equals 1.00 mg per
1.0 (ppm) Chlorine.
Reading NCl
3
Absent NCl
3
Present
A Free Cl Free Cl
B – A NH
2
Cl NH
2
Cl
C– B NH
2
Cl + 1
2
NCl
3
D Free Cl + 1
2
NCl
3
2(D – A) NCl
3
C – D NHCl
2
Should monochloramine be present with nitrogen trichloride, which is
unlikely, it will be included in Reading D, in which case NCl
3
is
obtained from 2(D – B). Chlorine dioxide if present, is included in
Reading A to the extent of one-fifth of its total chlorine content. For
the determination of chlorine dioxide and of mixtures of free chlorine,
chlorine dioxide, chlorite and chloramines, see Chlorine Dioxide and
Chlorite-DPD Method which follows.
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