Hach DR/850 Procedures Manual page 260

NITROGEN, AMMONIA,
9.
After the timer
beeps, place the blank
into the cell holder.
Tightly cover the sample
cell with the
instrument cap.
Sampling and Storage
continued
10.
Press:
ZERO
The cursor will move to
the right, then the
display will show:
0.00 mg/L NH3-N
Collect samples in clean plastic or glass bottles. Most reliable
results are obtained when samples are analyzed as soon as
possible after collection.
If chlorine is known to be present, the sample must be treated
immediately with sodium thiosulfate. Add one drop of Sodium
Thiosulfate Standard Solution, 0.1 N, for each 0.3 mg of chlorine
present in a one liter sample.
To preserve the sample, adjust the pH to 2 or less with
concentrated sulfuric acid (about 2 mL per liter). Store samples at
4 °C or less. Samples preserved in this manner can be stored up to
28 days. Just before testing the stored sample, warm to room
temperature and neutralize with 5.0 N Sodium Hydroxide
Standard Solution. Correct the test result for volume additions;
see Correction for Volume Additions, in Section 1 for more
detailed information.
11.
Place the prepared
sample into the cell
holder. Tightly cover the
sample cell with the
instrument cap.
260
12.
Press:
READ
The cursor will move to
the right, then the result
in mg/L ammonia
nitrogen will
be displayed.
Note: Standard Adjust may
be performed using a
prepared standard (see
Standard Adjust in
Section 1).