Hach DR/850 Procedures Manual page 286

OXYGEN DEMAND, CHEMICAL,
PREPARE BLANK
5.
Prepare a blank (see
note) by substituting
0.50 mL of deionized
water for the sample.
Continue with step 9 of
this procedure.
Note: The reagent blank is
stable and can be reused.
Verify reagent blank quality
by measuring the
absorbance of the blank vs.
a clean COD vial filled
with deionized water. The
absorbance range should
be about 1.36-1.43.
6.
Place the vials in
the DRB 200 Reactor
that is preheated to 150
°C. Digest for 1 hour.
Note: Boiling sample in the
vials during digestion
indicates the vial is not
properly sealed; test results
will be invalid.
Note: Samples can be
digested up to 4 hours to
oxidize more resistant
organics. The prepared
blank must be treated in the
same manner.
Note: See DRB 200 user
manual for selecting pre-
programmed temperature
applications.
continued
2:00 minutes
7.
Remove the vials
and place them in a
cooling rack for two
minutes to air cool. Then
cool the vials to room
temperature in a cool
water bath or running
tap water. This usually
takes about three
minutes.
Note: Occasionally a vial
will develop a colorless
upper layer and a purple
lower layer. Invert the vial
several times to mix and
proceed. This will not affect
test results.
286
8.
Remove the vials
from the water and wipe
with a clean, dry
paper towel.
Invert the vials several
times to mix.