Hach DR/700 - Page 584

61-28
FORMALDEHYDE, continued
4. Accurately
measure 25 mL of
sample in a 50-mL
mixing cylinder (the
prepared sample).
Note: Wash glassware
with chromic acid
cleaning solution to
remove trace
contaminants.
Note: Time and
temperature are very
important in this test.
The sample should be at
25 ±1
o
C, and the times
specified must be
followed precisely. A
temperature controlled
water bath is
recommended for better
accuracy.
Note: For proof of
accuracy, use a 320 µg/L
formaldehyde standard
solution (preparation
given in Accuracy
Check) in place of the
sample.
5. Accurately
measure 25 mL of
formaldehyde-free
water in a second
50-mL mixing
cylinder (the blank).
Note: Obtain
formaldehyde-free water
by distilling water from
alkaline permanganate
(4 g sodium hydroxide,
2 g potassium
permanganate per
500 mL water). Discard
the first 50 to 100 mL of
distillate.
6. Add the contents
of one MBTH
Powder Pillow to the
blank. Stopper the
cylinder.