Hach DR/700 Procedures Manual page 518

IRON, TOTAL,
4.
Fill a 25-mL cell to
the 25-mL line with
sample.
Note: For proof of
accuracy, use a 0.4 mg/L
iron standard solution
(preparation given in the
Accuracy Check) in
place of the sample.
Note: Sample pH is
important in this test, see
pH discussion in
Interferences (Section 1).
continued
5.
Add the contents
of one TPTZ Iron
Reagent Powder
Pillow (the prepared
sample). Cap. Shake
for 30 seconds.
Note: A blue color will
develop if iron is
present.
57-12
3 minutes
6.
Wait 3 minutes.
Note: Continue with
steps 7 and 8 while the
timer is running.