Hach DR/700 Procedures Manual page 525

IRON, TOTAL,
INTERFERENCES
In the powder pillow procedure if the sample, (without TPTZ Iron
Reagent) has a color or turbidity greater than the blank of Step 8
(demineralized water plus TPTZ Iron Reagent), use the sample as the
blank.
A sample pH of less than 3 or greater than 4 after the addition of reagent
may inhibit color formation, cause the developed color to fade quickly
or result in turbidity. Adjust the sample pH in the sample cell before the
addition of reagent to between 3 to 8 by using a pH meter or pH paper
and adding dropwise an appropriate amount of iron-free acid or base
such as 1.0 N Sulfuric Acid Standard Solution or 1.0 N Sodium
Hydroxide Standards Solution. Make a volume correction if significant
volumes of acid or base are used; see Sampling and Storage, Volume
Additions (Section I).
Interference tests were performed using an iron concentration of 0.5 mg/L.
When interferences occurred, the color formation was inhibited or a
precipitate formed. The following do not interfere with the test when
present up to the levels listed:
Cadmium
3+
Chromium ( )
6+
Chromium ( )
Cobalt
Copper
Cyanide
Manganese
Mercury
Molybdenum
Nickel
Nitrite Ion
SUMMARY OF METHOD
The TPTZ Iron Reagent forms a deep blue-purple color with ferrous iron.
The indicator is combined with a reducing agent which converts
precipitated or suspended iron, such as rust, to the ferrous state. The
amount of ferric iron present can be determined as the difference between
the results of a ferrous iron test and the concentration of total iron.
continued
57-19
4.0 mg/L
0.25 mg/L
1.2 mg/L
0.05 mg/L
0.6 mg/L
2.8 mg/L
50.0 mg/L
0.4 mg/L
4.0 mg/L
1.0 mg/L
0.8 mg/L