Hach DR/850 Procedures Manual page 205

IRON, TOTAL,
Method Performance
Interferences
Summary of Method
continued
mL of standard added.
d) If these increases do not occur, see Standard Additions in
Section 1 for more Information.
Standard Solution Method
Prepare a 0.4 mg/L iron working solution as follows:
a) Pipet 1.00 mL of Iron Standard Solution, 100 mg/L Fe,
into a
250-mL volumetric flask.
b) Dilute to volume with deionized water. Prepare this
solution daily. Analyze this working solution according to
the above procedure. Results should be between 0.36 and
0.44 mg/L Fe.
Precision
In a single laboratory, using a standard solution of 1.00 mg/L Fe
and two representative lots of reagents with the instrument, a
single operator obtained a standard deviation of ±0.006 mg/L Fe.
Estimated Detection Limit
The estimated detection limit for program 38 is 0.03 mg/L Fe. For
more information on the estimated detection limit, see Section 1.
A sample pH of less than 3 or greater than 4 after reagent addition
may inhibit color formation, cause the developed color to fade, or
result in turbidity. Adjust the sample pH before reagent addition
to between 3 and 5 using a pH meter or pH paper. Drop by drop,
add an appropriate amount of acid (1.0 N Sulfuric Acid Solution)
or base (1.0 N Sodium Hydroxide Standard Solution). Make
volume corrections if significant amounts of acid or base are used
(see Correction for Volume Additions
in Section 1).
FerroMo Iron Reagent 1 contains a reducing agent combined with
a masking agent. The masking agent eliminates interference from
high levels of molybdate. The reducing agent converts
precipitated or suspended iron (rust) to the ferrous state. FerroMo
Iron Reagent 2 contains the indicator combined with a buffering
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