50-20
IRON, TOTAL, continued
SAMPLING AND STORAGE
Collect samples in acid-cleaned glass or plastic containers. No acid
addition is necessary if analyzing the sample immediately. To preserve
samples, adjust the pH to 2 or less with concentrated nitric acid (about 2
mL per liter). Preserved samples may be stored up to six months at
room temperature. Before analysis, adjust the pH to between 3 and 5
with 5.0 N Sodium Hydroxide Standard Solution. Correct the test result
for volume additions; see Sampling and Storage, Volume Additions
(Section I) for more information.
If only dissolved iron is to be determined, filter the sample before acid
addition using the labware listed under Optional Apparatus.
ACCURACY CHECK
Standard Additions Method
a) Snap the neck off an Iron Voluette Ampule Standard Solution, 50 mg/L.
b) Use the TenSette Pipet to add 0.1, 0.2, and 0.3 mL of standard to three
25-mL water samples and mix thoroughly. For AccuVac Ampuls,
collect 25-mL samples in 50 mL beakers.
c) Analyze each sample as described above. The iron concentration
should increase 0.2 mg/L for each 0.1 mL of standard added.
d) If these increases do not occur, see Standard Additions (Section I) for
more information.
Standard Solution Method
Prepare a 1.0 mg/L iron standard by diluting 1.00 mL of Iron Standard
Solution, 100 mg/L Fe, to 100 mL with demineralized water. Or, use the
TenSette Pipet to dilute 1.0 mL of an Iron Voluette Ampule Standard
Solution (50 mg/L) to 50 mL in a volumetric flask. Prepare this solution
daily.
STATISTICAL EVALUATION
A single operator repetitively tested samples of two laboratory prepared
solutions, using one DR/700, matched sample cells and two
representative lots of testing reagents. Testing 1.5 mg/L Fe
3+
concentration samples, the standard deviation was ±0.008 mg/L Fe
3+
.
Testing zero concentration samples, the limit of detection was
0.007 mg/L Fe
3+
. The limit of detection was calculated as three times the
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