Hach DR/850 Procedures Manual page 198

IRON, TOTAL,
Sampling and Storage
Accuracy Check
continued
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 nitric acid
(about 2 mL per liter). Preserved samples may be stored up to six
months at room temperature. Adjust the pH to between 3 and 5
with 5.0 N Sodium Hydroxide Standard Solution before analysis.
Correct the test result for volume additions; see Correcting for
Volume Additions in Section 1 for more information. If only
dissolved iron is to be determined, filter the sample before
adding the acid.
Standard Additions Method
a) Snap the neck off a 50 mg/L Iron PourRite Ampule
Standard Solution.
b) Use the TenSette Pipet to add 0.1, 0.2, and 0.3 mL of
standard, respectively, to three 25-mL samples and mix
thoroughly.
c) For analysis using AccuVac Ampuls, transfer solutions to
dry, clean 50-mL beakers to facilitate filling of the ampuls.
For analysis with powder pillows, transfer only 10 mL of
solution to the 10-mL sample cells.
d) Analyze each standard addition sample as described
above. The iron concentration should increase 0.2 mg/L
for each 0.1 mL of standard added.
e) If these increases do not occur, see Standard Additions in
Section 1 for troubleshooting 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 deionized water.
Or, dilute 1.00 mL of an Iron PourRite Ampule Standard Solution
(50 mg/L) to 50 mL in a volumetric flask. Prepare this solution daily.
Run the test following the procedure for powder pillows or
AccuVac Ampuls. Results should be between 0.90 mg/L and 1.10
mg/L Fe.
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