Hach DR/850 Procedures Manual page 449

ZINC,
continued
Sampling and Storage
Accuracy Check
Method Performance
Collect samples in acid-washed plastic bottles. For storage,
adjust the pH to 2 or less with nitric acid (about 2 mL per liter).
The preserved samples can be stored up to six months at room
temperature.
Adjust the pH to 4 to 5 with 5.0 N sodium hydroxide before
analysis. Do not exceed pH 5, as zinc may be lost as a
precipitate. Correct the test result for volume additions; see
Sampling and Storage, Volume Additions, in Section 1 for more
information.
If only dissolved zinc is to be determined, filter the sample before
the acid addition.
Standard Additions Method
a) Snap the neck off a Zinc PourRite Ampule Standard, 25
mg/L Zn.
b) Use the TenSette Pipet to add 0.1, 0.2, and 0.3 mL of
standard to three 25-mL samples. Mix each thoroughly.
c) Analyze each sample as described above. The zinc
concentration should increase 0.1 mg/L for each 0.1 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 0.50 mg/L zinc standard solution by diluting 5.00 mL
of Zinc Standard Solution, 100 mg/L as Zn, to 1000 mL with
deionized water in a Class A 1000-mL volumetric flask. Prepare
this solution daily. Use this solution as the sample and perform
the zinc procedure as described above.
Precision
In a single laboratory, using a standard solution of 1.50 mg/L Zn
and two representative lots of reagent with the instrument, a
single operator obtained a standard deviation of ±0.02 mg/L Zn.
Estimated Detection Limit (EDL)
The EDL for program 97 is 0.02 mg/L Zn. For more information
on derivation and use of Hach's estimated detection limit, see
Section 1.
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