24
SECTION 2, continued
2.4.1.3 Preparing StablCal Stabilized Standards in Sealed Vials
Sealed vials that have been sitting undisturbed for longer than a month
must be shaken to break the condensed suspension into its original
particle size. Start at step 1 for these standards. If the standards are used
on at least a weekly interval, start at step 3.
When using <0.1-FNU* StablCal Standards do not shake or invert.
1. Shake the standard vigorously for 2–3 minutes to suspend particles.
2. Allow the standard to stand undisturbed for 5 minutes.
3. Gently invert the vial of StablCal 5 to 7 times.
4. Prepare the vial for measurement using traditional preparation
techniques. This usually consists of oiling the vial (see
Section 3.1.1 on page 35) and marking the vial to maintain the same
orientation in the sample cell compartment (see Section 3.2.2 on
page 39). This step will eliminate any optical variations in the
sample vial.
5. Let the vial stand for one minute. The standard is now ready for use
in the calibration procedure, Section 2.4.3 on page 27.
2.4.2 Formazin Primary Standards
Perform the procedure in Section 2.4.2.1 on page 24 to prepare a
4000-FNU standard, or order a 4000-FNU stock solution
(Cat. No. 2461-49). Prepare the dilutions from the 4000-FNU stock
solution by following the instructions in Section 2.4.2.4 on page 26.
2.4.2.1 Preparing Formazin Stock Solution
Dilute formazin standard solutions from a 4000 FNU stock solution
equivalent to Hach Cat. No. 2461-49. The prepared stock solution is
stable for up to one year when properly prepared. An alternative to
purchasing the 4000 FNU stock solution is preparing a stock solution
as follows:
1. Dissolve 5.000 grams of reagent grade hydrazine sulfate
(N
2
H
4
•H
2
SO
4
) in 400 mL of distilled water.
2. Dissolve 50.000 grams of pure hexamethylenetetramine in 400 mL
of distilled water.
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