40
SECTION 3, continued
3.6.2 Formazin Primary Standards
Perform the procedure in Section 3.6.2.1 to prepare a 4000-NTU
standard. Alternately, order a 4000-NTU stock solution from Hach by
specifying Cat. 2461-49. Prepare the dilutions from the 4000-NTU
stock solution by following the instructions in Section 3.6.2.4.
3.6.2.1 Preparing Formazin Stock Solution
Dilute formazin standard solutions from a 4000 NTU 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 NTU 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.
3. Pour the two solutions into a 1000-mL volumetric flask and dilute
to the mark with distilled water.
4. Let the solution stand for 48 hours at 25 °C (77 °F) to develop the
4000-NTU stock suspension. The standing temperature is critical
for correct formation of formazin polymers.
5. Mix the 4000 NTU suspension for at least ten minutes before use.
Then it can be diluted with distilled or demineralized water to
achieve a solution of the desired NTU value.
Instead of diluting a formazin stock solution, StablCal Stabilized
Formazin Standards may be used. Order the StablCal Calibration Set
for the 2100P Turbidimeter, Cat.No. 26594-00 (500-mL bottles),
Cat. No. 26594-10 (100 mL bottles), or Cat. No. 26594-05 (sealed
vials). (See OPTIONAL ACCESSORIES AND REAGENTS on page 74.)
3.6.2.2 Correcting for Turbidity of Dilution Water
The 2100P Turbidimeter automatically compensates for turbidity
contributed by dilution water when calculating the true value of the
lowest formazin standard. Use high quality distilled or deionized water
less than 0.5 NTU. The instrument will display E 1 after calibration if
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