25
SECTION 2, continued
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-FNU stock suspension. The standing temperature is critical
for correct formation of formazin polymers.
5. Mix the 4000 FNU 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 FNU value.
Instead of diluting a formazin stock solution, StablCal Stabilized
Formazin Standards may be used. (See OPTIONAL ACCESSORIES
AND REAGENTS on page 59.)
2.4.2.2 Correcting for Turbidity of Dilution Water
The 2100P ISO 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 FNU. The instrument will display “E 1” after calibration if
the dilution water turbidity is greater than 0.5 FNU. In this case, prepare
the water as directed below.
The value of the dilution water can be arbitrarily forced to zero using
the calibration procedure (see step 4 on page 31 ). This is not
recommended for most applications and, if done, should be done only if
the dilution water turbidity is less than 0.2 FNU.
2.4.2.3 Preparing Dilution Water
Note: Use the same dilution water for all dilutions and the sample blank.
Collect at least 1000 mL of high quality dilution water (distilled or
deionized water). The 2100P ISO Turbidimeter, as received from the
factory, is precalibrated and may be used to check the dilution water
turbidity. If the turbidity is greater than 0.5 FNU, filter the water with
the Sample Filtration and Degassing Kit (Cat. No. 43975-10) or
equivalent. When measuring low range turbidity, clean all glassware
with 1:1 hydrochloric acid and rinse several times with dilution water. If
the glassware is not used immediately, use stoppers to prevent
contamination from small particles.
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