Hach DR2400 - Page 284

Fluoride
Fluoride
Page 4 of 6
Fluoride_AccuVac_Other_SPD_Eng_Ody.fm
3. Turn on the water and maintain a steady flow through the condenser.
4. Measure 100 mL of sample into the distillation flask using a 100-mL
graduated cylinder. Add a magnetic stir bar and 5 glass beads.
5. For proof of accuracy, use a 1.0-mg/L Fluoride Standard Solution in place of
the sample.
6. Turn the stirrer power switch on. Turn the stir control to 5.
7. Using a 250-mL graduated cylinder, carefully add 150 mL of StillVer®
Distillation Solution into the flask. (StillVer Distillation Solution is a 2:1
mixture of concentrated sulfuric acid and water.)
Note: When distilling samples with high amounts of chloride, add 5 mg of Silver Sulfate
(Cat. No. 334-14) to the sample for every mg/L of chloride in the sample.
8. With the thermometer in place, turn the heat control to 10. The yellow pilot
lamp indicates the heater is on.
9. When the temperature reaches 180 °C or when 100 mL of distillate has been
collected, turn the still off (requires about 1 hour).
10. Dilute the distillate to a volume of 100 mL, if necessary. The distillate may
now be analyzed by the SPADNS or the fluoride ion-selective electrode
method.
Sample Collection, Storage and Preservation
Samples may be stored in glass or plastic bottles for at least seven days when
cooled to 4 °C (39 °F) or lower. Warm samples to room temperature before
analysis.
Accuracy Check
Standard Solution Method
A variety of standard solutions covering the entire range of the test is available
from Hach. Use these in place of sample to verify technique.
Minor variations between lots of reagent become measurable above 1.5 mg/L.
While results in this region are usable for most purposes, better accuracy may be
obtained by diluting a fresh sample 1:1 with deionized water and retesting.
Multiply the result by 2.
To adjust the calibration curve using the reading obtained with a Fluoride
Standard Solution:
1. Touch
Options on the current program menu. Touch Standard Adjust: Off.
2. Touch
On. Touch Adjust to accept the displayed concentration (the value
depends on the selected units). If an alternate concentration is used, touch the
number in the box to enter the actual concentration, then touch
OK. Touch
Adjust.
See Section 3.2.4 Adjusting the Standard Curve on page 40 for more information.