Hach DR2400 - Page 702

Selenium
Selenium
Page 4 of 8
Selenium_None_Other_DAB_Eng_Ody.fm
Interferences
Note: There are no positive inorganic interferences with this method.
Sample Collection, Storage, and Preservation
Collect samples in clean glass or plastic containers. Adjust the pH to 2 or less
with Nitric Acid (Cat. No. 152-49) (about 1.5 mL per liter). Preserved samples
can be stored for up to six months at room temperature. Correct the test result for
volume additions; see Section 3.1.3 Correcting for Volume Additions on page 29.
Distillation
Always perform this procedure under a fume hood! This distillation
involves the use of a strong acid and oxidizer at high temperatures. To avoid
personal injury, observe all laboratory safety precautions when operating the
distilling apparatus.
1. Measure 500 mL of sample into a 1000-mL beaker.
2. Add 1 mL of Methyl Orange Indicator Solution. Stir with a glass rod.
3. Use a dropper to add 0.1 N Hydrochloric Acid Standard Solution dropwise
until the solution becomes pink. Then add an additional 2 mL.
4. Use a pipet to add 5.0 mL Calcium Chloride Solution. Mix well.
5. Use a dropper to add 1-g/L Potassium Permanganate Standard Solution
drop-wise until the solution is purple.
6. Place the beaker on a hot plate. Evaporate the solution to approximately
250 mL. Periodically add 1-g/L Potassium Permanganate Solution to keep the
solution purple.
7. Any precipitate formed at this step is manganese dioxide, and may be
ignored.
8. Cool the solution. While cooling, set up the distillation apparatus for the
general purpose distillation as shown in the Hach Distillation Manual.
9. Pour the treated sample solution into the distillation flask. Add a stirring bar
to the flask.
10. Pipet 5.0 mL of 0.1 N Sodium Hydroxide Standard Solution into the flask.
Turn the stirrer power switch to ON. Set the stir control to 5.
11. Turn on the water and adjust so a constant flow is maintained through the
condenser. Set the heat control to 10.
12. When only a few milliliters are left in the distillation flask, turn the power
switch off. The distillate in the Erlenmeyer flask may be discarded.
13. Perform this step under a hood. When the flask has cooled, add 50 mL
of 19.2 N Sulfuric Acid Standard Solution to the flask. Add the contents of one
Potassium Bromide Powder Pillow to the flask.
Interfering Substance Interference Levels and Treatments
Ferric iron Up to 2.5 mg/L. Distill sample to eliminate interference.
Manganese Will not interfere.
Strong oxidizing agents (i.e.,
iodine, bromine, or chlorine)
Can react with the indicator to give low results. Distill sample to eliminate interference.