Hach DR/700 Procedures Manual page 343

ALUMINUM,
continued
used in the following pretreatment are replaced, even if the new reagent
has a matching lot number. When the pretreated sample has been
analyzed, subtract the aluminum concentration of the reagent blank from
the sample results.
c) Measure 50 mL of sample into the 125-mL erlenmeyer flask using the
graduated cylinder. Use a small amount of demineralized water to rinse
the cylinder contents into the flask.
d) Add 4.0 mL of 5.25 N Sulfuric Acid Solution.
e) Use a combination hot plate/stirrer to stir and boil the sample for at
least 30 minutes. Add demineralized water as needed to maintain a
sample volume of 20-40 mL. Do not boil dry.
f) Cool the solution to near room temperature.
g) Add 2 drops of Bromphenol Blue Indicator Solution.
h) Add 1.5 mL of 12.0 N Potassium Hydroxide Standard Solution using
a calibrated plastic dropper. Swirl to mix. The solution color should be
yellow or green, but not purple. If the color is purple, begin with Step a
again using an additional 1 mL of Sulfuric Acid Solution in Step d.
i) While swirling the flask, add 1.0 N Potassium Hydroxide Solution, a
drop at a time, until the solution turns a dirty green color.
j) Pour the solution into the graduated cylinder. Rinse the flask contents
into the graduated cylinder with demineralized water to bring the total
volume to 50 mL.
k) Use this solution in Step 4 of the ECR method.
Fluoride interference can be corrected by using Table 2.
-
Example: Is the fluoride concentration is known to be 1.00 mg/L F and
the ECR method gives a DR/2000 reading of 0.060 mg/L aluminum,
what is the true mg/L aluminum concentration?
Answer: 0.183 mg/L
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