Hach DR/850, Procedures Manual, Page: 54 / 460

54
SAMPLE PRETREATMENT
, continued
EPA Vigorous Digestion with Hot Plate for Metals Analysis Only
A vigorous digestion can be followed to ensure all organo-metallic bonds
are broken.
1. Acidify the entire sample with redistilled 1:1 Nitric Acid Solution to a
pH of less than two. Do not filter the sample before digestion.
2. Transfer an appropriate sample volume (see Table 8 ) into a beaker
and add 3 mL of concentrated redistilled nitric acid.
3. Place the beaker on a hot plate and evaporate to near dryness, making
certain the sample does not boil.
4. Cool the beaker and add another 3 mL of the concentrated redistilled
nitric acid.
5. Cover the beaker with a watch glass and return it to the hot plate.
Increase the temperature of the hot plate so that a gentle reflux occurs.
Add additional acid, if necessary, until the digestion is complete
(generally indicated when the digestate is light in color or does not
change color or appearance with continued refluxing).
6. Again, evaporate to near dryness (do not bake) and cool the beaker. If
any residue or precipitate results from the evaporation, add redistilled
1:1 hydrochloric acid (5 mL per 100 mL of final volume). See
Table 8 .
7. Warm the beaker. Add 5 mL of 5.0 N sodium hydroxide and
quantitatively transfer the sample with deionized water to a
volumetric flask. See
Table 8 below for the suggested final volume.
8. Adjust the sample to pH 4 by drop-wise addition of 5.0 N Sodium
Hydroxide Standard Solution; mix thoroughly and check the pH after
each addition. Dilute to volume with deionized water. Multiply the
result by the correction factor in
Table 8 . A reagent blank also should
be carried through the digestion and measurement procedures.
Table 8 Vigorous Digestion Volumes
Expected Metal
Concentration
Suggested Sample
Vol. for Digestion
Suggested Volume
of 1:1 HCl
Suggested Final
Volume After
Digestion
Correction Factor
1 mg/L 50 mL 10 mL 200 mL 4
10 mg/L 5 mL 10 mL 200 mL 40
100 mg/L 1 mL 25 mL 500 mL 500