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Laboratory Practices
2.3.1 EPA Mild Digestion with Hot Plate for Metals Analysis Only
2.3.2 EPA Vigorous Digestion with Hot Plate for Metals Analysis Only
Laboratory Practices
Page 16
The Hach Digesdahl system is a process that yields a digest suitable for the
determination of metals, total phosphorus and total Kjeldahl nitrogen (TKN). It
is rapid, convenient, and the method of choice.
For USEPA reporting purposes, USEPA-approved digestions are required.
USEPA presents two digestions (mild and vigorous) for metals analysis. These
are much more inconvenient and time consuming compared to the Hach
Digesdahl system. Other digestion procedures are required for mercury, arsenic,
phosphorus and TKN.
See Section 4 Sample Pretreatment by Digestion for more on sample digestion.
1. Acidify the entire sample at the time of collection with concentrated nitric
acid by adding 5 mL of acid per liter (or quart) of sample.
2. Transfer 100 mL of well-mixed sample to a beaker or flask. Add 5 mL of
distilled 1:1 hydrochloric acid (HCl).
3. Heat using a steam bath or hot plate until the volume has been reduced to
15–20 mL. Make certain the sample does not boil.
4. After this treatment, the sample may be filtered to remove any insoluble
material.
5. Adjust the digested 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.
6. Quantitatively transfer the sample with deionized water to a 100-mL
volumetric flask and dilute to volume with deionized water. Continue with
the procedure. This mild digestion may not suffice for all sample types. A
reagent blank also should be carried through the digestion and measurement
procedures.
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 3) 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 3.
5940022LabPrac.fm
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