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SAMPLE PRETREATMENT
Digestion
Digestion, required in several procedures, refers to the use of acid and
heat to break down a substance into components that can be analyzed.
This section has three different digestion procedures.
The Hach Digesdahl system is an absolute process that yields a digest
suitable for the determination of metals, total phosphorus and total kjeldahl
nitrogen (TKN). It is rapid and convenient. It is the method of choice.
For EPA reporting purposes, EPA-approved digestions are required.
EPA presents two digestions (mild and vigorous) for metals analysis.
These are much more inconvenient and time consuming compared to the
Hach Digesdahl system. Other tedious digestion procedures are
required for phosphorus and TKN.
Hach Digesdahl Digestion
In this procedure (pages 38-42) the sample is oxidized by a mixture of
sulfuric acid and hydrogen peroxide. Digestion of a dry sample requires
less than ten minutes, while liquid samples require about 1 minute/mL.
The digestion is done in a special flat-bottomed 100-mL volumetric
flask. Aliquots, sample portions, are taken for analyses using
colorimetric method, see Procedures (Section II).
EPA Mild Digestion with Hot Plate
For Metals Analysis Only
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 (HCI).
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.
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