Hach DR/850, Procedures Manual, Page: 53 / 460

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SECTION 2 SAMPLE PRETREATMENT
Digestion
Several procedures require sample digestion. Digestion uses chemicals
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 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 digesting
most samples analyzed by Hach methods.
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 phosphorus and TKN.
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 (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.