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GENERAL DIGESDAHL DIGESTION,
Metals Procedure
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Note: If analyzing aliquots smaller than 0.5 mL, pH adjustment is unnecessary.
1. Pipet the appropriate analysis volume into the appropriate mixing
graduate cylinder. See Sample and Analysis Volume Tables following
the specific digestion for liquids, solids or oils to determine the
analysis volume.
Note: Some methods require pipetting into a volumetric flask or a regular
graduate cylinder.
2. Dilute to about 20 mL with deionized water.
3. Add one drop of 2,4 Dinitrophenol Indicator Solution.
4. Add one drop of 8 N Potassium Hydroxide (KOH) Standard Solution,
swirling between each addition, until the first flash of yellow appears
(pH 3). If analyzing for potassium, use 5 N sodium hydroxide instead.
Do not use a pH meter if analyzing for potassium or silver.
5. Add one drop of 1 N KOH. Stopper the cylinder and invert several
times to mix. If analyzing for potassium, use 1 N sodium hydroxide
instead.
Note: Use pH paper to insure the pH is 3. If it is higher than 4, do not readjust
with acid; start over with a fresh aliquot.
6. Continue to add 1 N KOH in this manner until the first permanent
yellow color appears (pH 3.5-4.0).
Note: High iron content will cause precipitation (brown cloud) which will co-
precipitate other metals. Repeat this procedure with a smaller aliquot volume.
7. Add deionized water to the volume indicated in the colorimetric
procedure for the parameter you are analyzing. Fill a second graduated
mixing cylinder to the same volume with deionized water.
8. Continue with the colorimetric procedure for the parameter you
are analyzing.
continued
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