Hach Digesdahl 23130-20, Instrument Manual

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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.
TKN, Colorimetric Methods
Consult the spectrophotometer or colorimeter procedure to complete the
TKN analysis. The following is only a guide to use if a procedure is
not available.
1. Pipet an appropriate analysis volume into a graduated mixing cylinder.
2. Add one drop of TKN Indicator.
3. Add 8 N KOH Standard Solution, one drop at a time, swirling between
each addition, until the first flash of pale blue appears (pH 3).
4. Add one drop of 1 N KOH. Stopper the cylinder and invert several
times to mix.
Note: View the cylinder from the top against a white background. Compare the
cylinder against a second cylinder filled to the same volume with deionized water.
5. Continue to add 1 N KOH in this manner until the first permanent blue
color appears.
DIGESTION PROCEDURE FOR OILS,
continued
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