_Preparation of sample._—For the determination of organic nitrogen and
fats dry approximately 50 to 75 grams of the sludge or mud in a
porcelain dish first on the water bath and finally in the hot-water oven
until all the moisture has disappeared. Grind the dry material to a fine
powder and keep it in a glass-stoppered bottle.
_Reagents._—1. Sulfuric acid. Concentrated, nitrogen-free.
2. Copper sulfate solution. Ten per cent.
3. Potassium permanganate. Crystals.
_First procedure._—Weigh accurately 0.5 gram of dried sludge or 5.0
grams of dried mud and put it into a 500 cc. Kjeldahl flask. Digest it
with 20 cc. of sulfuric acid, or more if necessary, and 1 cc. of copper
sulfate solution to assist the oxidation. Boil for several hours until
the liquid becomes colorless or slightly yellow. Oxidize the residue
with 0.5 gram of potassium permanganate, and follow the “Procedure for
Sewage” (pp. 21–22).
_Second procedure._—The following method is convenient for routine work
at sewage disposal plants. After digestion as described in the first
procedure, cool, transfer to a glass-stoppered 100 cc. flask, dilute
with distilled water to 100 cc., and mix well. Transfer 50 cc. with a
pipette into another 100 cc. volumetric flask, and make this portion
alkaline with 50 per cent sodium hydroxide, testing a drop of the liquid
on a porcelain plate with phenolphthalein to insure neutralization. The
formation of a floc usually indicates that neutralization is complete.
Dilute the solution to 100 cc., pour it into a small glass-stoppered
bottle and permit it to stand until the next day. Nesslerize an aliquot
portion of the clear supernatant liquid, and calculate the percentage of
nitrogen in the material.