Labconco Two and Six-Place (Macro) Kjeldahl Distillation Systems 2125603 - Manual (Page 20)

Appendix C: Total Kjeldahl Nitrogen Method
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distillate receiving vessels, such as 250 ml Erlenmeyer flasks. The
equipment is available as a combined system, featuring both
digestion and distillation apparatus. The preheated burners should
bring 250 ml of water at 25° C to a rolling boil with in 5 minutes.
Condensation columns should cool distillate to 75° F (23.8°C).
3.3. Titration Class A Burets, 25-50 ml. for dispensing Standardized Acid
solution.
3.4. Analytical balance, sensitive to 0.0001grams.
4. Reagents
4.1. Purified water, nitrogen free.
4.2. Sulfuric acid, H
2
SO
4
concentrated, Specific Gravity of 1.84. (95-
98%, nitrogen free)
4.3. Copper sulfate, CuSO
4,
anhydrous. Nitrogen free. (catalyst)
4.4. Potassium sulfate, K
2
SO
4
. Nitrogen free. (boiling point elevator)
4.5. Sodium hydroxide NaOH, reagent grade Specific Gravity 1.3,
nitrogen free. 45% solution, (dissolve pellets, 450g NaOH in distilled
water and dilute to 1 liter).
4.6. Boiling stones, Antibumping agent, aluminum oxide stones, (Hengar
granules).
4.7. Mixed color indicator: 0.75 g of Methyl Red and 0.5 g Methylene
Blue in 300 ml of 95% ethanol. (Prepare fresh every 30 days)
4.8. Boric acid H
3
BO
3,
saturated solution, (dissolve granules, 40 g boric
acid, in purified water and dilute to 1 liter). Add 3 ml of the mixed
color indicator solution to the 1 liter of boric acid solution.
4.9. Sulfuric acid, H
2
SO
4,
standardized solution: (0.02 N) Prepare a stock
solution of approximately 0.1 N sulfuric acid by diluting 3 ml of conc.
H
2
SO
4
(sp. Gr. 1.84) to 1 liter with CO
2
-free distilled water. Dilute 200
ml of this solution to 1 liter with CO
2
-free distilled water.
Alternatively, premade standardized acid and base solutions with a
certified specification ranges are commercially available through most
Scientific Equipment Dealers. If the Sulfuric Acids standard solutions
are prepared, their normality must be determined analytically.
5. Digestion
5.1. The distillation condensation columns should be cleaned before use
by distilling a 1:1 mixture of distilled water and sodium hydroxide
until the distillate is ammonia-free. Then repeat this cleaning
procedure if the apparatus is out of service long enough to
accumulate ammonia (> 4 hours).
5.2. Digestion
5.2.1. Place a homogenous measured sample (0.250 – 1.000
gram) into a 500 - 800 ml Kjeldahl flask. Weigh or
measure the sample to the nearest 0.1 mg/ml.
5.2.2. Add to each flask:
20 ml sulfuric acid, 0.04 g CuSO
4,
15g K
2
SO
4
potassium
sulfate, 8-10 boiling stones. Place flask on the digestion
burner in inclined position (promotes acid refluxing), with