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Experiments Overview

Kjeldahl's determination of nitrogen

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Kjejdhal automatic Titrator with digester

Apparatus: Digestion tubes for digester, bottle-top dispensette, mortar and pestle.


Titration salt = NH4Cl.
Conc. H2SO4; conc. HCl.
Kjeldhal catalyst tablets.
Boric acid, HBO3, FW = 61.83
Sodium hydroxide, NaOH, FW = 40
Buffer tablets, pH4, pH 7.

Sample: Fertilizer (0.25g), or Mollusk (0.1g)

Solutions preparation:

1. 1% Boric acid. (dissolve 10g of salt in 1 liter distilled water)
2. 40% NaOH. (dissolve 400g in 1 liter distilled water)
3. 0.10M HCl. (use volumetric standard ampoule and dilute to req. vol.)
4. pH buffer 4 and 7 (use buffer tablets and make up to 100mls.)

Analytical notes:

1. Make up at least 2 liters of boric acid and NaOH solns. for each lab session.
2. Keep distilled water reservoir at least full during analysis.
3. Grind sample fine, put in bottle and label it.
4. Put one catalyst tablet into each digestion tube containing the weighed sample.
5. Add 8.0 mls. Conc. H2SO4 (use dispensette in fume hool) and leave to predigest overnight.
6. Digest in digester at 800 °C for 3 to 4 hrs. until sample colour becomes clear blue or green.
7. Make sure the water vacuum tap is turned ON, and never operate the digester when the water pressure is low (below 15 psi).

Operation of Instrument:

Follow User Guide: obtain help and advice from Demonstrator or Technician

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