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Kjeldahl Nitrogen Determination Apparatus

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OPERATION OF THE GERHARDT KJELDAHL
N - Determination Apparatus


1. Sample Digestion

  1. Weigh required amount of sample onto a clean dry watch glass. Use a rough balance for this purpose.

  2. Weigh sample and watch glass to the nearest mg on an analytical balance.

  3. Transfer the sample into a clean dry digestion tube. and reweigh the watch glass. Find the mass of sample trassfered.

  4. Add to each sample one catalyst tablet and 10 ml conc. sulphuric acid. (Care! Use dispensette set up in fume hood.)

  5. Place sample tubes into digestion heating block, put vapor removal manifold in place, and turn on water vacuum pump.

  6. Set heater to 400° and allow to digest until sample completely dissolves to give a clear light green to light blue solution. (1 - 2 hours @ 400°) Shut off heater.

  7. Lift sample rack and vapor removal manifold clear off the heating block, suspend on holding slot, and allow to cool to 35 - 40°C.
    Switch off heating block, and turn off water vacuum pump when samples are cooled.

2. Ammonia Distillation and Titration

  1. Place respective feed tubes into their reagent resevoirs: 0.1M HCl; 40% NaOH; 1% w/v Boric acid; and Distilled water supply for steam distillation. Clamp or wedge feed tubes to avoid their jumping out of the resevoirs during reagent feed.

  2. Remove and empty titration bowl by carefully turning the bowl through 90% and moving vertically downwards. Rinse electrodes and chamber with distilled water and replace.

  3. Switch on instrument and turn on water supply (from mains) to condenser.

  4. Reply "yes" to prompt for steam distillation.

  5. Reply "yes" to prompt for burette flush. Allow syringe to fill and empty twice and press STOP when syringe is filled for a third time.

  6. Press Boric acid switch and hold down for 5 - 6 seconds. Titration chamber should then be charged (filled) with boric acid.

  7. Depress spring-loaded lever on LHS of instrument while holding the sample tube in the steam distillation port. Remove the sample tube carefully, ensuring that the steam inlet tube is not bent when the lever is released.

    Replace with a tube containing a sample and ensure that it is properly seated and sealed when the lever is released.

  8. Calibration of electrode.

  • Press "cal". Select pH by pressing"no" until pH flashes, then press"yes"

  • Select "calbr". Instrument will now ask for pH buffer 7.0. Place buffer in a ~ 20 ml glass container vial to immerse sensing electrode. Avoid contacting the stirring paddle during this operation. Press "run".

  • After reading is stabilized, press "Ent". Instrument then asks for pH4.0 buffer. Rinse electrode with distilled water and immerse in pH 4.0 buffer in a vial. Press "run" and wait for reading to stabilize. The readout will indicate accecptability of calibration, and the slope of the electrode response.

3. Entering molarity of titrant HCl.

  1. Press "cal" and select "titrant" then "keyboard".

  2. Enter value of titrant and press "Ent"

4. Determination of % recovery

  1. Press "cal" and select "test"

  2. Weigh and transfer accurately an ammonium salt of known purity and composition e.g. ( 0.05g NH4Cl or 0.01g (NH4)2SO4) into a sample tube, which is then placed in the steam distillation port. Enter theoretical % N (26.17% for NH4Cl and 21.19% for (NH4)2SO4).

  3. Press "run". Steam distillation will start and end, dependent on program used. (Use program #6)

  4. At the end of the distillation, the sample tube followed by the titration chamber, will be autumatically emptied, and the latter recharged (refilled).

  5. The % recovery of the sample will be displayed.

5. Determination of N-content of samples

  1. Place sample tube (with digested sample) in steam distillation port.

  2. Press "sample". Press #6 when program # prompt is displayed.

  3. Enter sample number (1, 2, etc....).

  4. Enter sample weight.

  5. Press "run". Distillation/titration ensues, at the end of which % N for sample is displayed. Record value displayed.

  6. Replace sample tube with second sample and continue as before.

6. Shutting down of instrument

  1. After all samples have been analyzed, carefully remove the feed tubes from their respective resevoirs, and cap each resevoir securely.

  2. Rinse the outsides of the feed tubes with distilled water, and then place them into a 500 ml Erlenmeyer flask filled with distilled water.

  3. Switch off the instrument for about 5 seconds, and then ON again. Follow steps 2 (b - f).

  4. Follow steps 5 (b - e), entering fictitious values for (d). Stop distillation/titrations after about 1 minute.

    Press "emty" to empty sample tube.

  5. Remove sample tube, and rinse steam inlet tube with distilled water. Replace sample tube in distillation port.

  6. Switch off instrument and turn off water supply to condenser.



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Signature: Dhanlal De Lloyd, Chem. Dept, The University of The West Indies, St. Augustine campus
The Republic of Trinidad and Tobago.
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