Division 6 PROCEDURE

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1. 6.1To the Kjeldahl flask (5.1.) add boiling aid (5.2.) (eg. three glass beads), 15 g of potassium sulphate (4.1.), 1,0 ml of copper sulphate solution (4.2.), approximately 5 g of milk sample (weighed to the nearest 0,001 g) and 25 ml of sulphuric acid (4.3.). Use the acid to wash down any copper sulphate solution, potassium sulphate or milk on the neck of the flask, and gently mix the contents of the flask.
2. 6.2Heat each Kjeldahl flask on the digestion apparatus (5.5.), very gently at first so that all the black froth stays within the bulb. When the initial frothing has ceased and copious white vapour appears, boil vigorously (acid vapour will condense half-way up the neck of the flask) until no black particles remain and until the digest becomes clear pale blue-green in colour. Then boil gently for at least 1,5 hours. Note the following requirements:
3. 6.3When the Kjeldahl flasks are cool, add 300 ml of water (see note) to each so as to wash carefully down the neck of the flask, and mix the contents thoroughly ensuring that the crystals which have separated out are dissolved. Add some boiling aid (5.2.) to ensure uniform boiling. Then to each flask, add 70 ml of sodium hydroxide solution (4.4.) (see note) by gently pouring the solution down the inclined neck of the flask to form a bottom layer in the bulb; do not wet the top of the neck with the sodium hydroxide solution.
4. 6.4Immediately connect each Kjeldahl flask to a distillation apparatus (5.6.). Ensure that the tip of the condenser outlet-tube is immersed in 50 ml of boric acid solution (4.5.) together with 0,20 ml (5 - 6 drops) of indicator solution (4.6.) all contained in a conical flask (5.8.). Swirl the contents of each Kjeldahl flask to mix thoroughly and boil, but gently at first to prevent excessive frothing. When 100 to 125 ml of distillate have been collected, lower each conical flask until the tip of the condenser outlet-tube is approximately 40 mm above the 200 ml mark. Continue each distillation until irregular boiling (‘bumping’) starts and then immediately stop the heating. Disconnect each Kjeldahl flask and rinse the tip of each condenser outlet-tube with a little water, collecting the rinsings in the conical flask. Note the following requirements:
5. 6.5Titrate each distillate with standard volumetric solution (4.7.) until the pH is 4,6 ± 0,1, using a pH meter and if desired an automatic burette. Addition of an indicator helps to check whether the titration is proceeding correctly. Take each burette reading to the nearest 0.01 ml with the aid of a magnifying lens (5.10.) avoiding errors of parallax.
6. 6.6Carry out a blank test according to 6.1. to 6.5. inclusive, taking 5 ml of distilled water together with about 0,1 g of sucrose (4.8.) through the procedure instead of the milk sample.
7. 6.7Division 6.7
8. 6.7.1Check that no loss of nitrogen occurs as a result of excessive heat or mechanical leaks during distillation, by using a test portion of 0,15 g of ammonium oxalate or sulphate (4.9.) weighed to the nearest 0,001 g together with 0,1 g of sucrose (4.8.).
9. 6.7.2Division 6.7.2