Paragraph 7 PROCEDURE

Weigh to the nearest 0.0005 g, 1 g of the prepared sample and place in a small glass or porcelain mortar. Add about 10 drops of ammonium citrate solution (4.1) to moisten it and then break it up very carefully with a pestle. Add 20 ml ammonium citrate solution (4.1), mix to a paste and leave to settle for about 1 minute.

• Decant the liquid into a 500 ml graduated flask (5.2). Add 20 ml ammonium citrate solution (4.1) to the residue, grind as above and decant the liquid into the graduated flask. Repeat the process four times, so that by the end of the fifth time all the product can be poured into the flask. The total quantity of ammonium citrate solution used for these processes must be approximately 100 ml.

• Rinse the residue from the pestle and mortar into the graduated flask with 40 ml of distilled water.

• Stopper the flask and shake for three hours on the rotary shaker (5.1).

• Leave the flask standing for fifteen to sixteen hours and then shake it again under the same conditions for three hours. The temperature during the whole process should be kept at 20° ± 2° C.

• Make up to volume with distilled water and mix. Filter through a dry filter, discard the first portion of the filtrate and collect the clear filtrate in a dry flask.

Determine the phosphorus using Method 10 on an aliquot portion of the clear filtrate.