The Fertilisers (Sampling and Analysis) Regulations 1991

9g. EXTRACTION OF PHOSPHORUS BY JOULIE'S ALKALINE AMMONIUM CITRATE

SCOPE

1.  This method is for the determination of phosphorus soluble in Joulie’s alkaline ammonium citrate.

FIELD OF APPLICATION

2.  All the straight and compound phosphate fertilisers, in which the phosphate occurs in an aluminocalcic form.

PRINCIPLE

3.  Extraction by shaking vigorously with an alkaline solution of ammonium citrate of defined specification (and where appropriate in the presence of oxine), at about 20°C.

REAGENTS

Joulie’s alkaline solution of ammonium citrate:

4.—4.1 this solution contains 400 g of citric acid monohydrate and 153 g of NH₃ per litre. Its free ammonia content is approximately 55 g per litre. It can be prepared by one of the methods described below:

4.1.1 In a litre graduated flask, dissolve 400 g of citric acid monohydrate in approximately 600 ml ammonia solution (d = 0.925 g/ml), containing 200 g NH₃ per litre; this may be prepared by diluting 760 ml ammonia solution (d = 0.880 g/ml) from a freshly opened bottle with water to 1 litre. The citric acid is added successively in quantities of 50 to 80 g maintaining the temperature below 50°C. Make up the volume to 1 litre with ammonia.

4.1.2 In a 1 litre graduated flask, dissolve 400 g of diammonium citrate in 300 ml of water. Add 440 ml of ammonia solution (d = 0.925 g/ml) (see 4.1.1 above). Make up the volume to 1 litre with water.

Verification of the total ammonia content:

take a 10 ml sample of the citrate solution and place it in a 250 ml flask. Make up to the volume with distilled water. Determine the ammoniacal nitrogen content on 25 ml of this solution according to Method 2. In these conditions the reagent is considered to be correct when the volume of 0.5 N sulphuric acid consumed is between 17.7 and 18.0 ml (1 ml 0.5 N H₂SO₄-0.008516 g NH₃). If this is not so add 4.25 ml of ammonia (d = 0.925 g/ml) per 0.1 ml below the 18 ml indicated above.

4.2 8-Hydroxyquinoline (oxine), powdered.

APPARATUS

5.—5.1 Rotary shaker, 35-40 turns per minute.

PREPARATION OF THE SAMPLE

6.  See Method 1.

PROCEDURE

Extraction

7.—7.1 Weigh to the nearest 0.0005 g, 1 g of the prepared sample and place in a small mortar (glass or porcelain). Add about ten drops of ammonium citrate solution (4.1) to moisten it and break it up very carefully with a pestle. Add 20 ml ammonium citrate solution (4.1), mix to a paste and leave it to settle for about 1 minute.

• Decant the liquid in to a 500 ml graduated flask straining off particles which might have escaped the preceding moist disintegration. 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 pestle and mortar above 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.

Determination

7.2 Determine the phosphorus according to Method 10 on an aliquot part of the clear filtrate.

APPENDIX

8.  The use of oxine makes it possible to apply this method to fertilisers containing magnesium. This is recommended when the ratio of magnesium and phosphoric anhydride contents is higher than 0.03 (Mg/P₂O₅>0.03). If this is the case, add 3 g of oxine to the moistened sample for analysis. The use of oxine in the absence of magnesium is not, moreover, likely to interfere subsequently with the determination. In the known absence of magnesium, oxine may be omitted.