9a.EXTRACTION OF TOTAL PHOSPHORUS BY MINERAL ACIDS
SCOPE
1. This method is for the determination of phosphorus soluble in mineral acids.
FIELD OF APPLICATION
2. Subject to regulation 5(3), applicable only to the phosphatic fertilisers listed in Group 2(a) of Section A and Groups 1, 2 and 4 of Section B of the Table in Schedule 1 to the Fertilisers Regulations (Northern Ireland) 1992(1), and to phosphatic fertilisers listed in Groups 1(a), 1(b) and 2 of Section C of that Table which are not designated as “EEC fertiliser”.
PRINCIPLE
3. Extraction of the phosphorus in the fertiliser with a mixture of nitric acid and sulfuric acid.
REAGENTS
4.—4.1 Sulfuric acid (ρ = 1.84 g/ml).
4.2 Nitric acid (ρ = 1.40 g/ml).
APPARATUS
5.—5.1 A Kjeldahl flask, with a capacity of at least 500 ml, or a 250 ml round-bottomed flask with a glass tube forming a reflux condenser.
PREPARATION OF SAMPLE
6. See Method 1.
PROCEDURE
Extraction
7.—7.1 Weigh to the nearest 0.001 g, 2.5 g of the prepared sample and place it in a dry Kjeldahl flask. Add 15 ml water and swirl to suspend the substance. Add 20 ml nitric acid (4.2) and carefully add 30 ml sulfuric acid (4.1). When the initial violent reaction has ceased, slowly bring the contents of the flask to boiling and boil for 30 minutes. Allow to cool and then carefully add with mixing about 150 ml water and boil for 15 minutes.
Cool completely and transfer the liquid quantitatively to a 500 ml graduated flask. Make up to volume, mix and filter through a dry fluted filter, discarding the first portion of the filtrate.
Determination
7.2 Determine the phosphorus using Method 10 on an aliquot portion of the clear filtrate.
Note:
If the sample contains cellulose matter, the following procedure is suggested to avoid excessive frothing during digestion:
Weigh to the nearest 0.001 g, 2.5 g of the prepared sample and place it in a dry Kjeldahl flask. Add 30 ml sulfuric acid (4.1) and carefully boil until most of the organic matter has been destroyed. Allow to cool, add 15 ml water and 20 ml nitric acid (4.2); bring to the boil and continue boiling for 30 minutes. Continue as described in 7.1 from “Allow to cool and then …”.