1. This method is for the determination of phosphorus soluble in 2% citric acid (20 g per litre).
2. Only applicable to basic slag fertilisers in Group 2(a) of Section A and Groups 1, 2 and 4 of Section B of the Table in Schedule 1 of the Fertilisers Regulations 1990(1).
3. Extraction of phosphorus from the fertiliser with a 2% citric acid solution (20 g per litre) in given conditions.
4.—4.1 2% citric acid solution (20 g per litre), prepared from citrci acid and monohydrate.
Note: Verify the concentration of this citric acid solution by titrating 10 ml of the latter with a sodium hydroxide standard solution 0.1 N, using phenolpthalein as an indicator. If the solution is correct, the titre should be 28.55 ml.
5.—5.1 Rotary shaker: 35-40 turns per minute.
6. The analysis is carried out on the product as received after carefully mixing the original sample to ensure it is homogenous. See Method 1.
7.—7.1 Weigh to the nearest 0.001 g, 5 g of the prepared sample and plcae it in a dry flask with a sufficiently wide neck, with a capacity of 600 ml, allowing the liquid to be shaken thoroughly. Add 500 ml±1 ml of the citric acid solution (4.1) at 20±1°C. When adding the first mls of the reagent shake vigorously by hand to stop the formation of lumps and to prevent the substance sticking to the sides. Close the flask with a rubber stopper and shake it on the rotary shaker (5.1) for exactly 30 minutes at a temperature of 20±2°C.
Filter immedsiately through a dry fluted filter, into a dry glass receiver and discard the first 20 ml of the filtrate. Continue the filtering until a sufficient quantity of filtrate is obtained to carry out the phosphorus determination.
7.2 Determine the phosphorus according to Method 10 on an aliquot part of the clear filtrate.