9b.EXTRACTION OF TOTAL PHOSPHORUS BY MINERAL ACIDS
SCOPE
1. This method is for the determination of phosphorus soluble in 2% formic acid.
FIELD OF APPLICATION
2. Applicable only to soft natural phosphate.
PRINCIPLE
3. To differentiate between hard natural phosphates and soft natural phosphates, phosphorus soluble in formic acid is extracted under specified conditions.
REAGENTS
4.—4.1 Formic acid, 2% (20 g per litre): dilute 82 ml formic acid (concentration 98-100%; ρ = 1.22 g/ml) to 5 litres with distilled water.
APPARATUS
5.—5.1 500 ml graduated flask with a wide neck (eg Stohmann).
5.2 Rotary shaker, 35-40 turns per minute.
PREPARATION OF SAMPLE
6. See Method 1.
PROCEDURE
Extraction
7.—7.1 Weigh to the nearest 0.001 g, 5 g of the prepared sample and place it in a dry 500 ml graduated flask (5.1). While continuously rotating the flask by hand, add the formic acid (4.1) (at 20 ± 1°C) until it is approximately 1 cm below the graduation mark. Then make up to the volume. Close the flask with a rubber stopper and shake for 30 minutes (5.2). Filter the solution through a dry fluted filter, into a dry receiver, discarding the first portion of the filtrate.
Determination
7.2 Determine the phosphorus using Method 10 on an aliquot portion of the clear filtrate.