The Fertilisers (Sampling and Analysis) Regulations 1996

9e. EXTRACTION OF PHOSPHORUS BY ALKALINE AMMONIUM CITRATE (PETERMANN'S METHOD) AT 65°C

1.    SCOPE

This method is for the determination of phosphorus soluble in alkaline ammonium citrate.

2.    FIELD OF APPLICATION

Applicable only to precipitated dihydrated dicalcium phosphate (CaHPO4.2H2O).

3.    PRINCIPLE

Extraction of phosphorus at a temperature of 65°C with an alkaline solution of ammonium citrate (Petermann) under specified conditions.

4.    REAGENTS

Petermann’s solution

4.1  Characteristics:

Citric acid monohydrate, 173 g per litre.

Ammonia, 42 g per litre ammoniacal nitrogen.

pH, between 9.4 and 9.7.

Preparation from diammonium citrate:

Dissolve 941 g diammonium citrate in about 3,500 ml water in a 5 litre graduated flask. Stand the flask in a bath of running water, mix and cool. Add, in small amounts, 430 ml of ammonia solution (p=0.880 g/ml), from a freshly opened bottle (or an equivalent amount of diluted ammonia, for example if p=0.906 g/ml then 502 ml are required). Adjust the temperature to 20°C, make up to volume with water and mix.

Preparation from citric acid and ammonia:

Dissolve 865 g citric acid monohydrate in about 2,500 ml distilled water in a container of about 5 litres capacity. Place the container in an ice bath and add in small amounts, shaking continually, 966 ml of ammonia solution (p=0.880 g/ml), from a freshly opened bottle (or an equivalent amount of diluted ammonia, for example if p=0.906 g/ml, then, 1,114 ml are required) . Adjust the temperature to 20°C, transfer to a 5 litre graduated flask, make up to the mark with distilled water and mix.

Check the ammoniacal nitrogen content as follows:

Transfer 25 ml of the solution into a 250 ml graduated flask, make up to volume with distilled water and mix. Determine the ammoniacal nitrogen content on 25 ml of this solution using Method 2. If the solution is correct, 15 ml 0.25 M H2SO4 are required — Calculate the concentration of ammoniacal nitrogen in the reagent solution (1 ml 0.25 M H2SO4=0.007g nitrogen).

If the concentration of ammoniacal nitrogen is greater than 42 g/litre, ammonia can be expelled by a stream of inert gas or by moderate heating to bring back the pH to 9.7. Carry out a second determination.

If the concentration of ammoniacal nitrogen is less than 42 g/litre, calculate the volume of ammonia solution required to achieve this level (1 ml ammonia solution, p=0.880 g/ml contains approximately 0.22g ammoniacal nitrogen). For each ml of ammonia solution required add 0.173 g of citric acid.

Whenever corrections are made to this reagent solution, it is imperative that the final concentration of both the citric acid and ammoniacal nitrogen are as specified.

5.    APPARATUS

Water bath which can be maintained at a temperature of 65 +1 C.

500 ml graduated flask with a wide neck (eg Stohmann).

6.    PREPARATION OF SAMPLE

See Method 1.

7.    PROCEDURE

Extraction

Weigh to the nearest 0.001g, 1 g of the prepared sample and transfer to the 500 ml graduated flask (5.2). Add 200 ml alkaline ammonium citrate solution (4.1). Stopper the flask and shake vigorously by hand to avoid the formation of lumps and to prevent any adherence of the sample to the sides.

Place the flask in the water bath at 65 C and shake every 5 minutes during the first half an hour. After each shaking, raise the stopper to equilibrate the pressure. The level of water in the water bath should be above the level of solution in the flask. Allow the flask to remain in the water bath a further hour at 65 C and shake every ten minutes. Remove the flask, cool to a temperature of about 20 C, make up to volume of 500 ml with water. Mix and filter through a dry fluted filter paper, rejecting the first portion of filtrate.

Determination

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