Citric acid-insoluble P

ESTIMATION OF CITRIC ACID-INSOLUBLE P IN DICALCIUM

PHOSPHATE / ROCK PHOSPHATE

Aim 

         To estimate the citric acid – insoluble P content in the dicalcium phosphate / rock phosphate sample.

Principle

         The residue of the phosphate fertilizer remaining after leaching with distilled water for water-insoluble P estimation is extracted with ammonium citrate and the amount of P in the extract is estimated by the volumetric method.

Reagents

1.  1% ammonium citrate

2.  Conc. HNO3

3.  Dil. HNO3

4.  0.1619 N HNO3

5.  0.1619 N KOH

6.  20% ammonium molybdate

7.  Ammonium nitrate

8.  Ammonium hydroxide

9.  Phenolphthalein indicator

Procedure

         Dry the filter paper and the residue after extracting ammonium citrate soluble P2O5 and ignite it. Digest the ignited material in a conical flask using 10 to 15ml of 1:1 HCl and about 10ml of distilled water for an hour. Filter the contents collecting about 240mL of the filtrate in a 250mL volumetric flask . Use water for washing the ash , and make up the volume to 250mL.

Pipette out 25 ml of the extract into a 250 ml beaker and bring the solution to nitric acid medium by adding ammonium hydroxide till the solution becomes alkaline to litmus and then adding dilute nitric acid till the solution becomes acidic to litmus.  Add about 2 g of NH4NO3 and warm the contents to about 70oC on a water bath. Prepare the precipitant mixture by adding 10 ml of 20% ammonium molybdate to 10 ml of 7:3 (7 ml of conc. HNO3 + 3 ml of water) HNO3 in a 100 ml beaker.  Add 20 ml of the precipitant mixture slowly to the phosphate solution. Stir the contents gently without touching the sides of the beaker.  Keep the beaker on water bath at 70oC for half an hour and allow the canary yellow precipitate to settle, leaving a supernatant solution.

         Filter the supernatant solution through Whatman No.40 filter paper each time transferring the supernatant solution only and retaining as much precipitate as possible in the beaker.  Give washings with cold water alternately to the filter paper and the precipitate in the beaker using only 10 ml of water for each washing.  Add the next instalment of water to the filter paper only after the previously added water filtered completely.   Continue washings till the filtrate becomes free of acid (Collect half test tube full of filtrate and add a drop of 0.1619 N KOH and a drop of phenolphthalein Appearance of pink colour indicates free of acid).

         After making the precipitate acid-free, transfer the filter paper along with the precipitate to the same beaker in which precipitation was done and dissolve the yellow precipitate by adding known excess of 0.1619 N KOH.  Add 2-3 drops of phenolphthalein and titrate against 0.1619 N HNO3.  Calculate the amount of phosphorus in the solution and in-turn in the fertilizer from the amount of 0.1619 N KOH utilized to dissolve the ammonium phosphomolybdate precipitate.

Calculations

Total volume of the extract made up from the digested sample

:

250mL

Aliquot taken

:

25mL

Net amount of 0.01619N KOH consumed to dissolve the ammonium phosphomolybdate precipitate

:

Z mL

Therefore percentage of citrate insoluble P2O5

:

=

(Note : 0.1 N KOH and 0.1 NHNO3 can also be used instead of 0.1619 N acid and alkali. Then the factor for calculation will be 1 ml of 0.1 N KOH = 0.00 039 g of P2O5)