Citric acid-soluble P

ESTIMATION OF CITRIC ACID-SOLUBLE P IN DICALCIUM

PHOSPHATE / ROCK PHOSPHATE

Aim 

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

Principle

         The residue of the phosphate fertilizer remaining after leaching with distilled water for water-soluble 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

         Place the filter paper with the residue after leaching for water-soluble P estimation, in a 500 ml conical flask and loosely stopper it.  Add 100 ml of neutral ammonium citrate solution of sp. gravity 1.09 and keep the flask on a thermostat at 65oC for one hour, shaking the flask every 5 minutes.  Remove the flask from the thermostat and filter the contents immediately through a rapid filter paper (WhatmanNo.5).  Wash the residue on the filter paper with warm water (65oC).  Collect the filtrate in a 250 ml volumetric flask and make up the volume with distilled water.  Estimate the citrate-soluble P content in 25 ml of the extract by Pemberton method.

Calculations

Weight of fertilizer taken:1 g

Weight of the extract prepared:250 ml

Volume of the extract taken for P estimation:25 ml

Volume of 0.1619 N KOH added to dissolve the precipitate:x ml

Volume of 0.1619 N HNO3 used in back titration:y ml

Volume of 0.1619 N KOH utilized to dissolve the ammonium phosphomolybdate precipitate:x-y = z ml

1 ml of 0.1619 N KOH:0.0005 g of P2O5

z ml of 0.1619 N KOH:0.0005 x z  g of P2O5

This is present in 25 ml of aliquot

 In 250 ml of the ammonium citrate extract

:

 g of P2O5

This is present in 1 g of the fertilizer

In 100 g fertilizer ie., per cent of citric acid soluble P2O5 in the fertilizer

:

 g of 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 = 0.00039 g of P2O5)