ESTIMATION OF WATER-SOLUBLE P IN SUPERPHOSPHATE/

DICALCIUM PHOSPHATE / ROCK PHOSPHATE

Aim

         To estimate the content of water-soluble P in superphosphate / diammonium phosphate.

Principle

         Phosphatic fertilizer is leached with distilled water through Whatman No.40 filter paper. Phosphorous is then precipitated as ammonium phosphomolybdate in nitric acid medium, filtered, washed free of acid and dissolved in a known excess of alkali and the excess alkali is back titrated against nitric acid using phenolphthalein as indicator. From the volume of alkali consumed, P content of the sample is arrived at.

Reaction during precipitation

2H3PO4 + 24(NH4)2MoO4 + 42HNO3   → 2 (NH4)3PO4.12MoO3 + 42 NH4NO3 + 4H2O

Reaction during dissolution of precipitate

2(NH4)2PO4.12MoO3 + 46 KOH → 23 K2MoO4 + 2(NH4)2HPO4 + 22H2O

2(NH4)2HPO4  →  P2O5 + 4NH3 + 3H2O

Reaction during back titration

HNO3 + KOH* →KNO3 + H2O

      * Excess KOH not used in the dissolution of PO4 precipitate

    From the equation it is seen that two molecules of (NH4)3 PO4.12 MoO3 will be equivalent to one molecule of P2O5.

So, 46 molecules or 46 liters of 1N KOH   =142 g of P2O5

46 lit of N KOH=142 g of P2O5

1 lit of N KOH                                         =(142/46) g.  of P2O5 (or)

1000 ml of N KOH                                   =(142/46) g. of P2O5

1 ml of N/10 KOH=0.000309 g of P2O5

1 ml of 0.1619 N KOH                                           =0.0005 g of P2O5

    The nitric acid used in back titrating the excess of standard alkali is of the same strength as that of alkali i.e. 0.1619 N.

Reagents required

1.  Conc. HNO3

2.  0.1619 N HNO3

3.  0.1619 N KOH

4.  Ammonium nitrate

5.  Phenolphthalein indicator

Procedure

         Weigh accurately 1.0 g of superphosphate / DCP and transfer it to the funnel fitted with Whatman No. 40 filter paper and collect the filtrate in a 250 ml volumetric flask at the bottom. Leach the fertilizer with 10 ml portions of distilled water allowing each portion to drain before adding the next portion and collecting the leachate in the 250 ml volumetric flask.  Leaching must be continued all about 200 ml of leachate is collected in the volumetric flask within one hour.  Make-up the volume with distilled water after adding a few drops of concentrated sulphuric acid if the leachate is turbid.  Retain the filter paper along with the residue for the estimation of citrate-soluble phosphorus.

         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

Weight of fertilizer taken:1.0 g

Volume of the leachate made up:250 ml

Aliquot of the leachate taken for estimation:25 ml

Known excess  0.1619 N KOH added to the precipitate:x ml

Volume of 0.1619 N HNO3 used in back titration:y m l

Volume of 0.1619 N KOH actually used to dissolve the 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 fertilizer leachate 

\In 250 ml of the leachate=

 g of P2O5

This is present in 1 g of fertilizer

In 100 g SSP/DCP ie., % of P2O5 in the fertilizer

=

 g of P2O5

Things to Learn

i)           Phosphates are precipitated in HNO3 medium .Why?

ii)         What is the purpose of addition of solid NH4NO3?

iii)   How molybdic acid formation  is prevented during phosphate precipitation?

iv)   Write the principle involved in testing the filtrate for free of acid.

v)         Write the factor to convert P into P2O5.

vi)   Why do you employ 0.1619 N KOH and 0.1619 N HNO3 in P estimation?                

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