Procedure

PROCEDURE

Precipitation

  • Take 10 ml of aliquot (soluble ash) in a 250 ml beaker.
  • Add sufficient distilled water to increase the volume.
  • 10 ml of Ammonium molybdate is taken in a test tube and gently warmed.
  • Like wise the content of the beaker is gently warmed. (Caution:- A temperature rise of 40 º C is sufficient. Never heat or boil the contents of the beaker).
  • Add the ammonium molybdate slowly to the contents in the beaker with uniform stirring (Avoid violent stirring or scratching the bottom of beaker with glass rod).
  • Complete the precipitation by adding the reagent and allow it to stand.

Filtration and Washing

  • Keep a clean volumetric flask (as in Calcium experiment) with funnel and fix a Whatman (No 42) filter paper in the funnel.
  • Fix the filter paper well before filtration.
  • Transfer the supernatant fluid to the filter paper.
  • As the filtration proceeds, continue transferring the supernatant liquid in small portions. When all the supernatant liquid is thus transferred to the funnel, wait for complete filtration.
  • The next step is washing of the precipitate.

Reason for washing the precipitate

  • The reagent Ammonium molybdate is made by mixing concentrated HNO3 with Molybdic acid and Ammonium hydroxide. When the reagent is added, the excess HNO3, will be physically bound to the precipitate. Since the estimation involves a reaction between the precipitate and standard NaOH, the excess acid has to be removed by means of suitable washing.

Washing of the precipitate

  • KNO3 (3%) is used to wash the acid from the precipitate. KNO3 is amphoteric in nature (i.e. it behaves both as acid and alkali).
  • Use a 10 ml pipette to transfer 20ml of KNO3 to the beaker containing the precipitate. Mix well. Allow it to settle.
  • When the previous filtration is complete, transfer the supernatant fluid from the beaker to the funnel for filtration. (Don’t start filtration unless the previous filtration is complete). This is essential for quick washing and accurate results).
  • Continue the washing and filtration till the precipitate is free from acid.

Test

  • Take a blue litmus paper, wet it with distilled water, and allow a drop of filtrate to fall on the blue litmus paper. If it turns red, the precipitate is still acidic and further washing is needed.

Dissolving the precipitate in NaOH (0.1N)

  • When the precipitate is free from acid, gently take the filter paper and put it in the beaker.
  • Wash the sides of the beaker with distilled water.
  • Add sufficient distilled water to immerse the filter paper so that it is soaked in water. With the help of glass rod gently macerate the filter paper and make it into pulp. Now wash the sides of the beaker with distilled water.
  • The entire content is now yellowish in colour.
  • Add known amount of standard NaOH (0.1N) till the yellow colour disappears and the contents become colourless except for the white filter paper residue.
  • Stir well; add a drop or two of phenolphthalein indicator.
  • Pink colour appears indicating the presence of excess alkali.

Back titration

  • Out of the NaOH added certain amount of NaOH has titrated with phosphorus precipitate. Excess of alkali gives the pink colour. The amount of alkali neutralised by the precipitate is found out indirectly by back titrating the excess of alkali with standard HCl (0.1N). From the total amount of alkali added, the excess is reduced and the actual amount of NaOH (0.1N) reacted with precipitate is found out. The end point in this titration is colourless (i.e. from pink to colourless). Note the initial and final readings of the burette.

Reactions

  • Precipitation of Phosphorus

Precipitation_of_Phosphorus

  • Titration with NaOH (0.1N)

(NH4)3 PO4 12 MoO3 +23 NaOH ® 11 Na2MoO4 + (NH4)2 MoO4 + NaNH4 HPO4 + 11H2O

Equivalents

  • 1ml of 0.1N HCl = 1ml of 0.1N NaOH = 0.0001347g P

Calculation

% phosphorus in feed = 0. 0001347 x V x (100 / Z) x (100 / W)

Here,

V = Titre value (volume of 0.1 N HCl used),

Z = Volume of soluble ash taken for precipitation,

W = Weight of the sample.

Stepwise procedure

  • Take 10 ml of aliquot (soluble ash) in a 250 ml beaker. view video
  • Add enough distilled water to increase the volume. view video
  • Warm the contents. Note: Do not heat the contents. A temperature of 40 º C is enough.
  • Take 10 to 15 ml of Ammonium molybdate. Warm the reagent. Add slowly to the aliquot in the beaker. Stir well. view video
  • Allow it to settle
  • Transfer the supernatant fluid to the filtering funnel. view video
  • When all the supernatant fluid is transferred, add 20 ml of KNO3 (3%) to the beaker. Mix well. Allow it to settle. view video
  • Transfer the supernatant portion to the funnel. Repeat the process of washing till the precipitate is free of acid. view video
  • Transfer the filter paper to the beaker. Add enough distilled water to immerse the filter paper.
  • With the help of glass rod crush and make the filter paper into a pulp. Wash the sides of the beaker. view video
  • Add known amount of standard NaOH till the yellow colour disappears. view video
  • Add one drop of phenolpthalein, the colour will be pink. view video
  • Titrate with standard HCl. Note the volume of HCl required. view video
  • Calculate as shown above.
Last modified: Saturday, 27 August 2011, 10:40 AM