Fundamentals of Soil Science (1+1)

Exercise 12: Determination of P₂O5 and K₂O

Determination of P₂O₅

Material required

Equipment and glassware

• Porcelain dish
• Water bath
• Beaker (500ml)
• Shaker
• Red litmus paper
• Whatman No. 44 filter paper

Reagent

• HCl
• H₂SO₄
• HNO₃
• Ammonia solution
• Molybdate solution
• Soodium nitrate
• NaOH
• Phenolphthalein indicator

Procedure

Preperation of extract

• Take 20-50 ml of HCl extract in a 50 ml porcelain glazed dish. Evaporate on a water bath and ignite at a dull read heat. Rub with a pestle and ignite again.
• Extract the well ignited mass with hot water and filter free from chlorides. Keep the residues on the filter paper for the estimation of P₂O₅ and the extract for potash.

Determination of P₂O₅

• Digest the residue from which the potassium is removed for half an hour on a water bath with 30 ml of 10% H2SO4 and filter free from acid.
• Transfer the filterate to a 400 ml beaker.
• Add 5-10 ml HNO₃ and then add ammonia until a precipitate that forms dissolve on shaking.
• Dilute to about 100 ml and warm to about 25 – 30^oC and put a piece of red litmus paper which should turn blue.
• Now add drop wise drop HNO₃(1+10) till it turns red again. Add sufficient amount (30 ml) of molybdate solution to ensure complete precipitation.
• Shake vigorously for five minutes, allow to stand for at least half an hour.
• Filter by decantation using Whatman No. 44 filter paper.
• Wash the precipitate with 2 % solution of sodium nitrate or ice cold water. The washing should be continued till the filterate is free from acid.
• It can be tested by adding a drop of standard alkali and a few drops of phenolphthalein.
• The filterate should have pink colour.
• Spread the filter paper on the sides of the beaker in which the precipitation was done.
• Wash it with a jet of distilled water. Dissolve in a known amount of N/10 NaOH.
• Titrate with N/10 H₂SO₄, using phenolphthalein indicator till the pink colour just disappears.
• Put the filter paper in the beaker and add more acid till the pinkish color which reappeared on the addition of filter paper disappears.

Calculate results assuming 1 ml N/10 NaOH = 0.000309 g P₂O₅
Determination of K₂O
Reagent

• HCl
• H₂SO₄
• Glacial acetic acid
• Sodium chloride
• Sodium nitrate
• Cobalt nitrate
• Alcohol
• KMnO₄
• Evaporate the water extract on the water bath after the addition of few drops of HCl
• When cold take up the residue in 1.5 ml glacial acetic acid and 10 ml of saturated sodium chloride solution.
• Stir well and after 3 to 4 minutes, add 5 ml of 35% NaNO₂ solution stirring again.
• When solution is complete say after 5-10 minutes, but not longer, add 5ml of 20% cobalt nitrate solution rapidly with constant stirring. It would not take more than 2-3 seconds.
• Stir for 40 to 60 seconds. Cover and allow it to settle overnight in a cool place free from fumes of ammonia.
• Filter through a gooch crucible charged with asbestos or use sintered glss crucible No. 4.
• Transfer the precipitate to the crucible and wash 4 times with 6-10 ml of 35% alcohol each time.
• Fially wash the sides of the crucible with 3 small lots of about 2 ml each of cold water to remove alcohol.
• Pipette out a suitable amount i.e. 50 ml of 0.05N KMnO₄ in a 400ml beaker.
• Dilute to about 150 ml with water and add 5 ml of H₂SO₄(1+10). Add the crucible and the precipitate in this mixture and keep it submerged in the solution warm gently.
• If the colour appears to be discharged add further known quantity of KMnO₄ solution to ensure an excess. Heat nearly to boiling.
• Remove from the flame and after few minutes add excess of 0.05N oxalic acid (50ml).
• Warm until all oxides of manganese have dissolved.
• Titrate excess of oxalic acid with 0.05N KMnO₄ used for the oxidation of the precipitate.

Calculations

Amount of 0.25% solution of potassium sulphate taken = 10ml
Amount of 0.05 N KMnO₄ solution added = 50ml
Amount of 0.05 N oxalic acid solution added = 50ml
Amount of 0.05 N KMnO₄ solution required for the excess of oxalic acid = y ml
Amount of 0.05 N KMnO₄ solution used for reacting with precipitate= x- y ml

Amount of K= Amount of K₂O x 72 / 94