Estimation of phosphorous in edible oil is one of the most important parameter to be ascertained. Two different methods were tried for analysis of phosphorous in different soybean oils and values from the different methods were compared against each other. The first method was spectrophotometric method converting phosphorous to phosphomolybdate complex and measuring the absorbance by spectrophotometer at wave length of 650 nm. Second method was by atomic absorption spectrophotometer using graphite furnace and the wave length selected was 213.6 nm. Both the methods were found to be accurate and the results were within standard deviation.


  • Distilled water
  • Concentrated Nitric acid
  • Concentrated Hydrochloric acid
  • Zinc oxide
  • Potassium hydroxide (50%)  –  50g KOH is dissolved in 50ml of distilled water.
  • Concentrated  Sulphuric acid
  • Sodium molybdate  – 144ml of Conc. H2SO4 is added to 300ml of distilled water.  12.5g of Sodium molybdate is added. Then volume was made upto 500ml. Mixed thoroughly and allowed to stand for 24 hrs before use.
  • Hydrazine sulphate  –  0.150g of hydrazine sulphate was dissolved in 1L of distilled water.
  • Lanthanum Chloride  –  1000ppm solution.
  • Phosphorus standard stock solution : 1.0967g of dry Potassium dihydrogen phosphate is dissolved in 250ml of distilled water. This solution contains 1000 ppm of P.


  • Atomic Absorption Spectrophotometer with Graphite Tube Atomizer (Varian make)
  • UV/Vis Spectrophotometer (Perkin Elmer make)
Method A : By Spectrophotometer


3 – 4 g of sample was taken in silica crucible and 0.5g of ZnO was added to it. The crucible was heated until the mass was completely charred. It was placed in a muffle furnace at 575 ± 10 oC for 2 hrs. Then the crucible was removed and cooled to room temperature. 5 ml water & 5ml HCl was added to the ash. The solution was gently heated to boiling for 5 minutes. Volume was made up to 100ml volumetric flask after filtration. The solution was cooled to room temperature and neutralized to a faint turbidity by drop wise addition of 50% KOH solution. Conc. HCl was added drop wise until the precipitate was just dissolved. An extra 2 – 3 additional drop of Conc. HCl was added.

10 ml of the clear solution was pipetted out into the dry 50ml volumetric flask. 8 ml of hydrazine sulphate solution & 2 ml of sodium molybdate solution were added.  The flask was heated for 10 minutes in a vigorously boiling water bath. Then it was cooled to 25 ±  5 oC. After cooling, volume of the solution was made upto the mark.  Finally absorbance of the solution was measured at 650 nm using Spectrophotometer. A reagent blank was carried without using oil sample.

Standard graph was prepared by using the following concentration of 0.1, 0.2, 0.4, 0.6, 0.8 and 1.0 ppm of phosphorous. Excellent linearity was observed (r2 = 0.9999).



                                            10 x ( A  –  B )

Phosphorous (%)  =      ———————-

W  x  V

A = Phosphorous content of sample aliquot in mg

B = Phosphorous content of the blank aliquot in mg

W = Weight of sample in mg

V = Volume of solution taken for colour development.

Method B : Atomic  Absorption Spectrophotometer


About 20g of oil sample was taken in a crucible and carefully charred in a heating mantle by adding few drops of concentrated HNO3. Then the sample was placed in a muffle furnace at 550 oC for 2 hrs. After cooling the crucible, 5 ml HNO3 was added and digested on a heater. Finally the solution was filtered through a Whatman 41 filter paper and volume was made upto 50ml mark. To the final test solution, 10µl of 1000ppm Lanthanum Chloride solution was added and mixed well. The solution was ready for analysis using AAS with GTA.

Standard calibration was done using 10.0, 50.0, 100.0 & 250.0ppm of working standard solution by diluting the stock standard.

Standard Curve of Phosphorous by AAS


Analytical Data:



Though spectrophotometric method is conventional method for determination of Phosphorus in vegetable oil, a good reproducibility has been observed in recovery values using the alternative method by AAS with Graphite furnace atomization.


Contributed by: Mr. Sudip Roy and Mr. Bamdev Sinha