22.214.171.124 TM-58, Determination of Phosphoric Acid (P2O5) in Commercial Fertilizers
This method determines the percent of phosphoric acid (P2O5) in commercial fertilizers by colorimetry.
126.96.36.199.1 Reagents and Apparatus
1) An Atomic Absorption Spectrophotometer or a suitable colorimeter.
2) Nitric Acid (HNO3), 1.42 specific gravity.
3) Hydrochloric Acid (HCl), 1.19 specific gravity.
4) Potassium Phosphate, Monobasic (KH2PO4), Primary Standard Grade, dried at 105-110°C for several hours prior to use.
5) Ammonium Molybdate ((NH4)6MO7O24•4H2O), Reagent Grade.
6) p-Methylaminophenol Sulfate, Reagent Grade.
7) Sodium Acid Sulfite (NaHSO3), Reagent Grade.
8) Cuvets, optical glass, matched, 10mm path length. (Note: If available, an optical glass, automatic flow-through cuvet may be used.)
188.8.131.52.2 Preparation of Standard Solutions
Phosphate Stock Solution: Weigh 0.4394 g of dried, primary standard KH2PO4 into a 500- mL volumetric flask. Add 30 ml of HNO3 and 5 ml of HCl and boil until brown fumes have been expelled. Dilute to volume with distilled water.
5 ppm Working Standard: Pipette a 5-ml aliquot of the phosphate stock solution into a 200-ml volumetric flask and dilute to volume with distilled water. This solution is equivalent to 57.2% phosphoric acid.
4 ppm Working Standard: Pipette a 4-mL aliquot of the phosphate stock solution into a 200-mL volumetric flask and dilute to volume with distilled water. This solution is equivalent to 45.8% phosphoric acid.
Blank Working Solution: Boil a mixture consisting of 5 ml of HCl and 30 ml of HNO3 until brown fumes have been expelled, transfer to a 500-ml volumetric flask and dilute to volume with distilled water. Transfer a 4-ml aliquot to a 200-ml volumetric flask and dilute to volume with distilled water.
184.108.40.206.3 Preparation of Color Development Solutions
Acid Molybdate Solution: Pour, with stirring, a solution containing 16.62 g of ammonium molybdate in 156.9 ml of distilled water into a solution of 318.4 ml of HCl to which 29.6 ml of distilled water has been added.
Reducing Solution: Dissolve 1.0 g of p-methylaminophenol sulfate and 3.0 g of sodium acid sulfite in 100 ml distilled water.
Weigh, to the nearest 0.1 mg, 0.50 ± 0.02 g of sample into a 500-ml volumetric flask, adding 30 ml of HNO3 and 5 ml of HCl. Boil until brown fumes have been expelled, cool to room temperature and dilute to volume with distilled water. Transfer a 2-ml aliquot to a 100-ml volumetric flask and dilute to volume with distilled water. From this second dilution, transfer a 10-ml aliquot to a 20-ml scintillation vial and add a 2-ml aliquot of acid molybdate solution and a 2-ml aliquot of reducing solution. Allow to stand 1 hour for color development. At the same time that the sample vial is prepared, a blank vial, a 4 ppm phosphorus vial and a 5 ppm phosphorus vial must also be prepared and allowed to develop color along with the sample. The blank, 4 ppm phosphorus and 5 ppm phosphorus are prepared by the same procedure used for preparation of the sample, using 10-ml aliquots of the blank working solution, the 4 ppm phosphorus working standard and the 5 ppm phosphorus working standard. (Note: When making determinations on fertilizers with low percentages of phosphoric acid, it is necessary to adjust the sample weight so that the sample working dilution contains between 4 ppm and 5 ppm phosphorus.)
Calibrate the instrument using the blank solution, the 4 ppm phosphorus solution and the 5 ppm phosphorus solution, then determine the concentration of the sample solution. (Note: When using an Atomic Absorption Spectrophotometer for color measurements, the cuvet holder is placed over the burner in the light path and a Cu hollow cathode lamp is used with the instrument grating set at a wavelength of 640 nm.)
The method of calculating the % phosphoric acid will vary according to the make and model of instrument used. Report the results to the nearest 0.1% P2O5 as follows:
- % Phosphoric Acid (P2O5)