106.3.2.58 TM-58, Determination of Phosphoric Acid (P2O5) in Commercial Fertilizers

This method determines the percent of phosphoric acid (P₂O₅) in commercial fertilizers by colorimetry.

106.3.2.58.1 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.)

106.3.2.58.2 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.

106.3.2.58.3 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.

106.3.2.58.4 Procedure

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.)

106.3.2.58.5 Calculations

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)