An alkaline oxidation method for determination of Total Phosphorus in Soils
By: W.A. Dick and M.A. Tabutabai


Sodium Hypobromite Solution:
Add 3mL of bromine slowly to 100mL of 2M NaOH.

Formic Acid:  90% (Fisher Scientific)

Sulfuric acid (5N):  Add 145mL of concentrated H2SO4, AR quality, to about 700mL of H2O and dilute to 1 liter with H2O.

Ammonium Molybdate-Antimony Potassium Tartrate Solution (Reagent A)
Dissolve 12g of ammonium molybdate (J.T. Baker Chemical Co.) in 100mL of H2O. Add both solutions to 1 liter of 5N H2SO4, dilute to 2 liters with H2O, and mix the solution thoroughly. Store this reagent in a Pyrex glass bottle in a dark, cool place.

Ascorbic Acid Solution (Reagent B) :
Dissolve 1.056g of ascorbic acid (J.T. Baker Co.) in 200mL of Reagent A and mix the solution thoroughly. Prepare daily.

Standard Phosphate Solution:
Dissolve 0.2195g of potassium dihydrogen phosphate (KH2PO4) in about 700mL of H2O, and dilute to 1 liter. This contains 50ppm of orthophosphate P per mL. Should be refrigerated.

2M NaOH:
Put 80g NaOH in a liter volumetric flask. Dissolve the NaOH and dilute with 2D H2O [double-distilled deionized water?] to 1 liter.

1. ~.25g soil (<60 or 100 mesh).
2. Place soil in glass centrifuge tube.
3. Add 3mL of sodium hypobromite solution. IMPORTANT: Prepare hypobromite solution immediately before use. Can be used after several hours.
IMPORTANT: Add bromide slowly (approximately 0.5mL /min) and with constant stirring.
4. Swirl tubes for a few seconds and then let stand for 5 minutes.
5. Swirl again and place on sand bath (260-280 degrees C) for approximately 45 minutes. For first 30 minutes place gently into sand. Then push into sand (deep) for 15 minutes.
6. Remove and allow to cool.
7. Add 3mL of distilled H2O. Add 2mL of formic acid. Swirl gently.
IMPORTANT:  Make sure contents are in solution and not stuck to the sides or bottom of the tube.
IMPORTANT:  When swirling stopper the tubes. Be very careful. Gas will build up and the stopper may be “shot off”. Release the stopper every few seconds to allow for the gas to escape.
8. Add 25mL of 1.0N H2SO4. Stopper and shake carefully.
9. Place tubes in the centrifuge at 3000rpm for 10 minutes.
10. Aliquot 2mL from each tube into 25mL volumetric flask.
11. Add 4mL of ascorbic acid solution. (Reagent B)
12. Fill with distilled H2O to 25mL.
13. Stopper and shake contents.
14. After 30 minutes read on the Spec 20 at 720nm.
15. Record absorbance.

Soil Survey Laboratory Methods Manual. August 1992.  p.377.  U.S. Department of Agriculture.