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Laboratory Test Procedures - Colormetrics

Susan P. Needham Test Method for Detecting the Presence of Phosphoric Acid in Asphalt


Phosphoric acid is used to increase the high temperature Performance Grading of hot mix asphalt to help resist the effects of traffic during summer temperatures.  For quality control purposes, the amount of phosphoric acid present in a binder can be measured using X-ray fluorescence spectroscopy. However, x-ray fluorescence spectrometers can be cost prohibitive for some agencies.

The Turner-Fairbank Highway Research Center (TFHRC) has developed a very simple procedure that requires just a few simple chemicals and no specialized equipment. While the new procedure to detect phosphoric acid does not measure the amount of phosphoric acid in the binder, it clearly demonstrates whether phosphoric acid is or is not present, and can detect levels as low as 0.1 percent. 


This qualitative test method can be used to identify the presence of phosphoric acid in asphalt. It is simple to do and requires no specialized equipment. If phosphoric acid is present in the binder, a blue color is developed in less than five minutes. The method will detect phosphoric acid at concentrations as low as 0.1 percent. The method is not quantitative. It does provide positive confirmation of the presence of phosphoric acid.

The presence of recycled engine oil bottoms (REOB) in the asphalt will interfere with the test. These residues, derived from the processing of waste engine oil, contain phosphorous residues from the oil additives. These will also give a blue color and may falsely indicate the presence of phosphoric acid. Other tests need to be conducted to differentiate between phosphoric acid and REOB. These involve the measurement of phosphorous, calcium and zinc levels by a technique such as X-ray fluorescence spectroscopy (XRF). For further assistance, please contact the TFHRC chemistry department at 202-493-3305.

While the test is very safe, users should observe appropriate chemical hygiene, operating, and disposal procedures. The Material Safety Data Sheets (MSDS) for the reagents should be studied before proceeding.

Summary of Method:

This method is adapted from ASTM test method D515-88 “Standard Test Methods for Phosphorus in Water.” Butanol is used to extract some of the acid present from the asphalt. The extracted acid is then transferred to the water phase. Its presence is detected by reaction with ammonium molybdate, potassium antimonyl tartrate, and ascorbic acid to form a blue color.


  1. Disposable, plastic transfer pipettes.
  2. One-ounce cans, disposable test tubes, or small beakers. Disposable cans or glassware is recommended to avoid contamination. If glassware is reused, it must be washed with hot hydrochloric acid (1+ 3), and rinsed with distilled water. Commercial detergents should not be used to clean the glassware as they contain phosphates, which will interfere with the result.


  1. Antimonyl tartrate/Ammonium molybdate Solution: Dissolve 0.13g of potassium antimonyl tartrate hydrate [C8H4K2O12Sb2∙H2O] in 50 ml of distilled water. Add 5.6g of ammonium molybdate [(NH4)6Mo7O24∙4 H2O] and swirl until dissolved.
  2. 1N Sulfuric Acid Solution: [H2SO4]. This can be purchased in 1-liter polyethylene bottles.
  3. Stock Solution Mixture: Mix #1 and approximately 950ml of #2 above. This can be done by adding solution #1 to the 1 liter of 1N sulfuric acid if there is sufficient space in the bottle. The exact amount of sulfuric acid is not critical. This solution is stable for 1 year.
  4. Ascorbic Acid Color Reagent: Dissolve 0.50 g of L-Ascorbic Acid [C6H8O6] in 100 ml of the Stock Solution Mixture (see #3 above). This reagent is stable for a week if stored at 4 °C; otherwise, prepare the reagent fresh daily as needed.
  5. iso Butanol: [(CH3)2CHCH2OH] (n-butanol also can be used).


  • Heat the asphalt and pour 1 to 2 grams into a 1-ounce can or other small container.
  • Place the container in an oven for ten minutes, to ensure the asphalt is fluid.
  • Remove the container and immediately add 2 ml of butyl alcohol. Stir well.
  • Add 2 ml of distilled water and stir.
  • Add 2 ml of the Ascorbic Acid Color Reagent and stir.
  • If phosphoric acid is present in the asphalt, a blue color will develop in the bottom layer within 5-10 minutes. (Decant into a second 1-ounce can if unable to see the color.)
  • When new reagents are prepared, a blank (asphalt with no phosphoric acid) should be prepared and run simultaneously.