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University of California
Stable Isotope Laboratory
Earth and Planetary Sciences
552 Red Hill Road, E&MS C512
Santa Cruz, CA 95064

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Recipe for 100 % Phosphoric Acid

  • Download Phosphoric Acid Recipe hardcopy.


  • 1) The ingredients and equipment you will need for this procedure are:

    A: Ingredients needed:

    a. Four 500 gm. bottles of P2O5 (Phosphorous pentoxide).
    b. 4300 gm. (~5/8ths of a 4 liter bottle) of 85% H3P04 (Phosphoric Acid).
    c. 250 mg. Cr03 (Chromium trioxide).
    d. 3 ml. of 30% H202 (Hydrogen peroxide).

    B: Equipment needed:

  • Note: most of the equipment and ingredients needed for this recipe are found in a drawer below the Dionex ASE 200 or in the cabinet above it.

  • a. 1 heater I stirrer (lower cabinet below the water chiller) and a large diameter glass magnetic stir bar (located in C514 in the cabinet next to the drying oven).
    b. a glass encased (0-360C) thermometer. Note: Use a thermometer with the scale encased inside a glass sheath. The phosphoric acid will dissolve the scale markings on the thermometer if not protected.
    c. two 4 liter clean glass beakers.
    d. a scale to measure 4 kg. (Marine Analytical lab has a Mettler balance which measures to 2kg).
    e. A cylinder of argon or nitrogen gas. Usually you can run an Argon tube from the mass spectrometer room argon cylinder.
    f. a ring stand and three pronged clamp.
    g. glass stir rod ~18" long.
    h. aluminum foil.
    i. thermally insulating tape ( Waletex 1" heat protective tape found in a drawer below the Dionex in C514).
    j. Hydrometer (able to measure specific gravity = 1.92 g/cm3).

    2) In the acids fume hood, set up a 4 liter beaker on a heater/stirring plate and add 4300 gm. of 85% H3P04 (Phosphoric Acid). Use the scale in the Marine Analytical lab to weigh. Since the scale only weighs to 2kg, you will need to weigh out three aliquots to get 4.3kg. Use a ring stand and a clamp and suspend the 0-360íC thermometer into the beaker. Place the glass thermometer such that it will not be broken while adding and manually stirring in the Phosphorous Pentoxide during the next step.

    3) Dissolve 2010 gm. of P205 in 4300 gm. of 85% H3PO4 (Phosphoric Acid). Adjust glass magnet stir bar speed such that the magnet spinning and able to continue as the viscosity increases. Stir in P2O5 slowly allowing each bottle to be mostly dissolved before adding the next bottle. Initially adding heat will facilitate dissolution for the first bottles. However, be careful!, the reaction IS EXOTHERMIC! The temperature will rise to roughly 185C and then start cooling.

    4) Once the P205 has completely dissolved, add about 250 mg. of Chromium trioxide (CrO3), as an oxidant to drive oxidize organic matter. Caution: Chromium oxide is a poison.

    5) Cover the beaker in foil and insert the thermometer, which is held by the ring stand.

    6) Heat the solution to 200 plus/minus 5íC for seven hours (IMPORTANT STEP). Obey the timing and temperature requisites, the temperature should not get as high as 220C. The temperature will take time to reach 200C and will oscillate for many hours until it finally reaches equilibrium. This requires several hours of monitoring at intervals of every 15 minutes.

    7) After the seven hours, add 3 ml. of 30% H202 and increase the temperature to 220C. Heat at 220C for four hours. Note: in order for the heater to raise the temperature to 220C it will be necessary to wrap the 4 liter beaker with Waletex heat insulating tape.

    8) After 4 hours, cool the acid to room temperature while flowing argon or nitrogen gas on top of the surface. The breaker should still be covered with foil. This step can be left overnight.

    9) Measure the specific gravity, which should read 1.90-1.92 g/cm^3 @25íC if done correctly.

    10) Store in sealed 250 ml containers, sealed with parafilm.

    11) Optimally, the acid needs to age for about 60 days before it is usable.